[Action of insulin on contraction and electrical responses of rat skeletal muscle]

The effect of insulin on contractility of directly stimulated skeletal muscles was studied in experiments in isolated preparations of rat fast, extensor digitorum longus (m. EDL), slow, soleus (m. SOL) and mixed, diaphragm muscles. In addition (diaphragm only) characteristics of extracellularly recorded muscle fiber action potentials (APs) were evaluated before and after addition of insulin to a bath solution. Insulin (0.5-10 nM) decreased muscle twitch force. This negative inotropic effect of insulin was dose-dependent, with m. SOL appearing to be more sensitive to insulin than either m. EDL or diaphragm. Insulin did not affect strength of isotonic KCL- or caffeine-induced muscle contractures, but decreased second and increased first and third phases of extracellularly recorded muscle fiber APs. The analysis of the data obtained in this study and the data from the literature suggests changes in electrogenesis of the muscle fiber's t-tubular plasma membrane as a key element of the negative inotropic effect of the hormone on contractility of mammalian skeletal musculature. Possible mechanisms of such putative changes are discussed.