High-affinity taurine uptake in developing retina

A specific system for taurine transport is present at the early stages of development in both chick and rat retinas. The results obtained with taurine analogs indicate a high degree of specificity of taurine uptake. Two transport systems were detected for the adult rat retina: a high-affinity (K _m 21 M) and a low-affinity transport system (K _m 312 M). On the other hand, in the adult chick retina, only a low-affinity transport system (K _m 580 M) could be detected. Nevertheless, embryo chick retina accumulated [³H]taurine by two different kinetic mechanisms withK _m s of 242 M and 21 M for the low- and high-affinity processes, respectively. Taurine uptake systems were absolutely Na⁺ dependent. The sodium-dependence curve for taurine uptake was sigmoid. These mechanisms appear not to be mediated by a Na⁺ cotransport system. In spite of the differences observed in taurine uptake in both species, in each of them it closely parallels the changes brought about by the morphological and functional maturation of the retina.     

Alanine and taurine transport by the gill epithelium of a marine bivalve: Effect of sodium on influx

Summary Marine mussels can accumulate amino acids from seawater into the epithelial cells of the gill against chemical gradients in excess of 5×10⁶ to 1. Uptake of both alanine and taurine into gill tissue isolated fromMytilus californianus was found to be dependent upon Na⁺ in the external solution. Uptake of these amino acids was described by Michaelis-Menten kinetics, and a reduction in external [Na⁺] (from 425 to 213mm) increased the apparent Michaelis constants (alanine, from 8 to 17 m; taurine, from 4 to 39 m) without a significant influence on theJ _max's of these processes. Fivemm harmaline, an inhibitor of Na-cotransport processes in many systems, reduced both alanine and taurine uptake by more than 95%; this inhibition appeared to be competitive in nature, with an apparentK _i of 43 m for the interaction with alanine uptake. Increasing the external [Na⁺] from 0 to 510mm produced a sigmoid activation of alanine and taurine uptake withK _Na's of approximately 325mm. The apparent Hill coefficients for this activation were 7.3 and 7.4 for alanine and taurine, respectively. These data are consistent with uptake mechanisms which require comparatively high concentrations of Na⁺ to activate transport, and which couple several Na⁺ ions to the transport of each amino acid. These characteristics, in conjunction with the previously demonstrated low passive permeability of the apical membrane to amino acids, result in systems capable of i) accumulating amino acids from seawater to help meet the nutritional needs of this animal, and ii) maintaining the high intracellular amino-acid concentrations associated with volume regulation in the gill.     

Spontaneous and evoked release of [3H]taurine from a P2 subcellular fraction of the rat retina

The effects of spontaneous and evoked [³H]taurine release from a P₂ fraction prepared from rat retinas were studied. The P₂ fraction was preloaded with [³H]taurine under conditions of high-affinity uptake and then examined for [³H]taurine efflux utilizing superfusion techniques. Exposure of the P₂ fraction to high K⁺ (56 mM) evoked a Ca²⁺-independent release of [³H]taurine. Li⁺ (56 mM) and veratridine (100 M) had significantly less effect (8–15% and 15–30%, respectively) on releasing [³H]taurine compared to the K⁺-evoked release. 4-Aminopyridine (1 mM) had no effect on the release of [³H]taurine. The spontaneous release of [³H]taurine was also Ca²⁺-independent. When Na⁺ was omitted from the incubation medium K⁺-evoked [³H]taurine release was inhibited by approximately 40% at the first 5 minute depolarization period but was not affected at a second subsequent 5 minute depolarization period. The spontaneous release of [³H]taurine was inhibited by 60% in the absence of Na⁺. Substitution of Br^– for Cl^– had no effect on the release of either spontaneous or K⁺-evoked [³H]taurine release. However, substitution of the Cl^– with acetate, isethionate, or gluconate decreased K⁺-evoked [³H]taurine release. Addition of taurine to the superfusion medium (homoexchange) resulted in no significant increase in [³H]taurine efflux. The taurine-transport inhibitor guanidinoethanesulfonic acid increased the spontaneous release of [³H]taurine by approximately 40%. These results suggest that the taurine release of [³H]taurine is not simply a reversal of the carrier-mediated uptake system. It also appears that taurine is not released from vesicles within the synaptosomes but does not rule out the possibility that taurine is a neurotransmitter. The data involving chloride substitution with permeant and impermeant anions support the concept that the major portion of [³H]taurine release is due to an osmoregulatory action of taurine while depolarization accounts for only a small portion of [³H]taurine release.     

