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SUMMARY1. To explore how maximum velocity of shortening (Vmax) of fibres varies within one muscle and how Vmax varies with body size, we measured Vmax of muscle fibres from soleus muscle of a large animal, the horse.2. Vmax was determined by the slack test on skinned single muscle fibres at 15 'C during maximal activation (pCa = 5 2). The fibre type was subsequently determined by a combination of single-cell histochemistry and gel electrophoresis of the myosin light chains.3. Vmax values for the type I, IIA and IIB muscle fibres were 0 33+0 04 muscle lengths/s (ML/s) (±S.E.M., n = 6), 1-33+008 ML/s (n = 7) and 3-20+0-26 ML/s (n = 6), respectively. It is likely that the large range in Vmax is due to differences observed in the myosin heavy chains and light chains associated with the three fibre types.4. Comparison of Vmax over a 1200-fold range (450 kg horse vs. 0-38 kg rat) of body mass (Mb) suggests that slow fibres scale more dramatically (Mb-018) than do fast glycolytic fibres . This difference may enable the slow fibres to work at high efficiencies in the large animal while the fast fibres can still generate a large mechanical power when necessary.

The pale, soft, exudative (PSE) phenomenon in turkey pectoralis major (breast) muscle was studied using a combination of biochemical, meat quality, microscopic, and gel electrophoresis techniques. Breast muscle samples were collected from turkeys characterized by slow vs fast postmortem glycolysis assessed by muscle pH at 20 min after death. The PSE group was characterized by lower muscle ATP (P < .05) and higher lactate levels (P < .05) compared with the normal group. Excess water-holding capacity and cooking yield were significantly lower (P < .05) in the PSE group than in normal turkeys. Breast muscle of the PSE group was also lighter (P < .05) than that in the normal group as determined by Minolta L* values. The SDS-PAGE, Western blotting, and immunofluorescence microscopy revealed that phosphorylase, a soluble enzyme, became tightly associated with the myofibrils in muscle from the PSE group. Also, less myosin could be solubilized from PSE vs normal myofibril samples. The results indicate that irreversible myosin insolubility due to low pH and high-temperature conditions is decisive in the development of PSE turkey breast muscle.

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