CHAPTER
Endocrine Metabolism III:
Adrenal Glands
The adrenal glands, a pair of well-vascularized glands
positioned bilaterally above the cranial poles of the
kidney, consist of two embryologically, histologically,
and functionally distinct regions. The outer region of
each (
adrenal cortex)
accounts for about 80% of the
weight of the gland, is derived from the coelomic epithe-
lium of the urogenital ridge (mesodermal), and produces
steroid hormones. The inner core of each gland
(adrenal
medulla)
is derived from the neural crest (neuroectoder-
mal), represents a modified sympathetic ganglion that has
assumed an endocrine function, and synthesizes and se-
cretes catecholamines and enkephalins. Neural supply
by preganglionic sympathetic fibers from the splanchnic
nerve is mainly to the adrenal medulla. Blood is supplied
mainly to the adrenal cortex via the inferior phrenic, celiac,
and renal arteries and is drained from the adrenal medulla.
32.1 Adrenal Cortex
The adult adrenal cortex is divided into three zones his-
tologically: the outermost
zona glomerulosa,
the middle
zona fasciculata,
and the inner zona
reticularis.
All three
zones are exclusive steroid producers; together, they can
produce all classes of steroid hormones. Under normal
conditions the major products are
1 1
-hydroxylated C
21
A list of expanded acronyms appears as Appendix VIII.
steroids (corticosteroids) (Table 32-1). The two types of
corticosteroids are the
mineralocorticoids,
which regulate
sodium and potassium levels, and the
glucocorticoids,
which regulate carbohydrate metabolism. The major
mineralocorticoid, aldosterone (4-pregncn-l 1/3,21-diol-
3.18.20- trione; see Figure 30-1), is produced exclusively
by the zona glomerulosa, which contains the unique
mitochondrial enzyme, aldosterone synthase (CYP1 IB2).
The major glucocorticoid in humans and some other
species
is
cortisol
(or
hydrocortisone)
(4-pregnen-
11 /3,17«,21-triol-3,20-dione; see Figure 30-1), which is
produced exclusively by the zona fasciculata (major) and
zona reticularis (minor). These two inner zones contain
17-a-hydroxylase (CYP17) but lack enzymes to convert
corticosterone to aldosterone. Figure 32-1 reveals that
regional differences in steroid production exist because
of the enzyme distribution.
Aldosterone and cortisol are not the only adrenal
steroids with mineralocorticoid and glucorticoid activi-
ties. Corticosterone (4-prcgncn-l 1
/3,21 -diol-3,20-dione),
a glucocorticoid about one-fifth as active as cortisol,
has little mineralocorticoid activity relative to aldos-
terone; 11-deoxycorticosterone (DOC) (4-pregnen-21-ol-
3.20-
dione) has about one-tenth the mineralocorticoid
activity of aldosterone but is devoid of glucocorticoid ac-
tivity (Table 32-2). Both are intermediates in the biosyn-
thesis of aldosterone (Figures 30-2 and 32-1), but under
normal conditions, circulating corticosterone and DOC
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