Renal excretion of thiamin by the dog

¹ Department of Physiology, Faculty of Medicine, University of São Paulo, São Paulo, Brazil

Simultaneous thiamin and creatinine clearance determinations in unanesthetized dogs revealed a pattern of excretion characteristic of a substance actively excreted by the renal tubules. The maximum tubular excretory capacity per minute (Tm) calculated at high thiamin plasma levels was negative, unless a correction factor (FW) of 0.63, calculated from the clearance values, was applied. Under these conditions, the mean Tm value was 1463 µg/min. By means of perfusion experiments on isolated dog kidneys it was shown that there was very little or no protein binding of thiamin. In extraction-ratio determinations on unanesthetized dogs it was shown that thiamin was not destroyed by the kidney and that the extraction of thiamin and PAH were similar. At varying urine flows with high thiamin plasma levels, a correlation between water reabsorption and thiamin excretion could be observed, suggesting the occurrence of back-diffusion of thiamin under these conditions.