Glucose transport in the kidney

Characterization of an essential disulfide bond associated with the active site of the renal brush-border membrane D-glucose transporter

In a previous report (J. Biol. Chem. 258 (1983) 3565-3570) we have demonstrated that the disulfide-reducing agent dithiothreitol has two effects on the sodium-dependent outer cortical brush border membrane D-glucose transporter; the first results in a reversible increase in the affinity of the transporter for the non-transported competitive inhibitor phlorizin, while the second results in a partially reversible loss of phlorizin binding and glucose-transport activity. Evidence was presented that both of these effects are the result of the reduction of disulfide bonds on the transport molecule. In the present paper we extend our observations on the inactivation of the transporter by dithiothreitol. We provide evidence here (i) that the inactivation of the transporter by dithiothreitol is independent of the effect of the reducing agent on the affinity of the transporter, (ii) that this inactivation process is first-order in dithiothreitol and thus presumably due to the reduction of a single disulfide bond essential to the functioning of the transporter. (iii) that it is the reduction of this disulfide bond and not some subsequent conformational or other change in the transporter which results in its inactivation, (iv) that phlorizin and substrates of the transporter provide protection against inactivation by dithiothreitol and that the degree of protection provided correlates well with the known specificity and phlorizin-binding properties of the transporter, and (iv) that the reactivity of the transporter with dithiothreitol is pH-dependent, decreasing with increasing pH over the pH range 6.5-8.5. We conclude that this site of action of dithiothreitol is a single essential disulfide bond intimately associated with the glucose-binding site on the transport molecule.

Phloretin inhibition of glucose transport by the tapeworm Hymenolepis diminuta: a kinetic analysis

Phloretin non-competitively inhibits glucose and methionine absorption by Hymenolepis diminuta (Ki = 0.24 and 1.48 mM, respectively). Inhibition of glucose transport by phloretin is reversible. Phloretin and phlorizin binding sites on the surface of H. diminuta are distinct. Phloretin does not inhibit Na+ transport independent of glucose transport in H. diminuta. The data suggest that phloretin inhibits absorptive functions across the surface of H. diminuta via non-specific binding sites associated with the lipid portion of the worm outer membrane.

Glucose-dependent respiration in suspensions of rabbit cortical tubules

The effects of glucose on cellular respiration were examined in suspensions of rabbit cortical tubules. When glucose was removed from the bathing fluid, oxygen consumption (QO2) decreased from 18.6 +/- 0.8 to 15.7 +/- 0.5 nmol O2/mg protein X min (P less than 0.01). The transported but nonmetabolized analogue of glucose, alpha-methyl-D-glucoside (alpha MG), was found to support QO2 to the same extent as glucose. These observations were also evident in the presence of butyrate, a readily oxidized substrate of the renal cortex. Additional studies with nystatin and ouabain indicated that glucose-related changes in QO2 were the result of changes in Na, K-ATPase associated respiration. The effect of glucose was localized to the luminal membrane since phlorizin (10(-5) M), a specific inhibitor of luminal glucose-sodium cotransport, also significantly reduced QO2 by 10 +/- 1%. Phlorizin inhibition of QO2 was also evident in the presence of alpha MG but was abolished when glucose was removed from the bathing medium. Finally, measurement of NADH fluorescence showed that addition of glucose (5 mM) to a tubule suspension causes an oxidation of NAD. These data are all consistent with glucose acting to increase respiration by stimulating sodium entry at the luminal membrane (via glucose-sodium cotransport) followed by increased sodium pump activity and its associated increase in mitochondrial respiration.

Decreased Na+-gradient-dependent D-glucose transport in brush-border membrane vesicles from rabbits with experimental Fanconi syndrome

