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

Large conductance, Ca²⁺ -activated K⁺ (BK_Ca ) 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. BK_Ca channel blockers paxilline and iberiotoxin, as well as Ca²⁺ free solutions and divalent cation Ca_v channel blockers, eliminate the outwardly rectifying current, while NS1619 enhances it. In symmetrical 150 mm K⁺ , single channels had a conductance close to 250 pS, within the range of all known BK_Ca channels. In current clamped horizontal cells, BK_Ca channels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BK_Ca channel activation puts a ceiling on horizontal cell depolarization and regulates the temporal responsivity of the cells.

Large conductance, calcium-activated potassium (BK_Ca ) 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 +60 mV is carried primarily in BK_Ca channels in isolated horizontal cells of rats and mice. Whole-cell outward currents were maximal at +50 mV and declined at membrane potentials positive to this value. This current was eliminated by the selective BK_Ca channel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca²⁺ free solutions and divalent cation Ca_v channel blockers. It was activated by the BK_Ca channel activator NS1619 (30 μm). Single channel recordings revealed the conductance of the channels to be 244 ± 11 pS (n = 17; symmetrical 150 mm 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 BK_Ca current increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BK_Ca current activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BK_Ca current in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.