Volume-sensitive taurine transport in fish erythrocytes

Summary Taurine plays an important role in cell volume regulation in both vertebrates and invertebrates. Erythrocytes from two euryhaline fish species, the eel (Anguilla japonica) and the starry flounder (Platichthys stellatus) were found to contain high intracellular concentrations of this amino acid ( 30 mmol per liter of cell water). Kinetic studies established that the cells possessed a saturable high-affinity Na⁺-dependent -amino-acid transport system which also required Cl^– for activity (apparentK _m (taurine) 75 and 80 m;V _max 0.85 and 0.29 mol/g Hb per hr for eel (20°C) and flounder cells (10°C), respectively. This -system operated with an apparent Na⁺/Cl^–/taurine coupling ratio of 211. A reduction in extracellular osmolarity, leading to an increase in cell volume, reversibly decreased the activity of the transporter. In contrast, low medium osmolarity stimulated the activity of a Na⁺-independent nonsaturable transport route selective for taurine, -amino-n-butyric acid and small neutral amino acids, producing a net efflux of taurine from the cells. Neither component of taurine transport was detected in human erythrocytes. It is suggested that these functionally distinct transport routes participate in the osmotic regulation of intracellular taurine levels and hence contribute to the homeostatic regulation of cell volume. Volume-induced increases in Na⁺-independent taurine transport activity were suppressed by noradrenaline and 8-bromoadenosine-3, 5-cyclic monophosphate, but unaffected by the anticalmodulin drug, pimozide.     

Activation of adenosine A2 receptors enhances high K+-evoked taurine release from rat hippocampus: A microdialysis study

Summary The present study was designed to examine which type of adenosine receptors was involved in enhancement of high K⁺-evoked taurine release fromin vivo rat hippocampus using microdialysis. Perfusion with 0.5 or 5.0 mM adenosine enhanced high K⁺-evoked taurine release. Perfusion with 2M R(–)-N⁶-2-phenylisopropyladenosine (PIA), a selective adenosine A₁ receptor agonist, did not modulate taurine release. Perfusion with 1M 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective adenosine A₁ receptor antagonist, increased taurine release. On the other hand, perfusion with 20M 2-[4-(2-carboxyethyl)phenethylamino]-5-N-ethyl-carboxamideadenosine (CGS21680), a selective adenosine A_2A receptor agonist, enhanced taurine release, while perfusion with 1 mM 3,7-dimethyl-propagylxanthine (DMPX), an adenosine A₂ receptor antagonist, did not affect taurine release. These results demonstrate that adenosine enhances high K⁺-evoked taurine release via activation of adenosine A_2A receptors from both neurons and glial cells ofin vivo rat hippocampus.     

K+-evoked taurine efflux from cerebellar astrocytes: On the roles of Ca2+ and Na+

The ionic requirements for K⁺-evoked efflux of endogenous taurine from primary cerebellar astrocyte cultures were studied. The Ca²⁺ ionophore A23187 evoked taurine efflux in a dose-dependent fashion with a time-course identical to that of K⁺-induced efflux. The Ca²⁺-channel antagonist nifedipine had no effect upon efflux induced by 10 or 50 mM K⁺. In addition, verapamil did not antagonize 50 mM K⁺-evoked efflux except at high, non-pharmacological concentrations (>100 M), and preincubation with 2 M -conotoxin had no effect on 50 mM K⁺-evoked efflux. Similarly, preincubation with 1 mM ouabain had no effect on the amount of taurine released by K⁺ stimulation, but did accelerate the onset of efflux by 2–4 min. Although 2 M tetrodotoxin had no effect on K⁺-evoked release, replacing Na⁺ with choline abolished the taurine efflux seen in response to K⁺ stimulation. Together, these findings suggest that neuronal N- and L-type Ca²⁺- and voltage-dependent Na⁺-channels are not involved in the influx of Ca²⁺ which appears to be necessary for K⁺-evoked taurine efflux, and that in addition to Ca²⁺, extracellular Na⁺ is also required.     

