The effects of sodium substitution on action potential waveforms in cells with different upstroke velocity of sinoatrial area

Experiments were carried out on spontaneously beating strips of sinoatrial area of a rabbit with microelectrode technique. 20%, 50% and 70% Na+ replacement in outer cellular solution resulted in reversible reduction of upstroke velocity (dV/dt(max)) and overshoot in true pacemaker cells (similar to 3 V/s), latent pacemaker cells (similar to 17 V/s) and "conducting" cells (similar to 130 V/s). The decrease of these parameters more than twice in 50% Na+ solution in true pacemaker cells which are known to be insensitive to TTX (20 mu M) let us to come to the following conclusions: 1) in primary pacemaker cells there is a slow inward current which sensitive to the reduction of the transmembrane electrochemical Na+ gradient; 2) this current plays an important role in the upstroke mechanism of pacemaker action potentials in rabbit's SA node and is transferred by Na+ ions.