Effect of exercise on the serum level and urinary excretion of tetracycline, doxycycline and sulphamethizole.

The serum level and urinary excretion of sulphamethizole, tetracycline and doxycycline were studied in healthy volunteers subjected to intensive exercise and bed rest in a cross-over trial. Each group consisted of 7--8 subjects. The exercise or bed rest began 15 min before oral administration of the drug and was continued for the following 4 hours. During exercise serum drug concentration and the area under the serum concentration-time curve for each agent was significantly higher (p less than 0.05) than the corresponding values at rest. Exercise greatly suppressed the renal excretion of tetracycline and doxycycline, but the decrease alone appeared insufficient to account for the pronounced increase in serum drug concentration. Total drug excretion in urine was unchanged. Thus, it seemed most unlikely that overall absorption from the gastrointestinal tract had been altered by exercise. However, the rate of absorption appeared to be more rapid in the exercise than in the rest period. Marked haemoconcentration was not produced by the exercise. In addition to changes in absorption and elimination rates, alteration in the volume of distribution might contribute to the higher serum drug concentration during exercise. Therefore, the level of physical activity should be considered in the interpretation of pharmacokinetic data both in clinical practice and in pharmacokinetic studies.