Development of the action potential of the prenatal rat heart

Transmembrane action potentials were recorded from embryonic rat hearts at ages between 10½ days of gestation and birth (21 to 22 days). The resting potential averaged 30 to 40 mv at 10½ days and increased to around 80 mv by birth. Two periods of rapid increase in resting potential were noted: 1) from 10½ to 13½ days and 2) from 19½ days of gestation to birth. The maximal upstroke velocity of the action potential increased as the resting potential increased. The duration of the ventricular and atrial action potentials was inversely related to heart rate, and their rate sensitivity decreased with age. The atrial action potential lost its rate sensitivity by 13½ days of gestation and the ventricular action potential lost its by birth. At 10½ days diastolic depolarization was found in the area of the ventricle proximate to the A-V junction, the left and right atria, and the sinus venosus. The velocity of the diastolic depolarization was greatestin the sinus venosus and least in the ventricle. The diastolic depolarization decreased with growth and was lost by 13½ days of gestation.