INSULIN-INDUCED HYPERPOLARIZATION IN RETINAL CAPILLARY PERICYTES

Purpose. This study investigated the mechanism of insulin-induced memb rane voltage hyperpolarization in retinal capillary pericytes, which p ossess electrical membrane properties typical for smooth muscle cells and are supposed to regulate retinal microcirculation by a contractile mechanism. Methods. The mechanism of insulin-induced hyperpolarizatio n was studied in cultured bovine retinal capillary pericytes using con ventional microelectrodes. Results. Resting voltage averaged -28 +/- 0 .9 mV (mean +/- SEM, n = 45). Insulin (10(-9) to 10(-7) mol/1) induced a slow hyperpolarization in a dose-dependent fashion. Voltage change (DELTAV) was -3.1 +/- 0.4 mV (n = 14, P < 0.0001, = control) with an i nsulin concentration of 10(-8) mol/l. Blockade of potassium channels w ith Ba2+ (5 mmol/l) completely abolished the hyperpolarizing effect of insulin (n = 5). Apamin (10(-9) mol/l), a blocker of low-conductance Ca2+-activated potassium channels, also completely inhibited the insul in-induced hyperpolarization (n = 4). Blocking ATP-sensitive potassium channels with glibenclamide (10(-7) mol/l) did not reduce the hyperpo larizing action of insulin (DELTAV = -2.2 +/- 0.4 mV, n = 5, P = 0.29) . Equivalent hyperpolarizations were recorded when insulin was added i n the presence of ouabain (10(-4) mol/l) to inhibit the electrogenic N a+/-/K+/- ATPase (DELTAV = -3.5 +/- 1.0 mV, n = 4, P = 0.68). When per icytes were grown for 3 days in culture medium with elevated glucose c oncentrations (22.5 mmol/l), the resting membrane voltage and the insu lin-induced hyperpolarization were not significantly altered. Conclusi on. Insulin hyperpolarizes the membrane voltage of retinal pericytes p robably mediated by activation of apamin-sensitive Ca2+-activated pota ssium channels. Therefore, hormonal modulation of membrane voltage by insulin might be an important factor in the regulation of pericyte con tractility and retinal microcirculation under physiological conditions and in diabetes mellitus.