Tyrosine phosphorylation of human platelet plasma membrane Ca(2+)-ATPase in hypertension

Intracellular Ca(2+) is increased in the platelets of hypertensive individuals. Previously, we demonstrated that platelet plasma membrane Ca(2+)-ATPase (PMCA) activity inversely correlates with diastolic blood pressure and that inhibition of this Ca(2+) pump could explain the elevation of cytosolic Ca(2+) in hypertension. More recently, we discovered that PMCA is phosphorylated on tyrosine residues during thrombin-stimulated platelet aggregation and that this phosphorylation causes inhibition of PMCA activity. In the present work, we tested the hypothesis that tyrosine phosphorylation of PMCA in hypertensive patients could account for the observed inhibition of the Ca(2+) pump. Platelets were obtained from untreated hypertensive and normotensive volunteers. PMCA was immunoprecipitated from solubilized platelets, and tyrosine phosphorylation was quantified by chemiluminescence of immunoblots treated with anti-phosphotyrosine. PMCA content was measured on the same immunoblots by stripping and reprobing with anti-PMCA. Phosphorylation was reported as normalized phosphotyrosine chemiluminescence per nanogram PMCA (mean+/-SE). The average PMCA tyrosine phosphorylation for 15 normotensive subjects was 0.53+/-0. 09, whereas the average for 8 hypertensive individuals was 1.82+/-0. 25 (P<0.0005, Mann-Whitney U test). Age, gender, and systolic blood pressure did not correlate with PMCA phosphorylation. These results suggest that PMCA in platelets of hypertensive individuals is inhibited because of tyrosine phosphorylation, resulting in increased platelet intracellular Ca(2+), hyperactive platelets, and increased risk of heart attack and stroke.