Possible involvement of a chloride conductance in the transient outward current of whole-cell voltage-clamped ferret ventricular myocytes

Ca2+-induced current oscillations in rabbit ventricular myocytes

Transient currents are activated by spontaneous Ca2+ oscillations in rabbit ventricular myocytes. We investigated the ionic basis for these transient currents under conditions in which K+ currents would be expected to be blocked. Holding cells under voltage clamp at positive potentials leads to a rise in intracellular Ca2+ via reversal of the Na+-Ca2+ exchanger and subsequently to the initiation of spontaneous Ca2+ transients, presumably from a Ca2+-overloaded sarcoplasmic reticulum. The current transients associated with these Ca2+ transients reversed at about +10 to +15 mV under conditions of approximately symmetrical Cl-. In the absence of Cl-, this current was inward at all potentials examined over the range from -88 to +72 mV, consistent with a Na+-Ca2+ exchanger current. In the absence of Na+, the repetitive spontaneous Ca2+ transients could be initiated by a brief train of depolarizations to activate the inward Ca2+ current. Under such conditions, the current was found to reverse at -3 mV when the equilibrium potential of Cl- (ECl) was -2 mV, and the reversal potential shifted to -32 mV when internal Cl- was lowered, to make ECl -33 mV. Thus, in the absence of Na+, it appears that the current is exclusively a Ca2+-activated Cl- current. There is no evidence to indicate the presence of a Ca2+-activated cationic conductance. Further, our results demonstrate that the Ca2+-activated Cl- conductance can carry inward current at potentials more negative to ECl in rabbit ventricular myocytes and is therefore likely to contribute to the arrhythmogenic delayed afterdepolarizations that occur in Ca2+-overloaded cells.

Effects of chloride ion substitutes and chloride channel blockers on the transient outward current in rat ventricular myocytes

The Cai(2+)-insensitive transient outward current, ilo was studied at 20-24 degrees C in rat ventricular myocytes with the whole cell recording patch-clamp technique. The current was recorded before and after replacement of chloride by methanesulfonate or aspartate or in the absence and the presence of chloride channel blockers, SITS or 9-anthracene carboxylic acid. In control conditions (in the presence of external divalent cations, Ca2+ and Cd2+, Cd2+ being used to suppress Ca2+ current), ilo inactivation was composed of a fast and a slow component. When methanesulfonate was substituted for external Cl-, the peak current decreased to a variable extent, but the inactivation of the remaining current was still composed of a fast and a slow component. In contrast, the inactivation of the difference current was well fitted by a single exponential. The time to peak of the difference current was shorter than that of the current recorded either in the absence or the presence of methanesulfonate. Both activation- and steady-state inactivation-voltage curves were either unchanged (n = 4) or shifted by a few mV (5.5 mV, n = 14) towards positive potentials when methanesulfonate was substituted for Cl-. The current remaining in methanesulfonate reversed at potentials closed to EK. The difference current was composed of a peak and a steady-state component. The peak was suppressed by 4-aminopyridine whereas the steady-state component was not. The peak was also suppressed when pipette solution contained Cs+ instead of K+ but was still present when the Hepes concentration in both external and pipette media was increased 5-fold (50 mM vs. 10 mM). When aspartate was substituted for Cl- or when 2 mM SITS was added to the external solution (in the absence of Ca2+ and Cd2+ because aspartate is known to chelate Ca2+ ions and possibly other divalent cations), ilo was reduced to a similar extent in the two cases and the difference current was composed of a peak (inactivation fitted by a single exponential) and a steady-state component. The SITS-sensitive transient current reversed at a potential close to ECl. When 5 mM 9-anthracene carboxylic acid was added to external solution (in the presence of Ca2+ and Cd2+), the peak of the difference current was similar to that observed when Cl- was substituted by methanesulfonate. The difference current resulting from the substitution of methanesulfonate for chloride was not changed when the pipette solution contained either 50 mM EGTA (instead of 5 mM) or 10 mM EGTA and 10 mM BAPTA. The nature of Cs(+)- and 4-aminopyridine-sensitive transient outward current suppressed by chloride ion substitutes or chloride channel blockers is discussed.

Effects of extracellular Cl- on the inotropic response to alpha-adrenoceptor stimulation

This study was designed to determine if the sustained positive inotropic action of alpha-adrenergic stimulation is affected by the absence of extracellular chloride ion (Clo-). Atrial and papillary muscle were isolated from adult male rats, bathed in Krebs-Henseleit solution (30 degrees C) with and without Cl- (methane-sulfonate substitution), and stimulated at 0.5 Hz. Isometric developed tension was monitored during cumulative addition of phenylephrine, isoproterenol and Ca2+. The dose-dependent positive inotropic effects of isoproterenol and Ca2+ were not altered by the absence of Clo-. However, the magnitude of the response to phenylephrine was diminished in both tissues. In atrial muscle, the maximum positive inotropic effect of phenylephrine was reduced from 2.05 +/- 0.17 g in the presence of Clo- to 0.39 +/- 0.06 g in the absence of Clo-; control developed tension was 0.60 +/- 0.08 and 0.47 +/- 0.10 g in these two groups before exposure to the alpha-adrenoceptor agonist. In papillary muscle, control developed tension was 1.40 +/- 0.11 and 1.17 +/- 0.18 g in the presence and absence of Clo-, respectively; and the maximum inotropic responses to phenylephrine were 0.71 +/- 0.12 and 0.27 +/- 0.13 g. EC50 values for phenylephrine were not significantly affected by substitution for Cl-. Similar results were observed in a Hepes-buffered bathing solution without bicarbonate (HCO3-). These results indicate that the positive inotropic action of alpha-adrenergic stimulation is mediated in part by a mechanism requiring Cl-. Furthermore, data suggest that the antagonistic effect of Clo- removal is not mediated via Cl-/HCO3- exchange.