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Department of Internal Medicine III (M.E.-B.), University of Leipzig, 04103 Leipzig, Germany; Department of Biochemistry (J.P.H., G.P.V.), Queen Mary and Westfield College, London E14NS, England; National Institute of Child Health and Human Development (S.R.B.), National Institutes of Health, Bethesda, Maryland 20892; and Diabetes Research Institute at the Heinrich Heine University (W.A.S.), 40225 Düsseldorf, Germany
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Abstract |
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 Top Abstract I. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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I. Introduction: The Adrenal Functional Unit |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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As a result of intensive study in recent years, we now know that the regulatory mechanisms that account for such discrepancies are mainly located within the adrenal itself, and that several different components of the gland contribute to these functions. The adrenal produces a wide variety of hormones, neuropeptides, neurotransmitters, and cytokines, and it is evident that the colocalization of these different systems has a profound functional significance. The cells within the adrenal thus communicate with each other and adapt the function of the gland to different situations. The integrated control of adrenocortical function involves cortico-medullary interactions, the gland’s vascular supply, its neural input, the immune system, growth factors, and the intraglandular renin-angiotensin and CRH-ACTH systems. As well as directly regulating adrenocortical function, these systems influence each other and form complex intraadrenal regulatory circuits.
We here survey the mechanisms involved in the intra-glandular regulation of adrenocortical function and show how they combine to ensure that the gland functions as a unit.
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II. Interaction Between Adrenal Medulla and Adrenal Cortex |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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Adrenomedullary chromaffin cells originate from neural crest precursor cells that migrate into the adrenal "anlagen" and later differentiate into chromaffin cells in the adrenal medulla under the influence of adrenocortical steroids (1, 2). The main secretory products of these differentiated cells are the catecholamines epinephrine and norepinephrine. In addition, chromaffin vesicles contain numerous transmitters, neuropeptides, and proteins, which may be released together with the catecholamines (3).
A. Relationship between medullary and cortical cells
Within the adult tetrapod adrenal, cortical and chromaffin cells
are united in one gland. The organization of these two cell types
varies among different species. In nonmammalian species chromaffin
cells may be distributed in islets, as in amphibia and birds, or
concentrated toward one pole of the gland, as in reptiles (4). In
mammals, the conventional view holds that the two different endocrine
tissues are clearly separated into an outer steroid-producing cortex
and a central medulla (4, 5, 6, 7). However, this is an oversimplification,
and the distribution of these cells may not be so clearly demarcated,
but may instead involve closer contact than previously thought.
The occurrence of medullary cells in the zona glomerulosa of adult rats
was first observed nearly 30 yr ago (8, 9). Although slow to receive
general acceptance, it is now widely acknowledged that chromaffin cells
can be found in all zones of the adult adrenal cortex, either radiating
through the cortex from the medulla (8, 9, 10, 11, 12) or distributed as islets or
single cells. Medullary cells may also spread into the subcapsular
region, where they form larger nests of chromaffin cells (8, 11, 12, 13).
Conversely, cortical cells are also located in the medulla, where they
may form islets either surrounded by chromaffin tissue or retaining
some contact to the rest of the cortex. The adrenal medulla appears, in
part, to be peppered with cortical cells (12, 14) (Fig. 1
). This intimate intermingling of the
two cell types allows extensive contact zones for paracrine
interaction.
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illustrates the exocytosis of a
chromaffin vesicle from a medullary cell located in the rat zona
glomerulosa (15), demonstrating the possible paracrine action of a
chromaffin cell on an adrenocortical cell. The close anatomical
colocalization forms the basis for possible interactions of the two
endocrine systems.
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1. Epinephrine, norepinephrine, and serotonin (5-HT).
Neuroendocrine regulation of this type may depend on neurotransmitters
released from nerve endings in the adrenal cortex (see Section
III) or on the chromaffin cell products, epinephrine and
norepinephrine. These catecholamines are secreted in response to
splanchnic nerve stimulation and may influence adrenocortical function
(references in Table 1). For example, the
secretion of cortisol, aldosterone, and androstenedione was found to be
stimulated by perfusion of the isolated porcine adrenal glands with
epinephrine or norepinephrine (22, 24). In addition to this acute
effect, catecholamines have a long-term effect on corticosteroid
release from isolated adrenocortical cells, involving transcriptional
regulation of steroid enzymes (27, 28). Interestingly, epinephrine and
norepinephrine had the opposite effect in the frog adrenal, inhibiting
steroid secretion (29). These data provide evidence for a direct
influence of medullary products on cortical function.
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).
2. Neuropeptides. However, in addition to catecholamines,
adrenomedullary chromaffin cells produce, store, and secrete a whole
series of neuropeptides (Table 2). The first neuropeptides to be
discovered in the adrenal medulla were the enkephalins (37), which are
by number the most prominent neuropeptides in the chromaffin vesicles
(3). In addition, many other neuropeptides are costored with
adrenomedullary catecholamines (Table 2;
for review, see Refs. 3, and 38–40).