The effect of anhydro-4-epitetracycline on sodium gradient-dependent D-glucose transport of rabbit renal brush-border membrane vesicles was studied. The purity of isolated brush-border membrane vesicles as judged by enzyme activities was not different between normal control and anhydro-4-epitetracycline-administered rabbits. There was no difference in estimate of intravesicular volume, either. When NaCl was used for sodium gradient, the overshoot of D-glucose uptake into brush-border membrane vesicles isolated from anhydro-4-epitetracycline-treated rabbits was significantly smaller than that of normal control rabbits. In the cases of NaSCN or Na2SO4, the former was also smaller than the latter, but not significantly so. To avoid the possible effect of membrane potential on D-glucose uptake, the voltage-clamp method was applied. Even in the voltage-clamped condition, the overshoot of D-glucose uptake into vesicles from anhydro-4-epitetracycline-treated rabbits was decreased compared to that of normal rabbits. In vitro incubation of brush-border membrane vesicles with 20 mM anhydro-4-epitetracycline caused no alteration in sodium gradient-dependent D-glucose uptake. Our results demonstrate that there exists a disorder in sodium gradient-dependent D-glucose uptake of renal brush-border membrane in anhydro-4-epitetracycline-treated rabbits, and suggest that this disorder is one of the underlying mechanisms of experimental Fanconi syndrome.

Sodium gradient-driven transport processes in ATP-depleted renal tubules

The effects of directed Na+ gradients on proximal tubule cell transport processes were examined in suspensions of rabbit renal tubules depleted of ATP. Cells of high-Na+ content were generated by suspending the tubules in high-Na+ media, whereas low-Na+ cells were produced by incubating tubules in Na+-free media. Resuspension of the high-Na+ tubules in Na+-free media caused a fall in cell pH simultaneous with a fall in cell Na+. Resuspending the low-Na+ cells in Na+-replete media led to a rise in cell pH, in parallel with the rise in cell Na+. Removing HCO-3 and CO2 augmented and amiloride inhibited the increase in cell pH generated by the inward Na+ gradient. Low-Na+ tubules exposed to an inwardly directed Na+ gradient also concentrated the sugar analogue alpha-methylglucoside, and this uptake was blocked by phlorizin. These findings demonstrate the suitability of ATP-depleted renal tubules for the study of linked transport processes by providing evidence for the existence of the proximal luminal transport processes, Na+-H+ exchange, and Na+-sugar cotransport in this preparation.

Growth-dependent AIB and meAIB uptake in LLC-PK1 cells: effects of differentiation inducers and of TPA

Cultured pig kidney cells designated LLC-PK1, previously shown to acquire Na+-dependent concentrative transport of hexoses as the cells become growth arrested, also show Na+-dependent concentrative uptake of the amino acid analogs alpha-aminoisobutyric acid (AIB) and (methyl) meAIB. This A system-like transport is most active in sparse, growing cultures and becomes stepped down at confluence. The cell/medium equilibrium distribution ratio of the lipophilic cation tetraphenylphosphonium ion (TPP+) decreases in parallel fashion, suggesting that a decrease in membrane potential may be a major factor in the stepdown. Differentiation inducers (hexamethylene bisacetamide) and phosphodiesterase inhibitors (theophylline, methylisobutyl xanthine) accelerate the stepdown, but even in the presence of these compounds addition of the tumor promoter 12-0-tetradecanoylphorbol-13-acetate (TPA) results in the maintenance of a high level of AIB and meAIB uptake. In all these respects the changes in A system-like amino acid transport are the reciprocal of those seen for concentrative hexose transport, although the driving force appears to be the same for both systems. The TPA analogs phorbol and 4-0-methyl TPA which are inactive in tumor promotion are inactive in this system as well. In confluent, already stepped-down cultures, addition of TPA leads to a rapid (2-6 hour) stimulation of AIB and meAIB uptake. The enhancement is sensitive to cycloheximide and actinomycin D. The ouabain-sensitive fraction of meAIB uptake is not markedly changed in the TPA-enhanced uptake, nor is the TPP+ distribution ratio elevated in TPA-treated cells, making it unlikely that the TPA effect is through an alteration in the membrane potential.

Na+-dependent hexose transport in vesicles from cultured renal epithelial cell line

Apical membrane vesicles were prepared from cultured epithelia formed by LLC-PK1 cells using a calcium precipitation technique. alpha-Methylglucoside uptake into this vesicle preparation was markedly stimulated by sodium and inhibited by phlorizin. In addition, a transient "overshoot" of intravesicular alpha-methylglucoside concentration above its equilibrium value was observed under initial sodium gradient conditions. The specificity of this sodium-dependent hexose transporter closely resembled that found in the mammalian kidney brush border membrane, e.g., alpha-methylglucoside, D-glucose, and D-galactose apparently share the transporter while 2-deoxy-D-glucose, mannose, and fructose do not. Kinetic analysis of the sodium-dependent component of alpha-methylglucoside flux into LLC-PK1 apical membrane vesicles indicates the existence of single transporter with Km congruent to 2 mM and Vmax congruent to 3 nmol.min-1.mg protein-1. Measurement of alpha-methylglucoside uptake as a function of sodium concentration is consistent with a sodium:sugar stoichiometry of approximately 2:1.l There is a good correlation over time between the development of the concentrating capacity of the intact epithelium for alpha-methylglucoside and the transport properties of the vesicle preparation.

