Time course of postnatal changes in rat heart action potential and in transient outward current is different.

The rat ventricular action potential shortens after birth. The contribution of increases in the transient outward current (Ito) to postnatal action potential shortening was assessed by measuring Ito in isolated cells and by determining the effect of 2 mM 4-aminopyridine (4-AP) on the action potentials of papillary muscles. 4-AP had no effect on 1-day action potential duration at 25% repolarization (APD25), and 1-day cells had little Ito. In 8- to 10-day muscles, 4-AP caused a small, but significant, increase in APD25. Ito increased slightly between day 1 and days 8-10, but this increase was not significant. Most of the increase in Ito (79%) and in the response to 4-AP (64%) occurred between days 8-10 and adult; however, approximately 75% of the APD25 shortening took place by days 8-10. Thus, while Ito may contribute to repolarization in late neonatal and adult cells, the different time courses of action potential shortening and increases in Ito suggest that changes in Ito are unlikely to be responsible for most of the postnatal action potential shortening.