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). Some peptides
released from the adrenal medulla, however, may exert an inhibitory
influence on steroidogenesis. These include atrial natriuretic peptide,
which is thought to be involved in the regulation of aldosterone
secretion, somatostatin, dynorphin, substance P, neuropeptide Y (NPY),
and enkephalins (see Table 1 for references). However, some of these
apparently inhibitory peptides, such as substance P and the
enkephalins, are stimulatory under other conditions. The intact
architecture of the adrenal is necessary for the action of
adrenomedullary peptides that stimulate adrenocortical function via the
release of catecholamines from the medulla, i.e., pituitary
adenylate-cyclase activating peptide (PACAP) (44), vasoactive
intestinal peptide (VIP) (45), substance P (46), adrenomedullin (47),
and NPY (48) (Table 1). Some neuropeptides synthesized in chromaffin
cells, such as atrial natriuretic peptide and somatostatin, influence
preferentially the release of mineralocorticoids in the zona
glomerulosa. This correlates well with the morphological observations
confirming the occurrence of chromaffin cells in the subcapsular area
of the zona glomerulosa.
Coculture systems of bovine adrenomedullary chromaffin cells with
bovine adrenocortical cells have now supplied evidence for the
paracrine influence of chromaffin cells on adrenocortical cells. The
secretory products of chromaffin cells stimulate steroidogenesis both
when they are in direct contact with adrenocortical cells (Fig. 3) and also in a system in which the cell
types are separated by a semipermeable membrane (49).
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). If different medullary products are able to either
stimulate or inhibit adrenocortical function, mechanisms should exist
that differentially regulate their synthesis and release. In fact,
multiple populations of chromaffin cells exist within the medulla,
which vary in their peptide composition (51, 52, 53), depending on the
interactions between the calcium,
protein kinase A, and protein kinase C-signaling pathways (54, 55) and,
presumably, on the concerted action of different neurotransmitters
released from splanchnic nerves. These could act via various second
messenger pathways as shown for acetylcholine and VIP (55) or humoral
or immune factors (56, 57, 58, 59, 60), perhaps of either intraadrenal or
extraadrenal origin. Interestingly, interleukin (IL)-1
and tumor
necrosis factor (TNF)- differentially regulate Met-enkephalin, VIP,
neurotensin, and substance P biosynthesis in chromaffin cells (58).
Given the local production of these cytokines by adrenocortical cells
(see Section VI), a complete regulatory circuit may exist
within the adrenal, with the adrenal cortex releasing cytokines that
may influence the peptide composition of chromaffin granules.
Adrenomedullary secretory products in turn are able to differentially
stimulate or inhibit adrenocortical function.
C. Gap junctions in the adrenal cortex
Although the adrenal cortex and medulla are highly interwoven,
only a subpopulation of adrenocortical cells lies in direct contact
with adrenomedullary cells and therefore under the influence of
adrenomedullary secretory products. A prompt adrenocortical response to
stimulation (or inhibition), however, requires an effective cellular
communication system. Gap junctions occur within all zones of the
mammalian adrenal cortex (61, 65, 671, 672, 673), and active gap junctions
have been shown to form between adrenocortical cells in primary culture
(65, 66). Furthermore, gap junctions are induced relatively rapidly
after ACTH stimulation, suggesting their pivotal role in hormone
response. Gap junctions allow small, water-soluble molecules to pass
directly from the cytoplasm of one cell to the cytoplasm of the other,
and the intracellular signal may thus be propagated from stimulated to
unstimulated cells, as observed in dispersed rat adrenocortical cells
(67). Thereby, gap junctions could couple the cells both electrically
and metabolically and may help to coordinate the function of the gland,
especially in response to locally restricted stimulation.
D. Summary
In conclusion, the data presented in this chapter show that
the medulla and cortex are interdispersed to various degrees in
mammalian adrenals much as in nonmammalian vertebrates. In addition,
these two different tissues interact by complex regulatory circuits.
Increasing evidence exists that the close colocalization is the
prerequisite for paracrine interactions within the adrenal. Various
adrenomedullary secretory products, such as catecholamines, 5-HT, and a
whole series of neuropeptides, are involved in the regulation of
adrenocortical steroidogenesis, by either stimulating or inhibiting
adrenocortical function. Under basal conditions, stimulatory
influences seem to be dominant. However, the influence of
adrenomedullary secretory products on adrenocortical functions is
complex and probably varies in different physiological situations. The
complexity in the intraadrenal regulation of steroid secretion is
increased by the fact that several factors may interact by addition,
potentiation, or antagonism of their effects. In turn, adrenocortical
secretory products, the steroid hormones and cytokines released by the
cortex, influence the neuropeptide, protein, and catecholamine
expression in medullary chromaffin cells. It remains to be elucidated
how the different adrenomedullary secretory products are involved in
the adjustment of adrenocortical functions to the needs of the body
and to the maintenance of homeostasis under different
conditions.
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III. Innervation of the Adrenal Cortex |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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In addition to the well described afferent innervation of the adrenal cortex, there is also a less well documented efferent innervation. The presence of both chemoreceptors and baroreceptors in the adrenal gland has been reported (82, 83). This efferent innervation has been implicated in the contralateral hypertrophic response to adrenal damage (84).
B. The source of adrenocortical innervation
There is
evidence that the afferent innervation of the adrenal cortex derives
from two distinct sources (85): one source is the adrenal
medulla, and it has been suggested that the adrenal cortex may
originally have been a target organ for adrenomedullary postganglionic
nerves. During evolution, as the cortex and medulla became more
closely associated, the postganglionic fibers terminated entirely
within the adrenal gland, forming the chromaffin tissue and
innervating the cortical cells (86). The second source of
nerves supplying the adrenal cortex is not clear, but these nerves
appear to have their cell bodies outside the adrenal gland and enter
the gland along blood vessels (85). It has been
demonstrated that the adrenal nerves entering the gland contain both
pre- and postganglionic fibers (87, 88), and
presumably these postganglionic fibers supply the adrenal cortex.
