The force dependence of isometric and concentric potentiation in mouse muscle with and without skeletal myosin light chain kinase

The isometric potentiation associated with myosin phosphorylation is force dependent. The purpose of this study was to assess the influence of a pre-existing period of isometric force on the concentric force potentiation displayed by mouse muscles with and without the ability to phosphorylate myosin. We tested isometric (ISO) and concentric (CON) potentiation, as well as concentric potentiation after isometric force (ISO-CON), in muscles from wild-type (WT) and skeletal myosin light chain kinase-deficient (skMLCK−/−) mice. A conditioning stimulus increased (i.e., potentiated) mean concentric force in the ISO-CON and CON conditions to 1.31 ± 0.02 and 1.35 ± 0.02 (WT) and to 1.19 ± 0.02 and 1.21 ± 0.01 (skMLCK−/−) of prestimulus levels, respectively (data n = 6–8, p < 0.05). No potentiation of mean isometric force was observed in either genotype. The potentiation of mean concentric force was inversely related to relative tetanic force level (P/Po) in both genotypes. Moreover, concentric potentiation varied greatly within each contraction type and was negatively correlated with unpotentiated force in both genotypes. Thus, although no effect of pre-existing force was observed, strong and inverse relationships between concentric force potentiation and unpotentiated concentric force may suggest an influence of attached and force-generating crossbridges on potentiation magnitude in both WT and skMLCK−/− muscles.