Calcium-activated BKCa channels govern dynamic membrane depolarizations of horizontal cells in rodent retina.

Abstract

Large conductance, Ca2+ -activated K+ (BKCa ) channels play important roles in mammalian retinal neurons, including photoreceptors, bipolar cells, amacrine cells and ganglion cells, but they have not been identified in horizontal cells. BKCa channel blockers paxilline and iberiotoxin, as well as Ca2+ free solutions and divalent cation Cav channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150mm K+ , single channels had a conductance close to 250pS, within the range of all known BKCa channels. In current clamped horizontal cells, BKCa channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BKCa channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells. Large conductance, calcium-activated potassium (BKCa ) channels have numerous roles in neurons including the regulation of membrane excitability, intracellular [Ca2+ ] regulation, and neurotransmitter release. In the retina, they have been identified in photoreceptors, bipolar cells, amacrine cells and ganglion cells, but have not been conclusively identified in mammalian horizontal cells. We found that outward current recorded between -30 and +60mV is carried primarily in BKCa channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BKCa channel blocker paxilline (100nm), iberiotoxin (10μm), Ca2+ free solutions and divalent cation Cav channel blockers. It was activated by the BKCa channel activator NS1619 (30μm). Single channel recordings revealed the conductance of the channels to be 244±11pS (n=17; symmetrical 150mm K+ ) with open probability being both voltage- and Ca2+ -dependent. The channels showed fast activation kinetics in response to Ca2+ influx and inactivation gating that could be modified by intracellular protease treatment, which suggests β subunit involvement. Under current clamp, block of BKCa current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BKCa current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BKCa current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.