Involvement of calcium-activated potassium channels in the regulation of DNA synthesis in cultured Muller glial cells

Purpose. To determine the involvement of Ca²⁺-activated K⁺ channels of big conductance (BK) and of Ca²⁺ channels in the regulation of DNA synthesis in cultured guinea pig Muller cells. DNA synthesis was stimulated by elevated extracellular potassium, by serum, or by epidermal growth factor. Methods. Dissociated retinas from guinea pigs were cultured for 8 days. Just before confluence was achieved, the cultures were treated with the test substances in serum-free or serum-containing media. The rates of DNA synthesis were assessed by a quantitative bromodeoxyuridine immunoassay. The intracellular Ca²⁺ concentration was measured by the fura-2 fluorescence technique. Results. Blocking the BK channels with tetraethylammonium or by iberiotoxin had no effect at normal extracellular K⁺ (5.8 mM) but decreased the rate of DNA synthesis at higher extracellular K⁺ (10 or 25 mM). Epidermal growth factor-induced DNA synthesis was decreased by block of BK channels or by application of the Ca²⁺ channel blockers nimodipine and flunarizine. Application of epidermal growth factor elevated the intracellular Ca²⁺ concentration of cultured Muller cells. This elevation was diminished by co-application of iberiotoxin or of flunarizine. Conclusions. The activity of BK channels is necessary for elevated DNA synthesis in Muller cells when their membranes are depolarized and/or when the Ca²⁺ influx into Muller cells is increased by growth factors. BK channels may contribute to the maintenance of DNA synthesis by increasing mitogen-induced increase in intracellular Ca²⁺ concentration.