Carrier-mediated transport of glycyl-L-proline in renal brush border vesicles

Renal brush border vesicles prepared from rabbit were shown to transport intact glycyl-L-proline into the intravesicular space by a Na⁺-independent, carrier-mediated process. The kinetics of hydrolysis of glycyl-L-proline by the vesicles showed that this dipeptide was extremely resistant to hydrolysis. The intravesicular concentration of glycyl-L-proline was the same in both NaCl and KCl media. Dipeptide uptake into the vesicles appeared to be via carrier-mediated transport down a concentration gradient.     

Na+-independent l-arginine transport in rabbit renal brush border membrane vesicles

Na⁺-independent l-arginine uptake was studied in rabbit renal brush border membrane vesicles. The finding that steady-state uptake of l-arginine decreased with increasing extravesicular osmolality and the demonstration of accelerative exchange diffusion after preincubation of vesicles with l-arginine, but not d-arginine, indicated that the uptake of l-arginine in brush border vesicles was reflective of carrier-mediated transport into an intravesicular space. Accelerative exchange diffusion of l-arginine was demonstrated in vesicles preincubated with l-lysine and l-ornithine, but not l-alanine or l-proline, suggesting the presence of a dibasic amino acid transporter in the renal brush border membrane. Partial saturation of initial rates of l-arginine transport was found with extravesicular [arginine] varied from 0.005 to 1.0 mM. l-Arginine uptake was inhibited by extravesicular dibasic amino acids unlike the Na⁺-independent uptake of l-alanine, l-glutamate, glycine or l-proline in the presence of extravesicular amino acids of similar structure. l-Arginine uptake was increased by the imposition of an H⁺ gradient (intravesicular pH<extravesicular pH) and H⁺ gradient stimulated uptake was further increased by FCCP. These findings demonstrate membrane-potential-sensitive, Na⁺-independent transport of l-arginine in brush border membrane vesicles which differs from Na⁺-independent uptake of neutral and acidic amino acids. Na⁺-independent dibasic amino acid transport in membrane vesicles is likely reflective of Na⁺-independent transport of dibasic amino acids across the renal brush border membrane.     

Effect of papain treatment on dipeptide transport into rabbit intestinal brush border vesicles

Purified rabbit intestinal brush border membrane vesicles transport glycyl-L-proline into an osmotically responsive intravesicular space by a Na⁺- independent, carrier-mediated process. With short incubation, transport occurs mostly as the intact dipeptide, followed by hydrolysis. Pretreatment of the vesicles with papain results in a 60% reduction of L-alanine transport while glycyl-L-proline transport is stimulated by 40%. Papain treatment does not change the intravesicular volume, nor does it increase membrane permeability. Dipeptide transport into papain treated vesicles remains completely Na⁺- independent as it is in the control vesicles. Many dipeptides inhibit glycyl-L-proline transport into papain treated vesicles both in the presence and absence of a Na⁺ gradient.     

Evidence for a dipeptide transport system in renal brush border membranes from rabbit

Papain treatment of renal brush border vesicles was carried out as a successful first step towards the purification of the membrane components involved in dipeptide transport. The treated vesicles exhibited increased specific transport activity of glycyl-l-proline. In contrast, the specific transport activity of l-alanine in the treated vesicles was less than that in the control vesicles. Papain treatment resulted in the solubilization of 38% of protein, 55% of alkaline phosphatase, 90% of γ-glutamyltransferase and 95% of leucine aminopeptidase. There was no change in the intravesicular volume nor was there any increase in vesicular permeability. Glycyl-l-proline transport was Na⁺-independent in the control and papain-treated vesicles. Diamide reduced the Na⁺-dependent l-alanine transport while glycyl-l-proline transport remained unaffected in the presence of Na⁺. Many dipeptides inhibited glycyl-l-proline transport both in the presence and absence of Na⁺. The inhibition by dipeptides was greater than the inhibition by equivalent concentrations of free amino acids. These data demonstrate that renal brush border vesicles can efficiently handle dipeptides by a mechanism completely different from that of amino acid transport.     

