Arachidonate (20:4)

Prostaglandins Thromboxanes Leukotrienes

Unlike the hormones described above, they are not synthesized in advance and stored; they are produced, when needed, from arachidonate enzymatically released from membrane phospholipids by phospholipase A2 (see Fig. 10-18). The enzymes of the pathway leading to prostaglandins and thromboxanes (see Fig. 21-15) are very widely distributed in mammalian tissues; most cells can produce these signals, and cells of many tissues can respond to them through specific plasma membrane receptors. The eicosanoid hormones are paracrine hormones, secreted into the interstitial fluid (not primarily into the blood) and acting on nearby cells.

Prostaglandins promote the contraction of smooth muscle, including that of the intestine and uterus (and can therefore be used medically to induce labor). They also mediate pain and inflammation in all tissues. Many antiinflammatory drugs act by inhibiting steps in the prostaglandin synthetic pathway (see Box 21-2). Thromboxanes regulate platelet function and therefore blood clotting. Leukotrienes LTC4 and LTD4 act through plasma membrane receptors to stimulate contraction of smooth muscle in the intestine, pulmonary airways, and trachea. They are mediators of the severe immune response called anaphylaxis. ■

Steroid Hormones The steroid hormones (adrenocorti-cal hormones and sex hormones) are synthesized from cholesterol in several endocrine tissues.



Cortisol (glucocorticoid)


Aldosterone (mineralocorticoid)

Estradiol (sex hormones)

They travel to their target cells through the bloodstream, bound to carrier proteins. More than 50 corti-costeroid hormones are produced in the adrenal cortex by reactions that remove the side chain from the D ring of cholesterol and introduce oxygen to form keto and hydroxyl groups. Many of these reactions involve cy-tochrome P-450 enzymes (see Box 21-1). The steroid hormones are of two general types. Glucocorticoids (such as cortisol) primarily affect the metabolism of carbohydrates; mineralocorticoids (such as aldosterone) regulate the concentrations of electrolytes in the blood. Androgens (testosterone) and estrogens (such as estra-diol; see Fig. 10-19) are synthesized in the testes and ovaries. Their synthesis also involves cytochrome P-450 enzymes that cleave the side chain of cholesterol and introduce oxygen atoms. These hormones affect sexual development, sexual behavior, and a variety of other reproductive and nonreproductive functions.

All steroid hormones act through nuclear receptors to change the level of expression of specific genes (p. 465). Recent evidence indicates that they also have more rapid effects, mediated by receptors localized in the plasma membrane.

Vitamin D Hormone Calcitriol (1,25-dihydroxycholecal-ciferol) is produced from vitamin D by enzyme-catalyzed hydroxylation in the liver and kidneys (see Fig. 10-20a).

7-Dehydrocholesterol juV light

Vitamin D3 (cholecalciferol)

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