The Binding of d-Glucose to the Isolated Luminal Membrane of the Renal Proximal Tubule

Chloroquine, a novel inhibitor of amino acid transport by rat renal brush border membrane vesicles

Chloroquine is an antimalarial and antirheumatic lysosomotropic drug which inhibits taurine uptake into and increases efflux from cultured human lymphoblastoid cells. It inhibits taurine uptake by rat lung slices and affects the uptake and release of cystine from cystinotic fibroblasts. Speculations on its mode of action include a proton gradient effect, a non-specific alteration in membrane integrity, and membrane stabilization. In this study, the effect of chloroquine on the uptake of several amino acids by rat renal brush border membrane vesicles (BBMV) was examined. Chloroquine significantly inhibited the secondary active, NaCl-dependent component of 10µM taurine uptake at all concentrations tested, but did not change equilibrium values. Analysis of these data indicated that the inhibition was non-competitive. Taurine uptake was reduced at all osmolarities tested, but inhibition was greatest at the lowest osmolarity. Taurine efflux was not affected by chloroquine, nor was the NaCl-independent diffusional component of taurine transport. Chloroquine (1 mM) inhibited uptake of the imino acids L-proline and glycine, and the dibasic amino acid L-lysine. It inhibited the uptake of D-glucose, but not the neutralα-amino acids L-alanine or L-methionine. Uptake of the dicarboxylic amino acids, L-glutamic acid and L-aspartic acid, was slightly enhanced. With regard to amino acid uptake by BBMV, these findings may support some of the currently proposed mechanisms of the action of chloroquine but further studies are indicated to determine why it affects the initial rate of active amino acid transport.

Inhibitors of anion exchanger activity reduce sodium chloride-dependent taurine transport by brush border vesicles

The erythrocyte anion exchanger, or band 3 protein, catalyzes the exchange of chloride and bicarbonate in many cells. Anion exchanger activity is predominantly found in the basolateral membrane of the cortical collecting duct. Because proximal tubular renal taurine uptake across the brush border surface is dependent upon sodium and chloride, the effect of inhibitors of anion exchanger activity on renal taurine accumulation by rat renal brush border membrane vesicles was examined. The anion exchanger probes, DIDS (4,4-'diisothiosulfonic acid), and NBD-taurine (N-[7-nitrobenz-2-oxa-1,3-diazol-4-yl]-aminoethane sulfonic acid), and NBD-taurine (N-[7-nitrobenz-2-oxa-1,3-diazol-4-yl]-aminoethane sulfonic acid), were used as inhibitors of anion exchanger activity. DIDS, NAP-, and NBD-taurine all markedly inhibit the initial rate of NaCl-dependent accumulation of taurine by brush border membrane vesicles. NAP- and NBD-taurine accumulation is chloride dependent because inhibition was not found when uptake was performed in the presence of NaNO3 in place of chloride. In the presence of maximal inhibition of taurine uptake by NAP- or NBD-taurine, no additional inhibition was evident after incubation with DIDS. On the other hand, when a competitive inhibitor of taurine uptake, beta-alanine, was used, additional inhibition of taurine accumulation was found in the presence of NAP- or NBD-taurine (p < .01 respectively). These results suggest some interaction of the anion exchanger and the taurine accumulation process at the apical surface of the proximal tubule. Because NBD-taurine is a fluorescent probe, it may be possible to isolate membrane peptides that bind NBD-taurine and demonstrate fluorescence. Preliminary isoelectric focusing experiments of brush border protein incubated with NBD-taurine show fluorescence localized to several particular fractions. Hence this observation that inhibitors of the anion exchanger system block NaCl-dependent taurine uptake can potentially serve as a means of isolating the taurine transporter protein.

Efflux of taurine from renal brush border membrane vesicles: is it adaptively regulated?

