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 systolic 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.