Role of insulin in the regulation of membrane fluidity of erythrocytes in essential hypertension: an electron paramagnetic resonance investigation

In the present study, to determine a possible role of insulin in the regulation of membrane functions, we have examined the effects of insulin on the membrane fluidity of erythrocytes in patients with essential hypertension and normotensive subjects. Membrane fluidity of erythrocytes obtained from hypertensive and normotensive subjects were evaluated by means of an electron paramagnetic resonance (EPR) and a spin-labeling method. In an in vitro study, insulin increased the order parameter (S for 5-nitroxide stearate) and the peak height ratio (ho/h-1 for 16 nitroxide stearate) in the EPR spectra of erythrocyte membranes, which indicated that insulin decreased the membrane fluidity of erythrocytes. The effects of insulin on the membrane fluidity were potentiated in the presence of extracellular Ca2+, and in contrast, were antagonized by the Ca2+ channel blocker diltiazem. Furthermore, the effects of insulin alone and in combination with Ca2+ on the membrane fluidity were reduced in the erythrocytes from hypertensive subjects compared with the erythrocytes from normotensive controls. The high concentrations of glucose alone produced no significant effects on the membrane fluidity of erythrocytes. These results demonstrated that insulin might actively participate in the regulation of membrane fluidity of erythrocytes, which might be mediated by the intracellular Ca2+ kinetics.