Does the ?pacemaker current? generate the diastolic depolarization in the rabbit SA node cells?

Abstract

Small preparations of spontaneously beating rabbit sino-atrial node (SA node) were voltage clamped with the two-microelectrode technique. The effects of 0.25–5 mM Cs⁺ on the spontaneous pacing rate and the time-dependent inward “pacemaker” current,i _h, were studied. In the presence of 2 mM Cs⁺, the spontaneous pacing rate decreased only slightly even thoughi _h was strongly depressed at potentials negative to −60 mV Cs⁺ had little or no effect on other time-dependent currents observed with clamp pulses less negative than −50 mV. Since no voltage-dependence to the Cs⁺ effect oni _h could be measured (between −90 mV and −20 mV), it was considered unlikely that the lack of Cs⁺ effect on the rate of diastolic depolarization results from a voltage-dependent effect of Cs⁺ on thei _h channel. Adrenaline produced a marked positive chronotropic effect in Cs⁺-treated SA node cells. This effect was accompanied by marked enhancement of the slow inward current (i _si) with no change in the Cs⁺-blockedi _h current. These results are consistent with the idea thati _h plays a minor role in generation of pacemaker depolarization, and suggest a more prominent role ofi _si in the generation of diastolic depolarization in SA nodal cells.