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

We have reported previously that the sinoatrial node (SAN) in the newborn r abbit expresses a Na+ current (I-Na) with properties similar to the neurona l type-I isoform and that this current contributes to the net inward curren t flowing during diastolic depolarization. To characterize this current fur ther we conducted cell-attached single-channel experiments in isolated newb orn SAN myocytes. The Na+ channel was sensitive to divalent cation block an d had a single-channel conductance of 25.6 pS in the absence of divalent ca tions. Kinetic compatibility between single-channel and previous whole-cell data was confirmed by measuring the time constant of current decay. At pac emaker potentials, time constants were of the order of tens of milliseconds . Additional experiments indicated that this slow inactivation arises becau se the Na+ channels expressed in the neonatal SAN tend to re-open frequentl y at potentials in the pacemaker range. We suggest that this is the mechani sm by which a small tetrodotoxin (TTX)-sensitive current contributes to the total inward current flowing during slow diastolic depolarization in neona tal (but not adult) pacemaker myocytes.