Unitary Cl- channels activated by cytoplasmic Ca2+ in canine ventricular myocytes.
Recent whole-cell studies have shown that Ca(2+)-activated Cl- currents contribute to the Ca(2+)-dependent 4-aminopyridine-insensitive component of the transient outward current and to the arrhythmogenic transient inward current in rabbit and canine cardiac cells. These Cl(-)-sensitive currents are activated by Ca2+ release from the sarcoplasmic reticulum and are inhibited by anion transport blockers; however, the unitary single channels responsible have yet to be identified. We used inside-out patches from canine ventricular myocytes and conditions under which the only likely permeant ion is Cl- to identify 4-aminopyridine-resistant unitary Ca(2+)-activated Cl- channels, Ca2+ applied to the cytoplasmic surface of membrane patches activated small-conductance (1.0 to 1.3 pS) channels. These channels were Cl- selective, with rectification properties that could be described by the Goldman-Hodgkin-Katz current equation. Channel activity exhibited time independence when cytoplasmic Ca2+ was held constant and was blocked by the anion transport blockers, DIDS and niflumic acid. Ca2+ (ranging from pCa > or = 6 to pCa 3) applied to the cytoplasmic surface of inside-out patches increased, in a dose-dependent manner, NPo, where N is the number of channels opened and Po is open probability. At negative membrane potentials (-60 to -130 mV), an estimate of the dependence of NPo on cytoplasmic Ca2+ yielded an apparent Kd of 150.2 mumol/L. At pCa 3, an average channel density of approximately equal to 3 microns-2 was estimated. Calculations based on these estimates of cytoplasmic Ca2+ sensitivity and channel current amplitude and density suggest that these small-conductance Cl- channels contribute significant whole-cell membrane current in response to changes in intracellular Ca2+ within the physiological range. We suggest that these small-conductance Ca(2+)-activated Cl- channels underlie the transient Ca(2+)-activated 4-aminopyridine-insensitive current, which contributes to phase-1 repolarization, and under conditions of Ca2+ overload, these channels may generate transient inward currents, contributing to the development of triggered cardiac arrhythmias.
Collier
M L
ML
Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno 89557-0046, USA.
Levesque
P C
PC
Kenyon
J L
JL
Hume
J R
JR
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D003593:Cytoplasm / Q000737:chemistry*; D004285:Dogs; D004553:Electric Conductivity; D005260:Female; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009544:Niflumic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques
D018118:Chloride Channels; D009544:Niflumic Acid; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D002118:Calcium