[Postnatal variations in the electrophysiological characteristics of the mouse heart]

Electrocardiograms were recorded in 13 mice, from birth up to 57 days of age; during this period the heart rate, at rest, showed a marked increase (from 310 to 797 beats/min), occurring in two waves. Recordings of the intracellular electrical activity in the atria disclosed a different temperature dependence of the resting membrane potential, action potential duration and Vmax in the neonatal mouse as compared with the adult one. The results are consistent with the suggestion that a decrease of the 'slow inward' conductance/K+ outward conductance ratio still occurs during a postnatal period.