| [ Summary ] |
Phosphate loading causes a decrease in calcium excretion even when there is an increase in volume. The effect primarily results from an increase in calcium reabsorption in the distal tubule secondary to an increase in parathyroid hormone secretion, triggered by an initial drop in plasma ionized calcium. Conversely, phosphate depletion is associated with hypercalciuria, which is only partially corrected by the administration of PTH. The apparent discrepancy may be explained by a difference in plasma phosphate levels. Why can PTH not affect calcium transport in low phosphate levels ? In contrast, the elevation of plasma calcium decreased fractional Pi reabsorption in a PTH-independent (and calcitonin-independent) fashion. Thus, calcium and phosphate reabsorption seem to be coordinated to prevent precipitation. The exact mechanisms are in dispute. |