Ca 2+ ‐activated Cl − current in rabbit sinoatrial node cells

The Ca 2+ ‐activated Cl − current ( I Cl(Ca) ) has been identified in atrial, Purkinje and ventricular cells, where it plays a substantial role in phase‐1 repolarization and delayed after‐depolarizations. In sinoatrial (SA) node cells, however, the presence and functional role of I Cl(Ca) is unknown. In the present study we address this issue using perforated patch‐clamp methodology and computer simulations. Single SA node cells were enzymatically isolated from rabbit hearts. I Cl(Ca) was measured, using the perforated patch‐clamp technique, as the current sensitive to the anion blocker 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonic acid (DIDS). Voltage clamp experiments demonstrate the presence of I Cl(Ca) in one third of the spontaneously active SA node cells. The current was transient outward with a bell‐shaped current‐voltage relationship. Adrenoceptor stimulation with 1 μ m noradrenaline doubled the I Cl(Ca) density. Action potential clamp measurements demonstrate that I Cl(Ca) is activate late during the action potential upstroke. Current clamp experiments show, both in the absence and presence of 1 μ m noradrenaline, that blockade of I Cl(Ca) increases the action potential overshoot and duration, measured at 20 % repolarization. However, intrinsic interbeat interval, upstroke velocity, diastolic depolarization rate and the action potential duration measured at 50 and 90 % repolarization were not affected. Our experimental data are supported by computer simulations, which additionally demonstrate that I Cl(Ca) has a limited role in pacemaker synchronization or action potential conduction. In conclusion, I Cl(Ca) is present in one third of SA node cells and is activated during the pacemaker cycle. However, I Cl(Ca) does not modulate intrinsic interbeat interval, pacemaker synchronization or action potential conduction.