Comparison of the Effects of Calcium and Magnesium on Parathyroid Hormone Secretion Rate in Calves*

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Abstract
Parathyroid hormone (PTH) secretion rate was measured in 7 calves by using a technique which involved RIA [radioimmunoassay] of parathyroid venous blood collected during timed intervals and measured volumetrically. Infusion of solutions of NA2EDTA and mgCl2 into the jugular vein was used to alter plasma Ca and Mg concentrations. In 4 calves, elevation of plasma Mg concentration rapidly decreased the PTH secretion rate that had been stimulated by induced hypocalcemia. In 3 calves, equimolar and opposite changes in plasma Ca and Mg concentrations were induced by simultaneous infusions of Na2EDTA and MgCl2. Despite the equimolar increase in plasma Mg concentration, PTH secretion was increased in response to decline in plasma Ca concentration. In 3 experiments, the concentration of each cation was kept constant during periods in which the concentration of the other cation was varied. The effect of variations in plasma Mg concentration on PTH secretion rate was approximately 1/3-1/2 as great as that observed with changes in plasma Ca concentration. The effect of Mg on PTH secretion rate is similar to that of Ca, but not equipotent.

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