1. Fifty-two normotensive and essential hypertensive subjects were studied. Intracellular free calcium concentration ([Ca2+](i)) was measured in lymphocytes (37 subjects) and platelets (18 subjects) by means of the fluorescent indicators, quin 2 and fura-2. In 31 subjects, plasma ionized calcium concentration was also measured. 2. There was a positive correlation between platelet [Ca2+](i) and sytolic blood pressure (r = 0.485, P < 0.05), diastolic blood pressure (r = 0.542, P < 0.02) and mean blood pressure (r = 0.534, P < 0.02). 3. No statistically significant relationship was observed between plasma ionized calcium and blood pressure. 4. No relationship was found between lymphocyte [Ca2+](i) and blood pressure, or between lymphocyte [Ca2+](i) and plasma ionized calcium. 5. There was no relationship between [Ca2+](i) of lymphocytes and platelets measured simultaneously from the same subject. 6. These findings reconcile previous conflicting reports and show a relationship between platelet but not lymphocyte [Ca2+](i) and blood pressure in man.
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