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.     

Identification of a high affinity taurine transporter which is not dependent on chloride

Taurine transport by lactating gerbil mammary tissue has been examined. Taurine uptake is, mediated by a high-affinity system which is specific for -amino acids. The uptake of taurine is Na⁺-dependent but appears not to be obligatorly dependent upon Cl^–. Thus, replacing Na⁺ with choline almost abolished taurine uptake. Substituting Cl^– with NO ₃ ^– had no effect whereas SCN^– induced a small but significant increase in taurine influx. Taurine uptake was Na⁺-dependent under conditions where Cl^– had been replaced with NO ₃ ^– . However, it is apparent that the Na⁺-dependent taurine transport system requires the presence of a permeable anion because replacing Cl^– with gluconate markedly reduced taurine uptake. Cell-swelling, induced by a hyposmotic challenge, increased the efflux of taurine from gerbil mammary tissue via a pathway sensitive to niflumic acid.Abbreviations Tris (Tris(hydroxymethyl)aminomethane - BES (N,N-bis[2-hydroxyethyl]-2-aminoethane sulphonic acid)     

Characterization of the Na+-dependent taurine influx in flounder erythrocytes

About 92% of the taurine influx in flounder erythrocytes at physiological conditions in vitro (330 mosmol·l^-1, 145 mmol·l^-1 Na⁺, 0.30 mmol·l^-1 taurine) is Na⁺-dependent. This influx is highly specific for taurine. The -amino compounds hypotaurine and -alanine were the only compounds which mimicked the inhibitory effect of taurine on influx of [¹⁴C]taurine, the former more than the latter. Counterexchange of taurine was also mediated by the taurine transporters. Reduction of osmolality per se did not affect the activity of these transporters. Non-linear regression analysis of the influx values revealed the presence of two different influx systems: a system with high affinity and low capacity and another with low affinity and high capacity. However, we cannot exclude the possibility that this influx of taurine was mediated by only one transporter which operated in different modes depending on the extracellular Na⁺ concentration. On the assumption that the Na⁺-dependent influx was mediated by two separate systems, the maximal velocity of the low capacity system was 2.55 nmol·g dry weight^-1·min^-1 at 145 mmol·ll^-1 extracellular Na⁺. This capacity was about 50% lower than that of the high capacity system. The Michaelis constants were 0.013 and 1.34 mmol·l^-1, respectively. Reduction of the extracellular Na⁺ concentration reduced maximal velocity and the affinity to taurine of both transport systems. At 10 mmol·l^-1 Na⁺ or lower concentrations the high capacity system did not seem to operate. The activation method suggested that each taurine molecule transported by the high capacity system was accompanied by two Na⁺. The stoichiometry of the low capacity system was 1 taurine: 1 Na⁺. The Hill-coefficient for both transport systems was 1.00.Abbreviations cpm counts per minute - dw dry weight - GABA -amino-n-butyric acid - K _m Michaelis constant - pK _b basic dissociation constant - SD standard deviation - -ABA Dl--amino-n-butyric acid - V _max maximal velocity - ww wet weight     

The relationship between sodium and high-affinity taurine uptake in hypothalamic crude P2 synaptosomal preparations

