Dietary calcium attenuates platelet aggregation and intracellular Ca²⁺ mobilization in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) are known to be blood pressure sensitive to dietary calcium. The effects of dietary calcium on platelet aggregation and intracellular Ca²⁺ mobilization were assessed by turbidimetric methods and fura-2 methods, respectively, in washed platelets of SHR. Ca²⁺ ATPase activity was examined in aortic membrane fractions. Six weeks of dietary calcium supplementation attenuated the increase of systolic blood pressure (SBP 199 ± 16 v 170 ± 9 mm Hg, P < .001) and thrombin- induced platelet aggregation (84.5 ± 3.7 v 73.7 ± 7.4%, P < .004) at 9 weeks of age. The ionomycin-induced intracellular calcium ([Ca²⁺](i)) peak in the absence of external Ca²⁺, which reflects [Ca²⁺](i) storage size, and thrombin-evoked [Ca²⁺](i) release from [Ca²⁺](i) storage were decreased by 2.0% Ca diet (472 ± 55 v 370 ± 23 nmol/L, P < .001, 339 ± 29 v 278 ± 33 nmol/L, P < .002). In addition, SBP was positively correlated with platelet aggregation (r = 0.703, P = .0088), thrombin-evoked [Ca²⁺](i) (r = 0.739, P = .0044), and ionomycin-induced [Ca²⁺](i) (r = 0.591, P = .0415), respectively. However, there was no significant effect of dietary calcium on Ca²⁺-ATPase activity in aortic membranes. These results suggest that dietary calcium supplementation had a beneficial effect on platelets of SHR by attenuating [Ca²⁺](i) mobilization from [Ca²⁺](i) storage. The hypotensive effect of dietary calcium might be associated with attenuated [Ca²⁺](i) mobilization in SHR.