Renal excretion of tetracycline is transiently decreased during short-term heat exposure

Renal blood flow is known to be reduced during intensive external heating, which may have clinical relevance for renal drug elimination as well. The effects of heat exposure in a sauna bath on tetracycline pharmacokinetics were studied in 8 healthy volunteers in an open, randomized crossover study. The subjects were given a single oral dose of tetracycline (500 mg) both in a control session and in a sauna bathing session. The heat exposure consisted of three 10-minute stays in a sauna bath (temperature 76-87 degrees C, relative humidity 27-33%) starting 20 minutes after drug administration. The stays in the steam room were separated by two 5-minute cooling periods at 23 degrees C. Venous blood samples for determination of plasma tetracycline concentrations were taken 15 min before the drug intake, 20, 40, 60 min, and 2, 3, 4, 6, 8, and 24 h after it. The control session at room temperature (23 degrees C) followed the same sampling protocol. No statistically significant differences in tetracycline plasma concentrations, Cmax, Tmax, or AUC0-24h were seen. In addition, urine was collected (0-2 h, 2-5 h, 5-8 h, and 8-24 h) for determination of the amount of tetracycline excreted unchanged. Urinary tetracycline excretion was transiently (2-5 h after drug intake) reduced in the sauna session (P < 0.05 vs. control, Wilcoxon). The other collection periods and the total urinary excretion of tetracycline (24 h) did not differ from the control session. It is concluded that urinary tetracycline excretion was transiently decreased during short-term heat exposure, but otherwise the effects of external heating on tetracycline pharmacokinetics were negligible.