Further studies of proximal tubular brush border membrane D-glucose transport heterogeneity

The properties of two sodium-dependent D-glucose transporters previously identified in renal proximal tubule brush border membrane (BBM) vesicles are studied. The low-affinity system, found in BBM vesicles from the outer cortex (early proximal tubule), is shown to be associated with the high-affinity phlorizin binding site typically found in renal BBM preparations. The high-affinity system, found in BBM vesicles from the outer medulla (late proximal tubule), is almost two orders of magnitude less sensitive to inhibition by phlorizin and is apparently not associated with high-affinity phlorizin binding. The sodium/glucose stoichiometry of the outer medullary transporter is found to be 2:1 by two independent methods. Previous measurements have established that the stoichiometry of the outer cortical system is 1:1. It is suggested that this arrangement of transporters in series along the proximal tubule enables the kidney to reabsorb glucose from the urine in an energy-efficient fashion. The bulk of the glucose load is reabsorbed early in the proximal tubule at an energetic cost of one Na+ per glucose molecule. Then in the late proximal tubule a larger coupling ratio and hence a larger driving force is employed to reabsorb the last traces of glucose from the urine.

Development of Na+-dependent hexose transport in a cultured line of porcine kidney cells

LLC-PK1 cells in culture do not concentrate alpha-methylglucoside (alpha-meG) during their early growth phase but develop the capacity to concentrate this hexose as the growth rate decreases in confluent cultures. The concentrating ability is dependent on the Na+ electrochemical gradient and is inhibited by phlorizin with KI,0.5 approximately 0.2 microM. The development of the concentrative capacity can be accelerated by the Friend cell inducer hexamethylene bisacetamide (HMBA) and by the phosphodiesterase inhibitors dibutyryl cAMP, theophylline, and 1-methyl-3-isobutylxanthine (MIX). In cultures treated with any of these differentiation-accelerating chemicals, the development of alpha-meG concentrating capacity is severely inhibited by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) but not by inactive (in tumor promotion) analogs of TPA. In all cases, an early event in the development of alpha-meG accumulating capacity is an elevated intracellular cAMP concentration; however the results suggest that this increase in cAMP may be necessary but not sufficient to induce the differentiated hexose-accumulating capacity.

D-Glucose transport across the apical membrane of the surface epithelium in Nereis diversicolor

Epidermal D-glucose transport was investigated in vivo in the brackish-water polychaete worm Nereis diversicolor. Transfer across the apical membrane is rate-limiting to D-glucose uptake, but the cuticle and/or mucus presents some resistance to D-glucose diffusion between bulk solution and transporting membrane. Maximal D-glucose influx is about 10(-12) mol sec-1 per cm2 of apical plasmalemma. Under natural conditions (approximately 1 microM D-glucose in the medium), backflux from the epidermal transport pool is negligible, but a significant paracellular outflux may occur. D-glucose influx across the apical membrane is Na+-dependent and completely inhibitable by phlorizin and harmaline; phloretin is less effective, and cytochalasin B has no effect. Influx is moderately depressed by KCN and iodoacetate, alpha-methyl-D-glucopyranoside is an effective substitute of D-glucose in transport. Animals acclimated to a low salinity, in which epidermal salt transport takes place, show a marked decrease of D-glucose transport capacity. On transfer of animals from a high to a low salinity, or vice versa, the corresponding change of influx occurs after a time-lag of at least an hour. Permeability of the epidermis to simple diffusion of D-glucose is 8 X 10(-8) cm sec-1 (on basis of gross epidermal area).