The uptake of the beta-amino acid taurine by rat renal brush border membrane vesicles (BBMV) adapts to changes in dietary sulfur amino acid intake. Initial rate Vmax "upregulates" after ingestion of a low methionine and taurine diet (LTD) and "downregulates" after a high taurine diet (HTD). This is reflected in vivo by hypotaurinuria after a LTD (90% reduction in excretion) and an 18-fold increase in urine taurine after a HTD. This study was performed to determine whether taurine efflux from BBMV is adaptively regulated by external taurine concentration or by diet. Vesicles were preloaded with varying concentrations of radiolabelled and unlabelled taurine and a 150 mM concentration of various salts. Efflux conditions were: taurine and 150 mM salt inside and 150 mM salt outside. The efflux of five concentrations of taurine (10-500 microM) was linear over 6 min, reached equilibrium by 21 min, and was dependent upon intravesicular taurine content. The kinetic characteristics of efflux (E) were significantly different from influx (I): Km = 109.8 +/- 5.8 (E) versus 23.6 +/- 4.2 (I), P < 0.001 [time of linearity = 360 s (E) vs. 20 s (I)]. Efflux of taurine was dependent on the presence of both sodium and chloride in the system, but neither external taurine content (0.100 microM, 1,000 microM) nor external beta-alanine altered initial efflux. Feeding rats a normal diet, LTD, or fasting altered taurine uptake but not efflux. Efflux does not appear to play a role in the adaptive regulation of taurine transport found in all mammalian species.

Oxalate binding to rat intestinal brush-border membrane in pyridoxine deficiency: a kinetic study

Oxalate bound specifically to the intestinal brush-border membrane (BBM) of pyridoxine-deficient rats, but not to BBM of control rats. The binding of oxalate to intestinal BBM of pyridoxine-deficient rats was rapid, reversible, dependent on concentration of oxalate, temperature sensitive and competitively inhibited by oxalate analogues. Kinetic analysis of the oxalate binding data revealed induction of two distinct classes of receptor site for oxalate. The high-affinity oxalate binding sites, reached saturation at 60-70 nM oxalate, had a Kd of 24.29 nM and the number of binding sites were 30 pmoles (i.e., 1.8.10(13) molecules). The low-affinity oxalate binding sites, could not be saturated under experimental conditions upto 1 microM oxalate. It had a Kd of 487.5 nM and the number of binding sites were 156 pmoles (i.e., 9.4.10(13) molecules). The apparent energy of activation was 19 kcal/mol. The half-saturation concentration of inhibitor (IC50) of oxalate was 0.4.10(-5) M, while all other structural analogues of oxalate had higher IC50 values. Among the competitive inhibitors tested IC50 was in the following order, pyruvate greater than maleate greater than oxaloacetate greater than glyoxylate greater than parabonate greater than oxalate. These kinetic characteristics indicate involvement of a membrane protein in oxalate binding and transport in rat intestinal brush-border membrane in pyridoxine deficiency.

Na(+)-dependent amino acid uptake by human placental microvillous membrane vesicles: importance of storage conditions and preservation of cytoskeletal elements

Human placental microvillous membrane vesicles (MMV) were purified by precipitation of non-microvillous membrane with MgCl2. Two aspects of MMV preparation were found to be important to the interpretation of amino acid transport studies: (1) storage conditions, and (2) preservation of cytoskeletal elements. In non-frozen MMV, MeAIB uptake was stimulated by an inward Na+ gradient and showed 'overshoot'. The initial Na(+)-dependent uptake rate was concentration-dependent with a Vmax of 640 +/- 80 pmol/mg/30 sec and a Km of 0.44 +/- 0.77 mM. Na(+)-stimulated cysteine uptake (65 +/- 23 pmol/mg/30 sec), previously thought to be very low or absent in the human placenta, was comparable to MeAIB, although there was no 'overshoot'. Cysteine uptake was partially stimulated by Li+. In general, freezing and storage at either -80 degrees C or -196 degrees C markedly reduced Na(+)-dependent uptake of several amino acids, compared to vesicles stored at 4 degrees C. The greatest reduction was seen with storage at -80 degrees C, especially with cysteine. There was no effect of storage temperature on Na(+)-independent amino acid uptake. For frozen vesicles, there was no difference in uptake for 12 versus 60 h storage. Removal of cytoskeletal proteins with the chaotropic agent, KSCN, resulted in greater enrichment of MMV marker proteins, but the preparation lost the capacity for active MeAIB uptake. These data, especially with regard to storage conditions, highlight the importance of precise definition of preparation and storage conditions when interpreting results of amino acid uptake by human placental MMV.

