Effect of myosin light chain kinase overexpression on twitch force potentiation in skeletal muscle

Abstract

Repetitive low-frequency stimulation results in potentiation of twitch force development in fast-twitch skeletal muscle. Studies in skeletal muscle myosin light chain kinase (skMLCK) knockout mice show that frequency-dependent twitch potentiation is due in part to myosin regulatory light chain (RLC) phosphorylation by the calcium/calmodulin-dependent skMLCK. We generated transgenic mice that express skMLCK under control of the human skeletal muscle actin promoter. Skeletal muscle from transgenic mice exhibited a 23-fold increase in skMLCK protein expression and a 15-fold increase in enzymatic capacity as compared to skeletal muscle from wild-type mice. In extensor digitorum longus (EDL) muscle from wild-type mice, low-frequency (10 Hz) electrical stimulation led to a progressive potentiation of twitch force. Maximal twitch force potentiation was observed in wild-type EDL after 15 sec of stimulation. Electrical stimulation of transgenic EDL muscle resulted in a similar maximal response; however, maximal twitch-force occurred more rapidly. Time to half-maximal potentiation was 6.8±0.4 sec compared to 2.7±0.1 sec (p<0.001) for wild-type and transgenic mice, respectively. The rate and extent of RLC phosphorylation was also greater in transgenic compared to wild-type EDL. Surprisingly, a dramatic increase in skMLCK expression had only a modest positive effect on frequency-dependent twitch potentiation and RLC phosphorylation. Previous studies suggest that calmodulin may be limiting for smooth muscle activation of MLCK, therefore calmodulin may also be limiting for activation of skMLCK. (This work supported by NIH).