Effect of plasma calcium concentration on the metabolic clearance rate of parathyroid hormone in the dog.

Low perfusate calcium concentrations reportedly enhance the degradation of intact bPTH in isolated rat liver and dog kidney. Depending on the magnitude of such effects, they might affect the metabolic clearance of intact PTH in vivo. Therefore we have investigated the effect of acute and chronic hypocalcemia and hypercalcemia on the MCR of intact PTH in vivo in dogs. The t 1/2 of endogenous PTH was unchanged after 5, 30, and 480 min of hypocalcemia and ranged from 1.2 to 2.8 min. The MCR of bPTH infused to constant plasma levels in TPTX dogs also did not change with short- or long-term changes in the plasma calcium concentration by approximately 7 mg/dl. The MCR averaged 13 ml/min per kilogram for fully biologically active bPTH and 6 ml/min per kilogram for oxidized bPTH. Thus neither acute nor chronic hypocalcemia or hypercalcemia at extremes observed in disease states significantly altered the MCR of PTH. We conclude that changes in plasma intact PTH concentrations during changes in plasma calcium accurately reflect PTH secretion rate.