Intraadrenal nerve fibers, originating in the medulla and
innervating the outer layers of the cortex, have been described in the
rat adrenal gland in particular. The most extensively studied
neuropeptide has been VIP. Hökfelt and co-workers (89) described an intrinsic adrenal VIP-ergic neural system
with cell bodies in the medulla and varicose fibers in the zona
glomerulosa. This finding has been confirmed by others (90) and extended to include reports of other intrinsic adrenal
peptidergic nerves, including CGRP-, substance P-, and neuropeptide
Y-containing nerves (91, 92, 93). In addition to a wide range of neuropeptides,
catecholamines (85, 94) and acetylcholine
(95) have been identified as transmitters in nerves
supplying the adrenal cortex of different mammalian species (Table
3; for reviews see Refs. 38 and 96), although the
source of this innervation is often not clear. The term
‘intrinsic’ is frequently used to describe nerve fibers
innervating the adrenal gland. This term is most properly used to
describe neurons that are wholly contained within the adrenal gland,
whose cell bodies can be clearly identified, and should not be
extended to include other, less defined, forms of
innervation. Unfortunately, it is not always clear in reports of
adrenocortical innervation whether the innervation is
intrinsic.
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D. Role of the splanchnic nerve in
regulating adrenocortical neural function
It is not clear to
what extent the splanchnic nerve regulates adrenocortical nerve
activity. Although it has been found that stimulation of the
splanchnic nerve causes the release of several neuropeptides into the
adrenal venous effluent, it is not possible to determine what
proportion might be contributed by the cortical innervation, compared
with the adrenal medulla. As may be seen in Section I, the
same neurotransmitters found in nerve fibers innervating the cortex
(Table 3) are also present in the chromaffin cells of
the adrenal medulla (Table 2). There have been,
however, studies that have directly addressed the question of the role
of the splanchnic nerve in regulating cortical innervation, although
the results are difficult to interpret.
Hökfelt and colleagues (89) found that the VIP-ergic intraadrenal nerves were independent of splanchnic nerve activity: after extrinsic adrenal denervation (achieved by cutting the splanchnic nerve and periarterial nerves and removing the celiac ganglion) VIP-immunoreactive fibers persisted in the rat adrenal cortex, and indeed no difference was reported in levels of immunoreactive VIP between control and adrenal denervated rats. Holzwarth (90), on the other hand, found that the VIP immunoreactive adrenal nerves with cell bodies in the medulla were regulated by splanchnic nerve activity, with an increase in VIP immunostaining after splanchnic nerve ligation. After adrenal enucleation and cortical regeneration, however, the number of VIP-ergic fibers in the cortex was greatly reduced. A recent study, in which immunoreactive cortical and medullary VIP were measured, reported that, while medullary VIP content was significantly decreased after splanchnic nerve section, there was no effect on cortical VIP (98). The problem with all of these studies is that in each, neuropeptide content is measured at a single point in time since dynamic studies of this type of system are not accessible using current methodology. The question that is posed by these data is: What does a change in neuropeptide content signify? If cytochemical studies can demonstrate a population of neurons that disappears after splanchnic nerve section, then it seems reasonable to conclude that those neurons are regulated by the splanchnic nerve. However, when the amount of a transmitter present increases after nerve ligation, it is questionable whether this reflects increased synthesis or decreased release of the transmitter (85). The converse is also true: if the amount of neuropeptide present decreases, is it because the nerve has degenerated or has its activity increased?
It is difficult to distinguish between intrinsic and extrinsic adrenocortical nerves and equally difficult to determine the influence of the splanchnic nerve on adrenocortical innervation. Most of our understanding of the role of the adrenocortical innervation in regulating adrenocortical function has, therefore, come from functional studies.
E. Influence of adrenal
innervation on adrenocortical function
Several aspects of
adrenocortical function have been shown to be influenced by adrenal
innervation. Adrenal growth in response to damage of the contralateral
adrenal gland (or in response to unilateral adrenalectomy), termed
‘compensatory adrenal hypertrophy,’ has been shown to be
mediated by both adrenal afferent and efferent nerves, via the
hypothalamus (84). This effect is independent of
splanchnic nerve activity (84) and is not associated with
an increase in adrenal neuropeptide content (98).
Most functional studies, however, have investigated the effects of splanchnic nerve section on adrenal responses. An account of the role of the splanchnic nerve in the regulation of adrenal blood flow is given in Section IV. Several studies in different species, including dog, calf, and sheep, suggest that splanchnic nerve activity regulates adrenocortical sensitivity to ACTH stimulation: sectioning the splanchnic nerve decreases the adrenal response to ACTH (21, 99), while stimulation enhances it (20, 100). In the pig, isolated perfused adrenal, splanchnic nerve stimulation increases the secretion of aldosterone, cortisol (22), and androstenedione (24). It is likely that the effects of stimulating the nerves supplying the adrenal gland are mediated by the local release of neurotransmitters, either from nerve endings or from medullary cells.
Adrenal innervation has also been implicated in regulating the diurnal variation in cortisol secretion (16, 17).
