EFFECT OF A REPEATED BOUT OF ECCENTRIC EXERCISE ON CREATINE KINASE-MM ISOFORMS 369

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The increase in creatine kinase (CK) after a damage-inducing exercise is blunted upon performance of a subsequent bout of similar exercise. CK-MM1(muscle isoenzyme), the pure gene product, is initially released and then converted to structurally modified isoforms. MM2 and MM3, before being cleared by the reticuloendothelial system. This study investigated the release and clearance of MM isoforms following two bouts of eccentric exercise. Eight male subjects performed 50 forced lengthening contractions of the forearm flexor muscles six days apart. Blood samples were collected pre-exercise and at hours 2, 6, 24, 48, 72, 96, and 120 post-exercise for each bout. Total CK (TCK) levels were analyzed spectrophotometrically and MM isoforms were separated using flatbed isoelectric focusing. Densitometric quantification of each MM isoform was acquired as a percentage of total MM enzymatic activity and then converted into absolute activity (IU/L). The MM1:MM3 ratio was calculated for each time point and used to indicate clearance. Values <1 indicate clearance is faster than release, >1 indicate release is greater, and = 1 indicate the rates are equal. Post-bout 1, TCK was significantly elevated peaking at 96h (3530±927 IU/L) (p<0.01). Following bout 2 there was no increase in TCK, demonstrating the typical repeated bout effect. MM1 peaked at 72h after bout 1 (993±424 IU/L), but also showed an increase after bout 2 (166±44). The MM1:MM3 ratio was significantly lower after bout 2(0.58±0.12) when compared to bout 1 (1.61±0.35) (p<0.01). Despite no increase in TCK after bout 2, there was still a slight increase in MM1 indicating that some release of CK occurred. Accelerated clearance of CK appears to be the main factor explaining the blunted response of this enzyme following a repeated bout of exercise.