Inward current of the rabbit sinoatrial node cell

Summary Inward currents of the rabbit sinoatrial node cell were examined in voltage-clamp experiments using the two-microelectrode technique. A fast and slow inward current could be separated from each other. The slow inward current was blocked by Mn and D 600, but it was insensitive to TTX. On the contrary, the fast inward current was blocked by TTX, but not by Mn and D 600. Both the fast and slow inward current disappeared on Na removal.The fast inward current system was fully inactivated by holding the membrane potential positive to –40 to –50 mV, while the slow inward current system was recorded with the holding potential up to –20 mV. The voltage dependence of the inactivation of the 2 inward current systems and their dependence on [Na]₀ suggests that the rising phase of the spontaneous action potential in the S-A node cell is produced mainly by Na current through the slow inward current system.