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
(τ0 = 1.1 ms) consistent with a single open
state; and (6) closed time distribution fit with two exponentials
(τc1 = 0.7 ms, τc2 = 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.