Calcium intake by rats: influence of parathyroid hormone, calcitonin, and 1,25-dihydroxyvitamin D.

We examined the contribution of the primary hormones of calcium homeostasis to the control of calcium intake in the rat. Male Sprague-Dawley rats with 50 mM CaCl2 solution as their only source of calcium received subcutaneous hormone infusions for 13 days. Parathyroid hormone (PTH; 40, 80, or 160 ng/h) produced sustained dose-related decreases in CaCl2 intake. High doses of calcitonin (CT; 32 or 64 ng/h) increased CaCl2 intake transiently, and low doses (4, 8, or 16 ng/h) had no effect. 1,25-Dihydroxyvitamin D [1,25(OH)2D] in doses > 1 ng/h initially increased CaCl2 intake, but the effects of moderate doses (2 or 4 ng/h) tended to dissipate, and the sustained effect of high doses (8 or 16 ng/h) was to reduce CaCl2 intake. Infusions of combinations of the hormones had effects consistent with their individual actions: there was no evidence for synergy. Based on changes in plasma hormone concentrations, it appeared that most of the infusions had effects within the physiological range. Consistent with hypotheses that calcium appetite is mediated by circulating calcium, PTH and CT infusions produced reciprocal changes in plasma calcium concentrations and CaCl2 intake. However, the finding that 1,25(OH)2D elevated both plasma calcium concentrations and CaCl2 intake raises the possibility that one or more of the hormones may mediate calcium appetite directly.