Monovalent cation conductance of the sustained inward current in rabbit sinoatrial node cells

Under the whole cell clamp, superfusion of the rabbit sinoatrial node cells with a Na+-free solution suppressed the sustained inward current (I-st), and the L-type Ca2+ current (I-Ca,I-L) could be recorded on depolarization less negative than -40 mV from the holding potential of -80 mV. On the other hand, replacement of Ca2+ with Mg2+ in the external solution suppressed inward-going I-Ca,I-L and isolated I-st. Under this condition, I-st measured as a nicardipine-sensitive current showed an activation threshold between -60 and -70 mV. The conductance sequence of I-st for monovalent ions was determined as Na+ > Li+ much greater than K+ congruent to Cs+ by replacing the external Na+ with these alkali metal ions. The contribution of I-st to the diastolic depolarization is discussed.