The capacity of mdx mouse diaphragm muscle to do oscillatory work

Mdx mice are commonly used as a model for Duchenne muscular dystrophy (DMD) because both are characterized by a complete absence of the expression of dystrophin in skeletal muscle, that is they are homologous. Dystrophin is a cytoskeletal protein located on the inner face of the sarcolemma associated with cytosolic filament integral membrane glycoproteins. Dystrophin has been proposed as a stabilizer of the sarcolemma during contraction (Petrof et al. 1993), but its exact function is unknown (Koenig et al. 1988). The absence of the protein dystrophin in skeletal limb muscle of mdx mice is associated with constant progressive muscle degeneration and necrosis. The degeneration of myofibres activates satellite cells and initiates a regenerative response. In limb muscles of mdx mice an equilibrium between degeneration and regeneration is achieved. In contrast, in diaphragm muscles of mdx mice, the regeneration does not compensate for the degeneration and a progressive decrease in muscle function occurs similarly to that observed in skeletal limb muscle of boys with DMD (Stedman et al. 1991; Dupont-Versteegden & McCarter, 1992; Petrof et al. 1993; Louboutin et al. 1993; Lefaucheur et al. 1995; Lynch et al. 1997). One consequence of the dystrophic process in diaphragm muscles of mdx mice, as with most myopathies, is the increase in the amount of connective tissue (Stedman et al. 1991). The increase in collagen within the diaphragm muscle causes a decrease in muscle elasticity and a marked increase in passive resting tension (Marshall et al. 1989; Stedman et al. 1991). We hypothesized that the mdx condition would have a marked effect on the ability of diaphragm muscle from mdx mice to do active net work (sometimes called work during miometric contractions) and generate power. This hypothesis was tested using the work-loop technique developed by Josephson (1985). The work-loop technique uses imposed length changes to simulate in vivo muscle action. In vivo, skeletal muscle must spend some time lengthening and some time shortening. During shortening the diaphragm muscle does positive work. During lengthening of the diaphragm, work is done on the muscle by structures to which it is attached. Thus, net work per cycle during cyclic contraction and relaxation depends on both the positive work during shortening and the negative work that the muscle absorbs during lengthening. Figure 1 shows traces of length and force as a function of time and the work-loop generated by these traces. If we start at rest length and integrate over 100 % of one length change cycle, then positive work occurs during shortening from 25 to 75 % of the cycle, negative work occurs from 0 to 25 % and from 75 to 100 % of the cycle. where F is force and dl is displacement or the imposed length change. Figure 1 Traces to show conditions used to generate standard work-loops (4 Hz cycle frequency, 0.25 duty cycle), to define duty cycle and to define phase By definition, a muscle with high passive tension is one in which there is a large change in F for a standard passive length change (Woledge et al. 1985). Therefore, a muscle with high passive tension will have high values of negative work. Thus we would predict that because diaphragm muscles from mdx mice have high passive tension (Marshall et al. 1989; Stedman et al. 1991) they should have high values of negative work. One important functional consequence of the high value of negative work per cycle is a lower value of net work. Furthermore, because the relation between passive force and length tends to be exponential rather than linear (Woledge et al. 1985), we would expect that diaphragm muscle from mdx mice would have even higher values of negative work and thus even lower values of net work at long muscle lengths or at large muscle strains. The purpose of the present study was to test these expectations.