322
Chapter 11
hormone called
angiotensin II,
which acts on plasma mem-
brane receptors in the adrenal cortex to activate the inositol
trisphosphate second messenger pathway. Note that this is
different from the more common cAMP-mediated mecha-
nism by which steroid hormones are produced, as previously
described. Once synthesized, aldosterone enters the circula-
tion and acts on cells of the kidneys to stimulate sodium and
water retention, and potassium and hydrogen excretion in
the urine.
Cortisol
and corticosterone are called
glucocorticoids
because they have important effects on the metabolism of glu-
cose and other organic nutrients. Cortisol is the predominant
glucocorticoid in humans, and so we will not deal with cor-
ticosterone in future discussions. In addition to its effects on
organic metabolism, cortisol exerts many other effects, includ-
ing facilitation of the body’s responses to stress and regulation
of the immune system (Section D).
Dehydroepiandrosterone (DHEA) and androstenedione
belong to the class of hormones known as
androgens,
which
also includes the major male sex hormone,
testosterone,
produced by the testes. All androgens have actions similar to
those of testosterone. Because the adrenal androgens are much
less potent than testosterone, they are of little physiological
signifi cance in the adult male. They do, however, play roles in
the adult female, and in both sexes in the fetus and at puberty,
as described in Chapter 17.
The adrenal cortex is not a homogeneous gland but is
composed of three distinct layers (
Figure 11–6
). The outer
layer—the zona glomerulosa—possesses very high concen-
trations of the enzymes required to convert corticosterone to
aldosterone, but lacks the enzymes required for the formation
of cortisol and androgens. Thus, this layer synthesizes and
secretes aldosterone but not the other major adrenal cortical
hormones. In contrast, the zona fasciculata and zona reticu-
laris have just the opposite enzyme profi le. They, therefore,
secrete no aldosterone but much cortisol and androgen.
In certain disease states, the adrenal cortex may secrete
decreased or increased amounts of various steroids. For
example, an absence of the enzymes for the formation of
cortisol by the adrenal cortex can result in the shunting of
the cortisol precursors into the androgen pathway. (Look
at Figure 11–5b to imagine how this might happen.) In a
woman, one result of the large increase in androgen secre-
tion would be
masculinization
.
Zona
glomerulosa
Zona
fasciculata
Cortex
Medulla
Zona
reticularis
Aldosterone
Cortisol
and
androgens
Epinephrine
and
norepinephrine
Cortex
Medulla
Figure 11–6
Section through an adrenal gland showing both the medulla and the zones of the cortex, as well as the hormones they secrete.
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