Endocrine Control of Growth and Metabolism: Study Notes

Review of Endocrine Principles

The Hypothalamic-Pituitary Control System

The hypothalamic-pituitary axis is a central regulatory system for endocrine function, integrating signals from the brain and peripheral organs to control hormone secretion.

• Feedback Patterns: Hormone levels are regulated by negative and positive feedback loops.

• Hormone Receptors: Target cells express specific receptors for hormones, determining cellular response.

• Cellular Responses: Hormones trigger diverse effects, including gene expression, enzyme activation, and metabolic changes.

• Modulation of Target Cell Response: Sensitivity and responsiveness can be altered by receptor number or affinity.

• Endocrine Pathologies: Disorders arise from hormone excess, deficiency, or abnormal tissue responsiveness.

Adrenal Glucocorticoids

Anatomy and Function of the Adrenal Glands

The adrenal glands are paired organs located atop the kidneys, composed of two distinct regions: the adrenal medulla and adrenal cortex.

• Adrenal Medulla: Secretes catecholamines (e.g., epinephrine), mediating rapid fight-or-flight responses.

• Adrenal Cortex: Secretes steroid hormones, including aldosterone, glucocorticoids, and sex hormones.

• Embryological Origin: Medulla and cortex derive from different tissues, merging during development.

Adrenal Cortex Steroid Hormones

The adrenal cortex is organized into three zones, each producing specific hormones.

• Zona Glomerulosa: Secretes aldosterone (regulates sodium and potassium balance).

• Zona Fasciculata: Secretes glucocorticoids (mainly cortisol).

• Zona Reticularis: Secretes androgens (sex hormones).

• Crossover Effects: Structural similarity among steroids can cause one hormone to bind to another's receptor, leading to overlapping symptoms.

Cortisol Secretion and Regulation

Cortisol is the principal glucocorticoid, essential for life and regulated by the hypothalamic-pituitary-adrenal (HPA) pathway.

• HPA Pathway: CRH (hypothalamus) → ACTH (anterior pituitary) → Cortisol (adrenal cortex).

• Negative Feedback: Cortisol inhibits CRH and ACTH secretion.

• Diurnal Rhythm: Cortisol peaks in the morning, diminishes at night, and increases with stress.

Metabolic Effects of Cortisol

Cortisol mediates long-term stress responses and is catabolic, promoting glucose production and breakdown of proteins and fats.

• Gluconeogenesis: Stimulates glucose synthesis in the liver.

• Protein Catabolism: Breaks down skeletal muscle proteins for gluconeogenesis.

• Lipolysis: Releases fatty acids for energy; glycerol used for gluconeogenesis.

• Immune Suppression: Inhibits cytokine release and antibody production.

• Calcium Balance: Decreases intestinal absorption and increases renal excretion, leading to net calcium loss.

• Brain Effects: Alters mood, memory, and learning.

Cortisol as a Therapeutic Drug

Cortisol and synthetic glucocorticoids are used to treat inflammation and prevent organ transplant rejection, but prolonged use can suppress endogenous ACTH and cause adrenal atrophy.

• Immunosuppressant: Used for allergies, autoimmune diseases, and transplant rejection.

• Therapeutic Caution: Must taper off to avoid adrenal insufficiency.

Cortisol Pathologies

Disorders of cortisol secretion include hypercortisolism (Cushing’s syndrome/disease) and adrenal insufficiency (Addison’s disease).

• Hypercortisolism: Causes hyperglycemia, muscle wasting, central obesity, mood changes.

• Cushing’s Disease: Pituitary tumor secreting ACTH.

• Addison’s Disease: Autoimmune destruction or enzyme defects causing hyposecretion.

• Congenital Adrenal Hyperplasia: Excess androgens cause masculinization in newborn girls.

CRH and ACTH: Additional Functions

Immune and Behavioral Effects

CRH and ACTH, beyond their endocrine roles, influence immune responses and behavior.

• Immune System: Cytokines stimulate HPA pathway; immune cells have ACTH/CRH receptors.

• Mind-Body Interaction: Stress and mental state affect immune and endocrine function.

• CRH Family: Includes urocortin, involved in food intake, labor onset, mood disorders.

• POMC Processing: Produces ACTH, β-endorphin (pain blocker), and MSH (melanocyte-stimulating hormone).

Melanocortin Receptors (MCRs)

Melanocortin hormones (MSH, ACTH) act on five identified receptors, affecting skin pigmentation, feeding, and inflammation.

• MC2R: Responds only to ACTH (adrenal cortex).

• MC1R: Skin melanocytes; responds to α-MSH and ACTH (Addison’s disease causes skin darkening).

• MC4R: Depresses feeding behavior.

• MC3R: CNS/peripheral tissues; regulates energy homeostasis.

• MC5R: Exocrine glands, skeletal muscle; regulates gland secretion and lipid production.

Thyroid Hormones

Anatomy and Synthesis

The thyroid gland, located at the base of the throat, produces hormones essential for metabolism and development.

• Cell Types: C cells (calcitonin) and follicular cells (thyroid hormones).

• Hormone Structure: Amines derived from tyrosine, contain iodine.