F. Effects of transmitter substances
on adrenocortical function
Many studies have directly
addressed the question of the functional role of the different
neurotransmitters identified in neurons supplying the adrenal
cortex. The results of these studies have implicated the various
neurotransmitters in a wide range of effects. Many have been shown to
directly stimulate steroidogenesis by dispersed adrenal cells, or by
the intact perfused rat adrenal preparation. Others appear to
stimulate the growth of the adrenal cortex, or to modulate the
adrenocortical response to humoral stimulation. This subject has been
recently reviewed by several groups (38, 76, 96, 101, 102, 103, 104). Clearly it is difficult to distinguish
between the effects of medullary chromaffin cells and direct adrenal
innervation, as the same transmitters may be produced by both. The
effects of different transmitters are summarized in Table 1.
G. Summary
It is clear that the
adrenal cortex receives a direct innervation, at least partly derived
from the splanchnic nerve. The neurotransmitters identified in nerves
supplying the cortex have a variety of actions, from direct effects on
growth and steroidogenesis, to modulation of the actions of humoral
stimuli on the adrenal cortex. This innervation also has a role in
regulating the vasculature (see Section IV). It would appear
that adrenocortical innervation has a role in fine tuning the
functions of the adrenal cortex.
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IV. The Vascular System of the Adrenal Gland |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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) and receives a high
proportion of the cardiac output relative to its size. The rat
adrenal, for example, comprises approximately 0.02% of the total body
weight, but receives around 0.14% of the cardiac output (105). The vascular supply to the adrenal gland has been
described in several species, and while the general arrangement of
blood vessels is similar, it is clear that there are some important
variations between species, particularly in relation to the degree of
independence of cortical and medullary blood supply (43, 106, 107, 108). In
general, the adrenal gland is supplied by small arterioles that arise
from the aorta, and from the renal and inferior phrenic
arteries. Blood is supplied to a network of arterioles in the
connective tissue capsule, referred to as the subcapsular arteriolar
plexus, from which it is distributed into two types of vessel: the
thin-walled sinusoids that supply the cortex and the medullary
arteries that convey blood directly to the adrenal medulla. The
numbers of these medullary arteries vary greatly among species (63), with the dog having a relatively large number and the rat
very few (109). This is likely to be the reason for
differences in the regulation of adrenal cortical and medullary blood
flow (43).
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ACTH is well known to stimulate increases in adrenal blood flow (105, 116, 117, 118). Although it has been suggested that ACTH exerts its effects by constricting medullary arteries and diverting blood into the cortical sinusoids (106), studies in the perfused rat adrenal preparation have suggested that this is not the case: ACTH administration results in an overall increase in the flow of perfusion medium, suggesting a decreased vascular resistance within the gland (62). A mechanism has been proposed to account for the adrenal vascular effects of ACTH: mast cells are known to be present within the adrenal capsule, particularly where it is penetrated by the adrenal arteries. Mast cells contain histamine and 5-HT, both potent vasoactive compounds, which are released by the action of ACTH and cause adrenal vasodilatation. Evidence in support of this hypothesis was obtained by using disodium cromoglycate, an inhibitor of mast cell degranulation. This agent abolishes the vascular effects of ACTH, suggesting that mast cell degranulation is an essential step in the adrenal vascular response to ACTH (119, 120).
The recently identified secretory products of the vascular endothelium—nitric oxide, endothelin-1, and adrenomedullin—have also been implicated in the local regulation of adrenal blood flow. Decreasing endogenous nitric oxide production by administration of an inhibitor of nitric oxide synthase, such as L-NAME causes a decrease in the rate of blood flow through both the adrenal cortex and medulla in the dog (121) and a decrease in perfusion medium flow rate through the perfused rat adrenal gland (122). Perfusing the rat adrenal gland with medium lacking L-arginine, the substrate for nitric oxide synthesis, has a similar vasoconstrictive effect, which is reversed by administration of L-arginine (122), suggesting that nitric oxide exerts a tonic vasodilatory effect in this tissue. The presence or absence of nitric oxide does not significantly affect the vascular action of ACTH (122).
Endothelin also appears to regulate adrenal vascular tone. Administration of an endothelin receptor (ETA) antagonist causes adrenal vasodilation, suggesting that endothelin-1 exerts a tonic vasoconstrictor effect on the adrenal vasculature (122). Adrenomedullin stimulates perfusion medium flow in the intact perfused rat adrenal model (47). The interactions between these agents, which produce an integrated control of adrenal blood flow, remain to be elucidated.
B. Relationship between blood flow and steroid secretion
A relationship between the rate of blood flow through the adrenal
gland and the rate of glucocorticoid secretion was proposed in the
early 1950s by Hechter and co-workers (123). Further
studies in dogs, rats, and calves have shown that, in the presence of
submaximal concentrations of ACTH, there is a clear relationship
between adrenal blood flow and steroid secretion (123, 124, 125, 126). It has been shown that,
under these conditions, increased blood flow causes an increase in the
rate of presentation of ACTH to the adrenal gland, and it has been
suggested that it is the rate of ACTH presentation, rather than the
absolute concentration of ACTH present, that is the major determinant
in the adrenocortical response (124). However, a
relationship between adrenal blood flow and steroid secretion exists
even in the absence of ACTH (62, 127). In the
isolated perfused rat adrenal preparation, which has been used to
address this question, there is a significant correlation between
perfusion medium flow rate and corticosterone secretion that is seen
in the absence of ACTH and is found when flow rate is changed either
mechanically or by the use of vasodilators. Clearly, increased flow
rates in this preparation have implications for the delivery of oxygen
and the removal of steroid products, but this is an effect that is
unique to the intact adrenal gland: collagenase-dispersed cells
superfused on a column simply do not respond in the same way to
changes in flow (62). This suggests that the effect of
changes in blood (or perfusion medium) flow on steroid secretion are
mediated by an element present in the intact gland that is lost when
cells are dispersed. This factor is most likely to be one of the
secretory products of the cells of the vascular endothelium.
