Since the serum level of the enzyme creatine (phospho)kinase (CPK) is frequently used for diagnostic purposes and since circadian variations of this parameter have been observed under different stress conditions, it had to be determined whether they are induced by an endogenous circadian rhythm or whether they are activity-related masking effects. For this purpose, 15 untrained male test persons were examined for a period of 25h each under different conditions: lying constantly in a climatic chamber, everyday activity conditions, resting conditions with intermittent bicycle ergometer tests (1.4 W/kg body weight) between 09:00 and 11:30 or between 15:00 and 17:30. In addition, 5 sportsmen were included in the study under resting conditions and the bicycle ergometer tests in the forenoon. The serum CPK was determined at intervals of 2h or 4h, respectively. Under constant resting conditions, the mean serum CPK level decreased continuously (p < .001, repeated measures analysis of variance [Rep. Mes. ANOVA]), and the final values amounted to approximately 80% of the initial values. In contrast, under everyday conditions, the mean CPK level rose during the activity phase and decreased again to below the initial values during subsequent rest (p < .001, Rep. Mes. ANOVA). In the ergometer tests, the CPK blood level rose in each case after the activity phase and fell again continuously in the following resting period. However, in the calculation of the deviations from the linear trend, it was shown that certain daily variations in the CPK values can be detected under constant resting conditions; however, the amplitude was much smaller than the amplitude under everyday activity conditions. All in all, it can be stated that serum CPK levels are mainly dependent on the physical load conditions, and that the daily changes observed under everyday conditions disappear almost entirely under resting conditions. The daily variation of the serum creatine kinase level under everyday conditions can thus be regarded as a masking effect. These results are of practical significance for diagnosis to the extent that the physical activity that has taken place before the determination of the CPK level must be taken into account in the interpretation of the values. According to the results obtained so far, the CPK serum level may be used in chronobiology as an indicator of masking by physical activity.
