In vivo and in vitro studies of the role of HDL in platelet activation in normotensive and hypertensive subjects.

The influence of high density lipoproteins (HDL) on platelet function were examined in vivo and in vitro. Intraplatelet, ionised calcium concentrations ([Ca2+]i) were measured using the fluorescent dye Quin-2. In vitro, platelet responses to AVP were measured in the presence or absence of HDL. Pre-treating platelets from normal volunteers with HDL in vitro lowered the [Ca2+]i response to AVP but did not significantly affect basal values. In vivo, plasma lipoprotein profiles in normotensive (13 men, 17 women) and hypertensive (13 men, 11 women) subjects were compared with basal and AVP-stimulated platelet [Ca2+]i values. Plasma [HDL] were not related to basal or stimulated [Ca2+]i. Plasma cholesterol concentrations were higher in the hypertensive patients largely because low density lipoprotein [LDL] were raised in male hypertensive patients. Plasma [LDL] and total plasma cholesterol correlated negatively with basal [Ca2+]i (but not with AVP-stimulated values) only in this subgroup. In hypertensive patients, AVP-stimulated [Ca2+]i (but not basal values) were lower than normotensive controls. From in vitro studies, it may be concluded that the anti-atherogenic properties of HDL might be mediated by inhibition of platelet reactivity. However, ex vivo a negative influence of LDL on basal (not stimulated) [Ca2+]i appears to be paramount.