Potentiation of force by extracellular potassium and posttetanic potentiation are additive in mouse fast-twitch muscle in vitro

The influence of moderately elevated extracellular potassium concentration ([K + ]) on muscle force has marked similarities to that of posttetanic potentiation (PTP) in that twitch force may be enhanced whilst high-frequency force is depressed. The purpose of this work was to test whether K + -induced potentiation is mechanistically related to PTP via skeletal myosin light-chain kinase (skMLCK)-catalyzed phosphorylation of the myosin regulatory light chains (RLC). To do this, we assessed the influence of elevated [K + ] on the force response at various frequencies in extensor digitorum longus (EDL) muscles isolated from wild-type and skeletal myosin light-chain kinase (skMLCK −/− ) absent mice. Changing [K + ] of the incubation medium from 5 to 10 mmol increased isometric twitch force by a similar amount in wild-type and skMLCK −/− muscles (~ 13% in both genotypes) (all data n = 7–8, P