Cotransport Of Na Research Articles

Bicarbonate-dependent changes of intracellular sodium and pH in identified leech glial cells.

A new triple-barrelled ion-sensitive microelectrode was used to investigate the importance of bicarbonate for the regulation of intracellular Na+ and pH (Nai and pHi, respectively) of neuropile glial cells in the central nervous system of the leech Hirudo medicinalis. Addition of CO2/HCO3- produced an increase of the Nai activity and an intracellular alkalinization, indicating bicarbonate accumulation in the glial cells. Changes of external pH (from 7.4 to 7.0 and 7.8) produced large and rapid shifts of pHi and Nai and of the membrane potential in the presence, but not in the absence, of bicarbonate. Thus, acid/base transport and Na+ movements across the glial membrane into and out of the cells were accelerated severalfold in CO2/HCO3(-)-buffered saline as compared to a CO2/HCO3(-)-free, HEPES-buffered saline. The results suggest that the electrogenic, reversible, cotransport of Na+ and HCO3- in the glial cell membrane [3,9] can produce significant changes in intraglial pH and Na activity, and can carry a significant fraction of the total Na+ flux across the cell membrane.

Mechanisms of pH recovery from intracellular acid loads in the leech connective glial cell.

We used double-barrelled, neutral carrier, pH-sensitive microelectrodes to study the mechanisms by which the intracellular pH (pHi) is regulated in the connective glial cells of the medicinal leech. In HEPES-buffered, nominally CO2/HCO3(-)-free solutions the recovery of pHi from intracellular acidosis is Na(+)-dependent and reduced by at least half in the presence of amiloride, suggesting the action of Na+:H+ exchange. The rate of pHi recovery by this mechanism can be increased by raising the extracellular buffering power or by increasing extracellular pH. The presence of CO2/HCO3(-)-greatly increases the rate of pHi recovery from intracellular acidosis. This CO2/HCO3(-)-stimulated recovery is also dependent on external Na+, largely Cl(-)-independent, inhibited by DIDS, and accompanied by membrane hyperpolarization. This is consistent with it being mediated by the electrogenic cotransport of Na+ and HCO3- into the cells. A Cl(-)-dependent component to Na(+)- and HCO3(-)-dependent regulation is most easily explained by the added presence of a Na(+)-dependent exchange of HCO3- and Cl-.

Voltage-dependent phosphate transport in osteoblast-like cells

Phosphate ion (Pi) in sufficient concentrations is crucial for bone mineralization. The osteoblast (OB) may be responsible for the transport of Pi into the bone interstitium, where mineralization occurs. We previously characterized a Na(+)-dependent Pi transporter (NaPi) in the osteoblastic UMR-106-01 cell line. In the present study, the alteration of Na(+)-dependent Pi transport by changes in membrane potential was investigated. Depolarizing the cells with increasing concentrations of ambient K+ and valinomycin resulted in a progressive decline in Na(+)-dependent Pi uptake to a maximum of 28% at a membrane potential of -18 mV compared to control Na(+)-dependent Pi uptake at a membrane potential of approximately -60 mV. Hyperpolarizing the cells with SCN- increased Na(+)-dependent Pi uptake over control by 50% at an SCN- concentration of 70 mM. Determination of membrane potential by using the fluorescent probe, DiSC3(5), showed that the addition of Pi to cells in Na(+)-containing medium resulted in a small depolarization. These data show that NaPi activity can be altered by membrane potential changes and that the initiation of Na(+)-dependent Pi uptake is associated with depolarization of the plasma membrane of UMR-106-01 cells. Taken together, the cotransport of Na+ and Pi results in the movement of a net positive charge into the cell.

Glucose stimulation of ouabain-resistant efflux of Na+ from rat pancreatic islets.

