Contribution of Ca2+ transporters to relaxation in intact ventricular myocytes from developing rats

The relative contributions of Ca2+ transporters to intracellular Ca2+ concentration ([Ca2+](i)) decline associated with twitch relaxation were analyzed in intact ventricular myocytes from developing and adult rats. This was accomplished by estimation of individual integrated Ca2+ fluxes with the use of kinetic parameters calculated from [Ca2+](i) measurements during twitches and caffeine-evoked contractures, and from myocardial passive Ca2+ buffering data. Our main findings were the following: 1) twitch relaxation and [Ca2+](i) decline were significantly slower during the first postnatal week than in adults, 2) inhibition of sarcoplasmic reticulum (SR) Ca2+ accumulation resulted in faster [Ca2+](i) decline in young cells than in adult cells, 3) the contributions of the SR Ca2+ uptake and Na+/Ca2+ exchange (NCX) to twitch relaxation increased from similar to75 to 92%, and decreased from 24 to 5%, respectively, from birth to adulthood, and 4) Ca2+ transport by the sarcolemmal Ca2+-ATPase was apparently increased in neonates. Our data indicate that despite a marked increase in NCX contribution to cell relaxation in immature rats, the SR Ca2+ ATPase appears to be the predominant transporter responsible for relaxation-associated [Ca2+](i) decline from birth to adulthood.