Two uptake systems for taurine transport in a rat hypothalamic crude synaptosomal preparation were identified. The true transport constants were, for the high-affinity uptake system,K _m=240 M andV (maximum velocity)=400 nmol/g protein/min, and for the low-affinity uptake system.K _m=5290 M and V=1640 nmol/g protein/min. The initial velocity of high-affinity taurine uptake by the crude synaptosomal preparation was studied as a function of sodium and taurine concentration. Hill plots were constructed from these data. The requirement of high-affinity taurine uptake on a sodium gradient was examined by utilizing monensin, and the metabolic poisons, 2,4-dinitrophenol and ouabain. The major findings are as follows: 1) One sodium ion is co-transported with each taurine molecule; 2) the high-affinity uptake process is driven by the sodium concentration gradient across the membrane; 3) sodium increases the maximal velocity rather than the affinity of the high-affinity taurine carrier for the taurine molecule; 4) one taurine molecule is transported per carrier for both the high- and low-affinity taurine uptake systems; and 5) high-affinity taurine uptake is an energy-dependent process.     

Methylmercury modulates GABAA receptor complex differentially in rat cortical and cerebellar membranes in vitro

Effects of methylmercury (MetHg) on the specific [³H]flunitrazepam binding were studied in rat cortical and cerebellar P₂-fractions in vitro. MetHg did not affect significantly the specific [³H]flunitrazepam binding in unwashed P₂-fraction but increased it marginally (by 16%) at 100 M in washed P₂-fraction, in both brain regions.Muscimol (3 M), a GABA_A agonist, stimulated the [³H]flunitrazepam binding by 30% to 50% depending on the brain region. In washed cerebellar membranes the enhancing response of muscimol was 10 to 14% lower after preincubation of the tissue with MetHg but in cerebral cortex MetHg did not modulate the muscimol response at all. The results indicate that Met-Hg may have region specific effects on GABA_A receptors in vitro and the effect may depend on the occupational state of the GABA binding domain of the receptor complex.     

Effects of zinc ex vivo on taurine uptake in goldfish retinal cells

Background

Taurine and zinc exert neurotrophic effects in the central nervous system. Current studies demonstrate that Na⁺/Cl^- dependent neurotransmitter transporters, similar to that of taurine, are modulated by micromolar concentrations of zinc. This study examined the effect of zinc sulfate ex vivo on [³H]taurine transport in goldfish retina.

Methods

Isolated cells were incubated in Ringer with zinc (0.1–100 µM). Taurine transport was done with 50 nM [³H]taurine or by isotopic dilution with taurine (0.001–1 mM) and 50 nM [³H]taurine.

Results

Zinc reduced the capacity of taurine transport without changes in affinity, and caused a noncompetitive inhibition of high affinity taurine transport, with an EC₅₀= 0.072 µM. The mechanism by which zinc affects taurine transport is unknown at the present.

Conclusions

There may be a binding site of zinc in the transporter that affects union or translocation of taurine, or possibly the formation of taurine-zinc complexes, rather than free zinc, could affect the operation of the transporter.

     

Isethionate and taurine transport sites at brain cell membranes: Influx studies with brain slices

The influx of [¹⁴C]isethionate (ISA) into rat brain slices was studied with and without taurine. This influx was relatively rapid, but took place largely by a non-saturable, passive mechanism, which transferred much less ISA into the brain cells than taurine. Taurine inhibited the influx of ISA competitively (K _m=50 and 100 mol/l) at low ISA concentrations, and ISA that of taurine non-competitively (V=200 and 400–700 mol×min^–1×kg^–1 wet weight) at high taurine concentrations. It thus appears that ISA and taurine may have a small number of common transport sites at brain cell membranes, but these are apparently of little significance for the total transport of ISA.     

Ionic permeabilities of the plasma membrane of isolated intact bovine rod outer segments as studied with a novel optical probe

