Involvement of anion channel(s) in the modulation of the transient outward K+ channel in rat ventricular myocytes

The cardiac Ca2+-independent transient outward K+ current (I-to), a major repolarizing ionic current, is markedly affected by Cl- substitution and anion channel blockers. We reexplored the mechanism of the action of anions on Ito by using whole cell patch-clamp in single isolated rat cardiac ventricular myocytes. The transient outward current was sensitive to blockade by 4-aminopyridine (4-AP) and was abolished by Cs+ substitution for intracellular K+. Replacement of most of the extracellular Cl- with less permeant anions, aspartate (Asp(-)) and glutamate (Glu(-)), markedly suppressed the current. Removal of external Na+ or stabilization of F-actin with phalloidin did not significantly affect the inhibitory action of less permeant anions on Ito. In contrast, the permeant Cl- substitute Br- did not markedly affect the current, whereas F- substitution for Cl- induced a slight inhibition. The Ito elicited during Br- substitution for Cl- was also sensitive to blockade by 4-AP. The ability of Cl- substitutes to induce rightward shifts of the steady-state inactivation curve of Ito was in the following sequence: NO3- > Cl- approximate to Br- > gluconate(-) > Glu(-) > Asp(_). Depolymerization of actin filaments with cytochalasin D (CytD) induced an effect on the steady-state inactivation of Ito similar to that of less permeant anions. Fluorescent phalloidin staining experiments revealed that CytD-pretreatment significantly decreased the intensity of FITC-phalloidin staining of F-actin, whereas Asp(-) substitution for Cl- was without significant effect on the intensity. These results suggest that the Ito channel is modulated by anion channel(s), in which the actin cytoskeleton may be implicated.