Elevated skeletal muscle creatine kinase MB isoenzyme levels in marathon runners
A. J. Siegel, L. M. Silverman and W. J. Evans
Mean serum creatine kinase MB isoenzyme (CK-MB) activity in 108 trained
marathon runners after competition was 98 +/- 66 (SD) units/L measured by a
quantitative electrophoretic technique (normal, less than 5 units/L), or
7.9% of total CK activity. These levels in asymptomatic runners were
comparable with peak serum values reported in patients during acute
myocardial infarction. Elevated serum levels of CK-MB in runners can arise
from skeletal muscle through exertional rhabdomyolysis, from silent injury
to myocardium, or from a combined tissue source. To investigate this
directly, we analyzed skeletal muscle obtained by needle biopsy for CK
isoenzymes from 25 trained male marathon runners and ten sedentary male
subjects. The MB isoenzyme accounted for 8.9% +/- 1.3% (SD) of total CK
activity per gram of total protein in the skeletal muscle of runners and
3.3% +/- 0.7% (SD) in control tissue, which was significant. Total CK
activity was not statistically different between the two groups. Similar
relative concentrations of CK-MB in skeletal muscle (8.9%) and serum after
competition (7.9%) strongly suggest that elevated serum CK-MB activity in
asymptomatic runners arises from a noncardiac or skeletal muscle source.
