Sarcolemmal Na+-Ca2+ exchange and sarcoplasmic reticulum Ca2+ uptake in several cardiac preparations

Calcium transport mediated by the sarcolemma (SL) Na+-Ca2+ exchanger and by the sarcoplasmic reticulum (SR) Ca2+ pump were studied in crude membranes from ventricular tissue of newborn and young rat, adult frog, guinea pig, chick, hamster, Rhesus monkey, rabbit, and pig. Transport activities were evaluated per mass of membrane protein and heart tissue. Relative to unit of ventricular mass Na+-Ca2+ exchange activity (at [Ca2+] = 75 microM) is highest in newborn rat, frog, guinea pig, and chick amounting to about 17.7, 17.5, 16.2, and 10.1 mumol Ca2+/kg wet weight/2s, respectively. Considerable lower values were obtained in the other species. The lowest Na+-Ca2+ exchange rate was found in 40-day old rat amounting to 2.4 mumol Ca2+/kg wet weight/2s. Parallel determination of active SR Ca2+ uptake in the same membrane preparations reveals that the quantitative relationship between the two different Ca transport processes is not invariant with respect to species and stage of differentiation. The data suggest that Ca2+ handling in small-sized cardiac cells with poorly developed SR (newborn rat, frog) depends to a greater extent on Ca2+ fluxes via the SL Na+-Ca2+ exchanger as compared to differentiated cardiac cells of adult mammalian and avian species. By contrast, the contribution of SR Ca2+ transport to cardiac Ca2+ movements becomes more predominant in differentiated cardiac cells.