C. Effects of vascular endothelial cell products on steroid
secretion
Within the adrenal cortex the arrangement of blood
vessels is such that nearly every adrenocortical cell is directly
adjacent to a vascular endothelial cell (Fig. 4). In
addition to their actions on the adrenal vasculature, the secretory
products of endothelial cells exert significant effects on adrenal
function, which appear to be independent of their vascular effects
(Table 4). Endothelin-1 has been shown to stimulate
aldosterone secretion in several species, including frog (128), cow (129, 130), rabbit (131), rat (132, 133), and human (132). In the calf it appears that endothelin acts through the
ETA receptor subtype to stimulate aldosterone secretion
(130). In the rat, however, it is not clear which
receptor subtype is involved in aldosterone secretion since both
subtypes are present in the zona glomerulosa (134, 135, 136, 137); some authors report no
effect of ETA antagonists (135), while others
have found that both receptor subtypes are involved (137, 138) or that ETA receptors mediate
most of the effect of endothelin (137). In the human
adrenal cortex, both endothelin receptor subtypes are present in the
zona glomerulosa (139, 140) while only
ETB receptors are present in the inner zones (139, 140).
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Endothelin-1 also stimulates glucocorticoid secretion by inner zone cells from rat and human adrenals (132, 136) by interacting with ETB receptors (134, 136). There is evidence that endothelin-1 is released from vascular endothelial cells in response to shear stress and increased perfusion pressure (144, 145). Since the rat adrenal gland releases endothelin-1 in response to increased perfusion medium flow rates, and also in response to ACTH, which is a vasodilator in this tissue (146), it may be concluded that endothelin-1 mediates at least some of the effects of increased blood flow on steroid secretion (63, 132).
It has been suggested that nitric oxide also has effects on adrenal steroid secretion. The presence or absence of L-arginine, the substrate for nitric oxide synthesis, does not alter rat adrenal glucocorticoid responsiveness to ACTH stimulation, although the basal rate of steroid output is much lower in the absence of endogenous nitric oxide synthesis (122). More recently it has been reported that the nitric oxide synthase inhibitor, L-NMMA (L-NG monomethyl arginine), inhibits the aldosterone response to ACTH stimulation in rat capsular tissue (147), although L-NAME does not have this effect in the perfused rat adrenal (122). In intact rats the administration of inhibitors of nitric oxide synthesis attenuated the adrenal response to angiotensin II in anephric rats but did not affect basal aldosterone secretion (148).
There is some disagreement in the literature as to whether adrenomedullin stimulates (47, 149, 150) or inhibits (151, 152) aldosterone secretion. Nussdorfer and co-workers (47, 151) reported inhibition of stimulated aldosterone secretion using collagenase-dispersed adrenal cells but stimulation of basal aldosterone using an intact adrenal preparation. It has been reported, however, that adrenomedullin stimulates aldosterone and cAMP production by rat adrenal capsular tissue and zona glomerulosa cells (149, 153). The stimulatory effect of adrenomedullin appears to be mediated by specific adrenomedullin receptors (153), while its inhibitory actions appear to be mediated by the CGRP receptor (151). Differential expression of adrenomedullin and CGRP receptor may explain the discrepancies between the findings of different research groups. The related peptide PAMP (proadrenomedullin N-terminal 20 peptide) may have more potent inhibitory effects (154), although stimulatory actions have also been reported (153). It has also been shown that adrenomedullin inhibits angiotensin-stimulated aldosterone secretion in human adrenal cells (155) but stimulates inner zone function in the rat adrenal (47), although this effect may be secondary to vascular actions.
It has been established that adrenomedullin is produced within the adrenal gland, by both zona glomerulosa and chromaffin cells (153), although it is not yet clear how its secretion is regulated. Like endothelin-1, adrenomedullin may also have a role in mediating the steroid response to vascular events.
D. Summary
The regulation of
adrenal blood flow is complex and probably involves a range of humoral
and local mediators. There is clearly a close relationship between
vascular events and steroid secretion, which may be mediated by the
vascular endothelium in a paracrine manner. At present, the most
likely agent is endothelin-1, which appears to satisfy the criteria
for such a mediator. The evidence regarding adrenomedullin remains
unclear at present, although that which is currently available
suggests that the effects of this peptide differ according to the
receptor subtype it activates.
