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Steroid Hormones
- Steroid hormones make up the third family of hormones.
- Steroid hormones are primarily produced by the adrenal cortex and the gonads (testes and ovaries), as well as by the placenta during pregnancy.
- In addition, vitamin D is enzymatically converted in the body to an active steroid hormone.
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The general process of steroid hormone synthesis
- The general process of steroid hormone synthesis is illustrated in Figure 11.6a.
- In both the gonads and the adrenal cortex, the hormone-producing cells are stimulated by the binding of an anterior pituitary gland hormone to its plasma membrane receptor.
- These receptors are linked to Gs proteins, which activate adenylyl cyclase and cAMP production.
- The subsequent activation of protein kinase A by cAMP results in phosphorylation of numerous intracellular proteins, which facilitate the subsequent steps in the process
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Steroid hormone continued
- All of the steroid hormones are derived from cholesterol, which is either taken up from the extracellular fluid by the cells or synthesized by intracellular enzymes.
- The final steroid hormone product depends upon the cell type and the types and amounts of the enzymes it expresses.
- Once formed, the steroid hormones are not stored in the cytosol in membrane-bound vesicles, because the lipophilic nature of the steroids allows them to freely diffuse across lipid bilayers.
- As a result, once they are synthesized, steroid hormones diffuse across the plasma membrane into the circulation.
- Because of their lipid nature, steroid hormones are not highly soluble in blood. Consequently, the majority of steroid hormones are reversibly bound in plasma to carrier proteins such as albumin and various other specific proteins.
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Hormones of the Adrenal Cortex
- The five major hormones secreted by the adrenal cortex are aldosterone,
- cortisol, corticosterone, dehydroepiandrosterone (DHEA), and androstenedione (Figure 11.6b).
- (b) The five hormones shown in boxes are the major hormones secreted from the adrenal cortex.
- Dehydroepiandrosterone (DHEA) and androstenedione are androgens—that is, testosterone-like hormones. Cortisol and corticosterone are glucocorticoids, and aldosterone is a mineralocorticoid that is only produced by one part of the adrenal cortex
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Hormones of the adrenal cortex: Aldosterone
- Aldosterone: is known as a mineralocorticoid because its effects are on salt (mineral) balance, mainly on the kidneys’ handling of sodium, potassium, and hydrogen ions.
- Briefly, production of aldosterone is under the control of another hormone called angiotensin II, which binds to plasma membrane receptors in the adrenal cortex to activate the inositol trisphosphate second-messenger pathway.
- This is different from the more common cAMP-mediated mechanism by which most steroid hormones are produced, as previously described.
- Once synthesized, aldosterone enters the circulation and acts on cells of the kidneys to stimulate Na+ and H2O retention, and K+ and H+ excretion in the urine.
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Hormones of the adrenal cortex: cortisol
- Cortisol and the related but less functional steroid corticosterone are called glucocorticoids because they have important effects on the metabolism of glucose and other organic nutrients.
- Cortisol is the predominant glucocorticoid in humans and is the only one we will discuss.
- In addition to its effects on organic metabolism, cortisol exerts many other effects, including facilitation of the body’s responses to stress and regulation of the immune system.
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Hormones of the adrenal cortex: DHEA and androstenedione
- Dehydroepiandrosterone (DHEA) and androstenedione belong to the class of steroid hormones known as androgens; this class also includes the major male sex steroid testosterone, produced by the testes.
- The adrenal androgens are much less potent than testosterone, and they are usually of little physiological significance in the adult male.
- They do, however, have functions in the adult female and in both sexes in the fetus and at puberty.
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The adrenal cortex
- The adrenal cortex is composed of three distinct layers (Figure 11.7).
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- The cells of the outer layer—the zona glomerulosa— express the enzymes required to synthesize corticosterone and then convert it to aldosterone but do not express the genes that code for the enzymes required for the formation of cortisol and androgens.
- Therefore, this layer synthesizes and secretes aldosterone but not the other major adrenocortical hormones.
- In humans, the zona fasciculata primarily produces cortisol and the zona reticularis primarily produces androgens, but both zones produce both types of steroid.
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In certain diseases, the adrenal cortex may secrete decreased or increased amounts of various steroids
- For example, the absence of an enzyme required for the formation of cortisol by the adrenal cortex can result in the shunting of the cortisol precursors into the androgen pathway.
- One example of an inherited disease of this type is congenital adrenal hyperplasia (CAH).
- In CAH, the excess adrenal androgen production results in virilization of the genitalia of female fetuses; at birth, it may not be obvious whether the baby is phenotypically male or female.
- Fortunately, the most common form of this disease is routinely screened for at birth in many countries and appropriate therapeutic measures can be initiated immediately.
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