Insulin-induced hyperpolarization in retinal capillary pericytes.
OBJECTIVE
This study investigated the mechanism of insulin-induced membrane voltage hyperpolarization in retinal capillary pericytes, which possess 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 hyperpolarization was studied in cultured bovine retinal capillary pericytes using conventional microelectrodes.


RESULTS
Resting voltage averaged -28 +/- 0.9 mV (mean +/- SEM, n = 45). Insulin (10(-9) to 10(-7) mol/l) induced a slow hyperpolarization in a dose-dependent fashion. Voltage change (delta V) was -3.1 +/- 0.4 mV (n = 14, P < 0.0001, = control) with an insulin concentration of 10(-8) mol/l. Blockade of potassium channels with Ba2+ (5 mmol/l) completely abolished the hyperpolarizing effect of insulin (n = 5). Apamin (10(-9) mol/l), a blocker of low-conductance Ca(2+)-activated potassium channels, also completely inhibited the insulin-induced hyperpolarization (n = 4). Blocking ATP-sensitive potassium channels with glibenclamide (10(-7) mol/l) did not reduce the hyperpolarizing action of insulin (delta V = -2.2 +/- 0.4 mV, n = 5, P = 0.29). Equivalent hyperpolarizations were recorded when insulin was added in the presence of ouabain (10(-4) mol/l) to inhibit the electrogenic Na+/-/K+/-ATPase (delta V = -3.5 +/- 1.0 mV, n = 4, P = 0.68). When pericytes were grown for 3 days in culture medium with elevated glucose concentrations (22.5 mmol/l), the resting membrane voltage and the insulin-induced hyperpolarization were not significantly altered.


CONCLUSIONS
Insulin hyperpolarizes the membrane voltage of retinal pericytes probably mediated by activation of apamin-sensitive Ca(2+)-activated potassium channels. Therefore, hormonal modulation of membrane voltage by insulin might be an important factor in the regulation of pericyte contractility and retinal microcirculation under physiological conditions and in diabetes mellitus.
Berweck
S
S
Department of Clinical Physiology, Universitatsklinikum Steglitz, Freie Universität Berlin, Germany.
Thieme
H
H
Lepple-Wienhues
A
A
Helbig
H
H
Wiederholt
M
M
D000255:Adenosine Triphosphate / Q000502:physiology; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002196:Capillaries / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D002417:Cattle; D002478:Cells, Cultured; D007328:Insulin / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects*; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000502:physiology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D012171:Retinal Vessels / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology
D007328:Insulin; D015221:Potassium Channels; D001030:Apamin; D010042:Ouabain; D000255:Adenosine Triphosphate; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium