Developmental changes of glycine transport in the dog

Renal imino acid and glycine transport system ontogeny and involvement in developmental iminoglycinuria

Renal maturation occurs post-natally in many species and reabsorption capacity at birth can vary substantially from the mature kidney. However, little is known regarding the maturation of amino acid transport mechanisms, despite the well-known physiological state of developmental iminoglycinuria. Commonly seen during early infancy, developmental iminoglycinuria is a transient version of the persistent inherited form of the disorder, referred to as iminoglycinuria, and manifests as a urinary hyperexcretion of proline, hydroxyproline and glycine. The transporters involved in developmental iminoglycinuria and their involvement in the improvement of renal reabsorption capacity remain unknown. qPCR (quantitative real-time PCR) and Western blot analysis in developing mouse kidney revealed that the expression of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 was lower at birth (approx. 3.4-, 5.0-, 2.4- and 3.0-fold less than adult kidney by qPCR respectively) and increased during development. Furthermore, immunofluorescence confocal microscopy demonstrated the absence of apical expression of Slc6a18, Slc6a19, Slc6a20a and the auxiliary protein collectrin in kidneys of mice at birth. This correlated with the detection of iminoglycinuria during the first week of life. Iminoglycinuria subsided (proline reduction preceded glycine) in the second week of life, which correlated with an increase in the expression of Slc6a19 and Slc6a20a. Mice achieved an adult imino acid and glycine excretion profile by the fourth week, at which time the expression level of all transporters was comparable with adult mice. In conclusion, these results demonstrate the delayed expression and maturation of Slc6a18, Slc6a19, Slc6a20a and Slc36a2 in neonatal mice and thus the molecular mechanism of developmental iminoglycinuria.

1-Aminocyclopropanecarboxylic acid: mouse to man interspecies pharmacokinetic comparisons and allometric relationships

1-Aminocyclopropanecarboxylic acid (ACPC) is a partial agonist at the strychnine-insensitive glycine recognition site on the N-methyl-D-aspartate (NMDA) receptor complex in the mammalian central nervous system with preclinical activity in animal models of neuroprotection and psychiatric illnesses. The pharmacokinetics of ACPC were studied in the rat, beagle dog, cynomolgus monkey, and human, and compared also with data from the literature for the mouse. Plasma elimination half-lives were 1.5, 2.5, 2.9, 20.3, and 5.9 h for the mouse, rat, monkey, dog, and human, respectively. Volume of distribution at steady state (0.74 L/kg) was constant across all species. Clearance values were consistent with glomerular filtration rates for all species, except in the dog, where clearance is consistent with 84% tubular reabsorption. Allometric relationships for clearance and half-life versus body weight confirmed the predictability of the pharmacokinetics of ACPC in the mouse, rat, monkey, and human. Continued clinical evaluation of ACPC as a neuroprotective agent and in a variety of psychiatric illnesses is warranted.

Developmental changes of renal brushborder membrane ionic permeability

Renal brush-border membrane vesicles (BBMV) show an age-dependent increase in height of the Na(+)-gradient driven overshoot for glucose and proline uptake. Conversely, early uptake of 22Na+ is more rapid in BBMV from kidney of 7-day-old vs. adult rats. To understand the mechanisms responsible for these observations, ionic permeability characteristics of BBMV from different aged animals were determined using an electrical potential sensitive fluorescent dye, diS-C3(5). Absolute and relative ionic permeabilities were determined after a 3-h incubation in 100 mM KCl. Intravesicular K+ ([K+]in), a measure of absolute K+ permeability, was calculated from the extravesicular K+ at which valinomycin produced no potential difference (PD). [K+]in was significantly lower in vesicles from 7-day, compared to adult (P < 0.01). While Cl- permeability, relative to that of K+ was similar, PNa+/PK+ decreased significantly with age (P < 0.05, 7 day vs. adult). In the presence of an inwardly directed NaCl gradient, the lower PNa+ relative to PCl- of the adult vesicles would result in a less positive intravesicular charge, which would therefore augment Na(+)-solute co-transport. Fluorescence polarization studies also show that lipids from BBM vesicles of 7-day-old rats are more fluid than those from adult. These differences are likely due to developmental lipid compositional changes, which influence membrane transport and permeability characteristics. These findings would explain, in part, the age-dependent alterations of renal BBMV solute transport.

