Anion and cation modulation of the guinea-pig ventricular action potential during β-adrenoceptor stimulation

Abstract

Modulation of the ventricular action potential by β-adrenergic activation of Ca²⁺, K⁺ and cyclic adenosine monophosphate (cAMP)-dependent Cl⁻ channels was assessed in enzymatically isolated guinea-pig ventricular myocytes. The effectiveness and relative selectivity of 9-anthracene carboxylic acid (9-AC), as an antagonist of cAMP-dependent Cl⁻ channels was also tested. Membrane currents and action potentials were recorded using the conventional whole-cell variant of the patch-clamp technique or with the amphotericin B perforated-patch technique. The β-adrenergic agonist isoproterenol either increased or decreased action potential duration depending on whether the dominant effect was on inward Ca²⁺ currents or on outward K⁺ or Cl⁻ currents. When Ca²⁺ and K⁺ channel modulation was prevented by nisoldipine and low temperature respectively, β-adrenergic activation of Cl⁻ channels caused a significant reduction in action potential duration and a slight depolarization of the membrane potential. The β-adrenergic-mediated effects were reversed by the Cl⁻ channel blocker, 9-AC. In the absence of β-adrenergic stimulation, 9-AC had no detectable effects on action potentials or Ca²⁺ currents. These results suggest that β-adrenergic activation of Cl⁻ channels is a potent mechanism for regulation of action potential duration and that 9-AC may be a useful, relatively specific, pharmacological tool for evaluating the physiological role of cAMP-activated Cl⁻ channels in heart. 9-AC also reversed the ability of isoproterenol to antagonize prolongation of action potential duration by the class III antiarrhythmic agent E-4031.