The physiological role of the muscle histidyl dipeptides carnosine and anserine in contractile function of various types of muscle fibres in vivo is poorly understood. Ten adult male Wistar rats were randomly assigned to two groups: control and supplemented for 10 weeks with beta-alanine, the precursor of carnosine (~640 mg/kg BW/day). Thereafter, contractile properties and fatigability of isolated fast fatigable (FF), fast resistant to fatigue (FR), and slow motor units (MUs) from the medial gastrocnemius were determined in deeply anaesthetized animals. The fatigue resistance was tested with a 40 Hz fatigue protocol followed by a second protocol at 40 Hz in fast and 20 Hz in slow units. In the supplemented rats, histidyl dipeptide concentrations significantly increased (P < 0.05) by 25% in the red portion of the gastrocnemius and carnosine increased by 94% in the white portion. The twitch force of FF units and maximum tetanic force of FR units were significantly increased (P < 0.05) and the half-relaxation time was prolonged in slow units (P < 0.05). FF units showed less fatigue during the first 10 s and FR units between 10 and 60 s during the 40 Hz fatigue test. In slow units, forces declined less during the first 60 s of the 20 Hz test. In conclusion, this in vivo experiment demonstrates that an elevation in muscle histidyl dipeptide content elicits beneficial changes in MU contractile characteristics and fatigue resistance. Carnosine and anserine seem to play an important yet divergent role in various MUs.
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