1. Integrating flame photometry was employed for measuring the mobilization of sodium from rat pancreatic islets after substitution of extracellular Na+ by N-methylglucamine. 2. Glucose accelerated the initial loss of sodium both in the absence and presence of ouabain (1 mM). In the latter case the effect was maximal at 5 mM of the sugar. 3. Amiloride (0.1 mM), an inhibitor of Na(+)-H+ exchange, prevented the effect of glucose on the ouabain-resistant Na+ efflux, increasing the rate of outward transport in the absence of the sugar. 4. Extracellular K+ and arginine (10 mM) mimicked the action of glucose in promoting a ouabain-resistant mobilization of sodium. 5. Whereas the hypoglycaemic sulphonylurea tolbutamide (100 microM) did not modify the outward transport of Na+, the ouabain-resistant component of this process was partially suppressed after bumetanide (100 microM) inhibition of the chloride-dependent co-transport of Na+ and K+. 6. It is suggested that the glucose-induced lowering of the steady-state content of islet sodium involves an increased outward transport mediated at least in part by mechanisms other than stimulation of the Na(+)-K+ pump.

Excitatory amino acid-stimulated uptake of 22Na+ in primary astrocyte cultures

In this study we have found that L-glutamic acid, as well as being taken up by a Na+-dependent mechanism, will stimulate the uptake of 22Na+ by primary astrocyte cultures from rat brain in the presence of ouabain. By simultaneously measuring the uptake of 22Na+ and L-3H-glutamate a stoichiometry of 2-3 Na+ per glutamate was measured, implying electrogenic uptake. Increasing the medium K+ concentration to depolarize the cells inhibited L-3H-glutamate uptake, while calculations of the energetics of the observed L-3H-glutamate accumulation also supported an electrogenic mechanism of at least 2 Na+:1 glutamate. In contrast, kinetic analysis of the Na+ dependence of L-3H-glutamate uptake indicated a stoichiometry of Na+ to glutamate of 1:1, but further analysis showed that the stoichiometry cannot be resolved by purely kinetic studies. Studies with glutamate analogs, however, showed that kainic acid was a very effective stimulant of 22Na+ uptake, but 3H-kainic acid showed no Na+ -dependent uptake. Furthermore, while L-3H-glutamate uptake was very sensitive to lowered temperatures, glutamate-stimulated 22Na+ uptake was relatively insensitive. These results indicate that glutamate-stimulated uptake of 22Na+ in primary astrocytes cultures cannot be explained solely by cotransport of Na+ with glutamate, and they suggest that direct kainic acid-type receptor induced stimulation of Na+ uptake also occurs. Since both receptor and uptake effects involve transport of Na+, accurate measurements of the Na+ :glutamate stoichiometry for uptake can only be done using completely specific inhibitors of these 2 systems.

Active absorption of NH4+ by rat medullary thick ascending limb: inhibition by potassium.

These experiments were designed to determine the relative contributions of active NH4+ transport and voltage-driven NH4+ diffusion to direct NH4+ absorption by the medullary thick ascending limb of the rat. Medullary thick ascending limbs were perfused in vitro with solutions containing 25 mM HCO3 and 4 mM total ammonia. Under steady-state conditions, the lumen-positive transepithelial voltage (VT) was not sufficient to account for the observed decrease in lumen NH4+ concentration, consistent with active absorption of NH4+. Flux calculations based on VT and measured NH4+ permeability (6 x 10(-5) cm/s) indicate that the majority (at least 65%) of total ammonia absorption is due to active transport of NH4+. The remainder of NH4+ absorption can be accounted for by voltage-driven diffusion. Increasing the potassium concentration from 4 to 24 mM in perfusate and bath markedly inhibited total ammonia absorption but did not affect VT, NH4+ permeability, or HCO3 absorption. These results are consistent with inhibition of the active component of NH4+ absorption by potassium. The active NH4+ absorption is likely mediated by cotransport of Na+, NH4+, and Cl- across the apical cell membrane. Inhibition of active NH4+ absorption by an increase in potassium concentration may be due, in part, to competition between NH4+ and K+ for a common binding site on the Na+ -K+ -2Cl- cotransport system.

The mechanism of the 3H-noradrenaline releasing effect of various substrates of uptake1: multifactorial induction of outward transport.

