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

Pericytes are considered to contribute to the regulation of retinal microcirculation which is impaired in diabetic retinopathy. Single, large-conductance, Ca²⁺-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 4.2×10^–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 (P_o) 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 1.5×10^–5 mol/l) blocked from the intracellular side. Tetraethylammonium induced a dose-dependent block from the outside only with a halfmaximal blocking concentration of 2.5×10^–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 channelP _o 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. These data indicate that cultured retinal pericytes possess a BK channel. The activity of the channel increases after incubation with elevated glucose concentrations, which could indicate altered regulation of the channel under these conditions. The implications of altered function of BK channels are discussed with respect to haemodynamic changes observed in diabetic retinopathy.