MHC AND SARCOPLASMIC-RETICULUM PROTEIN ISOFORMS IN FUNCTIONALLY OVERLOADED CAT PLANTARIS MUSCLE-FIBERS

To determine whether the adaptations in myosin heavy chain (MHC) isofo rm expression after functional overload (FO) are accompanied by commen surate adaptations in protein isoforms responsible for relaxation [sar co(endo)plasmic reticulum (SR) Ca2+-adenosinetriphosphatase (SERCA) an d phospholamban (PHL)] in single muscle fibers, the isoforms of MHC an d SERCA and the presence or absence of PHL were determined for eat pla ntaris fibers 3 mo after FO. In control plantaris the relative MHC iso form composition was 23% type I, 21% type IIa, and 56% type IIb. FO re sulted in a shift toward slower isoforms (33% type I, 44% type IIa, an d 23% type IIb). In the deep region of the plantaris the proportions o f type I MHC and hybrid MHC fibers (containing type I and II MHCs) wer e 40 and 200% greater in FO cats, respectively. FO resulted in a 47% i ncrease in the proportion of fibers containing only the slow SERCA iso form (SERCA2) and a 41% increase in the proportion of fibers containin g PHL. The proportions of fibers containing type I MHC, SERCA2, and PH L in control and FO plantaris were linearly correlated. These data sho w that adaptations in MHC isoform expression are accompanied by commen surate adaptations in sarcoplasmic reticulum protein isoforms in singl e muscle fibers after FO.