The Endocrine System
323
Hormones of the Gonads
Compared to the adrenal cortex, the gonads have very dif-
ferent concentrations of key enzymes in their steroid path-
ways. Endocrine cells in both the testes and the ovaries lack
the enzymes needed to produce aldosterone and cortisol. They
possess high concentrations of enzymes in the androgen path-
ways leading to androstenedione, as in the adrenal cortex. In
addition, the endocrine cells in the testes contain a high con-
centration of the enzyme that converts androstenedione to tes-
tosterone, which is therefore the major androgen secreted by
the testes (
Figure 11–7
). The ovarian endocrine cells synthe-
size the female sex hormones, which are collectively known as
estrogens
(estrone and estradiol).
Estradiol
is the predomi-
nant estrogen present during a woman’s lifetime. The ovarian
endocrine cells have a high concentration of the enzyme aro-
matase, which transforms androgens to estrogens (see Figure
11–7). Thus, estradiol, rather than testosterone, is the major
steroid hormone secreted by the ovaries.
Very small amounts of testosterone do leak out of ovar-
ian endocrine cells, however, and very small amounts of estra-
diol are produced from testosterone in the testes. Moreover,
following their release into the blood by the gonads and the
adrenal cortex, steroid hormones may undergo further con-
version in the blood or organs. For example, testosterone is
converted to estradiol in some of its target cells. Thus, the
major male and female sex hormones—testosterone and estra-
diol, respectively—are not unique to males and females. The
relative concentrations of the hormones, however, are quite
different in the two sexes.
Finally, endocrine cells of the corpus luteum, an ovar-
ian structure that arises following ovulation, secrete another
major steroid hormone,
progesterone.
This steroid is criti-
cally important for uterine maturation during the menstrual
cycle, and for maintaining a pregnancy (Chapter 17).
Hormone Transport in the Blood
Most peptide and all catecholamine hormones are water-
soluble. Therefore, with the exception of a few peptides,
these hormones are transported simply dissolved in plasma
(
Table 11–2
). In contrast, the poorly soluble steroid hor-
mones and thyroid hormones circulate in the blood largely
bound to plasma proteins.
Even though the steroid and thyroid hormones exist in
plasma mainly bound to large proteins, small concentrations
of these hormones do exist dissolved in the plasma. The dis-
solved, or free, hormone is in equilibrium with the bound
hormone:
Free hormone + Binding protein
34
Hormone-protein complex
The total hormone concentration in plasma is the sum of
the free and bound hormones. It is important to realize, how-
ever, that only the
free
hormone can diffuse out of capillar-
ies and encounter its target cells. Therefore, the concentration
of the free hormone is what is biologically important rather
than the concentration of the total hormone, most of which
Cholesterol
Androstenedione
Aromatase
Aromatase
(see Fig. 11–5)
Estrone
Estradiol
Secreted
by ovaries
Secreted
by testes
Testosterone
Figure 11–7
Gonadal production of steroids. Only the ovaries have high
concentrations of the enzyme (aromatase) required to produce the
estrogens estrone and estradiol.
Table 11–2
Categories of Hormones
Chemical
Class
Major Form in
Plasma
Location of
Receptors
Most Common
Signaling Mechanisms*
Rate of Excretion/
Metabolism
Peptides and
catecholamines
Free (unbound)
Plasma membrane
1. Second messengers
(e.g., cAMP, Ca
2+
, IP
3
)
Fast (minutes)
2. Enzyme activation by receptor
(e.g., JAK)
3. Intrinsic enzymatic activity of receptor
(e.g., tyrosine autophosphorylation)
Steroids and thyroid
hormone
Protein-bound
Intracellular
Intracellular receptors directly alter gene
transcription
Slow (hours
to days)
*The diverse mechanisms of action of hormones were discussed in detail in Chapter 5.
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