The Influence of Posttetanic Potentiation on Neuromuscular Efficiency in Mouse Fast Twitch Muscle at 25°C

Abstract

Within skeletal muscle the release of calcium is responsible for the initiation of muscle contraction. In addition intracellular Ca2+ also induces the protein skeletal muscle myosin light-chain kinase (skMLCK) to phosphorylate the regulatory light chain (RLC) of fast myosin isoforms. For a short time following RLC-phosphorylation a potentiated state is induced within muscle fibres in which force generation and other contraction dynamics are augmented. The intent of our study was to examine the effect of tetanic stimulation (>100Hz) induced potentiation on the efficiency of neuromuscular contraction (Work output: # of Pulses). Concentric contractions were used in which muscles shortened 1.10 -> 0.90 Lo, at ~70% maximal shortening velocity (Vmax). The fast twitch extensor digitorum longus muscles were excised and mounted in vitro (25oC) to examine the effect of NME on whole muscle function. Unique to our lab were the use of skMLCK-/- mice which are unable to phosphorylate their myosin-RLC, and thus display no magnitude of posttetanic potentiation. These models were used as a negative control for potentiation compared to the wild type EDL. NME was tested during series of submaximal tetani at five frequencies (10, 25, 40, 55, 80 Hz) before and after muscles were exposed to the conditioning stimulus (4 x 400 msec, 100 Hz, over 10 seconds). Neuromuscular efficiency was found to be increased at all frequencies for both wild type (P<0.001) and skMLCK-/- (P<0.002) genotypes following the CS (n=12). NME potentiation was significantly impacted by the expression of skMLCK and test frequency. At optimal frequency wild type EDLs displayed a 92% increased relative NME compared to the 33% seen in the skMLCK-/- genotype showing the importance of RLC-phosphorylation to contractile enhancement. Work values preceding the CS were not significantly different at any frequency in either genotype (P = 0.236). The presence of RLC phosphorylation is physiologically significant in enhancing force output as well as improving neuromuscular efficiency following PTP.