Ca2+-activated K+ current at presynaptic terminals of goldfish retinal bipolar cells

Properties of the Ca2+-activated K+ current (I-K(Ca)) were investigated in bipolar cells isolated from the goldfish retina. Pharmacological experiments and single channel current recordings demonstrated that I-K(Ca) represented currents through BK channels, which were confined mostly to the presynaptic terminal. The ensemble noise analysis of I-K(Ca), which was evoked following the activation of presynaptic Ca2+ current, revealed that the single channel conductance and open probability (P-o) were approximately 50 pS ([K+](o) = 2.6 mM, [K+](i) = 140 mM) and 0.6 at 0 mV, respectively. To estimate [Ca2+](i) at the cytosolic side of BK channels, activation of I-K(Ca) was examined in Ca2+-loaded bipolar cells bathed in Co2+ solution. [Ca2+](i) was monitored using furaptra fluorimetry. It was found that [Ca2+](i) ranged between 10 and 20 mu M when P-o was 0.6. A high concentration of BAPTA (> 20 mM) was required to suppress I-K(Ca). Under this condition, channel number was reduced without changing P-o. Therefore, it is likely that some BK channels are co-localized with Ca2+ channels in presynaptic terminals of retinal bipolar cells. (C) 1997 Elsevier Science Ireland Ltd.