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.

Different effects of three aldosterone treatments on plasma aldosterone and salt intake.

Adrenalectomized male rats received a nominal dose of 47.6 micrograms/day aldosterone for 14 days by daily injections, osmotic minipumps, or controlled-release pellets. Plasma aldosterone concentrations were barely detectable (< 20 pg/ml) 24 h after rats received aldosterone by injection, remained constant at 200 pg/ml in rats with osmotic minipumps, and dropped from > 500 to 75 pg/ml during the first week after implantation of controlled-release pellets. For the most part, the effects of the different treatments on NaCl intake were related to their effects on plasma aldosterone levels according to a U-shaped function. However, NaCl intake was dissociated from plasma aldosterone levels when treatment first began or was discontinued. NaCl intake may be a function of the amount of aldosterone delivered but not necessarily plasma aldosterone concentration.

Independence of salt intake induced by calcium deprivation from the renin-angiotensin-aldosterone system.

We investigated whether the elevated NaCl intake shown by calcium-deprived rats is mediated by the renin-angiotensin-aldosterone system. First, we looked for manifestations of altered renin-angiotensin-aldosterone system activity during the progression of calcium deficiency. There were no differences between control and calcium-deprived rats in plasma aldosterone concentrations, plasma renin activity, plasma sodium concentrations, sodium balance, or blood pressure. Second, we used selective pharmacological antagonists to examine whether disruption of the renin-aldosterone-angiotensin system influenced salt intake. Blockade of aldosterone receptors with spironolactone (25 mg.kg-1 x day-1 sc for 7 days) had no effect on NaCl intake of control or calcium-deprived rats. Angiotensin AT1 receptor blockade with losartan potassium (0.5-10 mg/kg orally) had no effect on NaCl intake of control or calcium-deprived rats but doses > 0.5 mg/kg decreased NaCl intake of adrenalectomized rats. Taken together, these findings indicate that the renin-angiotensin-aldosterone system does not mediate the increased NaCl intake produced by calcium deficiency. The appetite for salt produced by calcium deficiency involves a different physiological substrate from most other models of NaCl intake.

Independence of salt intake from the hormones regulating calcium homeostasis.

Rats deprived of dietary calcium increase voluntary intake of NaCl solutions. We investigated whether the major hormones controlling calcium homeostasis are responsible for this increase in salt intake. Removing endogenous sources of calcitonin and parathyroid hormone by thyroidectomy and/or parathyroidectomy had no effect on NaCl intake. The surgically compromised rats and their intact controls drank similar amounts of NaCl in response to manipulations of diet calcium content. Despite normal NaCl intakes, rats with parathyroidectomy had low plasma calcium concentrations and a strong appetite for 50 mM CaCl2 solution. Chronic infusion of parathyroid hormone into rats with thyroparathyroidectomy decreased NaCl intake. Intact rats fed an American Institute of Nutrition (AIN)-76A-based vitamin D-deficient diet increased NaCl intake slightly and showed a strong appetite for CaCl2, but other rats maintained normocalcemic by the addition of calcium, phosphorus, and lactose to the vitamin D-deficient diet had normal NaCl and CaCl2 intakes. Chronic infusions of 1,25-dihydroxyvitamin D3 into intact rats had no effect on NaCl intake. Taken together, these results indicate that the increase in NaCl intake produced by calcium deprivation is not mediated by changes in circulating levels of calcium, calcitonin, parathyroid hormone, or 1,25-dihydroxyvitamin D3. Furthermore, the major calcium-regulating hormones are not involved in the control of "spontaneous" NaCl intake in the rat.