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Institute of Medical Biochemistry, University of Aarhus, Denmark.

1. The characteristics of renal transport of beta-alanine by luminal membrane vesicles isolated from either the proximal convoluted part (pars convoluta) or the proximal straight part (pars recta) of rabbit proximal tubule were investigated. 2. In vesicles from pars convoluta two transport systems have been characterized: (1) a Na+-dependent system with intermediate affinity (half-saturation 2.7 mM), and (2) a Na+-independent system, which in the presence of a H+ gradient (extravesicular greater than intravesicular) can drive the uphill transport of beta-alanine into these vesicles. This is the first demonstration of H+-beta-alanine co-transport across luminal membrane of rabbit kidney proximal convoluted tubule. 3. By contrast, in membrane vesicles from pars recta, transport of beta-alanine was strictly dependent on Na+ and occurred via a dual transport system, namely a high-affinity (half-saturation 0.16 mM) and a low-affinity system (half-saturation 9.3 mM). 4. The demonstration of competition between the Na+-gradient-dependent uptake of beta-alanine and taurine, without appreciable inhibition by alpha-amino acids in vesicles from pars convoluta as well as from pars recta, strongly suggests that the luminal membrane of proximal tubule has transport systems for the reabsorption of beta-amino acids which are distinct from alpha-amino acid transport systems.

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