Effects of inaktin on calcium metabolism in the rat

Experiments were designed to investigate further the alterations in calcium metabolism caused by inaktin, a thiobarbiturate that impairs parathyroid hormone action in rats (1981). Treatment with an anesthetic dose of inaktin induced a drop in serum calcium without any variation in immunoreactive parathyroid hormone serum level and slowed body calcium turnover as studied with 45Ca, but was without effect on blood pH or partial pressures of oxygen and carbon dioxide. In contrast, calcium metabolism in rats was unchanged after treatment with an anesthetic dose of pentothal, another thiobarbiturate anesthetic. The effect of inaktin on body calcium turnover was dose-dependent and significant even at non-anesthetic levels. A marked slowing of the fast phenomena accounting for the initial dilution of the tracer in the animal was observed, accompanied by a corresponding decrease in rapid 45Ca uptake into bone, but not other tissues. These results show that inaktin slows calcium turnover, especially the fast renewal of calcium in bone. These effects were not correlated with anesthesia or immobilization, and we suggest that inaktin-induced changes in calcium metabolism involve an impairment of hypocalcemia-induced parathyroid hormone secretion, in addition to the impairment of parathyroid hormone effects previously reported. Furthermore, the present findings suggest that inaktin might be a useful tool for investigation of the rapid mobilization of bone calcium, which is as yet not well understood.