Rates of Force Generation in Drosophila Fast and Slow Muscle Types Have Opposite Responses to Phosphate

Abstract

Much of the functional diversity of striated muscle that serves to meet different locomotory demands is due to different isoforms of myosin. (1965), who were among the first to investigate the physiological relevance of myosin isoforms, demonstrated that different speeds of contraction in various muscle types correlate with actin-activated myosin ATPase. (1985), (1987), and (1988), among others, extended these studies by showing muscle speeds correlate with myosin isoform composition. Recently, (2002) directly proved that myosin isoforms are the prime determinants of fiber kinetic differences by substituting the slow embryonic myosin (denoted EMB) for the native, fast adult IFM myosin (denoted IFI) in the indirect flight muscle (IFM) (Fig 1). Substituting the EMB isoform transformed the IFM from a muscle that generates maximum oscillatory work at high frequencies, to one that generates more work, but at much lower frequencies (Fig. 2, after Swank et al., 2002). The isoform substitution enhances calcium-activated isometric tension (T_o) nearly 3-fold, but reduces maximum oscillatory power (P_max, equal to oscillatory work times frequency) to only ∼25% that of IFI fibers (Swank et al., 2002). In EMB fibers T_max achieved at a frequency (∼20 Hz) that is considerably lower than that at which P_max is achieved in IFI fibers (∼150 Hz, i.e., the wing beat frequency at 15°C). At the resonant frequency of the flight system (i.e., at ∼ 150 Hz), no power is produced, thereby explaining the loss of flight ability in the EMB lines. Thus substitution of EMB for IFI is akin to converting the muscle from a fast fiber type to a slow fiber type.