UC San Diego

Mutations that affect basal levels of myosin regulatory light chain (RLC) phosphorylation have been implicated in the development of familial hypertrophic cardiomyopathy. Regulatory light chains, one of the two light chain components of the myosin molecule, play a significant role in stabilizing the myosin lever arm during force transmission. While phosphorylation of RLCs by myosin light chain kinases have been implicated in modulating force development in cardiac muscle, their detailed mechanism and precise role in cardiac function is not well understood as compared to their well-studied counterparts in skeletal muscle. In this study, the effects of RLC phosphorylation on cardiac force generation were assessed by comparing force generation results from wild type mice (̃30% phosphorylated RLC) with those from a non- phosphorylatable RLC knock-in strain MLC2v (̃0% phosphorylated RLC). Intact papillary muscles were isolated from mouse hearts, and placed within a tissue culture chamber system that enabled measurement of force within a physiologic environment, i.e. with controlled oxygen and superfusate delivery. This system enabled measurements of force generation before and after muscles were subject to an isometric stretch protocol (90% Lmax). RLC phosphorylation at baseline had no statistically significant effect on systolic force generation; however there was a profound difference in active stress between wild-type and MLC2v knock-in papillary muscles after isometric stretch. These results suggest that RLC phosphorylation potentiates the immediate force response to cardiac lengthening, i.e. the Frank Starling mechanism; therefore RLC phosphorylation may involve cross-bridge dynamics and compensatory protein phosphorylation mechanisms that are more complex then previously assumed.