| Abstract: | Plasma immunoreactive calcitonin (iCT) levels generally are not elevated in chronic, nonmalignant, hypercalcemic states. Normocalcitoninemia might occur despite increased CT secretion if hypercalcemia itself accelerated the clearance of CT from the circulation. Therefore, we have measured the metabolic clearance rate (MCR) of human CT (hCT), infused to constant plasma levels in thyroparathyroidectomized dogs, under conditions of acute and chronic hypo- and hypercalcemia. Acute increases of plasma calcium were without significant effect on the MCR of hCT, but chronic hypercalcemia, induced by dihydrotachysterol treatment, reduced the MCR of hCT by 30% (P less than 0.05) and glomerular filtration rate (GFR) by 40% (P less than 0.02). There was a significant, positive correlation (r = 0.60, P less than 0.02) between GFR and the MCR of CT. The data suggest that failure to detect elevated iCT levels in chronic hypercalcemia is not the result of an increased MCR of CT. In fact, hypercalcemia should exaggerate the effect of increased CT secretion by decreasing the renal clearance of the hormone. |
