Hypercalcemia in rats bearing growth hormone- and prolactin-secreting transplantable pituitary tumors.

We evaluated the effects of chronic massive elevations of serum GH and PRL on calcium metabolism in rats bearing the MStT/W15 and 7315a transplantable pituitary tumors. MStT/W15 tumor rats manifest elevated serum GH and PRL levels, hypercalcemia, hypercalciuria, and elevated serum levels of PTH and 1,25-dihydroxyvitamin D. The hypercalcemia was not reversed by dexamethasone or propranolol treatment, but was ameliorated by starvation. Parathyroidectomy produced hypocalcemia in the MStT/W15 tumor rats, confirming the parathyroid dependence of the hypercalcemia. The 7315a tumor produced a milder degree of hypercalcemia, along with elevated serum levels of PRL, ACTH, and corticosterone; serum GH was normal. In high concentrations, PRL and/or GH may stimulate the secretion of PTH as well as enhance dietary calcium absorption, in part through the mediation of 1,25-dihydroxyvitamin D.