Transport of d-glucose by brush border membranes isolated from the renal cortex

The transport of d-glucose by brush border membranes isolated from the rabbit renal cortex was studied. At concentrations less than 2 mM, the rate of d-glucose uptake increased linearly with the concentration of the sugar. No evidence was found for a “high-affinity” (μM) saturable site. Saturation was indicated at concentrations of d-glucose greater than 5 mM. The uptake of d-glucose was stereospecific and selectively inhibited by d-galactose and other sugars. Phlorizin inhibited the uptake of d-glucose in the presence and absence of Na⁺. The glycoside was a potent inhibitor of the efflux of d-glucose. Preloading the brush border membrane vesicles with d-glucose, but not with l-glucose, accelerated exchange diffusion of d-glucose. These results demonstrate that the uptake of d-glucose by renal brush borders represents transport into an intravesicular space rather than solely binding. The rate of d-glucose uptake was increased when the Na⁺ in the extravesicular medium was high and the membranes were preloaded with a Na⁺-free medium. The rate of d-glucose uptake was inhibited by preloading the brush border membranes with Na⁺. These results are consistent with the Na⁺ gradient hypothesis for d-glucose transport in the kidney. Thus, the presence of a Na⁺-dependent facilitated transport of d-glucose in isolated renal brush border membranes is indicated. This finding is consistent with what is known of the transport of the sugar in more physiologically intact preparations and suggests that the membranes serve as an effective model system in examining the mechanism of d-glucose transport in the kidney.     

Transport of carnosine by mouse intestinal brush-border membrane vesicles

The characteristics of carnosine (β-alanyl-l-histidine) transport have been studied using purified brush-border membrane vesicles from mouse small intestine. Uptake curves did not exhibit any overshoot phenomena, and were similar under Na⁺, K⁺ or choline⁺ gradient conditions (extravesicular > intravesicular). However, uptake of histidine showed an overshoot phenomenon in the presence of a Na⁺-gradient. There was no detectable hydrolysis of carnosine during 15 min of incubation with membrane vesicles under conditions used for transport experiments. Analysis of intravesicular contents further showed the complete absence of the constituent free amino acids of carnosine, and indicates that intact carnosine is transported. Studies on the effect of concentration on peptide uptake revealed that transport occurred by a saturable process conforming to Michaelis-Menten kinetics with a K_m of 9.6 ± 1.4 mM and a V_max of 2.9 ± 0.2 nmol / mg protein per 0.4 min. Uptake of carnosine was inhibited by both di- and tripeptides with a maximum inhibition of 68% by glycyl-l-leucyltyrosine. These results clearly demonstrate that carnosine is transported intact by a carrier-mediated, Na⁺-independent process.     

Na+-dependent transport of glycine in renal brush border membrane vesicles: Evidence for a single specific transport system

The uptake of glycine in rabbit renal brush border membrane vesicles was shown to consist of glycine transport into an intravesicular space. An Na⁺ electrochemical gradient (extravesicular>intravesicular) stimulated the initial rate of glycine uptake and effected a transient accumulation of intravesicular glycine above the steady-state value. This stimulation could not be induced by the imposition of a K⁺, Li⁺ or choline⁺ gradient and was enhanced as extravesicular Na⁺ was increased from 10 mM to 100 mM. Dissipation of the Na⁺ gradient by the ionophore gramicidin D resulted in diminished Na⁺-stimulated glycine uptake. Na⁺-stimulated uptake of glycine was electrogenic. Substrate-velocity analysis of Na⁺-dependent glycine uptake over the range of amino acid concentrations from 25 μM to 10 mM demonstrated a single saturable transport system with apparent K_m = 996 μM and V_max = 348 pmol glycine/mg protein per min. Inhibition observed when the Na⁺-dependent uptake of 25 μM glycine was inhibited by 5 mM extravesicular test amino acid segregated dibasic amino acids, which did not inhibit glycine uptake, from all other amino acid groups. The amino acids d-alanine, d-glutamic acid, and d-proline inhibited similarly to their l counterparts. Accelerative exchange of extravesicular [³H]glycine was demonstrated when brush border vesicles were preloaded with glycine, but not when they were preloaded with l-alanine, l-glutamic acid, or with l-proline. It is concluded that a single transport system exists at the level of the rabbit renal brush border membrane that functions to reabsorb glycine independently from other groups of amino acids.     

