The importance of the stomach in mediating histamine-induced hypocalcemia in the rat.

The effect of histamine on serum calcium homeostasis was studied in the rat. After the intravenous administration of 0.5-1.0 mg histamine base to fasted Holtzman rats weighing 80-100 g, a significant lowering of serum calcium (Ca) level occurred 30 min after injection (decrease in Ca, 1.4-1.9 mg/100 ml), but normocalcemia returned at 60 min. Repeat intravenous injections of histamine 1.0 mg resulted in repeated lowering of the serum Ca level. Hypophosphatemia did not accompany the hypocalcemia. Thyroparathyroidectomy (TPTX) did not eliminate the calcium-lowering effect of histamine in acute TPTX animals but did so in more chronic TPTX animals in which the mean serum Ca was 7.6 mg/100 ml or less. Gastrectomy, however, completely eliminated the calcium-lowering effect of histamine given in doses of up to 2 mg/rat (20 mg/kg of body weight), despite the presence of an intact thyroid gland. These studies support the role of a gastric factor and not the thyroid secretion of calcitonin in mediating this response in the rat.