Fragmentation analysis of normal and dystrophic avian muscle

Stiffness and contractile properties of avian normal and dystrophic muscle bundles as measured by sinusoidal length perturbations

Both tension and stiffness as a function of muscle length were measured under relaxing conditions on isolated small bundles of chemically skinned myofibers from normal and dystrophic chicken pectoral muscles. It was shown that the dystrophic muscle was stiffer than normal muscle and developed more tension for the same amount of stretch. A fraction of stiffness was not removed by extraction with either 0.6 M KI or with 5 M guanidine HCl mixed with 1% mercaptoethanol. The stiffness of dystrophic muscle was also unaffected by treatment with bacterial collagenase under conditions that destroyed the stiffness of tendon. Nyquist plots of normal and dystrophic muscles during calcium-activated isometric contraction were very similar and were characteristic of fast-twitch muscle, as evidenced by three clear exponential processes. The normal appearance of the Nyquist plot of dystrophic muscle demonstrates that cross-bridge function is not altered, and the characteristic slowing of contraction and relaxation is not a consequence of a fast-to-slow transformation of muscle types. The increased stiffness of dystrophic muscle may be a very fundamental change in the biomechanics of dystrophy. We postulate that the stiffness is mediated by an altered form of collagen, which is collagenase-resistant by virtue of excessive crosslinking.

The interaction of unregulated actin and myosin in avian muscular dystrophy

The presence of an altered form of the heavy chain component of myosin subfragment-1 (S-1) in avian dystrophic pectoral muscle was confirmed by Triton-urea-acetic acid polyacrylamide gel electrophoresis. The potential functional significance of this altered form of S-1 was evaluated by measuring the ATPase activity of the unregulated acto-S-1 complex using all possible pairwise combinations of actin and S-1 from normal (N) and dystrophic (D) muscle. (NN, DD, ND, DN, where the first letter designates the actin and the second letter the S-1). With conventionally purified actin and S-1, NN not equal to DD not equal to ND not equal to DN, implying both N actin not equal to D actin and N S-1 not equal to D S-1 functionally. An alternate purification scheme for actin resulted in preparations from normal and dystrophic muscles of actin Mg-polymers with the same rheology (viscosity vs shear rate) and critical concentration for polymerization. When these actins were combined with more highly purified preparations of S-1, the adenosine triphosphatase (ATPase) activity of the acto-S-1 complex did not vary with changes in the pairwise composition and responded similarly to variation of the actin or adenosine triphosphate (ATP) concentration. In experiments with actin activation of intact myosin, no differences were observed between myosin from normal vs dystrophic muscle. The different isozymes of myosin present in normal and dystrophic chicken pectoral muscles are functionally equivalent as ATPases in their interactions with unregulated actin.

Myofibrillar creatine kinase in Duchenne and avian muscular dystrophy

The presence and activity of the fraction of creatine kinase (CK) which was associated with myofibrils and located in the M line of the sarcomeres was determined in normal and dystrophic avian muscle and in normal and dystrophic (Duchenne) human muscle. Myofibrils were isolated from homogenates of muscle and washed nine times so as to remove nonmyofibrillar CK. In myofibrils from dystrophic muscle the enzyme CK was localized to the M line using immunofluorescent techniques and was enzymatically active. These results suggest that in both avian and Duchenne muscular dystrophy, there is not a myofibrillar disorder of the phosphocreatine shuttle.