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I. CORTICOSTEROIDS

  • The corticosteroids include mineralocorticoids, glucocorticoids, and also the adrenal hormones.
  • The adrenal glands, approximately 4 grams each, are located at the superior poles of the two kidneys and are typically described as triangular or pyramidal in shape.
    • Adrenal Medulla (20% of the gland):
    • Functions in the sympathetic nervous system.
    • Produces epinephrine and norepinephrine, which are fight or flight hormones (catecholamines).
    • Adrenal Cortex (outer layer):
    • Produces corticosteroids.

II. SYNTHESIS AND SECRETIONS OF ADRENOCORTICAL HORMONES

A. ADRENAL CORTEX

  • The adrenal cortex is divided into three distinct layers:
    • Zona Glomerulosa (15% of cortex):
    • Produces mineralocorticoids, primarily aldosterone, which are the only cells capable of secreting aldosterone.
    • Secretion is stimulated by angiotensin II (from the renin-angiotensin-aldosterone system) and by extracellular fluid potassium concentration.
    • Zona Fasciculata (75% of cortex):
    • The thickest layer, producing glucocorticoids like cortisol and corticosterone, along with small amounts of adrenal androgens and estrogens.
    • Secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis via ACTH.
    • Zona Reticularis:
    • Secretes adrenal androgens such as dehydroepiandrosterone (DHEA) and androstenedione, along with small quantities of estrogens and glucocorticoids.
    • Regulated by ACTH and cortical androgen-stimulating hormone.

B. SYNTHESIS OF ADRENOCORTICAL HORMONES

  • Approximately 80% of the cholesterol used for steroid synthesis is derived from low-density lipoproteins (LDLs) present in plasma.
    • LDLs bind to specific receptors in the "coated pits" of adrenocortical cell membranes.
    • These pits undergo endocytosis, forming vesicles that combine with lysosomes to release cholesterol for steroid synthesis.
  • Rate-limiting step: Cholesterol is cleaved by the enzyme cholesterol desmolase to form pregnenolone, and all subsequent steps occur in mitochondria and the endoplasmic reticulum.
  • Synthesis of Aldosterone:
    • Requires pregnenolone, converted into progesterone by the enzyme 3-beta-hydroxysteroid dehydrogenase to proceed to aldosterone formation.
  • Congenital Adrenal Hyperplasia:
    • Results from a mutation in 21-hydroxylase impacting aldosterone and cortisol synthesis, leading to increased androgen production and masculinizing effects.

C. MINERALOCORTICOIDS

  • Aldosterone:
    • The most significant mineralocorticoid, accounting for ~90% of all mineralocorticoid activity.
  • Deoxycorticosterone:
    • Less potent, approximately 1/30 as potent as aldosterone.
  • Corticosterone:
    • Has slight mineralocorticoid activity.

D. GLUCOCORTICOIDS

  • Cortisol:
    • The most potent glucocorticoid, accounting for ~90-95% of glucocorticoid activity.
  • Corticosterone:
    • Contributes ~4% of glucocorticoid activity, weaker than cortisol.
  • Cortisone:
    • Almost as potent as cortisol.
  • Prednisone:
    • A synthetic glucocorticoid, 4x as potent as cortisol.
  • Methylprednisolone:
    • Synthetic and 5x as potent as cortisol.
  • Dexamethasone:
    • Synthetic, 30x as potent as cortisol.

E. ADRENOCORTICAL HORMONES IN PLASMA

  • Cortisol:
    • Approximately 90-95% binds to plasma proteins, primarily cortisol-binding globulin (transcortin) and, to a lesser extent, albumin, resulting in a long half-life of 60-90 minutes.
    • Normal blood concentration: 12 µg/100 mL (average secretory rate of 15-20 mg/day).
  • Aldosterone:
    • Only 60% binds to plasma proteins; 40% is free form.
    • Normal concentration: 6 ng/100 mL (average secretory rate of 0.15 mg/day).
    • Has a shorter half-life of about 20 minutes.

III. FUNCTIONS OF MINERALOCORTICOIDS

A. RENAL AND CIRCULATORY EFFECTS OF ALDOSTERONE

  • Excess aldosterone increases extracellular fluid (ECF) volume and arterial pressure, with a small effect on plasma sodium concentration.
    • Sodium reabsorption in renal tubules prompts osmotic water reabsorption: "where salt goes, water follows."
  • Aldosterone also increases potassium secretion in principal cells of the collecting tubules and distal tubules.
    • Hypokalemia (low potassium) leads to muscle weakness, while hyperkalemia (high potassium) results in cardiac toxicity and arrhythmias.
  • Blood Pressure Regulation:
    • Detection of decreases in blood pressure by juxtaglomerular cells stimulates renin secretion, leading to angiotensin II release and subsequent aldosterone secretion, enhancing sodium and water reabsorption, ultimately increasing blood pressure.
  • Aldosterone Escape:
    • Increased ECF volume and arterial pressure lead to compensatory kidney excretion of sodium and water, preventing high blood pressure in chronic cases.

