PHYSIOLOGY OF DEVELOPING MUSCLE

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.