Chapter 20: The Adrenal Medulla & Adrenal Cortex

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.

INTRODUCTION

There are two endocrine organs in the adrenal gland, one surrounding the other. The main secretions of the inner adrenal medulla (Figure 20–1) are the catecholamines epinephrine, norepinephrine, and dopamine; the outer adrenal cortex secretes steroid hormones.

The adrenal medulla is in effect a sympathetic ganglion in which the postganglionic neurons have lost their axons and become secretory cells. The cells secrete when stimulated by the preganglionic nerve fibers that reach the gland via the splanchnic nerves. Adrenal medullary hormones work mostly to prepare the body for emergencies, the so-called “fight-or-flight” responses.

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, which can exert effects on reproductive function. Mineralocorticoids and the glucocorticoids are necessary for survival. Adrenocortical secretion is controlled primarily by adrenocorticotropic hormone (ACTH) from the anterior pituitary, but mineralocorticoid secretion is also subject to independent control by circulating factors, of which the most important is angiotensin II, a peptide formed in the bloodstream by the action of renin.

ADRENAL MORPHOLOGY

The adrenal medulla, which constitutes 28% of the mass of the adrenal gland, is made up of interlacing cords ...