Kinetics and rectification of the slow inward current in the rabbit sinoatrial node cell

The slow inward current (is) in the rabbit sinoatrial node cell was studied by the conventional two-microelectrode voltage clamp technique. When is was measured as the difference between two records obtained before and after blocking is with D 600, the fully activated current (is)-voltage relation was non-linear; the conductance decreased in the negative potential range resulting in an almost constant amplitude of is negative to -10 mV. The degree of steady-state activation was about 1 at -5mV and 0 at -65mV. The recovery time course of is during repolarization was measured by varying the interval between two sequential depolorizing pulses with various holding potentials. The time constant of the exponential recovery time course was about 120 msec at -40 mV and decreased to about 40 msec at -70 mV. The steady-state conductance of is, calculated from the activation and inactivation curves, produced a large hump in the steady-state current voltage relation between -60 and -20 mV, which was not observed in the experiment. When the above kinetics were incorporated, the S-A node model failed to discharge the spontaneous activity. The activation and inactivation curves which can simulate the experimental I-V curve and the action potential were proposed.