Summary The permeability properties of the plasma membrane of intact rod outer segments purified from bovine retinas (ROS) were studied with the aid of the optical probe neutral red as described in the companion paper. The following observations were made: (1) Electrical shunting of ROS membranes greatly stimulated Na⁺ and K⁺ transport, suggesting that this transport reflects Na⁺ and K⁺ currents, respectively. The dissipation of a Na⁺ gradient across the plasma membrane occurred with a half-time of 30 sec at 25°C. (2) The Na⁺ permeability was progressively inhibited when the external Ca²⁺ concentration was raised from 1 m to 20mm. A similar Ca²⁺ dependence was observed for H⁺ and Li⁺ transport. The Na⁺ permeability was not affected when the total internal Ca²⁺ content of ROS was varied between 0.1 mol Ca²⁺/mol rhodopsin and 7 mol Ca²⁺/mol rhodopsin, or when the free internal Ca²⁺ concentration was varied between 0.1 and 50 m. (3) The K⁺ permeability was progressively stimulated when the external Ca²⁺ concentration was raised from 0.001 to 1 m, whereas a further increase to 20mm was without effect. A similar Ca²⁺ dependence was observed for Rb⁺ and Cs⁺ transport. (4) At an external Ca²⁺ concentration in the micromolar range the rate of transport decreased in the order: Na⁺>K⁺=H⁺>Cs⁺>Li⁺. (5) Na⁺ fluxes depended in a sigmoidal way on the external Na⁺ concentration, suggesting that sodium ions move in pairs. The concentration dependence of uniport Na⁺ transport and that of Na⁺-stimulated Ca²⁺ efflux (exchange or antiport transport) were very similar.     

Characterization of the carrier-mediated [3H]GABA release from isolated synaptic plasma membrane vesicles

Synaptic plasma membrane (SPM) vesicles were isolated under conditions which preserve most of their biochemical properties. Therefore, they appeared particularly useful to study the cytoplasmic GABA release mechanism through its neuronal transporter without interference of the exocytotic mechanism. In this work, we utilized SPM vesicles isolated from sheep brain cortex to investigate the process of [³H]GABA release induced by ouabain, veratridine and Na⁺ substitution by other monovalent cations (K⁺, Rb⁺, Li⁺, and choline). We observed that ouabain is unable to release [³H]GABA previously accumulated in the vesicles and, in our experimental conditions, it does not act as a depolarizing agent. In contrast, synaptic plasma membrane vesicles release [³H]GABA when veratridine is present in the external medium, and this process is sensitive to extravesicular Na⁺ and it is inhibited by extravesicular Ca²⁺ (1 mM) under conditions which appear to permit its entry. However, veratridine-induced [³H]GABA release does not require membrane depolarization, since this drug does not induce any significant alteration in the membrane potential, which is determined by the magnitude of the ionic gradients artificially imposed to the vesicles. The substitution of Na⁺ by other monovalent cations promotes [³H]GABA release by altering the Na⁺ concentration gradient and the membrane potential of SPM vesicles. In the case of choline and Li⁺, we observed that the fraction of [³H]GABA released relatively to the total amount of neurotransmitter released by K⁺ or Rb⁺ is about 28% and 68%, respectively. Since the replacement of Na⁺ by K⁺, Rb⁺, and Li⁺ causes different levels of membrane depolarization, and the replacement of Na⁺ by choline causes hyperpolarization of the vesicles, these results suggest that, in parallel to the [³H]GABA release, which is directly proportional to the level of membrane depolarization, this neurotransmitter can be released by decreasing the external Na⁺, which reflects an elevation of the Na⁺ concentration gradient (inout). Like veratridine-induced release, the depolarization-induced release of [³H]GABA by SPM vesicles is inhibited by Ca²⁺, which suggests that this divalent cation interfers with the cytoplasmic GABA release mechanism.Abbreviations used ATPase adenosine triphosphatase - GABA -aminobutyric acid - Mes 2 (N-morpholino)-ethanosulfonic acid - SPM synaptic plasma membranes - membrane potential     

Evidence that the low-affinity K+ transport system of Rhodobacter capsulatus is a uniporter. The effects of ammonium on the transporter

