Expression of myosin isoforms in denervated, cross-reinnervated, and electrically stimulated rabbit muscles.

The expression of myosin heavy (MyHC) and light (MyLC) chain isoforms was analyzed after denervation and cross-reinnervation by a fast nerve of the slow-twitch Semimembranosus proprius (SMp) muscle, and after denervation and electrical stimulation at low frequency of the fast-twitch Semimembranous accessorius (SMa) muscle of the rabbit. The control SMp (100% type I fibers) expressed 100% type I MyHC and 100% slow-type (1S', 1S and 2S) MyLC isoforms. Five month denervation did not alter significantly the MyHC expression of the muscle, but induced the expression of a new type 1 MyLC corresponding most probably to an embryonic MyLC. Five-month cross-reinnervation of the SMp by the fast SMa nerve induced a large change of its fiber type properties. As shown by immunocytochemistry, almost all fibers were stained by fast myosin antibody, but a high proportion of them co-expressed slow myosin. This result was in agreement with biochemical data showing that fast MyHC and MyLC isoforms became predominant. The control SMa (nearly 100% type II fibers) expressed almost 100% type II MyHC (70% type IIb and 22% IIx/d) and 100% fast-type (1F, 2F and 3F) MyLC isoforms. Five month denervation of the SMa induced a shift in its MyHC, with 98% type IIx/d and 2% type IIb isoforms, and no change in the proportions of its MyLC. Three month electrical stimulation at 10 Hz of the SMa transformed its fiber type composition. All fibers reacted with the slow myosin antibody and a minor proportion of them were stained by the fast myosin antibody. These observations were in agreement with the biochemical analysis showing a large predominance of the slow-type MyHC and MyLC isoforms. Taken together, these results obtained from rabbit muscles which are normally homogeneous in either fast-twitch or slow-twitch fiber types, further support the idea that the different myosin isoforms, particularly the MyHC, are differentially regulated by motor innervation. Type I MyHC is maintained in denervated SMp muscle, but is not expressed in denervated SMa. Type IIb isoform is the most sensitive to neural influence, as it disappears rapidly in denervated and electrically stimulated fast-twitch SMa muscle, and is barely expressed in cross-reinnervated slow-twitch SMp muscle. In contrast, type IIa and type IIx/d are less dependent upon motor innervation. In addition to the previous studies of d'Albis et al. analysis of these results leads us to conclude that, in the rabbit, sensitivity to motor innervation increases from the glycolytic to the oxydative types of fibers, in the order IIB > IIX/IID > IIA > I.