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 (Ist), and the L-type Ca2+ current (ICa,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 ICa,L and isolated Ist. Under this condition, Ist measured as a nicardipine-sensitive current showed an activation threshold between -60 and -70 mV. The conductance sequence of Ist for monovalent ions was determined as Na+ > Li+ >> K+ approximately = Cs+ by replacing the external Na+ with these alkali metal ions. The contribution of Ist to the diastolic depolarization is discussed.