EXPRESSION OF AN ALPHA-CARDIAC LIKE MYOSIN HEAVY-CHAIN IN DIAPHRAGM, CHRONICALLY STIMULATED, AND DENERVATED FAST-TWITCH MUSCLES OF RABBIT

An additional slow fibre type, type I alpha, is detected in diaphragm and appears in fast-twitch hindlimb muscles of rabbit under the influe nce of altered neuromuscular activity. Type Iu fibres were delineated from fibres expressing myosin heavy chain I beta (type I beta) by immu nohistochemistry with a monoclonal antibody raised against the alpha-c ardiac MHCI alpha. When stained for mATPase after acid and alkaline pr eincubations, some type I alpha fibres resembled type I beta and type IIA fibres, respectively. Some type I alpha fibres displayed dissimila r mATPase staining, indicating heterogeneity of this fibre population. The appearance of numerous type I alpha fibres in stimulated muscles, which in addition contain type IIA and type I beta fibres, suggested that they may be interspaced between types IIA and I beta. Electrophor esis under nondenaturing conditions disclosed an additional isomyosin both in normal diaphragm and stimulated muscles. This band displayed t he same mobility as the slowest isomyosin in rabbit masseter muscle. I t was recognized by the same monoclonal (anti-alpha-cardiac MHC) antib ody used for immunohistochemistry. Therefore, this isomyosin appeared to be very similar, but perhaps not identical to the alpha-cardiac MHC -based isomyosin, probably resulting from discrete differences in the MHC complement. This assumption agrees with additional findings sugges ting an even greater heterogeneity of the MHCs than generally assumed. In support of this, we show in atrium and masseter muscles the existe nce of an additional, electrophoretically distinct MHC isoform which m igrates in close vicinity to MHCI alpha.