Frequency-dependent effects of 4-aminopyridine and almokalant on action-potential duration of adult and neonatal rabbit ventricular muscle.

The effects of 4-aminopyridine (1 mM) and almokalant (1 microM) on action-potential duration of neonatal and adult rabbit ventricular multicellular preparations and plateau membrane currents of single ventricular myocytes were studied. In adult ventricular preparations, 4-aminopyridine increased action-potential duration in a frequency-dependent manner, with a greater effect at low stimulation frequencies ("reverse" use dependence). In neonatal preparations, the increase in action-potential duration by 4-aminopyridine was significantly smaller than in adults, and the effect was frequency independent. Almokalant increased the action-potential duration more in neonatal than in adult myocytes. The effect of almokalant was frequency independent between 0.5 and 2 Hz. The block of transient outward current and delayed rectifier current in single myocytes was quantitatively similar. We propose that differences in the kinetic behavior of the transient outward current between adult and neonatal ventricular preparations, slower inactivation, and recovery from inactivation in adults determine differences in the frequency-dependent changes induced by 4-aminopyridine and almokalant on action-potential duration.