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SUMMARY Intracellular free calcium, [Ca 2 + ],, was studied in platelets of essential hypertensive subjects and normotensive controls under basal conditions and after stimulation with epinephrine, norepinephrine, angiotensin II, ouabain, and thrombin, using the fluorescent calcium indicator quin 2. Basal [Ca 2+ ], was significantly higher in hypertensive subjects (n = 32) than in normotensive controls (n = 30; 167.4 ± 5.0 vs 143.2 ± 3.1 nmol/L; p<0.001). Epinephrine, norepinephrine, angiotensin II, and ouabain had no effect on platelet calcium, whereas thrombin induced a dose-dependent increase in [Ca 2+ ]| in both the presence and absence of extracellular calcium. This [Ca 2+ ]| increase in the presence of extracellular calcium, which depends mainly on calcium influx, was significantly higher (p<0.05) in platelets of hypertensive subjects at all thrombin concentrations (ranging from 0.025-0.1 U/ml), while the [Ca 2+ ]| increase in the absence of extracellular calcium, which depends only on release from intracellular stores, was similar in hypertensive subjects and controls. These results suggest that, in essential hypertension, there is not only increased platelet resting [Ca 2+ ]| but also an increase in agonist-mediated calcium influx, which appears to indicate a cell membrane abnormality in the platelets of subjects with essential hypertension.

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SummaryWe studied the inhibitory effects of the calcium channel blocker verapamil both on platelet aggregation and intracellular calcium [Ca2+]i in platelets loaded with a fluorescent indicator (quin 2).The inhibitory effects of verapamil on the platelet aggregation response to both thrombin and ionomycin were seen to be clearly dissociated from the verapamil-induced inhibition of the [Ca2+]i increase produced by these agonists. Verapamil-induced inhibition of platelet aggregation was also obtained when using the “calcium-independent” agonist phorbol-myristate acetate (PMA). It may be deduced that a calcium-independent mechanism plays a role in verapamil-induced inhibition of platelet aggregation. We postulate that this mechanism may operate via a protein-kinase C pathway.