Transport of β-alanine in renal brush border membrane vesicles

The findings that the equilibrium uptake of β-alanine decreased with increasing medium osmolarity and preincubation with β-alanine increased uptake of the amino acid indicate that the uptake of β-alanine by rabbit renal brush border membranes represents transport into membrane vesicles. A Na⁺ electrochemical gradient (extravesicular > intravesicular) stimulated the initial rate of β-alanine uptake about three times and effected a transient accumulation of the amino acid twice the equilibrium value. Stimulation of the uptake was specific for Na⁺. Gramicidin abolished the overshoot, presumably by dissipating the gradient by accelerating the electrogenic entrance of Na⁺ into the vesicle via a pathway not coupled to uptake of β-alanine. In K⁺-loaded vesicle, valinomycin enhanced the Na⁺ gradient-dependent uptake of β-alanine. These findings indicate that the Na⁺ gradient-dependent transport of β-alanine is an electrogenic process and suggest that the membrane potential is a determinant of β-analine transport. Uptake of β-aniline, at a given concentration, reflected the sum of contributions from Na⁺ gradient-dependent and -independent transport systems. The dependent system saturated at 100 μM. The independent system did not saturate. At physiological concentrations the rate of the Na⁺ gradient-dependent uptake was four times that in the absence of the gradient. The Na⁺ gradient-dependent rate of β-alanine uptake was strongly inhibited by taurine, suggesting that β-amino acids have a common transport system, α-Amino acids, i.e. l-arginine, l-glutamate, l-proline, and glycine, representing previously reported specific α-amino acid transport systems in the brush border membrane, did not inhibit the uptake of β-alanine. These findings indicate that the brush border membrane has a distinct transport system for β-amino acids.     

Phenylalanine uptake in isolated renal brush border vesicles

The uptake of l-phenylalanine into brush border microvilli vesicles and basolateral plasma membrane vesicles isolated from rat kidney cortex by differential centrifugation and free flow electrophoresis was investigated using filtration techniques.Brush border microvilli but not basolateral plasma membrane vesicles take up l-phenylalanine by an Na⁺-dependent, saturable transport system. The apparent affinity of the transport system for l-phenylalanine is 6.1 mM at 100 mM Na⁺ and for Na⁺ 13 mM at 1 mM l-phenylalanine. Reduction of the Na⁺ concentration reduces the apparent affinity of the transport system for l-phenylalanine but does not alter the maximum velocity.In the presence of an electrochemical potential difference for Na⁺ across the membrane (η_Na0 >η_Na1) the brush border microvilli accumulate transiently l-phenylalanine over the concentration in the incubation medium (overshoot phenomenon). This overshoot and the initial rate of uptake are markedly increased when the intravesicular space is rendered electrically more negative by membrane diffusion potentials induced by the use of highly permeant anions, of valinomycin in the presence of an outwardly directed K⁺ gradient and of carbonyl cyanide p-trifluoromethoxyphenylhydrazone in the presence of an outward-directed proton gradient.These results indicate that the entry of l-phenylalanine across the brush border membrane into the proximal tubular epithelial cells involves cotransport with Na⁺ and is dependent on the concentration difference of the amino acid, on the concentration difference of Na⁺ and on the electrical potential difference. The exit of l-phenylalanine across the basolateral plasma membranes is Na⁺-independent and probably involves facilitated diffusion.     

l-Alanine uptake in brush border membrane vesicles from the gill of a marine bivalve

