Characteristics of lysine transport by isolated rat renal cortical tubule fragments

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Ion dependence of cystine and lysine uptake by rat renal brush-border membrane vesicles

The shared transport system for uptake of L-cystine and L-lysine was examined in isolated rat renal brush-border membrane vesicles for the ionic requirements for activation of the system. No requirement for sodium was seen for either cystine or lysine influx. However, the efflux of lysine from the vesicle was stimulated by Na+. Therefore, the transport system appears to be asymmetric in its requirement for sodium. Two different divalent cations were used in the membrane isolations which resulted in different responses of cystine uptake to the electrogenic movement of K+ out of the vesicle. Membranes prepared by Mg-aggregation showed no stimulation of cystine influx by the imposition of a transient interior negative potential while vesicles prepared by Ca-aggregation did respond to electrogenic stimulation by an outwardly directed K-diffusion potential in the presence of valinomycin. Lysine influx was stimulated by electrogenic potassium efflux in both Mg-prepared and Ca-prepared membranes. No difference in sodium requirement for cystine influx was seen between the vesicles isolated by different cation-aggregation methods.

Electrogenic transport of neutral and dibasic amino acids in a cultured opossum kidney cell line (OK)

A study has been made of electrogenic cellular uptake of amino acids resulting in the depolarization of cell membrane potential (PDm) in confluent monolayers of an established opossum kidney (OK) cell line using conventional and pH-selective microelectrodes. Apical superfusion of neutral and dibasic amino acids rapidly depolarized the cell membrane, while application of acidic amino acids had no effect on PDm. The depolarization in response to L-phenylalanine and L-arginine was stereoselective, dose-dependent and saturable. 10 mmol/l of L-phenylalanine reduced PDm by 4.8 +/- 0.4 mV (n = 51) in a completely sodium-dependent way and the concentration necessary for half-maximal depolarization (C1/2) was about 1.5 mmol/l. On the other hand, the C1/2 for L-arginine was about 0.02 mmol/l. The maximal depolarization produced by L-arginine (measured at 10 mmol/l) amounted to 6.8 +/- 1.2 mV (n = 10) and this was not affected when extracellular sodium was replaced by choline (6.3 +/- 1.2 mV; n = 10). The depolarizations induced by L-phenylalanine and L-arginine were significantly additive (p less than 0.001). The intracellular pH of OK cells was 7.09 +/- 0.03 (n = 11) and did not change during L-arginine application. We conclude that (1) carrier-mediated uptake of neutral and dibasic amino acids into OK cells is at least partially electrogenic. (2) L-Phenylalanine is transported by a Na+-symport. (3) In contrast, L-arginine depolarizes PDm independently of extracellular sodium. (4) Electrogenic uptake of acidic amino acids is not detectable in OK cells.

Lysine and alanine transport in the perfused guinea-pig placenta

The characteristics of L-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional L-[3H]lysine uptake (%) (perfusate lysine 50 microM) was high on maternal (M = 87 +/- 1) and fetal (F = 73 +/- 2) sides. L-[3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05-8.00 mM) gave Km values of 1.75 +/- 0.70 mM and 0.90 +/- 0.25 mM at maternal and fetal sides, respectively; corresponding Vmax values were 1.95 +/- 0.38 and 0.87 +/- 0.10 mumol.min-1.g-1. At both sides, lysine influx (50 microM) could be inhibited (about 60-80%) by 4 mM L-lysine and L-ornithine and less effectively (about 10-40%) by L-citrulline, L-arginine, D-lysine and L-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM L-alanine (Km = 6.25 +/- 3.27 mM; Vmax = 2.62 +/- 0.94 mumol.min-1.g-1), but unchanged by equimolar L-phenylalanine or L-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either L-alanine or L-phenylalanine; (iii) L-lysine and L-alanine influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of L-[3H]lysine uptake by 4 mM L-homoserine was partially (60%) Na+-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na+-dependent, while lysine influx was partially Na+-dependent only at low concentrations (0.05-0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced L-[3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y+) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.