Summary
Haemodynamic shear forces have been reported to exert direct and indirect effects
on platelet reactivity. In vitro, they activate platelets leading to spontaneous or
facilitated aggregation. In vivo, they stimulate the production of endothelium-derived
antiaggregatory agents. This study was designed to evaluate in hypertensive patients,
before and after antihypertensive treatment, the possible role of these haemodynamic
forces, determined at the brachial artery level on the ex vivo platelet aggregatory
response to ADP and collagen. Platelet reactivity, evaluated by EC50 for ADP and collagen, was found to be related to blood velocity, shear rate and shear
stress (p <0.01 for each). These inverse 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 platelet cytosolic
Ca2+ and cyclic AMP.
The changes in shear forces and in aggregatory responses to ADP and collagen induced
by nitrendipine treatment for 6 months remained negatively correlated, confirming
the relationships existing between haemodynamic shear forces and platelet reactivity.
These results indicate that the shear antiaggregant effects, likely mediated by tlow-dependent
endothelium-derived factors, prevail over its direct platelet aggregating effects.