title: Calcium-activated BKCachannels 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. BKCachannel blockers paxilline and iberiotoxin, as well as Ca2+free solutions and divalent cation Cavchannel 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 BKCachannels. In current clamped horizontal cells, BKCachannels subdue depolarizing membrane potential excursions, reduce the average resting potential and decrease oscillations. The results show that BKCachannel 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 +60 mV is carried primarily in BKCachannels 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 BKCachannel blocker paxilline (100 nm), iberiotoxin (10 μm), Ca2+free solutions and divalent cation Cavchannel blockers. It was activated by the BKCachannel 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 BKCacurrent increased depolarizing membrane potential excursions, raising the average resting potential and producing oscillations. BKCacurrent activation with NS1619 inhibited oscillations and hyperpolarized the resting potential. These effects underscore the functional role of BKCacurrent in limiting depolarization of the horizontal cell membrane potential and suggest actions of these channels in regulating the temporal responsivity of the cells.
authors.affiliation: Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Sun
authors.affiliation: Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.|Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, USA.
authors.forename: Arlene A
authors.initials: AA
authors.lastname: Hirano
authors.affiliation: Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.|Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, USA.|Departments of Medicine, Ophthalmology and Stein Eye Institute, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Brecha
authors.affiliation: Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA.|Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, CA, USA.|Departments of Physiology & Biophysics and Ophthalmology & Visual Sciences, Dalhousie University, Halifax, Nova Scotia, Canada.
authors.forename: Steven
authors.identifier: 0000-0003-0496-1645
authors.initials: S
authors.lastname: Barnes
mesh_terms: D000200:Action Potentials; D000818:Animals; D001562:Benzimidazoles; D002478:Cells, Cultured; D007211:Indoles / Q000494:pharmacology; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D055351:Retinal Neurons / Q000378:metabolism* / Q000502:physiology
chemical_list: D001562:Benzimidazoles; D007211:Indoles; D051036:Large-Conductance Calcium-Activated Potassium Channels; D010455:Peptides; D026902:Potassium Channel Blockers; C086491:NS 1619; C048220:paxilline; C064719:iberiotoxin
keywords: K channel; mammalian retina; outward rectifier