PLATELET-AGGREGATION AND IN-VIVO SHEAR FORCES

Haemodynamic shear forces have been reported to exert direct and indir ect effects on platelet reactivity. In vitro, they activate platelets leading to spontaneous or facilitated aggregation. In vivo, they stimu late the production of endothelium-derived anti-aggregatory agents. Th is study was designed to evaluate in hypertensive patients, before and after antihypertensive treatment, the possible role of these haemodyn amic forces, determined at the brachial artery level on the ex vivo pl atelet aggregatory response to ADP and collagen. Platelet reactivity, evaluated by EC50 for ADP and collagen, was found to be related to blo od velocity, shear rate and shear stress (p < 0.01 for each). These in verse correlations of platelet aggregation with stress levels did not depend on age, body mass index, mean blood pressure, serum cholesterol and triglycerides or haematocrit. They were also independent of plate let CytoSoliC Ca2+ and cyclic AMP. The changes in shear forces and in aggregatory responses to ADP and collagen induced by nitrendipine trea tment for 6 months remained negatively correlated, confirming the rela tionships existing between haemodynamic shear forces and platelet reac tivity. These results indicate that the shear antiaggregant effects, l ikely mediated by flow-dependent endothelium-derived factors, prevail over its direct platelet aggregating effects.