Angiotensin II Infusion Affects Fluorescence Polarization and Cholesterol Content of Platelet Membranes

Abstract

Objective: We studied the modifications of the physicochemical properties and the lipid composition of platelet plasma membranes during angiotensin infusion.

Methods: Platelets were obtained during angiotensin II infusion from 8 primigravid women at 25 to 32 weeks' gestation. Fluorescence polarization and cholesterol concentration were measured at the beginning of the test and when the effective pressor dose (EPD, i.e., infusion rate at which diastolic blood pressure rises 20 mm Hg from baseline) was achieved.

Main Outcome Measures: The modification of cholesterol/phospholipid ratio and the subsequent modification of polarization of platelet membrane in response to increasing angiotensin II infusion rates were considered.

Results: Both fluorescence polarization and cholesterol concentrations of platelet membrane were significantly reduced (ANOVA for repeated measures P <. 001) at the EPD (polarization 0.275 ± 0.004 vs. 0.250 ± 0.003; cholesterol 110.0 ± 4.0 vs. 65.0 ± 5.2 nmol/L, respectively). The infusion of 1 g magnesium pyrrolidone carboxylate, active as calcium antagonist, restored only the baseline values but did not affect the values obtained at the EPD.

Conclusions: These results suggest a direct involvement of cholesterol in the modulation of the platelet membrane fluidity during angiotensin infusion, not mediated by intracellular free calcium availability, and, therefore, a more specific role for angiotensin II in the regulation of vascular reactivity during pregnancy.

Key Words:

• Angiotensin II
• Cholesterol
• Fluidity
• Magnesium pyrrolidone carboxylate
• Platelet membrane