The mechanism of action of indirectly acting sympathomimetic amines was studied in the rat vas deferens, after inhibition of vesicular uptake (by reserpine), of MAO (by pargyline) and of COMT (by U-0521). 1. Km-values for the neuronal uptake of 12 substrates were determined as the IC50 of the unlabelled substrate inhibiting the initial rate of neuronal uptake of 0.2 mumol/l 3H-(-)-noradrenaline. The IC50 ranged from 0.35 mumol/l (for(+)-amphetamine) to 44.3 mumol/l (for 5-HT). The Vmax (determined for 8 substrates) was substrate-dependent. 2. Tissues were loaded with 0.2 mumol/l 3H-(-)-noradrenaline and then washed out with amine-free solution. All 12 substrates of uptake1 induced an outward transport of 3H-noradrenaline, and equieffective concentrations were positively correlated with Km. Moreover, the EC50 for release greatly exceeded Km. It is proposed that this discrepancy between EC50 and Km is indicative of the fact that at least four factors (each one in strict dependence on Km) contribute to the initiation of outward transport of 3H-noradreanline: a) the appearance of the carrier on the inside of the axonal membrane (facilitated exchange diffusion), b) the co-transport of Na+, c) the co-transport of Cl- (both lowering the Km for 3H-noradrenaline at the inside carrier), and d) inhibition of the re-uptake of released 3H-noradrenaline (through competition for the outside carrier). 3. At least for amezinium, Vmax appears to limit the maximum rate of outward transport. 4. For some substrates (especially for the highly lipophilic ones) bell-shaped concentration-release curves were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the apical membrane ionic permeability of the rabbit proximal convoluted tubule

Basolateral membrane potential (psi BL), transepithelial potential (psi T), and the ratio of apical to basolateral membrane resistance (RA/RBL) were measured in rabbit proximal convoluted tubules (PCT) perfused in vitro. Analysis of RA/RBL changes using several luminal perfusates indicates that the cotransport of Na with glucose and alanine would represent 19% of the apical conductance in normal conditions; the cotransport of Na with acetate, citrate, sulfate, and phosphate would represent 7%, whereas Na, K, and Cl diffusion would represent 10, 4, and 0% of this apical conductance, respectively. On the other hand, psi BL values can also be analyzed using the equivalent circuit of the epithelium to obtain the apical membrane equivalent electromotive force (EA) in the presence of each perfusate. These values, as well as the preceding values obtained from RA/RBL measurements, indicate that in the absence of cotransported solutes the transference number for Na diffusion is several times larger than for K diffusion. Among the conductance pathways studied, the transference number sequence would be as follows: Na cotransport with alanine and glucose greater than Na cotransport with anions greater than Na diffusion greater than K diffusion greater than Cl diffusion. This study also suggests the presence of another important but unidentified apical ionic permeation pathway, since the total of the transference numbers obtained from RA/RBL analysis represents only 40% of the total apical membrane conductance and the absolute values of EA are difficult to account for using only the tested apical membrane permeation pathways.

Na+ gradient-dependent glycine uptake in basolateral membrane vesicles from the dog kidney.

The imposition of a Na+ gradient (extravesicular greater than intravesicular) stimulated the uptake of [3H]glycine measured over time in basolateral membrane vesicles from dog kidney over that measured in the presence of a choline+ gradient or measured under Na+-equilibrated conditions. Na+ gradient-dependent uptake of [3H]glycine was stimulated by an intravesicular-negative membrane potential. Efflux of [3H]glycine was enhanced by an intravesicular-positive membrane potential. Substrate velocity analysis of net Na+-dependent [3H]glycine uptake over the range of amino acid concentrations from 10 to 500 microM demonstrated a single saturable transport system with apparent Km = 84 microM and apparent Vmax = 143 pmol [3H]glycine X mg protein-1 X 15 s-1. Counterflow of [3H]glycine was demonstrated in the presence of Na+ when basolateral vesicles were preloaded with glycine but not with L-alanine or L-proline. These findings are consistent with carrier-mediated, electrogenic cotransport of Na+ and glycine in basolateral vesicles. Unlike the case for [3H]glycine, Na+ gradient-dependent uptake of neither L-[3H]alanine nor L-[3H]proline was observed in basolateral vesicles. Na+ gradient-dependent uptake of all three amino acids was demonstrated in brush border vesicles from the dog kidney. We conclude that variability exists between basolateral and brush border membranes in terms of the presence or absence of Na+-dependent transport systems for specific amino acids. This variability probably reflects differences between the functional significances of the Na+-dependent transport processes in the two membranes.