Renal amino acid transport: cellular and molecular events from clearance studies to frog eggs

This article reviews recent advances in the mechanisms of renal amino acid transport. Renal amino acid transport is necessary to efficiently reclaim approximately 450 mmol amino acids from the glomerular ultrafiltrate each day in man. In general, individual amino acids are transported across the epithelial membrane of the proximal tubule by a sodium (Na+) dependent mechanism. This cotransport process utilizes the energy of the Na+ gradient to enter the cell. The amino acid then exits the basolateral surface and Na+ is pumped out by the Na(+)-K(+)-ATPase located in the basolateral membrane. In addition to the cellular accumulation of amino acids across the luminal membrane, these compounds may be taken up by the cell from the basolateral surface. Most amino acids are transported both individually and in a series of seven group specific processes. Human disorders of amino acid transport have been described for six of the seven transport systems. The process of ontogeny of amino acid accumulation by the proximal tubule is a complex one and will be further discussed in this review. A number of factors including pH, ion dependency, electrogenicity of transport process, as well as a variety of hormonal factors, may contribute to the regulation of amino acid transport. Gene expression of several amino acid transporters has been successfully performed using the oocyte of the frog Xenopus laevis. Using this system, a number of transporters have been cloned. Such a strategy will permit the cloning of virtually all transporter molecules, and thus we can anticipate the elucidation of the structure of the transporters. However, for a comprehensive understanding of cytoskeletal interactions protein phosphorylation and phospholipid domains and their linkage to the primary structure of the transporter need to be studied. The future for research in this area is indeed a bright one.

Characteristics of L-proline and sodium transport in renal brush border membranes isolated from 7-day-old and adult rats

To explore the nature of differences in uptake by renal brush border vesicles from animals of different ages, vesicles were isolated from 7-day old and adult rats by a Mg-aggregation method. A number of criteria were compared in vesicles from the young and mature animals. The vesicles isolated from animals of both ages appear similar on electron microscopy, in response to osmotic changes, and in uptake kinetics for L-glucose. Despite these parameters which indicate no basic differences between the membranes of young and mature kidney, differences in proline and sodium handling are seen. When compared to the uptake pattern seen in vesicles from adult animals, the height of the sodium gradient-stimulated proline overshoot is diminished and sodium entry is faster in vesicles of the 7-day old rats. These are the same differences which were found in vesicles prepared by differential centrifugation from 7-day old animals. In addition, although sodium efflux was faster from vesicles of immature kidney and mirrored the faster sodium entry, proline efflux was slower. The data indicate a dissociation of proline and sodium fluxes in brush borders of the young rat kidney.

Sodium-coupled amino acid transport in renal tubule

Amino acids are reabsorbed from the tubular lumen by a saturable, carrier-mediated, concentrative transport mechanism driven by a Na+ electrochemical gradient across the luminal membrane. This process is followed by efflux mainly via carrier-mediated, Na+-independent facilitated diffusion across the basolateral membrane. Individual amino acids may have two or more Na+-dependent transport systems with different kinetic characteristics along the luminal membrane of the proximal tubule, thereby enabling very efficient amino acid reabsorption. Dual Na+-coupled transport pathways for some amino acids located in both the luminal and the peritubular membranes may operate in concert to provide the tubular epithelial cell with essential nutrients. One or more Na+ ions, H+, Cl- and in the case of acidic amino acids, K+ ion, may be involved in the translocation of the carrier complex. For most amino acids this process is electrogenic positive, favored by a negative cell interior. At least seven distinct, but largely interacting, Na+-dependent amino acid transport systems have been identified in the brush border membrane. A diet-induced adaptation in Na+-coupled taurine transport and acidosis-induced adaptive response in Na+-dependent glutamine transport are expressed at the luminal and the basolateral membrane surfaces, respectively. The aminoaciduria of early life may be related to a rapid dissipation of the Na+ electrochemical gradient necessary for amino acid reabsorption.

