DISTRIBUTION OF THE ISOPRENALINE-INDUCED CHLORIDE CURRENT IN RABBIT HEART

Current density of the isoprenaline-induced chloride current (I(Cl)) was measured in sino-atrial (SA) node cells and atrial and ventricular myocytes dissected enzymatically from the rabbit heart. In addition to the conventional voltage clamp method the whole-cell patch clamp method using nystatin was employed to avoid rundown of I(Cl) in dialysed cells. Isoprenaline (0.3-mu-M) failed to induce I(Cl) in the 20 atrial cells examined. The integrity of the beta-adrenergic system was established by recording the response of the Ca2+ current in the same cell. Both isoprenaline and acetylcholine failed to affect the background membrane conductance in the 20 SA node cells studied. Myocytes isolated from the epicardial region of the left ventricular wall showed relatively higher I(Cl) density (24.9 +/- 12.1-mu-S/mu-F) than those from the endocardial side (12.3 +/- 8.5-mu-S/mu-F). We conclude that beta-receptor-operated I(Cl) is insignificant in atrial and SA node cells.