Sodium-dependent transport of inorganic sulfate by rabbit renal brush-border membrane vesicles. Effects of other ions.

A Na+ gradient (extravesicular greater than intravesicular) increased the rate of inorganic sulfate (SO24-) uptake into renal brush-border membrane vesicles and energized the transient accumulation of the anion against its concentration gradient, indicating a secondary active transport system. Stimulation of SO24- uptake was specific for Na+. The anions, SO23-, S2O23-, SeO24-, MoO24-, CrO24-, and WO24-, but not HPO24-, cis-inhibited and trans-stimulated SO24- uptake, suggesting that these divalent anions shared the SO24- carrier. The Na+/SO24- co-transport and Na+. The apparent Km for SO24- was 0.6 mM at 100 mM Na+. The relationship between Na+ concentration and rate of SO24- uptake was sigmoidal. From a Hill analysis of the data a [Na+]0.5 of 36 mM and an n value of 1.6 were calculated. Comparisons of the effects of a K+ diffusion potential (inside positive), of a H+ diffusion potential (inside negative), of Na+ salts of anions of different conductances on the Na+-dependent uptakes of SO24- and D-glucose, and of the responses of a membrane potential-sensitive fluorescent probe concomitant with the uptakes indicate that Na+/SO24- co-transport was electroneutral. The simplest stoichiometry consistent with an electroneutral mechanism would be the co-transport of two Na+ and one SO24-. Na+ gradient-dependent SO24- uptake was enhanced by intravesicular Cl-. cis-Cl- inhibited the efflux as well as the influx of SO24-. These findings suggest that Cl- was an inhibitor of SO24- transport. Intravesicular K+ stimulated Na+ gradient-dependent SO24- uptake. The co-transport of Na+/SO24- appeared not to be coupled to the transmembrane flux of K+. It is hypothesized that the co-transport system contained an internal site activated by K+.

Effects of theophylline and cAMP on intestinal allosteric chloride transport in freshwater prawns

Active transmural Cl- transport across the intestine of the freshwater prawn Macrobrachium rosenbergii and the transmural potential difference resulting from the net movements of this ion and Na+ are significantly reduced or abolished in the presence of 10 mM theophylline or 1 mM dibutyryl adenosine 3',5'-cyclic monophosphate (DB-cAMP). The locus of theophylline or DB-cAMP action appears to be at the epithelial apical membrane where increased intracellular cAMP or other cyclic nucleotide such as guanosine 3',5'-cyclic monophosphate (cGMP) may interact directly or through another agent, such as the divalent cation Ca2+, to reduce or eliminate coupled Na+-Cl- influx into the cell from the gut lumen. Under control conditions, cotransport of Na+ and Cl- across the epithelial apical membrane of prawn intestine occurs by an allosteric carrier protein exhibiting sigmoidal influx kinetics for the two monovalent ions. Exogenous Ca2+ is a third binding ligand for this multisubunit protein, with the divalent cation serving as an allosteric activator of the transport system. Bilateral intestinal incubation with 10 mM theophylline abolished Cl- influx (control, 0.71 +/- 0.23; theophylline treated, 0.03 +/- 0.03 mumol X cm-2 X h-1). Addition of 1 mM DB-cAMP to intestinal incubation saline significantly (P less than 0.01) reduced the maximal Cl- influx velocity from 0.76 +/- 0.08 to 0.33 +/- 0.05 mumol X cm-2 X h-1 without altering the anion affinity constant (control, 102 +/- 5; DB-cAMP, 96 +/- 19 mM). In addition, the cyclic nucleotide had no effect on the Hill interaction coefficient of the transfer mechanism (control, 3.22 +/- 0.12; DB-cAMP, 3.42 +/- 0.68).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium transport in astrocytes.

