Developmental change in the function of sarcoplasmic reticulum.

This study investigates the developmental changes of sarcoplasmic reticulum function in rabbit heart. We used ryanodine to evaluate sarcoplasmic reticulum function in the isolated arterially perfused fetus, newborn and adult rabbit heart muscles. The magnitude of the negative inotropic effect of ryanodine was similar in the fetus and the 3-day-old newborn, and it was less than in the 7-day-old while the effect in the 7-day-old newborn was significantly less than in the adult. In the adult, and not in the newborn, ryanodine (10(-6) M) caused prolongation of half relaxation time and time to peak tension, and an increase in resting tension. In addition, ryanodine (10(-1) M was more effective in the adult, compared to the 3-day-old newborn, in the attenuation of the positive inotropism of staircase, hyperosmolarity and paired electrical stimulation. These data suggest that sarcoplasmic reticulum function dramatically changes during the perinatal period, and furthermore, suggests that the adult hearts are most likely dependent on Ca2+ release and uptake from sarcoplasmic reticulum, while the perinatal hearts are more dependent on transsarcolemmal Ca2+ influx than on sarcoplasmic reticulum Ca2+ release in excitation-contraction coupling.