• Colloid: Stores 2-3 months supply of thyroid hormones.

• Thyroglobulin: Precursor protein for hormone synthesis.

Thyroid Hormone Synthesis

Thyroid hormone synthesis involves iodide uptake, oxidation, and coupling to form T3 and T4.

• Iodide Uptake: Sodium-iodide symporter (NIS) concentrates iodide.

• Pendrin: Transports iodide into colloid.

• Thyroid Peroxidase: Oxidizes iodide, attaches iodine to tyrosine (MIT, DIT).

• Coupling: MIT + DIT = T3; DIT + DIT = T4.

• Release: T3/T4 freed from thyroglobulin, diffuse into plasma, bind to thyroid-binding globulin (TBG).

• Active Hormone: T3 is active; most plasma hormone is T4, converted to T3 in target cells by deiodinases.

TSH Regulation

Thyroid-stimulating hormone (TSH) from the anterior pituitary, controlled by TRH from the hypothalamus, regulates thyroid hormone synthesis and secretion.

• Metabolic Effects: Increases oxygen consumption and heat production.

• Growth Effects: Essential for growth and development in children.

Thyroid Pathologies

Thyroid disorders include hyperthyroidism and hypothyroidism, affecting metabolism, growth, and organ function.

• Goiter: Enlargement of the thyroid gland due to elevated TSH.

• Hyperthyroidism: Excess hormone; causes heat intolerance, weight loss, muscle weakness, rapid heartbeat, and psychological disturbances (e.g., Graves’ disease).

• Hypothyroidism: Deficiency; causes cold intolerance, slow metabolism, brittle nails, dry skin, slow growth, and mental impairment (e.g., cretinism in infants).

• Primary Cause: Often due to iodine deficiency.

Growth Hormone

Regulation and Effects

Growth hormone (GH) is essential for normal growth, acting through direct effects and stimulation of insulin-like growth factors (IGFs).

• Regulation: GHRH stimulates, somatostatin inhibits GH secretion.

• Binding Proteins: Extend GH half-life in plasma.

• Anabolic Effects: Promotes protein synthesis, lipolysis, decreased glucose uptake, and gluconeogenesis.

• Bone Growth: GH and IGFs stimulate bone growth; IGFs directly stimulate cartilage synthesis.

Growth Hormone Disorders

GH disorders are most evident in children, causing dwarfism or giantism. In adults, excess GH causes acromegaly.

• Dwarfism: GH deficiency or defective receptors.

• Giantism: GH excess before epiphyseal plate closure.

• Acromegaly: GH excess in adults; causes jaw lengthening, coarsening of facial features, and growth in hands and feet.

Example: Acromegaly is illustrated by the comparison of individuals with excess GH to those with normal GH levels, showing pronounced differences in facial and hand features.

Tissues and Bone Growth

Soft Tissue and Linear Bone Growth

Growth involves both soft tissue (weight) and linear bone (height), requiring hormones, protein, and calcium.

• Hypertrophy: Increase in cell size.

• Hyperplasia: Increase in cell number.

• Bone Structure: Compact (outer) and spongy (inner) bone.

• Epiphyseal Plates: Sites of linear growth in children; contain dividing chondrocytes.

• Remodeling: Continues throughout life.

Calcium Balance

Physiological Functions of Calcium

Calcium is vital for signaling, cell adhesion, blood coagulation, and neuronal excitability.

• Distribution: 99% in bones; remainder in extracellular fluid and cells.

• Plasma Concentration: About 2.5 mM.

• Hypocalcemia: Can cause asphyxiation.

• Hypercalcemia: Reduces neuronal excitability.

Calcium Homeostasis

Calcium balance is maintained by intake (dietary absorption) and output (renal and fecal excretion).

• Intake: Absorbed in small intestine; regulated hormonally.

• Output: Primarily through kidneys.

• Compartments: Extracellular fluid, intracellular stores, and bone matrix.

Hormonal Regulation of Calcium

Three hormones regulate calcium movement between bone, kidney, and intestine.

• Parathyroid Hormone (PTH): Increases plasma calcium by mobilizing bone calcium, enhancing renal reabsorption, and increasing intestinal absorption.

• Calcitriol (Vitamin D3): Enhances intestinal absorption; synthesized from dietary vitamin D.

• Calcitonin: Opposes PTH; released when plasma calcium is high.

Bone Remodeling and Disorders

Bone Cells and Remodeling

Bone remodeling involves osteoblasts (build bone) and osteoclasts (break down bone), maintaining calcium and phosphate homeostasis.

• Osteoblasts: Produce calcified bone matrix.

• Osteoclasts: Dissolve bone matrix, releasing calcium and phosphate.

• Phosphate: Key component of hydroxyapatite; involved in energy transfer and storage.

Osteoporosis

Osteoporosis is a metabolic disorder characterized by bone loss, where bone resorption exceeds deposition, resulting in fragile bones prone to fracture.

• Risk Factors: Age, hormonal imbalance, inadequate calcium/vitamin D, inactivity.

• Prevention: Adequate diet, weight-bearing exercise, hormone therapy.