Interaction of phlorizin and sodium with the renal brush-border membrane D-glucose transporter: stoichiometry and order of binding

The order and stoichiometry of the binding of phlorizin and sodium to the renal brush-border membrane D-glucose transporter are studied. The experimental results are consistent with a random-binding scheme in which the ratio of phlorizin- to sodium-binding sites is one-to-one. When the kinetics of phlorizin binding are measured as a function of increasing sodium concentration no significant variation is found in the apparent number of binding sites; however, the apparent binding constant for phlorizin decreases rapidly from approximately 16 microM at [Na] = 0 to 0.1 microM at [Na] = 100 mM and approaches 0.05 microM as [Na] leads to infinity. The experimental data are fit to a random carrier-type model of the coupled transport of sodium and D-glucose. A complete parameterization of the phlorizin binding properties of this model under sodium equilibrium conditions is given.

Glucose transport by short loops of Henle in the rat

1. Short loops of Henle were artificially perfused with saline solutions containing 5 or 0 mM-glucose in the presence and absence of phlorizin. 2. Net fluid reabsorption was greater when glucose but no phlorizin was present than in all other series. 3. Glucose was reabsorbed from glucose-containing perfusate and this was abolished by phlorizin. Secretion of glucose occurred into the perfusate which initially contained no glucose and this secretion was enhanced by phlorizin. 4. Sodium reabsorption was inhibited by phlorizin, when glucose was present, but enhanced by phlorizin when glucose was absent. 5. It can be shown that there is secretion of some osmotically active solute in all series. Its secretion is enhanced by phlorizin in the absence of glucose.

Participation of the ring oxygen in sugar interaction with transporters at renal tubular surfaces

The pulse-injection indicator-dilution technique in vivo has been used to study the interaction of 5-thio-D-glucose and methyl-beta-D-thiogalactopyranoside with renal tubular surfaces in dog kidney. (i) 5-Thio-D-glucose and methyl-beta-D-thiogalactopyranoside have nor antiluminal interaction. (ii) 37 +/- 5% of 5-thio-D-glucose is extracted at the luminal surface relative to simultaneously filtered creatinine. (iii) Luminal extraction of 5-thio-D-glucose is blocked by preloading with D-glucose and phlorizin. (iv) Methyl-beta-D-thiogalactopyranoside in contrast to D-galactose has not luminal interaction. It is concluded that 5-thio-D-glucose shares the glucose transporter at the luminal surface of the proximal tubule. The data also suggest that the ring oxygen participates in the interaction of pyranosides with luminal and antiluminal membrane carriers. At the luminal surface, its absence is quantitatively important while at the antiluminal surface it is apparently essential for the sugar-transporter interaction.

Active renal hexose transport. Structural requirements

The active transport of methyl beta-D-galactoside and some other analogs of D-glucose and D-galactose was studied in slices of rabbit and renal cortex. 1. The non-metabolizable methyl beta-D-galactoside accumulates in renal cortical cells against its concentration gradient. At 1 mM substrate concentration (O2, 35 degrees C, 60 min incubation) the gradient was 2.36 +/- 0.11 S.E. (n = 33). The Kt was 1.50 +/- 0.02 mM. The active transport of the substrate was inhibited by dinitrophenol, phlorizin, absence of Na+ and by ouabain. This inhibition was incomplete, suggesting that the sugar may enter the cells by two separate pathways, only one of which was coupled to the down-hill electrochemical Na gradient. 2. The structural requirements for the interaction between substrate and the carrier were defined: (a) by testing the transport behavior of some analogs (1,5-anhydro-D-glucitol; methyl beta-thio-D-galadtoside; 3-deoxy-D-glucose; 4-deoxy-D-glucose; 5-thio-D-glucose; 6-deoxy-D-glucose and methyl-alpha-6-deoxy-D-glucoside); and (b) by inhibition analysis of methyl beta-D-galactoside transport. The role of each hydroxyl of the sugar molecule was tested by using a total of 41 analogs modified on each C by replacing -OH by -H, -O-CH3, -F and in some instances also by -SH. 3. The carrier is shared by D-glucose, D-galactose and their methyl glycosides. A pyranose ring is mandatory. The D-glucoconfiguration is preferred for the interaction with the carrier. 4. Replacement of -OH by -H or -S practically abolished (on C1, C2, C3) or greatly reduced (on C4) the affinity of the analog for the carrier. This was also confirmed by demonstrating that 1-deoxy-, and 3-deoxy-glucose and the thio-galactoside were not actively transported and their entry into the cells was not markedly affected by phlorizin, dinitrophenol, ouabain or absence of Na+. 4-Deoxy-D-glucose was taken up and its transport was inhibited by agents affecting the transport of methyl beta-D-galactoside. 5. Replacement of -OH by -F did not abolish the affinity of the analogs for the carrier, indicating hydrogen bonding between the carrier and the oxygens at C1, C2, C3, and C4. 6. 5-Thio-D-glucose was not transported against its concentration gradient and also poorly interacted with the carrier as shown by inhibition analysis. Hydrogen bonding between the carrier and the pyranose ring oxygen is suggested. 7. 6-Deoxyglucose is a potent inhibitor of methyl beta-D-galactoside transport although it is not actively taken up by the tissue. It is concluded that a hydroxyl at C6 is required for transport, but is not mandatory for an interaction with the carrier. However, 6-deoxy-D-galactose was ineffective as inhibitor. 8. The specificity of the carrier involved in the renal active transport of D-glucose, D-galactose and their methyl glycosides resembles qualitatively, and mostly also quantitatively that described for intestinal transport of these sugars.

