DIETARY CALCIUM REDUCES BLOOD-PRESSURE, PARATHYROID-HORMONE, AND PLATELET CYTOSOLIC CALCIUM RESPONSES IN SPONTANEOUSLY HYPERTENSIVE RATS

Dietary calcium effects on blood pressure, parathyroid hormone (PTH), and platelet cytosolic calcium concentrations were investigated. The d ietary calcium (low, 0.2%; medium, 0.5%; and high, 2.0% wt/wt) was sup plemented in spontaneously hypertensive rats from 6 through 22 weeks o f age. Mean systolic blood pressure was decreased by age 12 weeks with calcium supplementation (low, 227 +/- 6 mm Hg; medium, 211 +/- 6 mm H g; and high, 182 +/- 7 mm Hg; P <.001). By the 10th week of age, the l ow calcium group had significantly (P <.05) more elevated (44 +/- 2.3 pg/ml) plasma PTH compared with the high calcium-supplemented group (1 5 +/- 4.5 pg/mL). Regression analysis showed a significant (P <.001) p ositive correlation (r = 0.3) between systolic blood pressure and PTH. The platelet cytosolic calcium concentration was determined using the fura-2 method. The basal calcium was 134 +/- 5.5 nmol/L for the low c alcium group and thrombin increased to 228 +/- 8 nmol/L (P <.0001; +70 % change). The normal calcium group had 202 +/- 8 nmol/L; thrombin inc reased to 239 +/- 10 nmol/L (P <.0026; +19% change). The high calcium group had basal levels 145 +/- 7 nmol/L, with thrombin stimulating to 212 +/- 8 nmol/L (P <.0001; +46% change). Although thrombin increased platelet cytosolic calcium concentration in all groups, normal and hig h dietary calcium groups had smaller percentage increases (51% and 24% lesser, respectively) compared with the low dietary calcium group. Th e blood pressure-lowering effects of dietary calcium is associated wit h blunting of thrombin-induced increases in platelet cytosolic calcium , suggesting the involvement of an intracellular second messenger syst em mediated by receptors.