The significance of Na+ in E-C coupling in muscle

Recent studies have revealed arguments in favour of the possible triggering role of Na+ ions in E-C coupling in skeletal muscle fibres of vertebrates: (i) Na+ is one of the four major inorganic cations widespreaded in the biosphere ununiformly (with gradient) distributed across plasmic membranes of all muscle fibres; (ii) there is correlation between contractile parameters and a pattern of transsarcolemmal Na+ distribution in skeletal muscles; (iii) "Na+ current-induced Ca2+ release" mechanism is corresponded to the criterions for intracellular mediators: a) excitation of plasmic membrane increases [Na+] in junctional space; b) increase of [Na+] in surroundings in vitro induces efflux of Ca2+ from SR; c) estimated rate of [Na+] increase in junctional space in vivo is exceeded the threshold that induces Ca2+ from the SR in vitro; d) there is endogenic system (Na+, K(+)-ATPase) of quick removal of Na+ from junctional gap of triads; e) pharmacological modification of Na+ current through T-tubule membrane leads to correlated changing in twitch response. A definite order of Na(+)- and Ca2+ transmembrane triggering fluxes involved in E-C coupling in fast skeletal muscle fibers provide a very protective intracellular functional system of Ca2+ regulation, coordinated in time and space, and garantees the most complete dependence of voluntary muscle contractions on the CNS control.