Preparation and properties of liposome-associated gentamicin

Gentamicin was successfully incorporated into neutral, anionic, and cationic liposomes, and the percentage of gentamicin incorporated was found to be a function of lipid concentration. Gentamicin did not leak out of the liposomes over a 3-week period at 4 degrees C. When liposome-associated gentamicin was administered intravenously to rabbits, its serum half-life was greatly prolonged. Intragastric administration of dipalmitoylphosphatidylcholine liposomes containing gentamicin resulted in the appearance of gentamicin in serum. Liposome-associated gentamicin, when administered intravenously, led to the appearance of gentamicin in the liver and spleen, which was not observed when rabbits were injected with free gentamicin.

Partial purification of the sugar carrier of intestinal brush border membranes. Enrichment of the phlorizin-binding component by selective extractions

The [3H] phlorizin-binding component of brush border vesicles was enriched in situ by negative purification. Several procedures, known to effect selective solubilization of membrane components, were used separately or in combination to remove proteins unrelated to the binding. Deoxycholate ruptured the vesicles and released 67% of their protein, thereby increasing the specific [3H] phlorizin-binding activity of the pellet three-to fourfold. Extracting the deoxycholate-pellets with either NaI or alkaline solutions released up to 38% of the deoxycholate-insoluble protein without significantly affecting phlorizin binding. The polypeptide composition of the membranes at the different stages was analyzed by NaDodSO4-polyacrylamide gel electrophoresis. A number of polypeptides present in the original vesicles could be ruled out as essential components of the [3H] phlorizin binding entity. Intact and deoxycholate-treated vesicles were subjected to proteolytic attack. Papain liberated sucrase and isomaltase from intact vesicles, but affected neither other Coomassie-stained bands nor phlorizin binding. Neither the protein composition nor the binding properties of sealed vesicles were influenced by trypsin or chymotrypsin. However, all the proteolytic enzymes tested on deoxycholate-treated membranes substantially reduced [3H] phlorizin binding and produced concomitantly the disappearance of several bands from the electrophoretic profile. Pretreatment of vesicles with papain, followed by deoxycholate extraction and incubation in alkaline media, increased the specific binding activity of the membranes up to ninefold by removing close to 90% of the protein. A limited number of polypeptides are suggested as possible candidates for the glycoside-binding site of intestinal brush borders.

Sodium dependence of the nerve growth factor--regulated hexose uptake in chick embryo ganglionic cells

Embryonic dorsal root ganglionic cells, when incubated in vitro in the absence of nerve growth factor (NGF) undergo a general metabolic degeneration which is preceded by certain changes in permeation properties. Previous studies demonstrated that NGF can rapidly modulate permeation properties which regulate the availability to the cell of an important energy source, glucose. Hexose uptake was determined by measuring the ability of the cells to accumulate [3H]labeled 2-deoxy-D-glucose. The work reported here shows that the NGF-dependent portion (about one-third) of the total specific hexose uptake was also dependent on the presence of Na+, with the apparent uptake constant (Kt) for deoxyglucose varying inversely with an external Na+ concentration of 70-140 mM; Vmax was unaffected in this range. Preincubation of ganglionic cells with 10 mM ouabain for 15-60 min, followed by a pulse with [3H]-deoxyglucose, also resulted in 50-95% reduction of the NGF-sensitive uptake. A similar pretreatment of cells with veratridine gave a 25-50% reduction in uptake. The NGF-controlled hexose uptake was also energy dependent, being diminished 50-95% after a 30-90 min preincubation with 2 mM 2,4-dinitrophenol. Uptake activities for other substrates (alpha-aminoisobutyric acid, uridine) which exhibited NGF regulation were likewise Na+-sensitive. These results indicate that availability of major energy substrates to NGF-dependent dorsal root ganglionic neurons is controlled by sodium gradients across their membranes. It is conceivable that NGF provides for maintenance and development of its target neurons by acting on such sodium gradients and, consequently, regulating the intake of essential nutrients.