Comparison of the rate of accumulation of ⁸⁶Rb⁺ by intact cells of Rhodobacter capsulatus during short periods of illumination with the Rb⁺-dependent membrane ionic current measured by electrochromism supports the view that both activities reflect the operation of a low-affinity K⁺ transport system. In experiments performed under similar conditions the ratio of ⁸⁶Rb⁺ uptake to charge uptake was approx. 1.0, suggesting that the transport system operates as a uniporter. The addition of NH _inf4 ^sup+ to a cell suspension led to an increase in membrane ionic current but failed to inhibit the accumulation of ⁸⁶Rb⁺ during illumination. The presence of K⁺ and NH _inf4 ^sup+ inhibited the increase in cellular ATP levels at the onset of illumination. This effect was prevented by Cs⁺. The results are considered within the context of the hypothesis (Golby et al. Eur J Biochem 187: 589–597) that NH _inf4 ^sup+ can be transported by the K⁺ carrier and in the context of an alternative hypothesis that NH _inf4 ^sup+ increases the affinity of the K⁺ transport system for its natural substrate and for Rb⁺.Abbreviations pH pH difference across the cytoplasmic membrane - electrical potential difference across the cytoplasmic membrane     

Role of the sodium pump and the background K+ channel in passive K+(Rb+) uptake by isolated cardiac sarcolemmal vesicles

Summary A simple procedure was developed for the isolation of a sarcolemma-enriched membrane preparation from homogenates of bullfrog (Rana catesbeiana) heart. Crude microsomes obtained by differential centrifugation were fractionated in Hypaque density gradients. The fraction enriched in surface membrane markers consisted of 87% tightly sealed vesicles. The uptake of⁸⁶Rb⁺ by the preparation was measured in the presence of an opposing K⁺ gradient using a rapid ion exchange technique. At low extravesicular Rb⁺ concentrations, at least 50% of the uptake was blocked by addition of 1mm ouabain to the assay medium. Orthovanadate (50 m), ADP (2.5mm), or Mg (1mm) were also partial inhibitors of Rb⁺ uptake under these conditions, and produced a complete block of Rb⁺ influx in the presence of 1mm ouabain. When⁸⁶Rb⁺ was used as a tracer of extravesicular K⁺ (Rb ₀ ⁺ 40 m K ₀ ⁺ =0.1–5mm) a distinct uptake pathway emerged, as detected by its inhibition by 1mm Ba²⁺ (K _0.5=20 m). At a constant internal K⁺ concentration (K _in ⁺ =50mm) the magnitude of the Ba²⁺-sensitive K⁺ uptake was found to depend on K ₀ ⁺ in a manner that closely resembles the K⁺ concentration dependence of the background K⁺ conductance (I _Kl) observed electrophysiologically in intact cardiac cells. We conclude that K⁺ permeates passively this preparation through two distinct pathways, the sodium pump and a system identifiable as the background potassium channel.     

Mechanisms of fusicoccin action: evidence for concerted modulations of secondary K+ transport in a higher plant cell

Fusicoccin (FC) has long been known to promote K⁺ uptake in higher plant cells, including stomatal guard cells, yet the precise mechanism behind this enhancement remains uncertain. Membrane hyperpolarization, thought to arise from primary H⁺ pumping stimulated in FC, could help drive K⁺ uptake, but the extent to which FC stimulates influx and uptake frequently exceeds any reasonable estimates from Constant Field Theory based on changes in the free-running membrane potential (V _m) alone; furthermore, unidirectional flux analyses have shown that in the toxin K⁺ (⁸⁶Rb⁺) exchange plummets to 10% of the control (G.M. Clint and E.A.C. MacRobbie 1984, J. Exp. Bot.35 180–192). Thus, the activities of specific pathways for K⁺ movement across the membrane could be modified in FC. We have explored a role for K⁺ channels in mediating these fluxes in guard cells ofVicia faba L. The correspondence between FC-induced changes in chemical (⁸⁶Rb⁺) flux and in electrical current under voltage clamp was followed, using the K⁺ channel blocker tetraethylammonium chloride (TEA) to probe tracer and charge movement through K⁺-selective channels. Parallel flux and electrical measurements were carried out when cells showed little evidence of primary pump activity, thus simplifying analyses. Under these conditions, outward-directed K⁺ channel current contributed appreciably to charge balance maintainingV _m, and adding 10 mM TEA to block the current depolarized (positive-going)V _m; TEA also reduced⁸⁶Rb⁺ efflux by 68–80%. Following treatments with 10 M FC, both K⁺ channel current and⁸⁶Rb⁺ efflux decayed, irreversbly and without apparent lag, to 10%–15% of the controls and with equivalent half-times (approx. 4 min). Fusicoccin also enhanced⁸⁶Rb⁺ influx by 13.9-fold, but the influx proved largely insensitive to TEA. Overall, FC promotednet cation uptake in 0.1 mM K⁺ (Rb⁺), despite membrane potentials which were 30–60 mVpositive of the K⁺ equilibrium potential. These results tentatively link (chemical) cation efflux to charge movement through the K⁺ channels. They offer evidence of an energy-coupled mechanism for K⁺ uptake in guard cells. Finally, the data reaffirm early suspicions that FC alters profoundly the K⁺ transport capacity of the cells, independent of any changes in membrane potential.Abbreviations and symbols E _K equilibrium potential for K⁺ - FC fusicoccin - Hepes 4-(2-hydroxyethyl)-1-piperazineeth-anesulfonic acid - G _m membrane (slope) conductance atV _m - I-V current-voltage (relationship) - apparent rate constant for exchange - K _i ⁺ , K ₀ ⁺ intracellular, extracellular K⁺ (concentration) - TEA tetraethylammonium chloride - V _m free-running membrane potential (difference)     