Sodium transport in astrocytes in homogeneous primary cultures from mouse brain cortex were investigated with radiotracer (22Na) and electrophysiological methods. The equilibrated Na+ content was 190 nmol X mg-1 protein and the influx and efflux rates were identical at about 560 nmol X mg-1 X min-1. No significant change was observed in Na+ efflux or influx when external K+ was raised from 5.4 to 12 or 54 mM, but the Na+ content decreased. Intracellular Na+ loading, evoked by previous exposure to ice-cold K+-free medium, double the Na+ efflux. Ouabain, a Na+-K+ exchange inhibitor, exerted a small, nonsignificant inhibition of Na+ efflux at both 5.4 and 12 mM K+ and caused a large increase in Na+ content. At 5.4 mM K+, amiloride, a Na+-H+ exchange inhibitor, decreased both influx and efflux of Na+ and caused an increase in Na+ content. Furosemide, an inhibitor of a cation-Cl- carrier, decreased both content and influx of Na+ slightly but had no significant effect on Na+ efflux. The effects of amiloride or furosemide on Na+ influx were abolished at elevated (12 and 54 mM) K+. Attempts to stimulate the Na+-K+ pump with elevated external K+ or internal Na+ produced no electrogenic component of the membrane potential, probably owing to the high K+ permeability. Based on the present results and earlier experiments on K+ influx, it is concluded that 1) the Na+-K+ pump of astrocytes under normal conditions transports more K+ than Na+; 2) intracellular Na+ loading increases Na+ efflux; 3) some Na+-H+ exchange and cotransport of Na+ and Cl- seem to occur at 5.4 mM K+; and 4) neither of the latter two transport mechanisms is enhanced at elevated K+ concentrations.

Na+-K+-Cl- co-transport in the intestine of a marine teleost.

A key element in the absorption of salt and water by various epithelia is the presence, in the apical or brush border membrane, of a mechanism for directly coupling the inward movements of Na+ and Cl− (ref. 1). In this way the downhill gradient for Na+ from lumen to cell, which is maintained by the cell's Na+/K+ pump, can provide the energy for cellular accumulation and transepithelial transport of Cl−. A requirement for K+ at the serosal surface of the epithelium has long been accepted as essential to the operation of the Na+/K+ pump and therefore to salt absorption. Until very recently, however, the effect of luminal K+ on NaCl absorption has not been explored. We show here that luminal K+ stimulates NaCl absorption in the intestine of a marine teleost, the winter flounder, Pseudopleuronectes americanus. Furthermore, K+ uptake across the brush border membrane depends on the presence of both Na+ and Cl− in the bathing medium and is inhibited by furosemide, which also inhibits the coupled uptake of Na+ and Cl− (ref. 2). Thus, flounder intestine seems to have a Na+ −K+ −Cl− co-transport system similar to that demonstrated in erythrocytes3–5, cultured tumour cells6, cultured kidney cells7 and squid giant axon8. Electrophysiological studies of the thick ascending limb of Henle's loop in mammalian kidney9 and the diluting segment in amphibian kidney10 also indicate that luminal K+ is required for salt absorption. Co-transport of Na+, K+ and Cl− may therefore be characteristic of a variety of cells and tissues, including salt-absorbing epithelia.

Chloride self-exchange in toad skeletal muscle in vivo and in vitro.

