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OBJECTIVES
After reading this chapter, you should be able to:
Name the three catecholamines secreted by the adrenal medulla and summarize their biosynthesis, metabolism, and function.
List the stimuli that increase adrenal medullary secretion.
Differentiate between C18, C19, and C21 steroids and give examples of each.
Outline the steps involved in steroid biosynthesis in the adrenal cortex.
Name the plasma proteins that bind adrenocortical steroids and discuss their physiologic role.
Name the major site of adrenocortical hormone metabolism and the principal metabolites produced from glucocorticoids, adrenal androgens, and aldosterone.
Describe the mechanisms by which glucocorticoids and aldosterone produce changes in cellular function.
List and briefly describe the physiologic and pharmacologic effects of glucocorticoids.
Contrast the physiologic and pathologic effects of adrenal androgens.
Describe the mechanisms that regulate secretion of glucocorticoids and adrenal sex hormones.
List the actions of aldosterone and describe the mechanisms that regulate aldosterone secretion.
Describe the main features of the diseases caused by excess or deficiency of each of the hormones of the adrenal gland.
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There are two endocrine organs in the adrenal gland, one surrounding the other. The main secretions of the inner adrenal medulla are the catecholamines epinephrine, norepinephrine, and dopamine; the outer adrenal cortex secretes steroid hormones.
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The adrenal cortex secretes glucocorticoids, steroids with widespread effects on the metabolism of carbohydrate and protein; and a mineralocorticoid essential to the maintenance of Na+ balance and extracellular fluid (ECF) volume. It is also a secondary site of androgen synthesis, secreting sex hormones such as testosterone.
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The adrenal medulla, which constitutes 28% of the mass of the adrenal gland, is made up of interlacing cords of densely innervated granule-containing cells that abut on venous sinuses. Two cell types can be distinguished morphologically: an epinephrine-secreting type that has larger, less dense granules; and a norepinephrine-secreting type in which smaller, very dense granules fail to fill the vesicles in which they are contained. In humans, 90% of the cells are the epinephrine-secreting type and 10% are the norepinephrine-secreting type. The type of cell that secretes dopamine is unknown.
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The adrenal cortex is divided into three zones: the outer zona glomerulosa is made up of whorls of cells that are continuous with the columns of cells that form the zona fasciculate, separated by venous sinuses. The inner portion of the zona fasciculata merges into the zona reticularis, where the cell columns become interlaced into a network. The zona glomerulosa makes up 15% of the mass of the adrenal gland; the zona fasciculata, 50%; and the zona reticularis, 7%. The adrenocortical cells contain abundant lipid, especially in the outer portion of the zona fasciculata. All three cortical zones secrete corticosterone, but the active enzymatic mechanism for aldosterone biosynthesis is limited to the zona glomerulosa, whereas the enzymatic mechanisms for forming cortisol and sex hormones are found in ...