Perturbation of renal amino acid transport by brush border membrane vesicles in the vitamin D-deficient rat

Vitamin D deficiency is characterized by secondary hyperparathyroidism, phosphaturia, bicarbonaturia, and generalized amino aciduria. While the site at which the phosphaturia ensues has been described to occur at the apical membrane of the renal proximal tubule, no studies are available for amino aciduria. Thus, weanling rats were fed five vitamin D-deficient diets for 4-6 weeks: (i) VLC, 0.02% Ca, 0.3% P; (ii) VLC + 1,25[OH]2D, same + 500 pmole ip for 2 days; (iii) LC, 0.45% Ca, 0.3% P; (iv) HC, 2.5% Ca, 0.3% P; and (v) VLP, 1.2% cA, 0.1% P. The normal diet contained 1.2% Ca, 0.7% P, and 2.5 micrograms% vitamin D. Amino acids, serum 25[OH]D, 1,25[OH]2D, and PTH, using a specific anti-rat PTH antibody, were measured. There were 4.65 +/- 1.1- and 10 +/- 1.39-fold increases in the urinary excretion of taurine and proline, respectively, irrespective of diet. Hypocalcemia, secondary hyperparathyroidism, and increased concentrations of urinary cAMP were demonstrated in all diets, except VLP. Taurinuria and prolinuria manifested at the renal brush border membrane. There was 21-25% and 26-39% attenuation in the peak of the overshoot of Na(+)-dependent uptake of taurine and proline, respectively, that was statistically significant as compared to that of normal diets (P less than 0.01). VLC resulted in a reduction in the Vmax of taurine (VLC, 78.26 +/- 6.88 vs normal, 115.4 +/- 6.26 pmole/mg protein/min, P less than 0.01) and proline (VLC, 402.06 +/- 31.26 vs normal, 589.49 +/- 37.42 pmole/mg protein/15 sec, P less than 0.01) uptake. Acute supplementation with pharmacological doses of 1,25[OH]2D normalized the Vmax of taurine and proline uptake, without affecting their renal excretion. The VLP diet induced and increase in the Km of taurine (VLP, 58.95 +/- 1.88 microM vs normal, 39.75 +/- 2.75 microM P less than 0.01) and proline (VLP, 116.75 +/- 8.87 microM vs normal, 76.82 +/- 7.27 microM P less than 0.01) uptake, without an associated perturbation in the Vmax of uptake. We conclude that the amino aciduria of vitamin D deficiency manifests at the apical membrane of the proximal tubule by an attenuation in the Na(+)-dependent uptake of amino acids. This is associated with a reduction in the initial rate of uptake or number of active transporters in the presence of secondary hyperparathyroidism and hypocalcemia, or a decrease in the affinity of the symport in the presence of P depletion. The data suggest the interplay of multiple factors in the causation of amino aciduria.

Cyclic AMP does not alter taurine accumulation by rat renal brush border membrane vesicles

Secondary hyperparathyroidism has been attributed to be responsible for the generalized aminoaciduria and phosphaturia of vitamin D deficiency. Since PTH acts in the kidney to generate cAMP, we explored the possibility that its synthetic analog, dbcAMP, would alter the renal transport of taurine (an amino acid lost in the urine in vitamin D deficiency) and Pi. Exposure of renal BBMV prepared from normal and vitamin D-calcium-deficient rats to dbcAMP at concentrations ranging between 10(-4) and 10(-7) M did not alter taurine uptake by these vesicles. Higher dbcAMP concentrations blunted uptake, but these concentrations reduced intravesicular volume, thus representing an artifact of osmolarity. Preincubation of BBMV with dbcAMP for times between 0 and 60 min at 0 or 25 degrees C also did not alter taurine accumulation. Hypotonic lysis of BBMV, allowing entry of the cyclic nucleotide, followed by isotonic resealing did not influence taurine uptake. The addition of potassium fluoride (to inhibit phosphodiesterase activity) and ATP (as an energy source) did not alter taurine accumulation at 60 sec. The uptake of Pi, which is influenced by PTH, was decreased by 25% following exposure to dbcAMP on the internal surface of the vesicle. These data indicate that the taurinuria observed in vitamin D deficiency is unlikely to be related to a PTH-induced increase in intracellular cAMP, unlike the changes in Pi transport, which is sensitive to cyclic nucleotides.

Renal adaptation to dietary amino acid alteration is expressed in immature renal brush border membranes

The transport of ions and solutes at the epithelial surface of the renal proximal tubule increases during periods of reduced dietary intake and decreases with dietary excess. We have used the sulfur-containing beta-amino acid, taurine, as a probe of this renal adaptive response to altered dietary sulfur amino acid intake to better understand the mechanisms of renal amino acid reabsorption. There exists an age-related precession of taurine uptake values by brush border membrane vesicles prepared from nursing rats from youngest to oldest. However, despite the immaturity of this transport mechanism, epithelial membranes become able to display a full renal adaptive response to altered sulfur amino acid intake sometime between the 7th and 14th day of life. This adaptive response is expressed in both "up regulation" and "down regulation" by means of a change in the initial rate of Na(+)-taurine cotransport. No changes in the lipid microenvironment of the membrane, as assessed by measurements of membrane fluidity, are evident. The lack of adaptation observed in 7-day-old pups may be due to immaturity of the Na+ transporting mechanism which energizes the uptake of amino acids.

