Pacemaker Current in Frog Atrium

Abstract

PACEMAKER currents have been investigated by voltage clamp studies in Purkinje fibres^1,2 but not in tissues from those regions of the heart where the natural pacemaker lies: the amphibian sinus and mammalian sino-atrial node. Repetitive activity can often be induced in normally quiescent frog atrial trabeculae by application of small depolarizing constant current pulses³. These currents impose on the atrial cells the low membrane potential characteristic of sinus muscle⁴, Moreover, the potential changes involved are very similar to those which may be recorded from spontaneously active sinus, successive action potentials being separated by phases of slow diastolic depolarization⁴ (Fig. 1a). It seems likely therefore that the membrane current controlling this diastolic depolarization in atrium will closely resemble that which generates the natural sinus pacemaker.