Effect of salmon calcitonin infusion on plasma concentrations of recently administered⁴⁵Ca

Summary

The effect of intravenous salmon calcitonin (SCT) infusion on the net rate of loss from plasma of recently injected⁴⁵Ca has been studied in conscious fasted rats, some of which had been previously thyroparathyroidectomized (TPTX). The⁴⁵Ca was injected 10 h prior to or 1 or 2 h after the initiation of the infusion. Salmon calcitonin (0.05–0.2 MRC mU/g body weight/h) infusion produced the expected hypocalcemia and hypophosphatemia. The drop in plasma calcium levels produced by the hormone was lowered further by the addition of phosphate to the infusate.

Plasma⁴⁵Ca changes, during SCT infusion, followed a biphasic pattern when the radionuclide injection preceded SCT infusion by 10 h. During the first part of the infusion, the rate of loss of⁴⁵Ca from plasma was greater in SCT-treated than in control rats. Later, this pattern reversed, and the rate of loss of⁴⁵Ca from plasma became slower in SCT-treated animals. If⁴⁵Ca was injected after SCT infusion was initiated only the second phase of the biphasic effect was observed; i.e., disappearance of⁴⁵Ca from plasma was slower in SCT-infused than in control rats.

These data are considered to support the hypothesis that rapid and opposing calcium fluxes exist between bone fluid and plasma, and that calcitonin acts by decreasing the calcium efflux from bone fluid. The calcium moved in this efflux system includes some that has only recently entered bone fluid from plasma. This is the explanation given for the initial decrease in plasma⁴⁵Ca concentrations following SCT infusion. The reverse effect in the second phase can be explained as the result of a secondary reduction in calcium influx to bone fluid from plasma.