Inhibition of alkaline phosphatase activity and D-glucose uptake in rat renal brush-border membrane vesicles by aminoglycosides

The binding of aminoglycoside antibiotics to, and their effects on, the plasma membrane were studied using isolated rat renal brush-border membrane vesicles. Dibekacin was noted to bind with brush-border membrane vesicles having a single class of many binding sites. 3H-labeled dibekacin binding was inhibited competitively by unlabeled dibekacin, gentamicin or amikacin. The inhibition constants obtained from the Dixon plots followed the order of gentamicin approximately equal to dibekacin greater than amikacin. The alkaline phosphatase activity of brush-border membrane vesicles was inhibited by gentamicin significantly, as was also observed by a histochemical study. Sodium-dependent D-glucose uptake by brush-border membrane vesicles was significantly inhibited by the addition of gentamicin.

Sodium-dependent glucose transport by cultured proximal tubule cells

The cotransport of sodium ion and alpha-methyl glucose, a non-metabolized hexose, was studied in rabbit proximal tubule cells cultured in defined medium. The rate of uptake of alpha-methyl glucose shows saturation kinetics, in which Km, but not Vmax, is dependent upon the Na+ concentration in the medium. The transport system was found to be of the high-affinity type, characteristic of the straight portion of the proximal tubule. Analysis of the rates of initial uptake within the context of a generalized cotransport model, suggests that two Na+ ions are bound in the activation of the hexose transport. The steady-state level of accumulation of alpha-methyl glucose also depends upon sodium concentration, consistent with the initial rate findings. The uptake of alpha-methyl glucose is inhibited by other sugars with the relative potencies of D-glucose greater than alpha-methyl glucose greater than D-galactose = 3-O methylglucose. L-Glucose, D-fructose, and D-mannose show no inhibition. Phlorizin inhibits the alpha-methyl glucose uptake with a Ki of 9 X 10(-6) M. Ouabain (10(-3) M) decreases the steady-state alpha-methyl glucose accumulation by 60%. In the absence of sodium, the accumulation of alpha-methyl glucose is 7-fold less than at 142 mM Na+, reaching a level comparable to the sodium-independent accumulation of 3-O-methyl-D-glucose. These findings are similar to those observed in the proximal tubule of the intact kidney.

Studies on renal adaptation to altered dietary amino acid intake: reduced renal cortex taurine content increases the Vmax of taurine uptake by brush border membrane vesicles

Rats were placed on a normal taurine diet (NTD), low taurine diet (LTD) or a high taurine diet (HTD) for 14 days. beta-Alanine was fed to half of the animals in each group and resulted in a lowered renal cortex taurine content. Brush border membrane vesicle (BBMV) uptake of taurine was higher after beta-alanine feeding and was associated with an increase in Vmax of uptake. beta-Alanine feeding to HTD animals also altered the Km of uptake, possibly since the load of sulfur amino acids (6% of diet) was high. As a control, glycine (3%) feeding for 8 days along with each diet did not alter the plasma or renal cortex content; BBMV uptake as well as Km and Vmax of taurine accumulation were minimally altered. Accordingly, ingestion of a non-sulfur-containing alpha-amino acid did not change beta-amino acid transport. This study provides evidence that whole body taurine homeostasis is maintained in the presence of a taurine-depleting agent (beta-alanine feeding) by an increase in the number of Na(+)-taurine uptake sites.

