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

摘要

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 (10m), Ca2+ free solutions and divalent cation Ca-v channel blockers. It was activated by the BKCa channel activator NS1619 (30m). Single channel recordings revealed the conductance of the channels to be 24411pS (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 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.