Properties of single outwardly rectifying Cl- channels in heart

Abstract

A variety of potentially important macroscopic Cl- currents have been described in the heart. Although the single-channel properties of the cAMP- dependent current (I(Cl.cAMP)) have been well described, the single-channel equivalents of the other forms of cardiac Cl- current remain unknown. Unlike I(Cl.cAMP), many of these currents show prominent outward rectification in the presence of symmetrical transmembrane Cl- gradients and sensitivity to disulfonic stilbene Cl- transport blockers. We used the patch-clamp technique to search for single Cl- channels in inside-out patches from rabbit atrial cell membranes, under conditions minimizing the chances of observing channels carrying Na+, Ca2+, or K+. Under symmetrical Cl- conditions, single-channel activity was seen in 14 (9%) of 155 patches. Channels showed strong outward rectification and a unitary conductance of 60±3 picosiemens (mean±SEM) at positive voltages. The current-voltage relation was not altered by replacement of cations by the impermeant cation N'-methyl-D-glucamine (NMDG) and shifted as expected for a Cl--selective channel when methanesulfonate was substituted for Cl-. The Cl- transport blockers DIDS (diisothiocyanatostilbene-2,2'-disulfonic acid, 100 μmol/L) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, 1 mmol/L) strongly and reversibly inhibited channel activity when added to the bath and caused channel flickering suggesting open-channel block. Ensemble- average currents showed no time dependence, and the form of the ensemble- average current-voltage relation was similar to that of macroscopic background Cl- current. We conclude that single Cl- channels showing outward rectification and sensitivity to disulfonic stilbenes are present in rabbit atrial cell membranes and may play a role in the regulation of cardiac electrical activity.