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V. The Intraadrenal CRH/ACTH System |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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A. Extrapituitary effect of CRH
The release of
glucocorticoids is regulated via the
hypothalamo-pituitary-adrenocortical axis, with CRH and ACTH as the
respective secretagogues. Ovine CRH was purified and sequenced in 1981
(159, 160), and initial data on a
dexamethasone-nonsuppressible, probably extrapituitary effect of CRH
on adrenocortical steroidogenesis were published by De Souza and Van
Loon in 1984 (161). Data from different experimental
models have verified that CRH influences adrenocortical
steroidogenesis independently from the HPA-axis. High doses of CRH had
a trophic effect on the adrenal cortex of hypophysectomized rats (162), and CRH exerted a substantial steroidogenic effect on
the adrenals in functionally hypophysectomized calves (163). In addition, CRH enhances the adrenal response to
ACTH. When CRH was combined with a subeffective dose of ACTH, a marked
dose-dependent increase in corticosterone release was observed from
rat adrenals (164). In healthy humans, cortisol secretion
was stimulated either by insulin or CRH. Interestingly, the ratio
between the cortisol increment and the ACTH increment was higher after
stimulation with CRH than with insulin (165), indicating
a direct effect of CRH on the adrenal or via other factors that
stimulate cortisol release.
Experiments in which the effect of
CRH was neutralized also gave some evidence for the extrapituitary
influence of CRH on the adrenal cortex. Immunoneutralization of CRH
reduced resting corticosterone levels in rats without inducing
concomitant reduction in plasma ACTH (166), and an
antibody to CRH reduced the adrenal response to ACTH in
dexamethasone-treated rats (164). In hypophysectomized
rats, a subcutaneous infusion of -helical-CRH or
corticotropin-inhibiting peptide, competitive inhibitors of CRH and
ACTH, respectively, evoked a further significant lowering of plasma
corticosterone concentration and markedly enhanced adrenal atrophy
(167).
Is the influence of CRH due to its direct effect of this peptide on the adrenal cortex, or is it mediated by ACTH or another peptide? A direct effect of CRH on adrenocortical steroidogenesis seems unlikely, as CRH had no effect on either isolated, dispersed adrenocortical cells (164) or on adrenocortical autotransplants deprived of chromaffin tissue (168, 169). However, in isolated perfused adrenal glands in situ, the perfusion rate was increased when CRH and a substimulatory dose of ACTH were given together, although neither CRH nor ACTH alone had an effect on the flow rate (164). Thus, the CRH-enhanced adrenal response to ACTH may result from their synergistic action on adrenal blood flow. In another experimental model, CRH enhanced corticosterone secretion in rat adrenal slices containing both cortex and medulla but had no effect on adrenocortical autotransplants that lack chromaffin tissue (168, 169). Although the exact mechanism is not yet clear, the effect is indirect rather than direct, and the intact architecture of the adrenal seems to be mandatory for the stimulatory action of CRH on the adrenal cortex. In this context, two possibilities suggest themselves. First, CRH may act with ACTH directly on intraadrenal blood vessels, and second, the medulla may respond to CRH by releasing a secretagogue that stimulates adrenocortical function.
B. Intraadrenal ACTH
Given that the adrenal medulla is necessary for the action of CRH
on the adrenal cortex, which medullary product mediates the effects of
CRH? The stimulatory effect of CRH on corticosterone secretion by rat
adrenal slices, containing both cortex and medulla, was annulled by
corticotropin-inhibiting peptide (168, 169),
indicating the involvement of intraadrenal ACTH. Indeed, ACTH
immunoreactivity was found in extracts of rat (170) and
human (171, 172, 173) adrenal
medulla, and in the adrenal venous effluent of hypophysectomized
calves in response to splanchnic stimulation (174). In
addition, adrenal fragments composed mainly of chromaffin cells
released ACTH in response to high concentrations of CRH (168, 169). In patients with proven pituitary ACTH
deficiency who received ACTH replacement therapy, subsequent CRH
administration induced a significant increase in plasma cortisol that
was preceded by a rise in plasma ACTH (175). Taken
together, these data suggest that the adrenal medulla is a source of
extrapituitary ACTH that can be stimulated by CRH.
C. Intraadrenal CRH and CRH receptors
Plasma levels
of CRH are probably too low to stimulate a significant release of ACTH
from the adrenal medullary. Within the adrenal itself, CRH-like
immunoreactivity with ACTH-releasing activity has been detected in the
medulla of cattle (176, 177), dogs (178), humans (171, 172, 173), and rats (170, 179, 180). This adrenal CRH has proved to be identical to the
hypothalamic CRH (171, 173) and to be released
in response to physiological stimuli, such as hemorrhage (181), splanchnic nerve stimulation (182),
K+-induced depolarization, nicotine (180), neurotensin (183), vasopressin (184), neuropeptide K and neurokinin A (185), and
IL-1ß (170, 180, 186). The
immunohistochemical localization of CRH revealed a distinct
CRH-immunoreactive subpopulation of medullary cells in canine, with
dense clusters of these cells located at the boundary between the
medulla and cortex (178, 181). In sheep,
immunostaining revealed a network of CRH-containing cells and varicose
fibers in the adrenal cortex. Mono- and bipolar cells were found
throughout the cortex, most densely at the corticomedullary junction
(187). These nerves were found to be associated with
medullary cells at the medullary-cortical interface, islands of
medullary cells in the cortex, or with rays of medullary cells
extending into the cortex (188). This provides strong
morphological evidence for a local intraadrenal CRH/ACTH system that
is involved in the regulation of adrenocortical function.
The adrenal CRH/ACTH system is completed by the demonstration of CRH receptors in the monkey adrenal where they were located exclusively in the adrenal medulla with no staining of adrenocortical cells (189, 190). These receptors are coupled to adenylate cyclase and stimulate the secretion of catecholamines and Met-enkephalin (189).
