Practice 4 structured short- and long-answer medical questions on Calcium Homeostasis and Disorders. Review model answers for exam revision.
Answer: #### Structural Role - **Skeletal Integrity**: Calcium is a critical component of the vertebrate skeleton, primarily found in bones and teeth as hydroxyapatite (Ca10(PO4)6(OH)2). This provides strength and rigidity. - **Cell Structure**: Calcium ions (Ca²⁺) maintain the integrity of cellular membranes, influencing rigidity, permeability, and viscosity. #### Regulatory Roles - **Passive Roles**: - **Enzyme Cofactor**: Essential for enzymes like lipase. - **Blood Clotting**: Integral to the coagulation cascade, facilitating the conversion of prothrombin to thrombin. - **Active Roles**: - **Intracellular Signaling**: Functions as a secondary messenger in signaling pathways. - **Vascular Function**: Regulates contraction and relaxation of vascular smooth muscle. - **Cell Movement**: Influences cell adhesion, platelet aggregation, and immune cell motility. - **Muscle Contraction**: Triggers the interaction of actin and myosin via release from the sarcoplasmic reticulum. - **Hormone Secretion**: Regulates exocytosis (e.g., insulin release from pancreatic beta cells). - **Nerve Transmission**: Critical for neurotransmitter release at synapses. ### Hormonal Regulation of Calcium
Answer: - **Source**: Secreted by chief cells of the parathyroid glands. - **Stimulus**: Low ionized serum calcium. - **Actions**: Increases bone resorption (osteoclast activity), enhances renal calcium reabsorption, promotes phosphate excretion, and stimulates renal activation of Vitamin D.
Answer: - **Synthesis**: Cholecalciferol (D3) from skin (UV) or Ergocalciferol (D2) from diet. Hydroxylated in the liver (25-OH D3) and then in the kidney (1,25-(OH)2 D3) via 1-alpha-hydroxylase (stimulated by PTH). - **Actions**: Increases intestinal calcium absorption and renal calcium reabsorption.
Answer: - **Source**: Parafollicular C cells of the thyroid gland. - **Actions**: Inhibits osteoclast activity and increases renal calcium excretion. It is generally not considered critical for daily calcium balance in humans. ### Disorders of Calcium Metabolism #### Hypocalcemia - **Causes**: Hypoparathyroidism, hypomagnesemia (impairs PTH secretion), Vitamin D deficiency, chronic kidney disease (impaired calcitriol synthesis), and malabsorption. - **Complications**: Tetany (muscle spasms, paresthesia, hyperactive reflexes). - **Treatment**: Calcium carbonate, calcium citrate, or calcium phosphate supplements. #### Hypercalcemia - **Definition**: Total calcium > 2.6 mmol/L or ionized calcium > 1.23 mmol/L. - **Causes**: Primary hyperparathyroidism (adenoma), malignancy (PTHrP production), Vitamin D intoxication, granulomatous diseases (sarcoidosis, TB), and thiazide diuretics. - **Consequences**: Metastatic calcification, renal stones (nephrolithiasis), and polyuria. - **Management of Severe Hypercalcemia**: - **Aggressive Rehydration**: IV fluids to promote renal clearance (target 4-5L urine output/day). - **Loop Diuretics**: Furosemide (after hydration) to enhance excretion. - **Bisphosphonates**: IV pamidronate or zoledronate to inhibit bone resorption (takes 1-3 days for effect). - **Dialysis**: If renal failure is present. - **Adjuncts**: Glucocorticoids (for sarcoidosis/lymphoma), Calcitonin (rapid but short-lived effect). ### Bone Diseases and Bisphosphonates - **Osteoporosis**: Decreased bone mass and increased resorption. Treated with bisphosphonates and HRT. - **Paget’s Disease**: Disorganized bone remodeling and hyperactive metabolism. Treated with calcitonin and bisphosphonates. - **Bisphosphonates Mechanism**: Synthetic analogs of pyrophosphate that bind hydroxyapatite and inhibit osteoclast-mediated resorption. They have poor oral absorption and are excreted unchanged by the kidneys.