Peritubular uptake and transepithelial transport of glycine in isolated proximal tubules

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.

Transepithelial transport and metabolism of glycine in S1, S2, and S3 cell types of the rabbit proximal tubule

In the first of two sets of experiments, the lumen-to-cell and cell-to-bath transport rates for glycine were measured in the isolated-perfused medullary pars recta (S3 cells) of the rabbit proximal tubule at multiple luminal glycine concentrations (0-2.0 mM). The lumen-to-cell transport of glycine was saturated, which permitted the calculation of the transport maximum of disappearance rate of glycine from the lumen (pmol.min(-1).mm tubular length(-1)), K(m) (mM), and paracellular leak (pmol.min(-1).mm tubular length(-1).mM(-1)) values for this transport mechanism; these values were 4.3, 0.3, and 0.03, respectively. The cell-to-bath transport did not saturate but showed a linear relationship to cellular glycine concentration, 0.58 pmol.min(-1).mm tubular length(-1).mM(-1). The second set of experiments characterized the transport rate, cellular accumulation, and metabolic rate of lumen-to-cell transported [(3)H]glycine in all segments (cell types) of the proximal tubule, pars convoluta (S1 cells), cortical pars recta (S2 cells), and medullary pars recta (S3 cells). These proximal tubular segments were isolated and perfused at a single glycine concentration of 11.2 microM. From the results of this study and previous work (Barfuss DW and Schafer JA. Am J Physiol 236: F149-F162, 1979), we conclude that the axial heterogeneity for glycine lumen-to-cell and cell-to-bath transport capacity extends to the medullary pars recta (S3 cells; S1 > S2 < S3 for lumen-to-cell transport and S1 > S2 > S3 for cell-to-bath transport). Also, we conclude that lumen-to-cell transported glycine can be metabolized and its metabolic rate displays axial heterogeneity (S1 > S2 > S3). The physiological significances of these transport and metabolic characteristics of the S3 cell type permits the medullary pars recta to effectively recover glycine from very low luminal glycine concentrations and makes glycine available for protective and maintenance metabolism of the medullary pars recta.

Distribution and properties of the glycylsarcosine-transport system in rabbit renal proximal tubule. Studies with isolated brush-border-membrane vesicles

The distribution and properties of the peptide-transport system in rabbit renal proximal tubule was examined with glycylsarcosine as the substrate and using brush-border-membrane vesicles derived from pars convoluta (outer cortex) and pars recta (outer medulla). The dipeptide was transported into these vesicles against a concentration gradient in the presence of an inward-directed H+ gradient, demonstrating the presence of a H+-coupled peptide-transport system in outer-cortical as well as outer-medullary brush-border membranes. Even though the transport was electrogenic and was energized by a H+ gradient in both membranes, the system was more active in outer medullary membranes than in outer cortical membranes. Kinetic analysis showed that, although the affinity of the transport system for glycylsarcosine was similar in both membrane preparations, the capacity of the system was significantly greater in outer medulla than in outer cortex. In addition, the pH profiles of the peptide-transport systems in these membrane preparations also showed dissimilarities. The greater dipeptide uptake in one membrane vis-à-vis the other may probably be due to the difference in the affinity of the transport system for H+ and/or the difference in peptide/H+ stoichiometry.

Cytoprotective effects of glycine and glutathione against hypoxic injury to renal tubules

Roles for both the tripeptide, GSH, and individual amino acids in modifying the cellular response to oxygen deprivation-induced injury have been suggested by prior work in kidney and other tissues, but the precise interrelationships have not been clearly defined. We have studied the effects of GSH, its component amino acids, and related compounds on the behavior of isolated renal proximal tubules in a well characterized model of hypoxic injury in vitro. GSH, the combination of cysteine, glutamate, and glycine and glycine alone, when present in the medium during 30 min hypoxia, a duration sufficient to produce extensive irreversible injury in untreated tubules, were protective. Significant effects were detected at 0.25 mM concentrations of the reagents, and protection was nearly complete at concentrations of 1 mM and above. Glutamate and cysteine alone were not protective. The exogenous GSH added to the tubule suspensions was rapidly degraded to its component amino acids. Treatment of tubules with GSH or cysteine, but not glycine, increased intracellular GSH levels. Oxidized GSH was protective. Serine, N-(2-mercaptopropionyl)-glycine, and a panel of agents known to modify injury produced by reactive oxygen metabolites were without benefit. These observations identify a novel and potent action of glycine to modify the course of hypoxic renal tubular cell injury. This effect is independent of changes in cellular GSH metabolism and appears to be unrelated to alterations of cell thiols or reactive oxygen metabolites. Further elucidation of its mechanism may provide insight into both the basic pathophysiology of oxygen deprivation-induced cell injury and a practical way to ameliorate it.

Disorders of proximal nephron function

The proximal nephron is responsible for reabsorbing 80 to 99 percent of several filtered solutes, including amino acids, glucose and bicarbonate. Separate, high-affinity sodium co-transport mechanisms are used. Increasing luminal concentration of each of these solutes stimulates its active transcellular reabsorption until there is saturation. Slightly less than half of the filtered chloride is reabsorbed, partly by passive mechanisms that are linked to the reabsorption of organic solutes and bicarbonate, as well as by less well defined independent cellular and/or paracellular mechanisms that appear to be sensitive to transepithelial osmotic pressure gradients. Proximal tubule reabsorption is isosmotic and isonatric, and about 50 to 60 percent of the filtered sodium and water in reabsorbed. Disorders or proximal nephron function include conditions in which luminal, cellular and/or peritubular factors affecting reabsorption are altered. Clinical disorders caused by modification of the luminal reabsorptive determinants include conditions in which tubular flow rate is increased or luminal composition is altered, as when non-reabsorbable solutes (mannitol) are filtered or when reabsorbable solutes (glucose) are filtered in concentrations exceeding their tubular transport capacity. Other disorders occur due to loss of affinity or capacity of the cellular active transport systems for specific solutes, such as amino acids (renal aminoacidurias), glucose (renal glycosurias) and bicarbonate (proximal renal tubular acidosis), or for all solutes (Fanconi syndrome). Finally, disorders due to changes in the peritubular factors affecting reabsorption include states of altered peritubular Starling forces or pH, which modify sodium chloride or sodium bicarbonate reabsorption, respectively.