The exchange of cellular Cl with 36Cl has been measured in saline-perfused hindlimb muscles of the pithed toad and compared with cellular Cl exchange in isolated muscles incubated in the vitro either in toad Ringer solution or in toad plasma. In the perfused hindlimb, the rate of 36Cl efflux from muscle cells [17.0 +/- 0.9 pmol Cl.(cm2 plasma membrane.s)-1] was only 40% as fast as that of the 36Cl influx. The discrepancy between Cl influx and efflux was accompanied by a cellular accumulation of Cl against the electrochemical gradient for this anion. Concurrently, the cells took up Na in amounts at least equal to the accumulated Cl. During this accumulation of Na and Cl, the mean resting potential remained constant at a value of -89.2 +/- 1.9 (SE) mV. Na and Cl were taken up by the muscle cells of the perfused hindlimb without a concomitant decrease in cellular K content; i.e., without evidence of inhibition of the Na-K pump. The rate constant for cellular 36Cl efflux from isolated toad muscles preincubated for 3 h in vitro in toad Ringer solution was about five times faster than that of muscles in the perfused hindlimb and similar in magnitude to published values for Cl fluxes in frog muscle. Cellular Cl efflux from muscles briefly preincubated in vitro for 15 min instead of 3 h was significantly slower than after prolonged preincubation. In vitro incubation of isolated toad muscles in toad plasma slowed the cellular 36Cl efflux to values approaching those measured in the perfused hindlimb, without comparably depressing the 36Cl influx. It is suggested that the uptake of NaCl by the cells of perfused hindlimb muscle may proceed by an electroneutral inward cotransport of Na and Cl on the same carrier.

Stimulation of the efflux of L-glutamate from renal brush-border membrane vesicles by extravesicular potassium.

The rate of efflux of L-glutamate from renal brush-border membrane vesicles was enhanced by Na+ and by extravesicular L-glutamate, but not by D-glutamate nor analogs of L-glutamate that do not share the Na+-L-glutamate co-transport system. These results suggest that efflux was mediated by the Na+-L-glutamate carrier. The efflux of L-glutamate was increased by extravesicular K+ or Rb+ but not by Li+, choline+, or Tris+. These findings, together with previous results showing that intravesicular K+ or Rb+ increased L-glutamate uptake and that a K+ gradient energized the concentrative uptake of the acidic amino acid in the absence of other gradients, provide evidence consistent with the hypothesis that the co-transport of Na+-L-glutamate is coupled to the transmembrane flux of K+.

Stoichiometry versus coupling ration in the cotransport of Na and different neutral amino acids

The stoichiometry of Na coupling to amino acid movement across the brush border membrane of the rabbit distal ileum has been determined under initial rate conditions. The coupling ratio, defined as the amino acid-dependent Na influx/the Na-dependent amino acid influx, was equal to unity for alanine, measured over a 10-fold range of Na and alanine concentrations. Coupling ratio values determined under a single set of conditions for a number of amino acids varied from 1 for serine to 4.6 for methionine. Reducing the methionine concentration from 12.5 to 1.5 mM caused the coupling ratio value to fall from 4.6 to 1.2. These results are explained by assuming a fixed stoichiometry of 1 : 1 under all conditions, with initial binding of the amino acid (A) to the Na-dependent carrier (E) but with some amino acids being able to cross on the Na-dependent carrier in the absence of Na. The variation in coupling ratio values can be used to calculate K A, the apparent dissociation constant of amino acid from the Na-dependent carrier in the absence of Na, and the ratio k 1 k 2 , where k 1 and k 2 are first-order rate constants for translocation of the complexes EA and EANa, respectively. This method of processing results has been defined as delta analysis. The value of K A for methionine is 3.6 ± 1.1 mM and the k 1 k 2 ratio is 1.01 ± 0.07. The constant coupling ratio value of 1 for alanine indicates that the value for K A is extremely high or that the k 1 value is extremely low.

