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The Adrenal Gland Theodore C Ch066.qxd 10/8/03 7:07 PM Page 603 Chapter 66 XI The Adrenal Gland Theodore C. Friedman Angiotensin II is the predominant regulator of aldos- Physiology terone secretion, but plasma potassium concentration, plasma volume, and ACTH levels also influence aldos- The adrenal glands lie at the superior pole of each terone secretion. ACTH also mediates the circadian kidney and are composed of two distinct regions: the rhythm of aldosterone; as a result, the plasma concen- cortex and the medulla. The adrenal cortex consists of tration of aldosterone is highest in the morning. Aldos- three anatomic zones: the outer zona glomerulosa, terone binds to the type I mineralocorticoid receptor. In which secretes the mineralocorticoid aldosterone; the contrast, cortisol binds to both the type I mineralocor- intermediate zona fasciculata, which secretes cortisol; ticoid and type II glucocorticoid receptors, although the and the inner zona reticularis, which secretes adrenal functional binding to the former receptor is limited by androgens. The adrenal medulla, lying in the center the intracellular enzyme 11 -hydroxysteroid dehydro- of the adrenal gland, is functionally related to the b genase (11 -HSD) type II, which catabolizes cortisol to sympathetic nervous system and secretes the cate- b inactive cortisone. The availability of cortisol to bind to cholamines epinephrine and norepinephrine in response the glucocorticoid receptor is modulated by 11 -HSD to stress. b type I, which interconverts cortisol and cortisone. The synthesis of all steroid hormones begins with Binding of aldosterone to the cytosol mineralocorticoid cholesterol and is catalyzed by a series of regulated, receptor leads to Na+ absorption and K+ and H+ secre- enzyme-mediated reactions (Fig. 66–1). Glucocorticoids tion by the renal tubules. The resultant increase in affect metabolism, cardiovascular function, behavior, plasma Na+ and decrease in plasma K+ provide a feed- and the inflammatory/immune response (Table 66–1). back mechanism for suppressing renin and, sub- Cortisol, the natural human glucocorticoid, is secreted sequently, aldosterone secretion. by the adrenal glands in response to ultradian, circa- Approximately 5% of cortisol and 40% of aldos- dian, and stress-induced hormonal stimulation by terone circulate in the free form; the remainder is bound adrenocorticotropic hormone (ACTH). Plasma cortisol to corticosteroid-binding globulin and albumin. has marked circadian rhythm; levels are highest in the Adrenal androgen precursors include dehy- morning. ACTH, a 39–amino acid neuropeptide, is droepiandrosterone (DHEA) and its sulfate and part of the pro-opiomelanocortin (POMC) precursor androstenedione. They are synthesized in the zona retic- molecule, which also contains -endorphin, - b b ularis under the influence of ACTH and other adrenal lipotropin, corticotropin-like intermediate-lobe peptide androgen-stimulating factors. Although they have (CLIP), and various melanocyte-stimulating hormones minimal intrinsic androgenic activity, they contribute to (MSH). The secretion of ACTH by the pituitary gland androgenicity by their peripheral conversion to test- is regulated primarily by two hypothalamic polypep- osterone and dihydrotestosterone. In men, excessive tides: the 41–amino acid corticotropin-releasing adrenal androgens have no clinical consequences; hormone (CRH) and the decapeptide vasopressin. Glu- however in women, peripheral conversion of excess cocorticoids exert negative feedback upon CRH and adrenal androgen precursor secretion results in acne, ACTH secretion. The brain-hypothalamic-pituitary- hirsutism, and virilization. Because of gonadal pro- adrenal (HPA) axis (Fig. 66–2) interacts with and influ- duction of androgens and estrogens and secretion of ences the function of the reproductive, growth, and norepinephrine by sympathetic ganglia, deficiencies of thyroid axes at multiple levels, with major participation adrenal androgens and catecholamines are not clinically of glucocorticoids at all levels. recognized. The renin-angiotensin-aldosterone system (Fig. 66–3) is the major regulator of aldosterone secretion. Renal juxtaglomerular cells secrete renin in response to a decrease in circulating volume and/or a reduction in Adrenal Insufficiency renal perfusion pressure. Renin is the rate-limiting enzyme that cleaves the 60-kD angiotensinogen, syn- Glucocorticoid insufficiency can be either primary, thesized by the liver, to the bioinactive decapeptide resulting from the destruction or dysfunction of the angiotensin I. Angiotensin I is rapidly converted to the adrenal cortex, or secondary, resulting from ACTH octapeptide angiotensin II by angiotensin-converting hyposecretion (Table 66–2). Autoimmune destruction of enzyme in the lungs and