During skeletal muscle development, morphological and biochemical changes occur that are likely to affect its functional performance. In particular the events leading to contraction following electrical excitation of the surface membrane (E-C coupling) may change as a result of maturation of the transverse tubular and sarcoplasmic reticulum (SR) system. The primary aim of this proposal is to investigate the development of the E-C coupling process in a mammalian preparation and compare its development with known structural and biochemical changes. The central hypothesis to be tested is that during muscle development the SR calcium release process is immature and therefore E-C coupling in the neonate is more dependent on extracellular calcium than in the adult. It is well established that in mature frog muscle the removal of extracellular calcium, under conditions that protect the surface membrane charge, has little effect on twitch and Sr calcium release. A calcium channel is present in the T-tubular and surface membranes whose function is not clear. This proposal considers the possibility that the calcium channel is important during muscle development and that it may provide a direct source of calcium for muscle contraction. Voltage clamp experiments will be used to determine the potential dependence of the calcium current in neonatal rat skeletal muscle. The results will be compared with those obtained in adult rat muscle to test whether the activation of the calcium channel changes during development. Optical techniques using calcium indicator dyes will be used to monitor calcium transients in neonatal muscle. Experiments are planned to study the effect of extracellular calcium removal or addition of calcium antagonists on the calcium transients to determine whether a sarcolemmal calcium influx during depolarization can directly contribute to the calcium transient. The voltage dependence of the calcium transient in the absence of extracellular calcium or with the calcium current blocked will be measured in neonatal muscle. These results will be compared with those obtained in adult to determine whether the SR calcium release process changes during development. These experiments will provide information regarding the development of E-C coupling and the relation between structure and function which is important not only for understanding muscle function in general but also for understanding changes in function that occur during disease or injury.