RESPONSES OF FATIGABLE AND FATIGUE-RESISTANT FIBERS OF RABBIT MUSCLE TO LOW-FREQUENCY STIMULATION

This study investigates early adaptive responses of fast-twitch muscle to increased contractile activity by low-frequency stimulation. Chang es in metabolite levels and activities of regulatory enzymes of carboh ydrate metabolism were investigated in rabbit tibialis anterior muscle after 24 h of stimulation. In addition, changes elicited during a 5-m in lasting acute stimulation experiment were compared between 24-h-pre stimulated and contralateral control muscles. Stimulation for 5 min re duced energy-rich phosphates and glycogen, and increased lactate in th e control muscle. A transient elevation of fructose 2,6-bisphosphate d emonstrated that activation of phosphofructokinase 2 was an immediate response to contractile activity. Prestimulated muscles displayed near ly normal values for ATP, phosphocreatine and glycogen, and did not au gment lactate. Increased activities of hexokinase and phosphofructokin ase 2 and permanently elevated levels of fructose 2,6-bisphosphate poi nted to enhanced glycolysis with glucose as the main fuel in the prest imulated muscle. Isometric tension of the control muscle decreased rap idly a few minutes after the onset of stimulation. In the prestimulate d 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 d istribution of rabbit tibialis anterior, these findings suggested that a large fibre fraction of the prestimulated muscle, possibly the glyc olytic type IID fibres, did not contract. Therefore, the possibility m ust be considered that the metabolite pattern of the 24-h-stimulated m uscle primarily reflected metabolic activities of the contracting, les s fatigable fibres, most likely type IIA and type II fibres. The sugge stion that a large fibre fraction did not produce force, in spite of m etabolic recovery, points to factors responsible of their refractorine ss to low-frequency stimulation other than metabolic exhaustion.