Inward current activated during hyperpolarization in the rabbit sinoatrial node cell.

Inward current activated by hyperpolarization (ih) was dissected from the K-current by the difference in its activation voltage range and the selective blocking effect of Ba2+ on the K-current. The ih shows little specificity to any particular ion, and its reversal potential was -25 mV. The current system can be expressed well by Hodgkin-Huxley type kinetics. The time constant of ih ranged from 2-4 s at about -70 mV, but it became shorter at about -10 mV. The ih began to activate at -50 mV and fully saturated at about -100 mV. The fully activated current-voltage relation shows no rectifying property. Activation and deactivation time courses were fitted by a single exponential with the same time constant at a given membrane potential. Although ih plays only a small role during the normal action potential in the isolated preparation, it plays a significant role in keeping the pacemaker cell at a low membrane potential