Responses of fatigable and fatigue-resistant fibres of rabbit muscle to low-frequency stimulation

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Abstract
This study investigates early adaptive responses of fast-twitch muscle to increased contractile activity by low-frequency stimulation. Changes in metabolite levels and activities of regulatory enzymes of carbohydrate metabolism were investigated in rabbit tibialis anterior muscle after 24 h of stimulation. In addition, changes elicited during a 5-min lasting acute stimulation experiment were compared between 24-hprestimulated and contralateral control muscles. Stimulation for 5 min reduced energy-rich phosphates and glycogen, and increased lactate in the control muscle. A transient elevation of fructose 2,6-bisphosphate demonstrated that activation of phosphofructokinase 2 was an immediate response to contractile activity. Prestimulated muscles displayed nearly normal values for ATP, phosphocreatine and glycogen, and did not augment lactate. Increased activities of hexokinase and phosphofructokinase 2 and permanently elevated levels of fructose 2,6-bisphosphate pointed to enhanced glycolysis with glucose as the main fuel in the prestimulated muscle. Isometric tension of the control muscle decreased rapidly a few minutes after the onset of stimulation. In the prestimulated muscles, tension was almost stable, but amounted to only 30%–40% of the initial tension of the control muscle. In view of the fibre type distribution of rabbit tibialis anterior, these findings suggested that a large fibre fraction of the prestimulated muscle, possibly the glycolytic type IID fibres, did not contract. Therefore, the possibility must be considered that the metabolite pattern of the 24-h-stimulated muscle primarily reflected metabolic activities of the contracting, less fatigable fibres, most likely type IIA and type I fibres. The suggestion that a large fibre fraction did not produce force, in spite of metabolic recovery, points to factors responsible of their refractoriness to low-frequency stimulation other than metabolic exhaustion.

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