Excitation-contraction coupling in neonatal and adult myocardium of cat

Neonatal cardiac cells were smaller in diameter, having a lower concentration of myofilaments than cardiac cells of the adult cat. The sarcoplasmic reticulum-content in 1-day-old neonates was less than in the adult, and there were no transverse tubules in the neonatal myocardium. Postextrasystolic potentiation and post-voltage clamp potentiation were significantly greater in the adult than in the neonate. Rate inotropisms consisted of a fast component (1st 6-8 beats) and a slow component (50-100 beats). The beat constants for the decay of postextrasystolic potentiation and of the fast component of a negative frequency staircase were the same in both neonate and adult. The restitution of contractility was much faster in the neonate than in the adult. Shortening of the action potential plateau suppressed twitch tension in the first beat with little further effect on subsequent shortened beats in the neonate. The structural and functional differences between the neonate and adult lead to the conclusion that two sources of activator calcium contribute to the development of tension in mammalian ventricle.