Velocity of actomyosin sliding in vitro is reduced in dystrophic mouse diaphragm

It has recently been suggested that dystrophin deficiency in mdx diaphragm muscle is associated with quantitative changes in the myosin molecular motor. In vitro motility assays were used to study the kinetics of actomyosin interactions between purified actin filaments and myosin molecules. Monomeric myosin was obtained from the diaphragm and limb (semitendinosus) muscles of 9-mo-old male mdx (mdx) and age-matched control mice. The sliding velocity (vo, mum/s) of fluorescent-labeled actin filaments moving over a myosin-coated surface (40 mug/ml) was measured. In diaphragm, vo was significantly slower in mdx than in control mice (1.2 +/- 0.1 mum s(-1) versus 1.9 +/- 0.1 mum s(-1), p < 0.001). Conversely, there was no significant difference in vo between control and mdx semitendinous muscles (2.4 +/- 0.1 mum s(-1) versus 2.5 +/- 0.1 mum s(-1)). As compared with control mice, mdx diaphragm exhibited a shift from IIX-MHC to IIA-MHC (p < 0.001) and a reduction in IIB-MHC (p < 0.01). Semitendinous muscle from control and mdx mice contained almost exclusively type IIB MHC. Our results are in good agreement with the proposal that myosin is altered in dystrophic mouse diaphragm.