Role of the Sarcoplasmic Reticulum in Contraction and Relaxation of Immature Rabbit Ventricular Myocytes

Previous indirect studies of newborn hearts have suggested a diminished functional role of the SR and a greater dependency upon trans-sarcolemmal Ca2+fluxes to directly elicit contraction and promote relaxation. We tested the hypothesis that the SR in newborn rabbit hearts is functionally incompetent by measuring contraction and relaxation in ventricular myocytes isolated from the hearts of 1–2-day-old (newborn), 10–12-day-old (juvenile) and >150-day-old (adult) rabbits. Electrically stimulated twitch characteristics were compared to those elicited by the rapid application of 10 m caffeine in the presence and absence of functional sarcolemmal Na–Ca exchange (disabled using a Na+- and Ca2+-free extracellular solution). During steady state, electrically-induced contractions were lower in amplitude in newborn and juvenile compared to adult myocytes (2.9±0.5 and 3.4±0.3v8.5±0.9% of resting cell length, respectively;n=24–29) and relaxation was slower in immature myocytes (t0.75values: newborn 250±20; juvenile 240±10; adult 130±20 ms,n=14–21). Contrary to our hypothesis, caffeine triggered sufficient SR Ca2+release from immature myocytes to elicit contractions of similar magnitude to adults (newborn 12.8±1.1; juvenile 14.0±0.9; adult 15.0±1.6% of resting cell length,n=25–29). The amplitude of indo-1 Ca2+transients during steady-state twitch was 36±12% of the maximal caffeine-induced Ca2+transient in newborns (n=6) and 59±4% in adults (n=6). Caffeine slightly prolonged relaxation in adult myocytes (t0.75=200±30 ms), but accelerated relaxation in newborn and juvenile myocytes (t0.75=180±20 and 150±30 ms, respectively). When both the SR and Na–Ca exchanger were disabled, the rate of relaxation (attributable to the sarcolemmal Ca2+-ATPase and mitochondrial Ca2+uniporter) of newborn and juvenile myocytes was significantly faster than in the adults (1660±210 and 3030±180v4530±310 ms, respectively;n=14–21). We conclude that neonatal and adult rabbit ventricular myocytes have comparable SR Ca2+load, but neonatal cells exhibit smaller fractional SR Ca2+release during steady-state contractions and greater Ca2+removal by sarcolemmal Na–Ca exchange during relaxation.