Glucose transport by horse kidney brush borders. I.--Transport properties of brush border membrane closed vesicles

Brush border membranes isolated from horse kidney cortex as closed right-side out vesicles show selective permeability when analyzed on sucrose and dextran gradients. These vesicles can actively accumulate D-glucose. The preservation of the glucose transport system is demonstrated by the following features: (a) the uptake and release rates of D-glucose are higher in the presence of a sodium gradient, showing that D-glucose transport is a sodium-dependent process; (b) this transport, specific for the D-isomer, is inhibited by phlorizin; (c) the D-glucose transport system is saturable; (d) no inhibition of D-glucose transport is found with C-mannose; (e) the D-glucose uptake is sensitive to osmotic variations.

Sugar uptake into brush border vesicles from dog kidney. II. Kinetics

The kinetics of D-glucose transport over the concentration range 0.07--20 mM have been investigated in a vesiculated membrane preparation from dog kidney cortex. 1. A sodium-dependent and a sodium-independent component of D-glucose uptake are observed. The sodium-dependent component is phlorizin sensitive (KI approximately 0.6 micron) and electrogenic. 2. The sodium-dependent component of D-glucose uptake yields non-linear Eadie-Hofstee plots consistent with the presence of high (GH) and low (GL) affinity sites (KH approximately 0.2 mM, KL approximately 4.5 mM, VL/VH approximately 7 at pH 7.4, 25 degrees C, 100 mM NaC1 gradient). Alternative explanations are cooperative effects of non-Michaelis-Menten kinetics. 3. The initial uptake of D-glucose increases as the intravesicular membrane potential become more negative but the numerical values of KH and KL show little, if any, change. 4. alpha-Methyl-D-glucoside transport is also sodium dependent and phlorizin sensitive (KI approximately 1.9 micron). 5. In contrast to the results for D-glucose, the sodium-dependent component of alpha-methyl-D-glucoside uptake exhibits a nearly linear Eadie-Hofstee plot consistent with a single carrier site with Km approximately 1.9 mM and Vmax approximately 27 nmol/min per mg protein at pH 7.4, 25 degrees C, 100 mM NaCl gradient. 6. The kinetics of D-glucose transport in newborn dog kidney are similar to those in the adult except that the low affinity (GL) system appears to be less well developed.

Sodium, phosphate, glucose, bicarbonate, and alanine interactions in the isolated proximal convoluted tubule of the rabbit kidney

Interactions among the transport systems involved with sodium, bicarbonate, glucose, phosphate, and alanine absorption in isolated segments of the rabbit proximal convoluted tubule were examined with radioisotopic techniques to measure glucose, phosphate, and fluid absorption rates. The composition of the perfusate and bath varied from normal, physiological fluids to fluids deficient in a single solute. The deletion of glucose from the perfusate increased the lumen-to-bath flux of phosphate from 5.51 +/- 1.15 to 8.32 +/- 1.34 pmol/mm-min (P less than 0.01). Similar changes occurred when glucose transport was inhibited by phlorizin 10 micron in the perfusate, The deletion of alanine from the perfusate increased the lumen-to-bath flux of phosphate from 6.55 +/- 1.08 to 9.00 +/- 1.30 pmol/mm-min (P less than 0.01) but did not affect glucose transport significantly, 80.1 +/- 10.1 vs. 72.5 +/- 5.4 pmol/mm-min. Replacement of intraluminal sodium with choline, elimination of potassium from the bath, and removal of bicarbonate from the lumen and bath each reduced glucose, phosphate, and fluid absorption. These data indicate that the proximal absorptive processes for glucose and for phosphate include elements that are dependent upon some function of sodium transport. Additionally, the effects on phosphate transport of deleting glucose or alanine occur independent of any changes in net sodium transport and are opposite the effects of deleting bicarbonate. These differences may relate to the observations that the transport of glucose and alanine is electrogenic while that of bicarbonate is not. Regardless of possible mechanisms, the data demonstrate that important changes in the absorption rates of different solutes handled significantly by the proximal convoluted tubule may occur in response to changes in specific components of proximal sodium transport.

