ENERGY-STATE AND MYOSIN HEAVY-CHAIN ISOFORMS IN SINGLE FIBERS 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 r abbit muscles, and in fibres isolated from tibialis anterior muscle un dergoing fast-to-slow conversion by chronic low-frequency stimulation (CLFS). In normal muscles, energy-rich phosphate contents and [ATP]/[A DP(free)] ratios could thus be assigned to different MHC-based fibre t ypes. Phosphocreatine (PCr) contents and [ATP]/[ADP(free)] ratios diff ered markedly between fast- and slow-twitch fibres, as well as within the fast fibre subtypes. Both magnitudes were approximately twofold hi gher in the fastest (type IIB) fibres as compared to the slowest (type I) fibres. According to PCr contents and [ATP]/[ADP(free)] ratios pur e and hybrid fibres were aligned in an order similar to that determine d 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-twi tch fibres. In both normal and stimulated muscles, PCr and [ATP]/[ADP( free)] ratios were correlated, indicating their equilibrium in the dif ferent fibre types. The relationship detected between MHC isoform expr ession and the [ATP]/[ADP(free)] ratio suggests that the drastic and p ersistent depression of the cellular energy state may act as an import ant signal initialing fast-to-slow transformation processes in muscle fibres.