title: Chloride efflux inhibits single calcium channel open probability in vertebrate photoreceptors: chloride imaging and cell-attached patch-clamp recordings.
abstract: The present study uses cell-attached patch-recording techniques to study the single-channel properties of Ca2+ channels in isolated salamander photoreceptors and investigate their sensitivity to reductions in intracellular Cl-. The results show that photoreceptor Ca2+ channels possess properties similar to L-type Ca2+ channels in other preparations, including (1) enhancement of openings by the dihydropyridine agonist, (-)BayK8644; (2) suppression by a dihydropyridine antagonist, nisoldipine; (3) single-channel conductance of 22 pS with 82 mM Ba2+ as the charge carrier; (4) mean open probability of 0.1; (5) open-time distribution fit with a single exponential (tau0 = 1.1 ms) consistent with a single open state; and (6) closed time distribution fit with two exponentials (tau(c1) = 0.7 ms, tau(c2) = 25.4 ms) consistent with at least two closed states. Using a Cl- -sensitive dye to measure intracellular [Cl-], it was found that perfusion with gluconate-containing, low Cl- medium depleted intracellular [Cl-]. It was therefore possible to reduce intracellular [Cl-] by perfusion with a low Cl- solution while maintaining the extracellular channel surface in high Cl- pipette solution. Under these conditions, the single-channel conductance was unchanged, but the mean open probability fell to 0.03. This reduction can account for the 66% reduction in whole-cell Ca2+ currents produced by perfusion with low Cl- solutions. Examination of the open and closed time distributions suggests that the reduction in open probability arises from increases in closed-state dwell times. Changes in intracellular [Cl-] may thus modulate photoreceptor Ca2+ channels.
authors.affiliation: Department of Ophthalmology, University of Nebraska Medical Center, Omaha 68198-5540, USA. wbthores@unmc.edu
authors.forename: W B
authors.initials: WB
authors.lastname: Thoreson
authors.forename: R
authors.initials: R
authors.lastname: Nitzan
authors.forename: R F
authors.initials: RF
authors.lastname: Miller
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000557:Ambystoma; D000818:Animals; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D002712:Chlorides / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology; D005456:Fluorescent Dyes; D008564:Membrane Potentials; D015737:Nisoldipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D020419:Photoreceptor Cells, Vertebrate / Q000502:physiology*
chemical_list: D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002712:Chlorides; D004095:Dihydropyridines; D005456:Fluorescent Dyes; D015737:Nisoldipine; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; C038806:1,4-dihydropyridine