Large conductance calcium-activated potassium channels in cultured retinal pericytes under normal and high-glucose conditions

Abstract

Pericytes are considered to contribute to the regulation of retinal microcirculation which is impaired in diabetic retinopathy. Single, large-conductance, Ca(2+)-dependent K+ channels (BK) were studied in cultured bovine retinal capillary pericytes using the patch-clamp method. In excised patches with symmetrical 135-mmol/l K+ solutions a single channel conductance of 238 +/- 9.9 pS was measured. With a K+ gradient of 4/135 mmol/l (extracellular/intracellular) the slope conductance averaged 148 +/- 2.9 pS at 0 mV. The mean permeability was 4.2 X 10(-13) cm3/s. The channel was highly selective for K+ with a permeability ratio for K+ over Na+ of 1/0.02. The mean open time and the open probability (Po) of the BK channel increased with depolarization and with increasing internal [Ca2+] showing a maximal sensitivity to Ca2+ between 10(-4) and 10(-5) mol/l Ca2+. Ba2+ (5 mmol/l), quinine (5 mmol/l), and verapamil (Michaelis constant 1.5 X 10(-5) mol/l) blocked from the intracellular side. Tetraethylammonium induced a dose-dependent block from the outside only with a half-maximal blocking concentration of 2.5 X 10(-4) mol/l. Charybdotoxin (10(-8) mol/l) blocked completely from the extracellular side. The channel activity was not changed by either internal adenosine triphosphate (ATP, 10(-4) mol/l) or the putative opener of ATP-sensitive K+ channels Hoe 234 (10(-6) mol/l). In cell-attached patches channel Po was less than 3%. After a 3-day incubation in culture medium containing an elevated glucose concentration (22.5 mmol/l) the channel activity in attached patches was markedly increased.(ABSTRACT TRUNCATED AT 250 WORDS)