Uptake of aminoglycoside antibiotics into brush-border membrane vesicles and inhibition of (Na+ + K+)-ATPase activity of basolateral membrane

The effects of aminoglycoside antibiotics on plasma membranes were studied using rat renal basolateral and brush-border membrane vesicles. 3',4'-Dideoxykanamycin was bound to the basolateral membrane and brush-border membrane vesicles. They had a single class of binding sites with nearly the same constant, and the basolateral membrane vesicles had more binding sites than those of the brush-border membrane. Dideoxykanamycin B was transported into the intravesicular space of brush-border membrane vesicles, but not into that of basolateral membrane vesicles. The (Na+ + K+)-ATPase activity of the plasma membrane fraction prepared from the kidney of rat administered with dideoxykanamycin B intravenously decreased significantly. Aminoglycoside antibiotics entrapped in the basolateral membrane vesicles inhibited (Na+ + K+)-ATPase activity, but those added to the basolateral membrane vesicles externally failed to do so. The activity of (Na+ + K+)-ATPase was non-competitively inhibited by gentamicin. It is thus concluded that aminoglycoside antibiotics are taken up into the renal proximal tubular cells across the brush-border membrane and inhibit the (Na+ + K+)-ATPase activity of basolateral membrane. This inhibition may possibly disrupt the balance of cellular electrolytes, leading to a cellular dysfunction, and consequently to the development of aminoglycoside antibiotics' nephrotoxicity.

Factors affecting the transport of beta-amino acids in rat renal brush-border membrane vesicles. The role of external chloride

The effect of a variety of ions and other solutes on the accumulation of the beta-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na+ effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K+ or Li+ do not increase taurine accumulation more than Na+-free mannitol, except that the combination of external K+ and Na+ in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN- and NO3-) or less permeant (SO4(2-)), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl- stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO3 and LiSO4. Internal LiCl, regardless of the final Na+ anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO3 or LiSO4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl- in taurine uptake in addition to Na+ effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H+ gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na+/H+ antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na+ concentrations. Taurine uptake is blocked only by other beta-amino acids and in a competitive fashion. D-Glucose and p-aminohippurate at high concentrations (greater than 10(-3) M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of D-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal beta-amino acid transport system in brush-border vesicles and indicate a role for external Cl- in this uptake system.

Renal adaptation to altered dietary sulfur amino acid intake occurs at luminal brushborder membrane

The beta-amino acid transport capabilities of rat renal epithelium were assessed using brushborder membrane vesicles (BBMV). Taurine, a metabolically inert sulfur-containing amino acid, was studied with emphasis on the renal adaptation to dietary sulfur amino acid alteration. Three isoproteinic diets were given to Sprague-Dawley rats: low-sulfur-amino-acid diet (LTD), normal-sulfur-amino-acid diet (NTD), and high-taurine diet (HTD). Our studies demonstrated that taurine is actively transported into membrane vesicles by a sodium-dependent transport system. This transport is enhanced by hyperpolarization with valinomycin and decreased by dissipation of the sodium gradient by gramicidin. On LTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were reduced. On HTD (compared to NTD), plasma taurine, urinary taurine, and fractional excretion of taurine were elevated. In vitro studies in BBMV from NTD animals revealed a Km of 40 microM and Vmax of 102 pmoles/mg protein/30 sec. Other beta-amino acids significantly inhibited BBMV taurine accumulation. BBMV taurine uptake was enhanced after LTD (compared to NTD) and diminished after HTD (compared to NTD). These studies indicate that a renal adaptation to dietary alterations in sulfur-containing amino acids occurs and that the luminal brushborder membrane participates in the adaptation. Renal adaptative mechanisms to dietary change may serve to help conserve amino acids during deprivation and to excrete amino acids during periods of excess.

Sodium gradient- and sodium plus potassium gradient-dependent L-glutamate uptake in renal basolateral membrane vesicles

A membrane preparation enriched in the basolateral segment of the plasma membrane was isolated from the rat renal cortex by a procedure that included separation of particulates on a self-generating Percoll gradient. The uptake of L-glutamate by the basolateral membrane vesicles was studied. A Na+ gradient (Na+]o greater than [Na+]i) stimulated the uptake of L-glutamate and provided the driving force for the uphill transport of the acidic amino acid, suggesting a Na+-L-glutamate cotransport system in the basolateral membrane. A K+ gradient ([K+]i greater than [K+]o) increased the uptake additionally. This effect was specific for K+(Rb+). The action of the K+ gradient in enhancing the uptake of L-glutamate had an absolute requirement for Na+. In the presence of Na+, but in the absence of a Na+ gradient. i.e., [Na+]o = [Na+]i, the K+ gradient also energized the concentrative uptake of L-glutamate. This effect of the K+ gradient was not attributable to an alteration in membrane potential. The finding of a concentrative uptake system for L-glutamate energized by both Na+ ([Na+]o greater than [Na+]i and K+ ([K+]o) gradients in the basolateral membrane, combined with previous reports of an ion gradient-dependent uphill transport system for this amino acid in the brush border membrane, suggests a mechanism by which L-glutamate is accumulated intracellularly in the renal proximal tubule to extraordinarily high concentrations.