Characteristics of lysine transport by isolated rat renal cortical tubule fragments

The uptake of L-lysine was examined in isolated renal cortical tubules. Lysine was actively taken up by the renal tubule cells isolated from 7-week-old rats. No metabolism of the transported lysine was found. There was no evidence for sodium-dependence of lysine uptake. Concentration dependence studies revealed that the lysine was taken up by one saturable transport system with a Km of 1.66 mmol/l and Vmax of 7 mmol/l intracellular fluid per 10 min. Lysine also entered by a non-saturable pathway. Arginine and ornithine inhibited the initial uptake of lysine. Cystine increased the efflux of lysine from preloaded renal cells via hetero-exchange, indicating that a common system exists for these two amino acids.

Age related changes in fluidity of rat renal brushborder membrane vesicles

Fluorescence anisotrophy of 1,6 diphenyl-1,3,5-hexatriene was determined in renal brushborder membranes prepared from rats 7, 14, 21 and 28 days of age, and adults, from 5 degrees C to 45 degrees C. There is a parallel relationship between temperature and mean fluorescence anisotrophy in the different age groups with a progressive decrease in fluidity with age. There is no phase transition apparent in membranes from any age group as evidenced by the lack of a fluorescence polarization "break point". There is also a linear relationship between limiting hindered anisotrophy and previously determined values for the height of the Na+-proline overshoot. This suggests that the physical characteristics of the renal brushborder membrane responsible for differences in fluidity are related to age-dependent transport alterations.

Characteristics of lysine uptake by isolated renal cortical tubule fragments from mature and immature dogs

The uptake of L-lysine was examined in isolated renal cortical tubule fragments from adult and 1-week-old dogs. Lysine uptake by adult tubules was initially more rapid than that by the immature tubules. This uptake by mature tubules reached a steady state after 30 min of incubation, while the newborn tubules still had not reached a steady state by 90 min of incubation. Because a steady state of lysine uptake was not attained with the immature tubules, their uptake of lysine exceeded that of the adult after 60 min of incubation. Kinetic studies revealed that lysine was taken up by one saturable transport system with a Km of 0.56 mM and Vmax of 6.18 mmol/liter intercellular fluid per 5 min in the adult and one saturable transport system in the 1-week-old with a Km of 0.38 mM and Vmax of 3.66 mmol/l intracellular fluid per 5 min. Lysine also entered the renal tubule cells in both age groups via a diffusional pathway with a kd of 0.35 min-1 in the adult and 0.30 min-1 in the newborn. Cystine competitively inhibited lysine uptake by adult dog tubules with a Ki of 0.61 mM. The other dibasic amino acids, ornithine and arginine, also inhibited lysine uptake in both the adult and the newborn.

Developmental aspects of proline transport in rat renal brush border membranes

Proline uptake by rat renal brush border membrane vesicles from animals 7 days of age and older has been examined to delineate developmental changes in membrane function that may underlie the physiological hyperprolinuria of young animals. Although the two proline transport systems normally present in adult membranes were found in membranes from young animals, the proline "overshoot" resulting from a sodium ion gradient is minimal and increases with age of the animal from which the membranes were isolated. This is associated with a severalfold faster entry of 22Na into vesicles of the 7-day-old animal compared to entry into membranes prepared from adult kidneys. The very rapid dissipation of the sodium gradient thus diminishing the driving force for transmembrane proline movement may explain the changes in proline overshoot observed in membranes from young animals. The altered sodium permeability is consistent with the fact that young animals have a generalized inability to reabsorb other amino acids whose transport is known to be sodium gradient stimulated.