Dietary calcium effects on blood pressure, parathyroid hormone (PTH), and platelet cytosolic calcium concentrations were investigated. The dietary calcium (low, 0.2%; medium, 0.5%; and high, 2.0% wt/wt) was supplemented in spontaneously hypertensive rats from 6 through 22 weeks of age. Mean systolic blood pressure was decreased by age 12 weeks with calcium supplementation (low, 227 +/- 6 mm Hg; medium, 211 +/- 6 mm Hg; and high, 182 +/- 7 mm Hg; P < .001). By the 10th week of age, the low calcium group had significantly (P < .05) more elevated (44 +/- 2.3 pg/ml) plasma PTH compared with the high calcium-supplemented group (15 +/- 4.5 pg/mL). Regression analysis showed a significant (P < .001) positive 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 calcium group and thrombin increased to 228 +/- 8 nmol/L (P < .0001; +70% change). The normal calcium group had 202 +/- 8 nmol/L; thrombin increased 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 high dietary calcium groups had smaller percentage increases (51% and 24% lesser, respectively) compared with the low dietary calcium group.(ABSTRACT TRUNCATED AT 250 WORDS)