Hypercalcemia in acute uremia and following citric acid administration: differential effect on parathyroid gland microtubule content.

Hypercalcemia, which occurs 4 hr after bilateral nephrectomy in normal rats, is not seen 4 hr after either bilateral ureterotomy or sham surgery. These results indicate that it is loss of renal mass per se, not the uremic syndrome, which is responsible for the hypercalcemia. Citric acid levels also increase 4 hr after nephrectomy, and a degree of hypercalcemia and hypercitricemia comparable to that which follows nephrectomy can be produced by administration of citric acid to normal rats. In an attempt to evaluate the role of the parathyroid gland in the development of hypercalcemia in these two situations, the microtubule content of parathyroid gland chief cells was determined by ultrastructural sterologic techniques 4 hr after either bilateral nephrectomy or citric acid administration. The results of these measurements indicate that parathyroid gland chief cell microtubule content increases after citric acid administration but not following bilateral nephrectomy. The significance of these results is not clear. However, since a previous study has suggested a correlation between increased microtubule content and increased secretory status in the chief cell, one may speculate that increased microtubule content resulting from citric acid administration may also be associated with increased parathyroid hormone secretion. By this formulation, citric acid-induced hypercalcemia would be secondary to increased parathyroid hormone secretion, but the transient hypercalcemia that occurs after nephrectomy would take place in the absence of an increase in parathyroid hormone secretion. In this latter instance, it is possible that loss of the kidney, a major site of parathyroid hormone removal from plasma, leads to an increase in circulating parathyroid hormone level, and hypercalcemia, in the absence of an increase in hormone secretion rate.