EFFECTS OF ANGIOTENSIN-II AND ENDOTHELIN-L ON PLATELET-AGGREGATION AND CYTOSOLIC PH AND FREE CA2+ CONCENTRATIONS IN ESSENTIAL-HYPERTENSION

The aims of this study were to determine the relations between platelet free calcium concentrations ([Ca2+](i)), intracellular pH (pH(i)), and aggregation and to assess the effects of angiotensin II (Ang II) and endothelin-1 on these platelet parameters in normotensive subjects and hypertensive patients. Seventeen normotensive subjects, 25 untreated hypertensive patients, and 34 treated hypertensive patients were studied. Platelet cytosolic free [Ca2+](i) and pH(i) were measured spectrofluorometrically using specific fluorescent probes (fura 2-AM and BCECF-AM, respectively) in unstimulated and Ang II- and endotheIin-1-stimulated platelets. Aggregation was measured by a turbidometric technique. Basal [Ca2+](i) (141+/-11 nmol/L) and pH (7.16+/-0.01) were higher (P<.05) in the untreated hypertensive group compared with the normotensive (118+/-9 nmol/L, 7.11+/-0.01, respectively) and treated hypertensive (121+/-11 nmol/L, 7.12+/-0.01, respectively) groups. In the combined normotensive and hypertensive groups, there were significant correlations between [Ca2+](i) and mean arterial pressure (r=.75, P<.01), pH(i) and mean arterial pressure (r=.72, P<.01), [Ca2+](i) and pH(i) (r=.71, P<.01), [Ca2+](i) and aggregation (r=.69, P<.02), and pH(i) and aggregation (r=.56, P<.05). Ang II stimulation significantly increased [Ca2+](i) and pH(i) in the untreated hypertensive and normotensive groups. The net change in [Ca2+](i) induced by Ang II was significantly higher (P<.05) in the untreated hypertensive group compared with the other groups (67+/-6 nmol/L for the untreated hypertensive group versus 54+/-5 and 29+/-8 nmol/L for the normotensive and treated hypertensive groups, respectively). In the presence of Ang II, thrombin-induced aggregatory responses were increased in all three groups, but the maximal response was significantly higher in the untreated hypertensive group compared with the other groups (P<.05). Endothelin-1 increased pH(i) through endothelin A-receptors (effect blocked by the specific antagonist BQ-123) but had no significant effect on [Ca2+](i) or aggregation. However, endothelin-1 blunted thrombin-induced platelet aggregation in normotensive subjects but not in hypertensive patients. In conclusion, increased Ang II-stimulated [Ca2+](i) and pH(i) in platelets of essential hypertensive patients may be associated with increased aggregatory responses. The stimulatory effect of endothelin-1 on pH(i) but not on [Ca2+](i) or aggregation suggests that in platelets endothelin-induced signaling pathways other than phospholipase C may be involved. The significant correlations between platelet aggregation, [Ca2+](i), pH(i), and blood pressure may suggest a possible link between altered platelet cation status and platelet function in hypertension.