D. Feedback
mechanisms
The central CRH/ACTH system is regulated via
feedback mechanisms. Similar feedback mechanisms have also been
described for the adrenal CRH/ACTH system. In rats, cortisol treatment
lowered adrenal ACTH secretion after 2 days of treatment, and adrenal
CRH content after 7 days (179). In contrast, the
intraadrenal content of CRH and ACTH immunoreactivity increased in
hypophysectomized rats in direct relation to the number of days after
hypophysectomy. ACTH infusion, at a rate that restored its normal
blood concentration, prevented the effect of hypophysectomy on
intraadrenal ACTH and CRH concentrations (191), and in
hypophysectomized calves, ACTH reduced adrenal CRH output (182). These findings suggest that the elimination of the
central CRH/ACTH system induces a marked increase in the activity of
the intraadrenal system, and that regulation of adrenal CRH and ACTH
is achieved through feedback inhibition through the end products ACTH
and glucocorticoids. Evidence exists that some neuropeptides
[e.g., PACAP (192, 193) and
neuromedin U8 (194)] may stimulate adrenocortical
steroid secretion by activating the intraadrenal CRH/ACTH
system.
E. Summary
In conclusion, a
complete CRH/ACTH system exists within the adrenal, with the adrenal
medulla and/or intraadrenal nerves as a source of CRH and adrenal
medullary chromaffin cells as the target for this local releasing
hormone and the source of ACTH. It is possible that the adrenal
CRH/ACTH system is important in the overall function of the gland, and
it would seem that as for other products of the medulla (see
Section I) the close interrelationship of adrenocortical and
chromaffin cells is of great importance.
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VI. Immune Cells and Cytokines in the Adrenal Gland |
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TopAbstractI. Introduction: The Adrenal...II. Interaction Between Adrenal...III. Innervation of the...IV. The
Vascular System...V. The
Intraadrenal CRH/ACTH...VI. Immune Cells and...VII. Peptide Growth Factors...VIII. The Intraadrenal Renin...IX. Clinical ImplicationsX. ConclusionsReferences |
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A. Source of cytokines
within the adrenal
It is generally accepted that plasma levels
of immune-derived cytokines are too low to account for a direct effect
on adrenal function. If secretory products of the immune system
influence adrenal function, these factors should therefore be produced
within the adrenal itself. The two main sources of cytokines within
the adrenal are the local immune cells and the adrenal cells
themselves.
1. Immune cells. The adrenal cortex is
extensively infiltrated by macrophages under normal conditions (205, 206). Macrophages, shown immunohistochemically
to be phagocytic, are found primarily in the zona reticularis close to
the medulla (206). In addition to their phagocytotic
functions, stimulated macrophages are able to secrete a variety of
products, such as cytokines, including IL-1, IL-6, and TNF (207), or peptides, such as VIP (208) and
transforming growth factor-ß (TGFß) (209, 210), that can influence adrenocortical function. In the
adrenal, macrophages have been shown to produce IL-1 (211), IL-6 (212), and TNF (213). Since different cytokines may be stimulatory or
inhibitory, the regulation of adrenocortical function by
monocytes/macrophages may depend on their differential release,
although how this is controlled is unclear. Supporting this view,
monocytes may stimulate cortisol production by human cells through a
non-ACTH factor (214), whereas
lipopolysaccharide-stimulated murine macrophages produce a factor(s)
that inhibit the action of ACTH on rabbit adrenocortical cells in
vitro (215, 216).
The intraadrenal regulatory loop may be completed by the sympathetic regulation of macrophages, through their high numbers of ß-receptors (217) and the inhibition of cytokine secretion by cortisol (218). The macrophage seems to be a key player in the bidirectional immune-adrenocortical communication within the adrenal.
Lymphocytes have also been reported to produce and secrete ACTH-like substances (219), although the levels of extrapituitary, lymphocyte-derived ACTH are probably too low to account for a significant stimulation of adrenal steroidogenesis, even in response to viral infection (220). However, at least one case has been reported in which ectopic ACTH production by a granulomatous mass led to Cushing’s syndrome (221). In adrenals from elderly patients, T lymphocyte infiltration seems to be a regular phenomenon (222), thus making T lymphocyte-derived ACTH more relevant to the regulation of adrenal function. Thus, lymphocytes may also exert a paracrine influence on adrenocortical function during aging or in pathological situations.
2. Cytokines produced by adrenal cells.
Adrenal cells themselves are also able to produce cytokines (Table
5), and although there is some conflicting evidence,
there is agreement that several cytokines are produced, predominantly
by adrenocortical cells.
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. Within the rat adrenal, both
of these cytokines were found in the zona glomerulosa, with only small
amounts in the zona reticularis/fasciculata (227, 228). The release of
these two cytokines is regulated by secretagogues for corticosteroids
such as angiotensin II (229) and ACTH (228, 230), as well as by typical
regulators of immune function such as IL-1, IL-1ß,
lipopolysaccharide (228, 231), or 5-HT (232). As in the rat, IL-6 is
produced by human adrenocortical cells, and its mRNA is expressed
mainly in the zona reticularis and in steroid-producing cells located
within the medulla with little staining in the outer zones of the
cortex (212). There is conflicting evidence concerning the production
of TNF in the human adrenal gland. Using RIA, TNF was found in
the fetal but not in the adult adrenal (233). In contrast, TNF mRNA
was localized by in situ hybridization in the
steroid-producing cells of the zona reticularis in adult adrenals
(213).
