title: Modulation of A-type potassium currents in retinal horizontal cells by extracellular calcium and zinc.
abstract: Extracellular Ca2+ and Zn2+ influence many aspects of retinal function. Here, we examined the effect of external Ca2+ and Zn2+ on potassium channels of retinal horizontal cells. When extracellular Ca2+ was lowered from 3 mM to 0.3 mM, horizontal cell transient outward currents elicited by voltage steps from resting membrane potential (-70 mV) were decreased by approximately 50%, whereas the sustained currents remained unchanged. This effect was due to a hyperpolarizing shift in the steady-state inactivation curve of A-type K+ currents when extracellular Ca2+ concentration was lowered. The mean half inactivation potential of the steady-state inactivation curves was hyperpolarized from -56.3 +/- 4.7 mV in 3 mM Ca2+ to -76.4 +/- 3.9 mV in 0.3 mM Ca2+. Neither the state-steady activation curve nor the kinetics of inactivation was significantly changed in low extracellular Ca2+. The addition of 30 microM Zn2+ restored peak outward currents in 0.3 mM Ca2+. The half inactivation voltages were depolarized from -70 +/- 2.8 mV in 0.3 mM Ca2+ to -56 +/- 2.6 mV in 0.3 mM Ca2+ plus 30 microM Zn2+. Taken together, the results indicate that external Ca2+ and Zn2+ maintain the activity of A-type potassium channels in retinal horizontal cells by influencing the voltage dependence of steady-state inactivation.
authors.affiliation: Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee 37235-1634, USA.
authors.forename: Dao-Qi
authors.initials: DQ
authors.lastname: Zhang
authors.forename: Ziyi
authors.initials: Z
authors.lastname: Sun
authors.forename: Douglas G
authors.initials: DG
authors.lastname: McMahon
mesh_terms: D000818:Animals; D001492:Bass / Q000502:physiology; D002118:Calcium / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques / Q000379:methods; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000166:cytology*; D051248:Retinal Horizontal Cells / Q000187:drug effects* / Q000502:physiology / Q000528:radiation effects; D015032:Zinc / Q000494:pharmacology*
chemical_list: D015221:Potassium Channels; D015032:Zinc; D002118:Calcium