β-Amino acid transport in isolated tubule segments

Using taurine as a model substrate, β-amino acid transport in isolated rat renal cortical tubule segments was studied. Heterogeneity of taurine accumulation was found in tubule segments and has previously been measured in renal cortex slices. In contrast to studies in cortical slices, initial rate kinetics could also be determined, and demonstrated uptake by two systems. Efflux of taurine from tubule cells was measured and showed 50% remaining in tubule segments after 30 min incubation in taurine-free medium. Concentrative uptake was prevented by NaCN and iodoacetate. Incubation with other β-amino acids resulted in decreased taurine accumulation. An important although unexpected finding was decreased taurine uptake by isolated tubules in the presence of Krebs-Ringer phosphate and Tris-HCl buffer as compared to Krebs-Ringer bicarbonate buffer. Isolated tubule preparations provide a rapid means of assessing β-amino acid transport including initial rate kinetics. However, when using tubules to study uptake, the buffer medium may play a critical role.