Sodium-linked transport of ethacrynic acid by rat liver: Possible significance for choleretic action

Uptake, biotransformation and biliary excretion of ethacrynic acid was investigated in the isolated rat liver and related to the choleretic effect of the drug. Ethacrynic acid transport from sinusoidal extracellular space into liver cells is mediated by a saturable, energy-dependent, and partially Na +-dependent transport mechanism. Extracellular sodium stimulates translocation of ethacrynic acid across the sinusoidal membrane by increasing the maximal velocity from 0.47 to 0.64 μmoles.min −1.g liver −1 without any major effect on the substrate affinity of the carrier. Within the cell, ethacrynic acid is rapidly and almost completely metabolized to its glutathione derivative which, in turn, is excreted into bile canaliculi by a saturable transport system. Canalicular excretion of metabolized ethacrynic acid is the rate-limiting step in hepatic transport of the choleretic drug ( V max0.15 μmoles.min −1.g liver −1. Both translocation steps are accompanied by an increase in transmembrane sodium fluxes. At the sinusoidal site, co-transport of Na + with ethacrynic acid and/or inhibition of (Na +-K +)-ATPase might be responsible for the net increase in intracellular Na + as observed in liver slices. Excretion of intracellular Na + into bile canaliculi is enhanced during canalicular transport of the ethacrynic acid glutathione adduct. The transepithelial Na + net movement induced by these events might be underlying the stimulatory effect of ethacrynic acid on bile secretion.

Amino acid transport in brush-border-membrane vesicles isolated from human small intestine.

Uptake of L-alanine and L-phenylalanine by purified bursh-border-membrane vesicles isolated from human small intestine was investigated by using a rapid-filtration technique. L-Alanine entered the same osmotically reactive space as D-glucose, indicating that transport into the vesicle rather than binding to the membranes was being observed. The uptake rate for L-alanine was higher in the presence of a Na+ gradient than in the presence of a K+ gradient. In the presence of a Na+ gradient, the lipophilic anion SCN- caused an increase in L-alanine transport, whereas the nearly impermeant SO42- anion decreased the uptake of L-alanine compared with its uptake in the presence of Cl-. The uptake of L-phenylalanine into the brush-border-membrane vesicle was also stimulated by Na+. The results indicate co-transport of Na+ and neutral amino acids inthe human intestinal brush-border membrane.

Co-transport of Na + and methyl-β-D-thiogalactopyranoside mediated by the melibiose transport system of [formula omitted

Na +-dependent transport of methyl-β-D-thiogalactopyranoside (TMG) mediated by the melibiose transport system was investigated in Escherichia coli mutants lacking the lactose transport system. When an inwardly-directed electro-chemical potential difference of Na + was imposed across the membrane of energy depleted cells, transient uptake of TMG was observed. Addition of TMG to cell suspensions under anaerobic conditions caused a transient acidification of the medium. This acidification was observed only in the presence of Na + or Li +. These results support the idea that TMG is taken up by a mechanism of Na +-TMG co-transport via the melibiose transport system in Escherichia coli .

Uptake of Taurocholic Acid into Isolated Rat‐Liver Cells

Binding and transport characteristics for uptake of taurocholic acid by isolated rat liver cells were studied. 1. An adsorption of taurocholate to the cell surface is terminated in less than 15 s. A Ks of 0.55 mM and a total binding capacity of 3.8 nmol/mg cell protein is determined. 2. The rate of uptake of taurocholate follows Michaelis-Menten kinetics with Km = 19 muM and V = 1.7 nmol/mg protein min. 3. There is a broad pH optimum for uptake between pH 6.5 -- 8.0. 4. The activation energy amounts to 29 kcal/mol. At high taurocholate concentration an unusual upward bend is observed in the Arrhenius plot. 5. Taurocholate uptake is competitively inhibited by taurochenodeoxycholate (Ki = 9 muM). It is noncompetitively inhibited by bromosulfophthalein (Ki = 3 muM). 6. At physiological taurocholate concentrations a 200-fold intracellular accumulation of taurocholate is observed. 7. Uptake is inhibited by about 75% by either antimycin A, carbonylcyanide m-chlorophenyl-hydrazone, ouabain. 8. Replacement of extracellular Na+ by either K+ or sucrose results in a 75% decrease of uptake. 9. It is concluded that taurocholate uptake is a carrier-mediated process, and suggested that the energy for intracellular accumulation is made available by cotransport of Na+.