Calcium Metabolism and Kinetics in Intact and Parathyroidectomized Cows Given Parathyroid Extract

Abstract

Nutritional balance and ⁴⁵Ca kinetic studies were conducted on 3 cows during periods of experimentally altered parathyroid status. In experiments on the intact cows and on two of the cows following parathyroidectomy, the hypercalcemic response to parathyroid extract administration was consistent, and the degree of the response was comparable to that of the dog (as defined by the U.S. Pharmacopoeia) provided the dose was expressed as a function of body weight, kg^0.75. Cessation of parathyroid extract administration was followed by hypocalcemia. The kinetic data indicated that the hypercalcemic response to parathyroid extract was associated with an increase in bone resorption, the subsequent decline in plasma calcium concentration with a decrease in calcium absorption from the gut. Fecal calcium was increased during the period of parathyroid extract administration and during the immediate postinjection period. Increased fecal calcium during the hypercalcemic period induced by parathyroid extract administration appeared to be due to both an increase of endogenous fecal calcium and decreased absorption, whereas in the hypocalcemic period the increased fecal calcium was due solely to decreased calcium absorption.