B. OTHER EFFECTS OF ALDOSTERONE

  • Stimulates sodium and potassium transport in sweat glands, salivary glands, and intestines to conserve body salt, especially in hot conditions.
  • Impaired sodium absorption in the absence of aldosterone can result in diarrhea.

C. CELLULAR MECHANISM OF ACTION

  • Aldosterone diffuses across cell membranes due to its lipid solubility.
  • Combines with a specific intracellular mineralocorticoid receptor, with the receptor complex entering the nucleus and inducing mRNA formation related to sodium and potassium transport.
  • Effects are not immediate, with delayed responses in sodium transport and cellular uptake taking thirty to forty-five minutes, respectively.

IV. REGULATION OF ALDOSTERONE SECRETION

  • Aldosterone secretion is regulated by:
    • Increased K+ concentration: Major stimulant.
    • Increased Angiotensin II: Major stimulant.
    • Decreased Na+ concentration: Minor inhibitor.
    • Increased Atrial Natriuretic Peptide (ANP): Secreted by cardiac atria and inhibits aldosterone secretion.

V. FUNCTIONS OF GLUCOCORTICOIDS

A. EFFECTS OF CORTISOL ON CARBOHYDRATE METABOLISM

  • Cortisol significantly increases liver gluconeogenesis (6- to 10-fold), enhancing glucose production via increased enzyme activity and mobilization of amino acids from muscle tissues while antagonizing insulin’s inhibitory effects on gluconeogenesis.
  • Chronic excess cortisol can lead to diabetes insipidus seen in Cushing's syndrome.
  • Decreases glucose utilization due to reduced GLUT4 transporter translocation, leading to insulin resistance.
  • Adrenal Diabetes: Caused by elevated blood glucose levels due to increased gluconeogenesis and reduced cellular glucose usage.

B. EFFECTS OF CORTISOL ON PROTEIN METABOLISM

  • Results in reduced protein stores in most tissues except the liver.
  • Increased liver and plasma proteins due to enhanced amino acid transport and liver protein synthesis.

C. EFFECTS OF CORTISOL ON FAT METABOLISM

  • Mobilizes fatty acids from adipose tissue, increasing plasma fatty acid concentration and altering cellular metabolism towards fatty acid usage.
  • Excess cortisol can lead to visceral obesity, characterized by fat deposition in specific body areas (e.g., abdomen, face).

D. IMPORTANCE IN RESISTING STRESS AND INFLAMMATION

  • Stress causes immediate ACTH release, increasing cortisol secretion.
  • Mobilizes amino acids and fats for energy and tissue repair during stress.

E. ANTI-INFLAMMATORY EFFECTS OF HIGH LEVELS OF CORTISOL

  • High cortisol levels block inflammation and have rapid healing effects, stabilizing lysosomal membranes and reducing immune responses, including fever reduction.

VI. REGULATION OF CORTISOL SECRETION

  • Controlled by ACTH via the hypothalamic-pituitary axis, functioning through negative feedback mechanisms.
  • Circadian Rhythm of Glucocorticoid Secretion:
    • Cortisol levels peak in the early morning and decline by evening, influenced by stress and daily activities.

VII. PRO-OPIOMELANOCORTIN (POMC)

  • A precursor to ACTH and several other hormones, including MSH and β-lipotropin, which are produced in the anterior pituitary and hypothalamus and processed to generate various peptides, affecting pigmentation and stress response.

VIII. ADRENAL ANDROGENS

  • Include dehydroepiandrosterone (DHEA), which is essential for male sexual development, and small amounts of estrogen necessary for the growth of pubic and axillary hair in both genders.

IX. ABNORMALITIES OF ADRENOCORTICAL SECRETIONS

A. HYPOADRENALISM (ADRENAL INSUFFICIENCY) - ADDISON’S DISEASE
  • Results from insufficient adrenocortical hormone production, often due to autoimmune atrophy.
  • Treatments include mineralocorticoid and glucocorticoid supplementation.
B. MINERALOCORTICOID DEFICIENCY
  • Impairs sodium reabsorption leading to hyponatremia and potentially causing shock.
C. GLUCOCORTICOID DEFICIENCY
  • Involves reduced glucose synthesis and muscle weakness, leading to vulnerability to stress.
D. MELANIN PIGMENTATION
  • Linked to ACTH overproduction, resulting in darkening skin, particularly in the mucous membranes.
E. ADRENAL CRISIS
  • A life-threatening condition in Addison's Disease, characterized by inadequate cortisol response to stress.
F. HYPERADRENALISM (CUSHING’S SYNDROME)
  • Results from excessive cortisol due to various causes, leading to obesity, hypertension, and immunosuppression.
G. PRIMARY ALDOSTERONISM (CONN’S SYNDROME)
  • Results from aldosterone-secreting tumors causing hypertension and marked hypokalemia.
H. ADRENOGENITAL SYNDROME
  • Associated with tumors that produce excess androgens, leading to masculinization effects, especially in females.