Depolarization of the neuronal membrane caused by Co-transport of taurine and sodium

C 1300 neuroblastoma cells were cultured and used to study the effect of sodium dependent taurine transport on the membrane potential. Measuring net accumulation of taurine and the depolarization caused by externally applied taurine, we found both processes become active at an external concentration of taurine of 1 mM or more. Net accumulation had K_m of 13 mM and a V_max of 126 nmol × mg of protein^–1×min^–1. The taurine induced depolarization of the neuroblastoma cell was parallelled by a 25 per cent decrease in its membrane impedance. The transport of taurine, the depolarization caused by taurine and the effect of taurine on the membrane impedance, all, had a similar dependence on the external sodium concentration. Our results on the depolarizing cotransport between taurine and sodium at the neuronal membrane, may illustrate an additional mechanism for the control of the electrical activity of neuronal cells.     

Renal handling of taurine,l-alanine,l-glutamate andd-glucose inOpsanus tau: studies on isolated brush border membrane vesicles

Summary Renal brush border membrane vesicles (bbmv) from the aglomerular toadfish (Opsanus tau), isolated by differential precipitation, were tested for their ability to actively translocate (i) taurine, known to be secreted by the kidney of several marine teleosts, and (ii)l-alanine,l-glutamic acid, andd-glucose, solutes that are normally reabsorbed in the filtering nephron. Vesicular taurine uptake displayed a Na⁺ dependence. Transport was greatest under conditions of an inward-directed Na⁺ gradient, but a significant stimulation by Na⁺ over K⁺ could also be observed in the absence of a salt gradient. At high extravesicular K⁺, the addition of valinomycin reduced taurine uptake. Na⁺-dependent³H-taurine flux was almost completely inhibited by non-labeled taurine (tracer replacement) or -alanine, but was unaffected byl-alanine. Replacement of medium chloride by SCN^– or NO ₃ ^– in the presence of Na⁺ resulted in significantly lower uptake rates under both anion gradient and anion equilibrium conditions, whereas Br^– could almost fully substitute for the stimulatory Cl^– action. These results indicate the presence of an electrogenic Na⁺-cotransport mechanism with specificity for -amino acids in the toadfish renal brush border. Whether the system under physiological conditions mediates reabsorption or secretion of taurine remains to be determined. Toadfish bbmv also translocatedl-alanine andl-glutamic acid in a Na⁺-dependent manner. Possible roles for these most likely reabsorptive transport systems in a non-filtering kidney are discussed.d-glucose uptake, however, appeared to occur via Na⁺-independent pathways, since it was not affected by phlorizin in the presence of Na⁺, or by Na⁺ replacement.Abbreviation bbmv brush border membrane vesicles