TIME-COURSE OF POSTNATAL CHANGES IN RAT-HEART ACTION-POTENTIAL AND INTRANSIENT OUTWARD CURRENT IS DIFFERENT

The rat ventricular action potential shortens after birth. The contrib ution of increases in the transient outward current (I-to) to postnata l action potential shortening was assessed by measuring I-to in isolat ed 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 (APD(25)), and 1- day cells had little I-to. In 8- to 10-day muscles, 4-AP caused a smal l, but significant, increase in APD(25). I-to increased slightly betwe en day 1 and days 8-10, but this increase was not significant. Most of the increase in I-to (79%) and in the response to 4-AP (64%) occurred between days 8-10 and adult; however, similar to 75% of the APD(25) s hortening took place by days 8-10. Thus, while I-to may contribute to repolarization in late neonatal and adult cells, the different time co urses of action potential shortening and increases in I-to suggest tha t changes in I-to are unlikely to be responsible for most of the postn atal action potential shortening.