Parathyroid hormone secretion in vivo. Demonstration of a calcium-independent nonsuppressible component of secretion.

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Abstract
The response of normal bovine parathyroid glands to hypercalcemia was assessed in vivo by radioimmunoassay of immunoreactive parathyroid hormone concentrations in parathyroid effluent blood obtained by surgical cannulation of both anesthetized and nonanesthetized calves. Hypercalcemia was induced for periods of 0.3-35 h by intravenous infusion of a solution of calcium chloride. Assessment of immunoreactivity in effluent and peripheral blood included measurements of selected samples by use of a radioimmunoassay specific for a site residing in the biologically active portion of the hormone molecule. In all instances, the concentration of immunoreactive parathyroid hormone in hypercalcemic venous effluent from a superior parathyroid gland exceeded that of the peripheral blood. Failure of hypercalcemia to suppress completely secretion by normal parathyroids indicates that a portion of parathyroid hormone secretion occurs independent of blood calcium concentration. Consequently, continued parathyroid hormone secretion despite hypercalcemia can no longer be regarded as a unique feature of parathyroid neoplasia.