EFFECTS OF AGE ON PHYSIOLOGICAL, IMMUNOHISTOCHEMICAL AND BIOCHEMICAL-PROPERTIES OF FAST-TWITCH SINGLE MOTOR UNITS IN THE RAT

1. Physiological, enzyme-histochemical, biochemical and morphometrical properties of fast-twitch single motor units were compared between young (3-6 months) and old rats (20-24 months) using the glycogen depletion technique. Monoclonal antibodies (mAbs) were used to identify the myosin heavy chain (MHC) composition in the muscle fibres of the motor unit (motor unit fibres) in order to facilitate correlative physiological, histochemical, biochemical and morphometrical studies. 2. Earlier observations on effects of age on contractile properties of fast-twitch motor units were confirmed and extended. That is, the duration of the isometric twitch, and the twitch and tetanus forces, were increased. Further, motor unit fibres were rearranged, occupying a larger territory and displaying an increased innervation ratio in old age, indicating a denervation-reinnervation process. 3. Motor units with muscle fibres expressing the novel IIX myosin heavy chain (MHC) were observed in both young and old animals, and they constituted the predominant motor unity type identified in the old animals. In contrast to the type IIX MHC motor units in the young animals, the type IIX MHC units in old age often contained muscle fibres which expressed either the type IIA or type IIB MHC, although type IIX MHC fibres were in the majority (so called 'IIX' MHC motor units), but motor units containing all these three fibre types were never observed. There were also single fibres co-expressing IIX and IIB MHCs in old age. 4. In the young animals the IIX MHC motor units had a higher (P < 0.001) resistance to fatigue (fatigue ratio 0.45 +/- 0.11) than the type IIB MHC units (0.03 +/- 0.05), a succinate dehydrogenase (SDH) activity (0.62 +/- .007) intermediate (P < 0.001) between those of type IIA muscle fibres classified according to myofibrillar ATPase activity after acid pre-incubation, i.e. type IIA ATPase, (0.84 +/- 0.13) and type IIB MHC motor unit fibres (0.20 +/- 0.04), and cross-sectional fibre areas (1650 +/- 320-mu-m2) which were similar to those of type IIA ATPase muscle fibres (1460 +/- 150-mu-m2) but smaller (P < 0.001) than type IIB MHC motor unit fibres (4650 +/- 1180-mu-m2). 5. In the old animals the type 'IIX' MHC motor units displayed physiological (fatigue ratio 0.05 +/- 0.11), biochemical (SDH absorbance 0.33 +/- 0.15) and morphometrical (fibre area 3640 +/- 780-mu-m2) properties resembling the type IIB MHC motor units (fatigue ratio 0.01 +/- 0.00, SDH absorbance 0.23 +/- 0.09 and fibre area 4210 +/- 1100-mu-m2), although there appeared to be a gradient of these properties within the 'IIX' MHC motor type which was related to the MHC composition of motor unit fibres. Furthermore, the type 'IIX' MHC motor unit fibres were not restricted to a particular region of the muscle and were observed in regions which in young animals were restricted only to type IIB MHC motor units. 6. In conclusion, the use of mAbs specific for MHCs has improved the differentiation of fast-twitch motor units and the novel IIX MHC motor unit type has been identified in both young and old animals. This motor unit type undergoes specific age-related alterations that cannot be detected with conventional enzyme-histochemical techniques. It is proposed that the 'IIX' MHC motor units represent a transitional motor unit type involved in an age-related fast-to-slow transformation process.