Activation of contractility and sarcolemmal Ca2+-ATPase by Ca2+ during postnatal development of the rat heart

The effects of 0.25-16.0 mM Ca2+ on the contractile force of isolated ventricular strips and sarcolemmal Ca2+-ATPase activity during postnatal development of the rat heart were studied. The half maximal concentrations for contractile activation of ventricular strips were 0.76 and 5.59 mM Ca2+ for adult and 3-day-old rats, respectively. The sensitivity towards Ca2+ began to change from newborn type to that of adult rat 2 weeks after birth and was almost completed after 4 weeks. No significant differences were found in half maximal activation of Ca2+-ATPase by Ca2+ between different age groups. Activation of contractility and Ca2+-ATPase by Ca2+ were linearly related in 30-day-old and adult rats but not in 3- and 10-day-old rats. The observed sensitivity change towards extracellular Ca2+ for contractile activation is suggested to be due to the development of transverse tubular system and sarcoplasmic reticulum during the first 4 weeks of postnatal development.