Interferon-
-inducing factor (IGIF, also called IL-1 or IL-18) is
a recently identified cytokine with pleiotropic functions. This
cytokine is also produced by rat adrenocortical cells, specifically in
the zona reticularis and fasciculata, and its gene expression is
induced by cold stress (234). Although no effect of this cytokine on
adrenocortical function has yet been shown, it may well turn out to be
an additional intra-adrenal cytokine that modulates adrenocortical
steroido-genesis.
B. Cytokines that influence the adrenal cortex
There is considerable evidence that cytokines directly influence
adrenocortical function (summarized in Table 6).
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and -ß directly affect adrenal
function, which has been demonstrated in different experimental models.
The involvement of an ACTH-independent mechanism in IL-1-induced
corticosteroid secretion was recently shown by Kapcala et
al. (235), who found that vagotomy attenuated plasma ACTH levels
that were stimulated by intraabdominal IL-1ß whereas the
corticosterone response was unaltered. This is in accord with a
stimulatory effect of IL-1 on the release of glucocorticoids in
hypophysectomized rats (186), from in situ perfused rat
adrenals (236), rat adrenal slices in vitro (186, 237), and
from isolated rat (237, 238, 239), bovine (240), and human (214) adrenal
cells. There are conflicting data on the mode of action of IL-1 on
adenocortical cells, but three possible mediators have been suggested:
1) prostaglandins, since the addition of indomethacin, a cyclooxygenase
inhibitor, completely abolished the stimulatory effect of IL-1 (236, 239, 240); 2) the intraadrenal CRH/ACTH system, since IL-1 has no
stimulatory action on adrenocortical preparations lacking chromaffin
cells, and its stimulatory effect is annulled by blockade of the
intraadrenal CRH/ACTH system (186); and 3) catecholamines released from
the medulla (237, 238). However, IL-1 receptors have not so far been
demonstrated on adrenomedullary cells (240, 241). In contrast to its
stimulatory action on glucocorticoids, IL-1 inhibits angiotensin
II-induced aldosterone secretion (242, 243). 2. IL-2. Like IL-1, IL-2 also stimulates the release of corticosterone from isolated rat adrenal cells (239). This effect is accompanied by increased cAMP and prostaglandin E2 accumulation. The occurrence of IL-2 receptors was immunohistochemically demonstrated in rat adrenal cells in culture (244), and in this study the expression of the IL-2 receptor was enhanced by incubation with IL-2, but attenuated by dexamethasone.
3. IL-6. IL-6 alone and in synergism with ACTH stimulates the release of corticosterone from rat adrenocortical cells (245), an effect that may be mediated by prostaglandins (239). In humans, IL-6 activates the HPA axis by stimulating the release of cortisol and ACTH. Interestingly, in these studies plasma levels of ACTH decreased after long-term application of IL-6, whereas cortisol remained elevated, suggesting a direct effect of IL-6 on the adrenal cortex (246, 247). The IL-6 receptor is located on human adrenocortical cells, predominantly in the zona reticularis and inner zona fasciculata, and IL-6 stimulates corticosteroid release from human adrenal cells in primary culture, with a prominent effect on the release of adrenal androgens (248, 249). The effect of IL-6 on adrenal androgen production is of interest for two reasons. First, in view of the discrepancy in plasma ACTH levels and androgen release at adrenarche, and in several other clinical situations (250), IL-6 may be a local factor in the production of C19-steroids. Second, as IL-6 is expressed in the zona reticularis and stimulates DHEA secretion, it may be involved in local immune functions of the adrenal.
4. TNF. TNF is a 17-kDa polypeptide hormone that is
produced by a wide variety of tissues (251). Together with IL-1 and
IL-6, TNF accounts for most of the HPA-axis-stimulating activity in
plasma (203). In the adrenal, however, TNF inhibits the angiotensin
II- and ACTH-induced release of aldosterone from rat adrenal cells
(243) as well as basal and ACTH-stimulated cortisol production. TNF
also influences P450 enzyme expression in human fetal adrenals (233, 252) with a consequent shift to androgen production (233). In contrast
to its inhibitory action on isolated adrenal cells, TNF had a
direct, ACTH-independent, stimulatory effect on corticosterone
secretion in rats with cholestasis due to bile duct resection (253). In
cultured human fetal adrenal cells, TNF inhibits basal and
ACTH-induced insulin-like growth factor (IGF) expression (254, 255),
indicating its involvement in growth and differentiation of the fetal
adrenal (see Section VII). The direct, intraadrenal
inhibitory effect of TNF therefore contrasts with its effects in the
intact animal, indicating a role of local, intraadrenal produced
TNF, which is different from the role of circulating inflammatory
TNF.
5. Interferons. Interferons are a family of polypeptides that
disrupt viral replication in infected cells. In addition, they have a
wide variety of effects in immunological processes (256). Two members
of this family have been shown to influence adrenal steroidogenesis.
Interferon- stimulates corticosterone secretion from rat adrenal
cells in primary culture (257). In contrast, interferon- inhibits
basal and ACTH-induced IGF-II gene expression in human fetal adrenal
cells (254,