Is hexokinase present in the basal lateral membranes of rat kidney proximal tubular epithelial cells?

The possible presence of hexokinase in basal lateral membranes from rat kidney proximal tubules was investigated. Basal lateral membranes were obtained from homogenates of rat kidney cortex by differential centrifugation and free flow electrophoresis. They were further purified by density gradient centrifugation. Hexokinase activity was measured as the phosphorylation of D-[U14C]glucose. Throughout the purification of the membranes, the specific activity of hexokinase decreased while that of (Na+ + K+)-ATPase increased. Hexokinase activity in all fractions could be quantitatively accounted for in terms of cytosolic and mitochondrial enzyme contributions. It is concluded that there is no hexokinase activity in basal lateral membranes from rat kidney.

Effects of phlorizin on glucose, water and sodium handling by the rat kidney

1. The effect of phlorizin on glucose, water and sodium handling by the kidney in anaesthetized rats was investigated, using clearance techniques, during infusion of saline (200 microliter min-1) or saline to which either low (0.1 mumole kg body weight-1 ml.-1) doses of phlorizin had been added. 2. Phlorizin increased the absolute and fractional excretion of glucose, urine osmolality and negative free water clearance; and reduced urine flow rate, glomerular filtration rate (GFR), absolute and fractional excretion of sodium, absolute excretion of sodium, absolute excretion of potassium and absolute and fractional rates of glucose reabsorption. 3. The data indicate that phlorizin has sites of action and effects additional to those on glucose transport in the proximal tubule. 4. Within each series there was a positive correlation between sodium and glucose reabsorption; but the rate of glucose reabsorption was different between each series even though the sodium reabsorption was not. 5. It is suggested that since both sodium and glucose reabsorption correlate with GFR, they may be related via GFR. 6. The data indicate that for the whole kidney any effect of glucose on sodium transport is small relative to total renal handling of sodium.

Effects of glucose on water and sodium reabsorption in the proximal convoluted tubule of rat kidney

1. The effects of glucose on sodium and water reabsorption by rat renal proximal tubules was investigated by in situ microperfusion of segments of proximal tubules with solutions containing glucose or no glucose, with and without phlorizin. 2. Absence of glucose did not significantly alter net water flux. Sodium flux was reduced by about 10% but this was not statistically significant. 3. In the absence of glucose in the perfusion fluid net secretion of glucose occurred. 4. Phlorizin reduced either net reabsorption or net secretion of glucose; and net water flux. 5. The data suggest that in later parts of the proximal convoluted tubule some sodium may be co-transported with glucose, but that this represents only a small fraction of the total sodium reabsorption. 6. It is suggested that the glucose carrier is reversible and in appropriate circumstances could cause glucose secretion. 7. Although phlorizin alters net water flux the underlying mechanisms are not clear. 8. The calculated osmolality of the reabsorbate was significantly greater than the perfusate osmolality and greater than plasma osmolality although this was not quite significant statistically.

Kinetics of Na+-dependent D-glucose transport

The kinetic parameters of the Na+-dependent glucose transport system have been determined in isolated membrane vesicles for D-glucose, Na+, and phlorhizin. The D-glucose flux measurements were carried out by the equilibrium exchange procedure at constant external and internal Na+ concentrations and zero potential. Equations were developed to extract information about Km and Vmax from uptake measurements into a vesicle population that is heterogeneous with respect to size (surface to volume ratio). The Km for D-glucose was 14 mM and independent of the Na+-concentration, while the Vmax was strongly Na+-dependent and increased 15-fold between 1 and 100 mM Na+. The Km of Na+ for activation of the Vmax was 18 mM. The calculated KI values for phlorhizin were 2.7 and 1.9 micrometer when determined under active and equilibrating D-glucose flux conditions, respectively.