Hypotensive effect of a high calcium diet in the Wistar rat

In order to study the effects of Ca supplementation upon systolic blood pressure, male weanling Wistar rats were fed rat chow or chow mixed with either CaCO3 (to 2.5% total Ca) or CaHPO4 (to 3.4% total Ca). In the first study rats receiving CaCO3 supplementation had lower systolic pressures after 6 weeks on the diet. Urinary P was reduced, fecal P was increased and urinary Ca was increased with this diet. Ionized Ca in blood was unaltered by Ca supplementation, but total Ca was decreased and plasma P was increased. In the second study with CaCO3, only a transient effect on blood pressure was found. The effects of the diet upon Ca and P metabolism, however, were similar to those found in the first study. In the third study, with CaHPO4 as the vehicle for Ca supplementation, blood pressure was consistently reduced in rats after 8 weeks on the high Ca diet. Plasma P, total Ca and whole blood ionized Ca were unaltered by this diet, but ionized Ca was reduced in plasma. The dietary intake of nutrients other than the Ca salt did not differ between control and and experimental groups in any of the experiments. Urinary Na and K were similar in control and experimental animals in all three experiments. These data suggest that neither increased ionized Ca, P depletion nor increased Na excretion are necessary conditions for expression of the hypotensive effect of supplementary Ca.