M I Sheikh

Demonstration of H+‐ and Na+‐coupled co‐transport of beta‐alanine by luminal membrane vesicles of rabbit proximal tubule.

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.

Na(+)‐ and H(+)‐gradient‐dependent transport of alpha‐aminoisobutyrate by luminal membrane vesicles from rabbit proximal tubule.

1. The characteristics of renal transport of alpha‐aminoisobutyrate (AIB) 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. Transport of AIB in vesicles from pars convoluta was mediated by both Na(+)‐dependent and Na(+)‐independent systems, which in the presence of an inwardly directed H+ gradient can drive the uphill transport of AIB into these vesicles. 3. By contrast, in luminal membrane vesicles from pars recta, transient accumulation of AIB was only dependent on Na+. Lowering pH without a H+ gradient (pHi = pH0 = 5.5) completely abolished the Na(+)‐dependent transient accumulation of AIB in these vesicle preparations. 4. Attempts to determine the stoichiometry of both the Na(+)‐AIB and H(+)‐AIB transporters located in these two segments of proximal tubule suggested that one Na+ and one H+ ion may be involved in the transport of AIB. 5. Sodium‐dependent uptake of AIB in vesicles from pars convoluta was competitively inhibited by L‐serine and L‐phenylalanine, whereas the presence of L‐proline, L‐alanine and glycine had no significant effect. By contrast, the H(+)‐gradient‐dependent uptake of AIB was drastically reduced (30% of the control value) by L‐proline, L‐alanine and glycine, while L‐serine and L‐phenylalanine had no significant effect. 6. On the other hand, pars recta vesicles exhibited a different transport specificity. L‐Phenylalanine, L‐serine, L‐alanine and glycine, but not L‐proline competitively inhibited the uptake of AIB, providing evidence for the existence of a common transport system for AIB, L‐phenylalanine, L‐serine, L‐alanine and glycine in this segment of rabbit proximal tubule.

Transport of leucine, isoleucine and valine by luminal membrane vesicles from rabbit proximal tubule.

1. Transport of L‐ and D‐isomers of leucine, isoleucine and valine by luminal membrane vesicles prepared from either the convoluted part (pars convoluta) or the straight part (pars recta) of rabbit proximal tubule was studied by a rapid filtration technique and by a spectrophotometric method using a potential‐sensitive carbocyanine dye. 2. Both types of renal membrane vesicle take up the amino acids in a Na(+)‐dependent, H(+)‐independent and electrogenic manner. The L‐isomers are transported with higher affinities than their corresponding D‐forms, of which only D‐leucine is taken up to a significant extent. 3. Membrane vesicles prepared from pars convoluta take up the L‐amino acids by a single and common system. Filtration studies showed that the Km values for L‐leucine and L‐valine transport are, on average, 0.23 and 0.83 mM, respectively. The values of KA (the concentration of amino acid producing a half‐maximal optical response) are comparable to those of Km, namely 0.18 mM for L‐leucine and 0.60 mM for L‐valine. KA for L‐isoleucine transport was found to be 0.19 mM. D‐Leucine is taken up by the same system but with a much lower affinity (KA = 7.2 mM). 4. Membrane vesicles prepared from pars recta possess two, and probably common, transport systems for the L‐isomers of the amino acids. The average Michaelis‐Menten constants were as follows: L‐leucine, K1m = 0.17 mM, K2m = 6.5 mM; L‐valine, K1m = 0.19 mM, K2m = 11.5 mM. The KA values were: L‐leucine, K1A = 0.12 mM, K2A = 7.4 mM; L‐valine, K1A = 0.18 mM, K2A = 10.0 mM; L‐isoleucine, K1A = 0.17 mM, K2A = 9.0 mM. D‐Leucine is taken up by a low‐affinity system only (KA = 6.5 mM), which seems to be the same as the low‐affinity system transporting the L‐forms of the amino acids.