other tissues. Angiotensin II is the adrenal glands (Addison’s disease) is the most a potent vasopressor and stimulates aldosterone pro- common cause of primary adrenal insufficiency in the duction but does not stimulate cortisol production. industrialized world, accounting for about 65% of 603 Ch066.qxd 10/8/03 7:07 PM Page 604 604 SECTION XI—Endocrine Disease Cholesterol 1 2 3 4 5 6 Pregnenolone Progesterone 11-Deoxy- Corticosterone Aldosterone Mineralocorticoids corticosterone (DOC) 7 7 3 4 5 17-Hydroxy- 17-Hydroxy- 11-Deoxy- Cortisol Glucocorticoids pregnenolone progesterone cortisol 7 7 3 9 Dehydroepian- D4-Androstene- Estrone Sex Steroids drosterone (DHEA) dione 8 8 9 Testosterone Estradiol Sex Steroids Enzyme Number Enzyme (Current and Trivial Name) 1 StAR; Steroidogenic acute regulatory protein 2 CYP11A1; Cholesterol side chain cleavage enzyme/desmolase 3 3b-HSD II; 3b-Hydroxylase dehydrogenase 4 CYP21A2; 21a-Hydroxylase 5 CYP11B1; 11b-Hydroxylase 6 CYP11B2; Corticosterone Methyloxidase 7 CYP17; 17a-Hydroxylase/17, 20 lyase 8 17b-HSD; 17b-Hydroxysteroid dehydrogenase 9 CYP19; Aromatase FIGURE 66–1 Pathways of steroid biosynthesis. Site Site Catecholamines, Liver Angiotensinogen (452 A.A.) Brain cytokines, growth – factors Kidney Prorenin Renin Angiotensin I (10 A.A.) Lung, Hypothalamus – CRH AVP – plasma Angiotensin-converting enzyme Angiotensin II (8 A.A.) Pituitary – ACTH Adrenal, Angiotensin II receptor vascular Adrenal Cortisol Adrenal Aldosterone FIGURE 66–2 The brain-hypothalamic-pituitary-adrenal axis. ACTH = FIGURE 66–3 The renin-angiotensin-aldosterone axis. A.A. = amino adrenocorticotropic hormone; AVP = arginine vasopressin; CRH = corti- acids. cotropin-releasing hormone. Ch066.qxd 10/8/03 7:07 PM Page 605 CHAPTER 66—The Adrenal Gland 605 TABLE 66–1 Actions of Glucocorticoids TABLE 66–2 Syndromes of Adrenocortical Hypofunction Maintain Metabolic Homeostasis Primary Adrenal Disorders Regulate blood glucose level, permissive effects on Combined Glucocorticoid and Mineralocorticoid Deficiency gluconeogenesis, increase glycogen synthesis Autoimmune Raise insulin levels, permissive effects on lipolytic hormones Isolated autoimmune disease (Addison’s disease) Increase catabolism, decrease anabolism (except fat), inhibit Polyglandular autoimmune syndrome, type I growth hormone axis Polyglandular autoimmune syndrome, type II Inhibit reproductive axis Infectious Mineralocorticoid activity of cortisol Tuberculosis Affect Connective Tissues Fungal Cytomegalovirus Cause loss of collagen and connective tissue Human immunodeficiency virus Affect Calcium Homeostasis Vascular Stimulate osteoclasts, inhibit osteoblasts Bilateral adrenal hemorrhage Reduce intestinal calcium absorption, stimulate parathyroid Sepsis hormone release, increase urinary calcium excretion, Coagulopathy decrease reabsorption of phosphate Thrombosis/embolism Maintain Cardiovascular Function Adrenal infarction Increase cardiac output Infiltration Increase vascular tone Metastatic carcinoma/lymphoma Permissive effects on pressor hormones, increase sodium Sarcoidosis retention Amyloidosis Hemochromatosis Affect Behavior and Cognitive Function Congenital Affect Immune System Congenital adrenal hyperplasia Increase intravascular leukocyte concentration 21-hydroxylase deficiency Decrease migration of inflammatory cells to sites of injury 3b-ol dehydrogenase deficiency Suppress immune system (thymolysis; suppression of cytokines, 20,22-desmolase deficiency prostanoids, kinins, serotonin, histamine, collagenase, and Adrenal unresponsiveness to ACTH plasminogen activator) Congenital adrenal hypoplasia Adrenoleukodystrophy Adrenomyeloneuropathy latrogenic Bilateral adrenalectomy cases. Usually both glucocorticoid and mineralocorti- Drugs: metyrapone, aminoglutethimide, trilostane, coid secretion are diminished in this condition and, if ketoconazole, o,p¢-DDD, RU-486 untreated, it may be fatal. Isolated glucocorticoid or Mineralocorticoid Deficiency without Glucocorticoid mineralocorticoid deficiency may also occur, and it is Deficiency becoming apparent that mild adrenal insufficiency Corticosterone methyloxidase deficiency (similar to subclinical hypothyroidism, discussed in Isolated zona glomerulosa defect Chapter 65) should also be diagnosed and treated. Heparin therapy Adrenal medulla function is usually spared. Approxi- Critical illness mately 70% of the patients with Addison’s disease have Converting enzyme inhibitors anti-adrenal antibodies. Tuberculosis used to be the most common cause of Secondary Adrenal Disorders adrenal insufficiency. However, its incidence in the Secondary Adrenal Insufficiency industrialized world has decreased since the 1960s, and Hypothalamic/pituitary dysfunction it now accounts for only 15 to 20% of cases of adrenal Exogenous glucocorticoids
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