Chloride efflux inhibits single calcium channel open probability in vertebrate photoreceptors: Chloride imaging and cell-attached patch-clamp recordings

The present study uses cell-attached patch-recording techniques to study th e single-channel properties of Ca2+ channels in isolated salamander photore ceptors and investigate their sensitivity to reductions in intracellular Cl -. The results show that photoreceptor Ca2+ channels possess properties sim ilar to L-type Ca2+ channels in other preparations, including (1) enhanceme nt of openings by the dihydropyridine agonist, (-)BayK8644; (2) suppression by a dihydropyridine antagonist, nisoldipine; (3) single-channel conductan ce of 22 pS with 82 mM Ba2+ as the charge carrier; (4) mean open probabilit y of 0.1; (5) open-time distribution fit with a single exponential (tau(0) = 1.1 ms) consistent with a single open state; and (6) closed time distribu tion fit with two exponentials (tau(cl) = 0 7 ms, tau(c2) -25.4 ms) consist ent with at least two closed states. Using a Cl--sensitive dye to measure i ntracellular [Cl-], it was found that perfusion with gluconate-containing, low C-l- medium depleted intracellular [C-l-]. It was therefore possible to reduce intracellular [Cl-] by perfusion with a low Cl- solution while main taining the extracellular channel surface in high Cl- pipette solution. Und er 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- so lutions. Examination of the open and closed time distributions suggests tha t the reduction in open probability arises from increases in closed-state d well times. Changes in intracellular [Cl-] may thus modulate photoreceptor Ca2+ channels.