Inward current activated during hyperpolarization in the rabbit sinoatrial node cell

Abstract

Inward current activated by hyperpolarizationi _h) 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. Thei _h 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 ofi _h ranged from 2–4 s at about −70 mV, but it became shorter at about −10 mV. Thei _h 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. Althoughi _h 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.