Summary Brush border membrane vesicles were prepared from mussel gills using differential and sucrose density gradient centrifugation. These vesicles contained both the maximal Na⁺-dependent alanine transport activity found in the gradient and the maximal activities of -glutamyl transpeptidase and alkaline phosphatase. Electron micrographs showed closed vesicles of approximately 0.1–0.5 m diameter. Transport experiments using these vesicles demonstrated a transient 18-fold overshoot in intravesicular alanine concentration in the presence of an inwardly directed Na⁺ gradient, but not under Na⁺ equilibrium conditions. A reduced overshoot (10-fold) was seen with an inwardly directed K⁺ gradient. Further studies revealed a broad cation selectivity, with preference for Na⁺, which was characteristic of alanine transport but not glucose transport in these membranes. The apparent amino acid specificity of the uptake pathway(s) was similar to that of intact gills and supported the idea of at least four separate pathways for amino acid transport in mussel gill brush border membranes. The apparent Michaelis constant for alanine uptake was approximately 7m, consistent with values forK _t determined with intact tissue.     

l-Arginine uptake into renal brush border membrane vesicles

The uphill uptake of l-arginine by renal brush border membrane vesicles was found to be energized by a Na⁺ gradient (extravesicular > intravesicular) in the presence of a membrane potential (inside negative). The uptake was specific for Na⁺. Either a K⁺-diffusion potential, generated by valinomycin, or a H⁺-diffusion potential, generated by the mitochondrial uncoupler, carbonyl cyanide p-trifluoromethoxyphenylhydrazone, provided the electrical driving force. The Na⁺ gradient-dependent l-arginine transport system was shared by specific basic amino acids and l-cystine, but not by d-arginine nor other classes of amino acids. The molecular structure of the basic amino acid recognized by the carrier was postulated.     

Hibernation enhancesd-glucose uptake by intestinal brush border membrane vesicles in ground squirrels

The ability to actively transport nutrients is maintained in intestinal tissues of hibernating ground squirrels compared with their active counterparts, and shows apparent upregulation in hibernators when transport rates are normalized to tissue mass. To identify the mechanisms responsible for the preservation of transport function during the extended fast of hibernation, we studiedd-glucose uptake into jejunal brush border membrane vesicles prepared from active and hibernating 13-lined ground squirrels. Hibernators were without food and showing regular bouts of torpor for at least 6 weeks before sacrifice. Electron micrographs indicated similar microvillus heights of jejunal enterocytes in the two activity states, whereas microvillus density was slightly greater in the hibernators. Glucose uptake into brush border membrane vesicles was inversely related to medium osmolarity, indicating negligible binding of substrate to brush border membrane vesicles surfaces, and intravesicular spaces were similar in hibernating and active squirrels. Glucose uptake showed strong Na⁺ dependency in both groups, with equivalent overshoot values in the presence of Na⁺. Kinetic analysis revealed a significant increase in the maximal velocity of transport (J _max) in hibernators (55.9±5.6 nmol·min^-1·mg^-1) compared with active squirrels (36.7±5.1 nmol·min^-1·mg^-1,P<0.05), with no change inK _m. Thus, the structure and absorptive capacity of the intestinal brush border persists in fasted hibernators, and the increase inJ _max for glucose uptake during hibernation likely contributes to the enhanced Na⁺-dependent glucose absorption previously observed at the tissue level.Abbreviations BBM brush border membrane(s) - BBMV brush border membranes vesicles - SGLT1 Na⁺-glucose transporter - 3-OMG 3-orthomethylglucose - J _max maximal velocity of transport - K _m transporter affinity for substrate - T _b body temperature     

Inhibition of Na+-coupled solute transport by calcium in brush border membrane vesicles