High- and low-affinity transport of D-glucose from blood to brain

Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D- and L-glucose. The Tmax and Km for the high-affinity system were 0.16 mmol 100 g-1 min-1 and 1 mM; for the low-affinity system, approximately 5 mmol 100 g-1 min-1 and approximately 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235 X 10(-7) cm s-1, a specific but nonsaturable diffusion pathway of permeability 85 x 10(-7) cm s-1, and a nonspecific passive diffusion pathway of permeability 2 x 10(-7) cm s-1.

In vitro effects of parathyroid hormone on kidney cortical slices: cAMP responses and concomitant inhibition of the Na+ gradient-dependent uptake of phosphate by brush border membrane vesicles isolated from the renal slices

Mouse renal cortical slices were incubated with parathyroid hormone (30 U/ml) for 2 min. Brush border membrane vesicles isolated from the treated slices had a decreased Na+ gradient-dependent uptake of phosphate. Concomitantly, the hormone elicited the activation of adenylate cyclase, the increase in tissue level of cAMP, and the enhancement of cAMP-dependent protein kinase.

Regulation of canine renal vesicle Pi transport by growth hormone and parathyroid hormone

Renal phosphate (Pi) reabsorption is increased by growth hormone (GH) and decreased by parathyroid hormone (PTH). Na+-stimulated Pi transport across the brush border membrane of the proximal tubule is the initial step in the process of Pi reabsorption. To determine whether changes in Pi reabsorption induced by GH or PTH are accompanied by changes in brush border membrane Na+-gradient-stimulated Pi transport, we examined the effect of in vivo GH and PTH administration and thyroparathyroidectomy on Pi transport by isolated brush border membrane vesicles prepared from canine kidney. In experiments in which the effect of PTH administration was examined, the same animal provided the control kidney (before PTH administration) and the experimental kidney (after PTH administration). The Na+-gradient Pi overshoot in vesicles isolated from normal, GH-treated and thyroparathyroidectomized dogs was increased after in vivo PTH administration. GH administration and thyroparathyroidectomy increased the height of the overshoot compared to normal. PTH administration decreased the apparent V value by 44% in vesicles from normal animals. The apparent V value was increased, compared to normal, by GH (34%) and thyroparathyroidectomy (57%). PTH administration decreased the apparent V in both the latter groups. GH administration to thyroparathyroidectomized dogs further increased the apparent V. Changes in the apparent V paralleled changes in Pi reabsorption in vivo induced by experimental manipulations. We conclude that changes in renal Pi reabsorption induced by GH were like those induced by PTH, accompanied by changes in the Na+-stimulated Pi transport system in the renal brush border membrane, and that the effect of PTH on vesicular Pi transport in GH-treated dogs did not differ from the effect on vesicles from normal animals.

Similarity in effects of Na+ gradients and membrane potentials on D-glucose transport by, and phlorizin binding to, vesicles derived from brush borders of rattit intestinal mucosal cells

Both the presence of sodium and of an electrical potential difference across the membrane have been found to be necessary in order to achieve optimal D-glucose-protectable phlorizin binding to brush border membranes from rabbit small intestine. The effect of delta approximately muNa on phlorizin binding shows a close similarity to that on D-glucose transport, confirming that phlorizin is indeed bound to the D-glucose transporting protein. Possible modulations of binding by a transmembrane potential are discussed on the basis of some models.

myo-Inositol binding and transport in brush border membranes of rat kidney

Using hypotonically treated brush border membranes, binding and transport of myo-inositol were examined. By hypotonic treatment, both total and non-specific uptake decreased significantly, but specific uptake was not affected. myo-Inositol release from membranes preloaded by incubation for 2 min was very rapid and about 98% of preloaded myo-inositol was released in 5 min of incubation. However, myo-inositol release from membranes preloaded by incubation for 20 min was fairly slow and 50% of myo-inositol remained in the membranes even after 10 min of incubation. Uptake of myo-inositol decreased by the increase of osmolarity in the medium. However, effect of osmolarity on the uptake was less significant when myo-inositol concentration was lower. Under conditions in which mainly binding occurred, myo-inositol binding to the membranes was measured. Two binding systems were demonstrated and high affinity site could bind 22 pmol/mg protein at most and the apparent Km value was 8.3 muM. Both binding and transport processes were dependent on Na+ and enhanced by Na+-gradient.