Single-channel properties of the sinoatrial node Na+ current in the newborn rabbit.

We have reported previously that the sinoatrial node (SAN) in the newborn rabbit expresses a Na+ current (INa) with properties similar to the neuronal type-I isoform and that this current contributes to the net inward current flowing during diastolic depolarization. To characterize this current further we conducted cell-attached single-channel experiments in isolated newborn SAN myocytes. The Na+ channel was sensitive to divalent cation block and had a single-channel conductance of 25.6 pS in the absence of divalent cations. Kinetic compatibility between single-channel and previous whole-cell data was confirmed by measuring the time constant of current decay. At pacemaker potentials, time constants were of the order of tens of milliseconds. Additional experiments indicated that this slow inactivation arises because the Na+ channels expressed in the neonatal SAN tend to re-open frequently at potentials in the pacemaker range. We suggest that this is the mechanism by which a small tetrodotoxin (TTX)-sensitive current contributes to the total inward current flowing during slow diastolic depolarization in neonatal (but not adult) pacemaker myocytes.