RENAL EXCRETION AND ACCUMULATION KINETICS OF 2-METHYLBENZOYLGLYCINE IN THE ISOLATED-PERFUSED RAT-KIDNEY

The effect of protein binding on kidney function has been studied by i nvestigating the renal accumulation and secretion of the hippurate ana logue 2-methylbenzoylglycine in the isolated perfused rat kidney in th e absence and presence of bovine serum albumin (BSA). Experiments were performed with either 2.5% pluronic or a combination of 2.2% pluronic and 2% BSA as oncotic agents; a wide concentration range (1-190 mu g mL(-1)) of 2-methylbenzoylglycine was studied. Tubular secretion appea red to be a function of the amount of unbound drug in the perfusate an d was best described by a model consisting of a high and low affinity Michaelis-Menten term. Parameters obtained after the analysis of renal excretion data were maximum transport velocity for the high affinity site (T-M,T-H) = 3.0 +/- 2.8 mu g min(-1), Michaelis-Menten constant f or tubular transport for the high affinity site (K-T,K-H) = 0.5 +/- 0. 8 mu g mL(-1), maximum transport velocity for the low affinity site (T -M,T-I) = 250 +/- 36 mu g min(-1) and Michaelis-Menten constant for tu bular transport for the low affinity site (K-T,K-L) = 62 +/- 17 mu g m L(-1). The compound accumulated extensively in kidney tissue, ratios u p to 175 times the perfusate concentration were reached. Accumulation data were best analysed by a two-site model similar to the model used to describe renal excretion. Calculated parameters were -1 theoretical maximum capacity of the high affinity site (R(M,H)) = 26 +/- 23 mu g g(-1), affinity constant for renal accumulation at the high affinity s ite (K-A,K-H) = 0.2 +/- 0.4 mu g mL(-1), theoretical maximum capacity of the low affinity site (R(M,L)) = 1640 +/- 1100 mu g g(-1) and affin ity constant for renal accumulation at the low affinity site (K-A,K-L) = 60 +/- 58 mu g mL(-1). The very high accumulation in kidney tissue could be explained by active tubular uptake, mediated by the secretory mechanisms involved, and dependent on the amount of free drug in the perfusate. This study shows that anionic drugs, subject to active secr etion, may reach high concentrations in tubular cells even at low plas ma concentrations.