COORDINATED EXPRESSION OF MYOSIN HEAVY-CHAIN ISOFORMS AND METABOLIC ENZYMES WITHIN OVERLOADED RAT MUSCLE-FIBERS

We studied the coordinated regulation of myosin heavy chains (MHC) and metabolic enzymes within individual overloaded adult rat plantaris fi bers. This was done using monoclonal antibodies raised against distinc t developmental and adult MHCs, and quantitative microphotometric succ inate dehydrogenase (SDH) and glycerol-3-phosphate dehydrogenase (GPDH ) enzyme assays. Overload shifted MHC expression in the order IIb --> IIx --> IIa --> alpha/I, with a tripling of cells coexpressing I and a lpha-MHC, and a transient reexpression of two embryonic MHC and the ne onatal isoform in preexisting myofibers. Overload caused a rapid, size -independent, 50% decrease in GPDH activity across all cell types, whi ch recovered by 6 wk. Fiber SDH activities varied according to MHC com position, such that overloaded fibers coexpressing IIa MHC displayed c ontrol slow fiber SDH levels, whereas cells expressing IIx and IIb MHC displayed a transient 30% increase in SDH that recovered by 6 wk. Our results suggest that during overload, fibers adapt progressively to t he new functional requirements and display more efficient cellular ene rgy utilization and delivery characteristics. The time course of adapt ations suggests a role for glycolytic enzymes in the initiation of the se transformations.