Postnatal developmental changes in the electrophysiological properties of cat right ventricular papillary muscles.

Cellular electrophysiological properties of cat right ventricular papillary muscles were studied during postnatal development using intracellular microelectrodes. Animals were studied in the neonatal period (less than 75 h of age), in infancy (16 to 18 days old) and in adult life (1.8 to 2.3 kg body weight). Resting membrane potential, action potential amplitude, Vmax and action potential duration increased significantly with age. The age related changes in action potential duration were independent of stimulation frequency. Conduction velocity was measured using two simultaneous impalements and increased significantly with age. Exposure to low [Ca]o (0.2 mmol X litre-1) caused a prolongation in action potential duration which was greater in infant and adult than neonatal myocardium, and was greatest at low stimulation frequencies. High [Ca]o shortened action potential duration and this was also greater in infant and adult tissue. Action potential duration was prolonged by low [K]o (0.5 mmol X litre-1) and shortened by high [K]o. Both of these effects appeared to be independent of age. Exposure to ouabain, 10(-8) and 10(-7) mol X litre-1 caused a dose related shortening of action potential duration. This effect was significantly greater in adult and infant than neonatal myocardium. The results indicate that the electrophysiological properties of the cat myocardium continue to develop during early postnatal life. These findings are discussed in relation to changes in morphology and transmembrane ionic gradients occurring at this time.