Energy state and myosin heavy chain isoforms in single fibres of normal and transforming rabbit muscles

Energy-rich phosphates, [ATP]/[ADP(free)] ratios, and the myosin heavy chain (MHC) complement were determined in single fibres from normal rabbit muscles, and in fibres isolated from tibialis anterior muscle undergoing fast-to-slow conversion by chronic low-frequency stimulation (CLFS). In normal muscles, energy-rich phosphate contents and [ATP]/[ADP(free)] ratios could thus be assigned to different MHC-based fibre types. Phosphocreatine (PCr) contents and [ATP]/[ADP(free)] ratios differed markedly between fast- and slow-twitch fibres, as well as within the fast fibre subtypes. Both magnitudes were approximately twofold higher in the fastest (type IIB) fibres as compared to the slowest (type I) fibres. According to PCr contents and [ATP]/[ADP(free)] ratios pure and hybrid fibres were aligned in an order similar to that determined by their contractile properties and myofibrillar ATPase activities. CLFS for up to 30 days induced pronounced decreases in PCr and [ATP]/[ADP(free)] which attained levels twofold lower than in normal slow-twitch fibres. In both normal and stimulated muscles, PCr and [ATP]/[ADP(free)] ratios were correlated, indicating their equilibrium in the different fibre types. The relationship detected between MHC isoform expression and the [ATP]/[ADP(free)] ratio suggests that the drastic and persistent depression of the cellular energy state may act as an important signal initialing fast-to-slow transformation processes in muscle fibres.