The effect of papain upon proline and sodium transport of rat renal brush-border membrane vesicles

Membrane fluidity and sodium transport by renal membranes from dogs with spontaneous idiopathic Fanconi syndrome

To comprehend the renal defect underlying the idiopathic Fanconi syndrome in the Basenji dog, we have used isolated renal brush border membrane vesicles to examine two factors that influence membrane nonelectrolyte transport processes, sodium flux and membrane fluidity. We have found that there is no significant difference in the rate of uptake of 100 mmol/L 22Na+ and conclude that the previously observed defects in the sodium gradient-stimulated overshoot of glucose and of proline are not related to an alteration in the flux of sodium at physiological concentrations. Since carrier proteins exist in a lipid milieu, alteration in the physical state of the lipid membrane can determine transport function. Renal brush border preparations from normal and affected animals were studied by measuring fluorescence polarization to assess differences in the physical state of the membranes using the fluorescent probe, DPH, which quantitates inner core membrane fluidity. Membranes from affected dogs consistently showed a higher fluidity as measured by eta, a parameter of DPH fluorescence polarization. Since membrane fluidity is related to lipid composition, the data suggest that there may be an important alteration in the lipids in renal membranes of affected animals.

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 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.

L-cystine transport by papain-treated rat renal brush-border membrane vesicles

In papain-treated rat renal brush-border membrane vesicles, cystine uptake was enhanced under sodium gradient conditions. This effect was not observed when sodium was equilibrated across the vesicle membrane or when sodium was completely absent from the incubation medium. The increased rate of cystine uptake occurred within the first two minutes of incubation and coincided with the period of increased flux of sodium known to occur after papain treatment. Under sodium gradient conditions, the Vmax of cystine uptake by treated vesicles was 65% greater while the Km was 25% lower than the value observed in untreated membranes. The increased cystine uptake after papain treatment occurred when medium cystine was in the electroneutral form. In the absence of a sodium gradient, cystine uptake by control membranes was insensitive to changes in membrane potential and this was unaltered after papain treatment. Exposure of the membranes to papain also resulted in a profound decrease in cystine binding which occurs in native membranes incubated with cystine. The fact that cystine uptake is unchanged under sodium equilibration and even enhanced under sodium gradient conditions suggests that the component of cystine binding is not essential for cystine transport and may represent non-specific binding to membrane proteins.

Inositol uptake by cultured isolated rat Schwann cells

The uptake of radiolabeled myo-inositol by Schwann cells isolated from the sciatic nerve of 2-4 day old rats was found to occur by a saturable, sodium-dependent phlorizin-inhibited mechanism with an estimated Km of 30 microM. The system was inhibited by galactose and glucose but not by galactitol. At high concentrations of myo-inositol, a diffusion-like process appeared to be functional. The characteristics of the saturable system are very similar to those of myo-inositol uptake by the endoneural fascicle preparation of sciatic nerve.