Intracellular Ca⁺⁺ is known to influence Na⁺ flux in luminal membranes. Abnormally elevated Ca⁺⁺ levels in some cells is believed to be the primary pathophysiologic defect in cystic fibrosis (CF). This in turn is thought to alter Na⁺ transport which accounts for certain clinical manifestations of this disease. Two Na⁺-dependent intestinal transport mechanisms have been reported to be suppressed or missing in CF. To examine whether alterations in cell Ca⁺⁺ may account for these findings, studies were performed to examine the influence of Ca⁺⁺ on Na⁺-solute co-transport across intestinal luminal membranes. Purified brush border membrane vesicles prepared from rat small bowel were preincubated in either Ca⁺⁺-free buffer or buffer containing 2.5 mM CaCl₂. Ca⁺⁺ loaded vesicles showed marked inhibition of Na⁺ co-transport of taurocholic acid, taurochenodeoxycholic acid, glucose and valine when compared to controls. The uptake of Na⁺ was also significantly reduced by intravesicular Ca⁺⁺. These data demonstrate that intravesicular Ca⁺⁺ inhibits Na⁺-coupled solute transport as well as Na⁺ influx across intestinal brush border membranes. These data suggest that intracellular Ca⁺⁺ may suppress Na⁺-dependent solute absorption in the intestine. Results presented here further support the theory that elevated intracellular Ca⁺⁺ may account for intestinal malabsorption and other altered transport phenomena reported in CF.     

Ion requirements for taurocholate transport by ileal brush border membrane vesicles.

The ion requirements for intestinal taurocholate transport were studied using vesicles prepared from the brush borders of guinea pig small intestines. For each experimental electrolyte, parallel uptake experiments were performed with vesicles from jejunal and ileal brush border membranes to differentiate between uptake by passive fluxes and non-specific binding and uptake by the ileal bile salt active transport system. Uptake of taurocholate prior to the addition of electrolyte was the same for vesicles prepared from jejunal and ileal tissue. During the presence of a sodium gradient (extravesicular concentration greater than intravesicular), only ileal vesicles displayed the enhanced uptake which is characteristic of the overshoot phenomenon. When NaCl was replaced by KCl or LiCl, the overshoot was not observed. Replacement of NaCl with NaCNS, Na₂SO₄, or NaSO₃C₂H₄OH, however, resulted in no significant difference in the initial uptake values observed in either the jejunal or ileal vesicles. This pattern of taurocholate transport independence of relative anion permeability differs from the pattern observed by others for the Na⁺ dependent transport of D-glucose by intestinal brush border membrane vesicles. This difference may be attributed in part to the fact that, unlike the situation with glucose, the binding of a taurocholate anion and a sodium cation by the hypothetical carrier would result in an electroneutral addition.     

Ketone body transport in renal brush border membrane vesicles

Ketone body uptake by renal brush border vesicles has been investigated. Ketone bodies enter into the brush border vesicles by a carrier-mediated process. The uptake is dependent on an Na⁺ gradient ([Na⁺]outside > [Na⁺]inside) and is electroneutral. The uptake is transport into an osmotically active space and not a binding artifact as indicated by the effect of increasing the medium osmolarity. A pH gradient (alkaline inside) also stimulates the ketone body uptake. Acetoacetate and 3-hydroxybutyrate share the same carrier as demonstrated by the accelerated exchange diffusion and mutual inhibitory effects.     

Effects of ethanol in vitro on rat intestinal brush-border membranes

Ethanol, at concentrations found in the intestinal lumen after moderate drinking, has been shown to inhibit carrier-mediated intestinal transport processes. This inhibition could occur by direct interaction with membrane transporters, dissipation of the energy producing Na⁺ electrochemical gradient and/or nonspecific alteration of membrane integrity. The latter alteration may be reflected by changes in membrane fluidity, chemical composition or vesicular size. These possibilities were examined with studies in purified brush border membrane vesicles of rat intestine. Ethanol inhibited concentrative Na⁺-dependent d-glucose uptake in a dose-dependent manner. In contrast, ethanol did not inhibit concentrative d-glucose uptake under conditions of d-glucose trans-stimulation in the absence of a Na⁺ electrochemical gradient. Ethanol also inhibited initial, concentrative Na⁺-dependent taurocholic acid uptake, as well as equilibrium uptake. That ethanol exerted a dual effect on transport by increasing membrane conductance for Na⁺ while decreasing intravesicular space was supported by direct studies of Na⁺ uptake. Morphometric analysis confirmed that ethanol-treated membranes had a decreased intravesicular size when compared to untreated membranes. Finally, membrane fluidity measured by EPR showed that ethanol had a significant fluidizing effect without producing qualitative changes in membrane proteins, as determined by SDS gel electrophoresis. These results suggest that ethanol inhibits carrier-mediated transport by dissipation of the Na⁺ electrochemical gradient and alteration of membrane integrity rather than by direct interaction with membrane transporters.     

