Ontogenetically determined changes in the duration of ventricular action potentials of the rabbit heart under various conditions of stimulation.

Using glass microelectrodes, we measured the duration of action potentials of the right ventricular papillary muscles and myocardium of rabbits aged 1--3 (newborn), 7, 10 and 30 days and adult, stimulated in different ways. In the steady state, the duration of action potentials of the myocardium of adult animals was longest on stimulation at 1.5 Hz frequency. Frequencies above and below this value shortened the action potentials. In newborn animals, the duration of action potentials was the same at all the stimulation frequencies used. After a two minutes' break, the first action potential of the myocardium of newborn animals was the longest (and the same length as in the steady state); in older age groups its duration shortened, in inverse proportion to age. After a two minutes' break, a steady state was reattained the soonest in newborn animals (the second action potential in the series was the same length as the first); with advancing age more action potentials were needed before a steady state was attained (in adult animals, at 1 Hz frequency, usually five). The results show that the ventricular myocardium does not acquire the ability to react to different and changing stimulation frequencies by a change in the duration of action potentials until during postnatal ontogenesis (mainly in the first month).