## Acute vs. Chronic Effects of PTH on Calcium Homeostasis **Key Point:** PTH acts on the kidney within minutes to hours to increase serum calcium, whereas bone resorption and intestinal absorption are slower, chronic effects. ### Mechanism of Acute Calcium Reabsorption PTH binds to PTH1R receptors on principal cells of the distal convoluted tubule (DCT) and collecting duct, leading to: 1. Activation of adenylyl cyclase → increased cAMP 2. Opening of calcium-selective channels in the apical membrane 3. Increased transcellular calcium reabsorption 4. Decreased urinary calcium excretion (hypercalciuria is prevented) **High-Yield:** The acute hypercalcemic effect of PTH (within 1–2 hours) is **entirely renal** — increased DCT reabsorption. Bone resorption takes 24–72 hours; intestinal absorption requires 1,25-dihydroxyvitamin D synthesis and takes 24–48 hours. ### Timeline of PTH Actions | Effect | Timeframe | Mechanism | Site | |--------|-----------|-----------|------| | **Acute calcium reabsorption** | Minutes–hours | Increased channel expression & cAMP | Kidney (DCT) | | **Bone resorption** | 24–72 hours | Osteoclast activation via RANKL | Bone | | **Intestinal absorption** | 24–48 hours | 1,25-dihydroxyvitamin D synthesis | Intestine | | **Phosphate excretion** | Minutes–hours | Decreased proximal tubule reabsorption | Kidney (PT) | **Clinical Pearl:** In acute hypocalcemia (e.g., post-thyroidectomy), PTH begins to correct serum calcium within 1–2 hours via renal mechanisms alone, before bone resorption contributes. **Warning:** Do not confuse the acute renal effect with the chronic bone effect. Osteoblasts do not directly resorb bone; they produce RANKL, which activates osteoclasts — a slower process.
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