To determine platelet Ca2+ transport entities involved in increased cytosolic Ca2+ in the platelets of hypertensive individuals, we studied the relations between blood pressure and Ca2+ transporters in platelet membranes from 22 white male volunteers 32 to 68 years old. We used thapsigargin, a specific inhibitor of the internal membrane Ca2+-ATPase, to differentiate between plasma membrane and internal membrane Ca2+-ATPases. Inositol 1,4,5-trisphosphate-mediated and Ca2+ ionophore (A23187)-induced Ca2+ release was also assayed in membrane preparations using rhod-2, a fluorescent Ca2+ indicator. Levels of glycoprotein Illa, a possible component of agonist-mediated Ca2+ influx, were measured by immunoblotting. The results show that plasma membrane Ca2+- ATPase is decreased as a function of diastolic blood pressure (P<.002), whereas the internal membrane Ca2+-ATPase is not (P<148). Neither activity is correlated with age or systolic blood pressure. However, inositol trisphosphate-mediated Ca2+ release is negatively correlated with age (P<.024) but not blood pressure. Glycoprotein IIIa levels and A23187-induced Ca2+ release were not related to age or blood pressure, demonstrating that inhibition of the plasma membrane Ca2+-ATPase was not a result of differences in the proportion of plasma membrane in the preparation or differences in intravesicular Ca2+ concentration. Inhibition of the plasma membrane Ca2+-ATPase could directly cause elevation of cytoplasmic Ca2+ and enhancement of platelet sensitivity.