Peptide transport in rabbit kidney. Studies with l-carnosine

l-Carnosine was shown to be transported into rabbit renal brush-border membrane vesicles by an Na⁺ - independent mechanism. The transport was competitively inhibited by glycyl-l-proline. Various di- and tripeptides inhibited l-carnosine transport, whereas free amino acids did not. Inhibition studies showed that blocking the free amino and carboxyl groups of the peptide reduced its affinity for the transport carrier. Under the conditions in which there was no detectable hydrolysis of l-carnosine in the medium, intravesicular contents showed a 30% hydrolysis of the peptide within the vesicles. Disruption of membrane vesicles with deoxycholate resulted in a 3-fold increase in l-carnosine hydrolyzing activity over untreated intact vesicles. Based on these observations, a model for peptide transport is proposed in which transport of the intact peptide across the membrane is followed by its partial or complete hydrolysis by a membrane peptidase whose active site is on the cytoplasmic side of the membrane.     

Peptide transport in intestinal and renal brush border membrane vesicles

A longstanding question about the possible dependence of transmembrane peptide transport on sodium has now been resolved. Recent studies with purified intestinal brush border membrane vesicles have shown that peptide transport across this membrane is Na⁺-independent and occurs by a non-concentrative mechanism. Similar studies with renal brush border membrane vesicles have established for the first time the presence of a peptide transport system in mammalian kidney. The essential characteristics of peptide transport in these two tissues are the same. However, it still remains to be seen whether a new mechanism other than the Na⁺-gradient, hitherto unrecognized, is involved in energizing the active transport of peptides in vivo in mammalian intestine and kidney.     

Glycodeoxycholate transport in brush border membrane vesicles isolated from rat jejunum and ileum

The transport of the bile salt, glycodeoxycholate, was studied in vesicles derived from rat jejunal and ileal brush border membranes using a rapid filtration technique. The uptake was osmotically sensitive, linearly related to membrane protein and resembled d-glucose transport. In ileal, but not jejunal, vesicles glycodeoxycholate uptake showed a transient vesicle/medium ratio greater than 1 in the presence of an initial sodium gradient. The differences between glycodeoxycholate uptake in the presence and absence of a Na⁺ gradient yielded a saturable transport component. Kinetic analysis revealed a $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value similar to that described previously in everted whole intestinal segments and epithelial cells isolated from the ileum. These findings support the existence of a transport system in the brush border membrane that: (1) reflects kinetics and characteristics of bile salt transport in intact intestinal preparations, and (2) catalyzes the co-transport of Na⁺ and bile salt across the ileal membrane in a manner analogous to d-glucose transport.     

Taurine transport by brush border membrane vesicles isolated from the flounder kidney

Summary Taurine transport was investigated in brush border membrane vesicles isolated from renal tubules of the winter flounder (Pseudopleuronectes americanus). Taurine uptake by the vesicles was greater in the presence of NaCl as compared to uptake in KCl. The Na⁺-dependent taurine transport was electrogenic and demonstrated tracer replacement and inhibition by -alanine and HgCl₂, indicating the presence of Na⁺-dependent, carrier-mediated taurine transport. In contrast to Na⁺-dependent taurine transport across the basolateral membrane, there was not a specific Cl^– dependency for transport in the brush border membrane. No evidence was obtained for Na⁺-independent carrier-mediated taurine transport. The possible involvement of the brush border Na⁺-dependent transport system in the net secretion of taurine from blood to tubular lumen in vivo (Schrock et al. 1982) is discussed.