The Concentration of Trimethoprim in Prostatic Fluid: Nonionic Diffusion or Active Transport?

Abstract

The diffusion of trimethoprim (TMP) from plasma to prostatic fluid (PF) and salivary fluid (SF) was studied in six dogs by progressive iv loading until vascular collapse and death occurred. Despite rising concentrations in serum(s), the PF/S ratios remained constant, excluding saturation of sites of active transport. Calculation of the theoretical nonionic diffusion gradients, based on the Henderson-Hasselbach equation and accurate determinations of pH of blood and PF, indicated that the observed PF/S ratios 5.9–7.0 were always less than the theoretical, thereby further excluding the need to invoke an active mechanism of transport. A single iv dose of TMP (4 mg/kg) in the dog with constant secretion of PF showed that maximal ion-trapping of TMP occurred in PF in 60–90 min.