Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

Umbach, Joy A.; Coady, Michael J.; Wright, Ernest M.

doi:10.1016/s0006-3495(90)82640-0

Cited by 110 publications

(86 citation statements)

References 18 publications

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“…The intracellular glucose concentration was 125 ± 23 µM (n = 5). This is slightly higher than what has been reported earlier (<50 μM; Umbach et al, 1990). …”

Section: Resultscontrasting

confidence: 67%

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Asif

Ali

Nur

et al. 2010

Biosensors and Bioelectronics

129

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In this article, we report a functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose. To adjust the sensor for intracellular glucose measurements, we grew hexagonal ZnO nanorods on the tip of a silver-covered borosilicate glass capillary (0.7 µm diameter) and coated them with the enzyme glucose oxidase. The enzyme-coated ZnO nanorods exhibited a glucose-dependent electrochemical potential difference versus an Ag/AgCl reference micro-electrode. The potential difference was linear over the concentration range of interest (0.5 µM -1000 µM). The measured glucose concentration in human adipocytes or frog oocytes using our ZnO nanorod sensor was consistent with values of glucose concentration reported in the literature; furthermore, the sensor was able to show that insulin increased the intracellular glucose concentration. This nanoelectrode device demonstrates a simple technique to measure intracellular glucose concentration.

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“…The intracellular glucose concentration was 125 ± 23 µM (n = 5). This is slightly higher than what has been reported earlier (<50 μM; Umbach et al, 1990). …”

Section: Resultscontrasting

confidence: 67%

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Asif

Ali

Nur

et al. 2010

Biosensors and Bioelectronics

129

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“…Phlorizin is a specific high-affinity competitive inhibitor of Na + /glucose cotransport from the external membrane surface, with an apparent inhibitory constant K i ≈ 10 µM and a true K i of 0.76 µM (Umbach, Coady & Wright, 1990;Birnir et al, 1991;Parent et al, 1992a;Hazama et al, 1997;Hirayama et al, 1996). Fig.…”

Section: Resultsmentioning

confidence: 99%

Kinetics of the Reverse Mode of the Na+/Glucose Cotransporter

2005

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This study investigates the reverse mode of the Na + /glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of α-methyl-D-glucopyranoside (αMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na + -and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The currentvoltage relationship saturated at positive membrane voltages (30-50 mV), and approached zero at − 150 mV. The half-maximal concentration for αMDG-evoked outward current ( ) was 35 mM (at 0 mV). In comparison, for forward sugar transport was 0.15 mM (at 0 mV). was similar for forward and reverse transport (≈35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: αMDG (1.0) > D-galactose (0.84) > 3-O-methyl-glucose (0.55) > D-glucose (0.38), whereas for forward transport, specificity was: αMDG ≈ D-glucose ≈ D-galactose > 3-Omethyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na + /sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

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“…1B), 3 mM PFA induces a small upward deflection of the steadystate holding current (I PFA ). This is interpreted as evidence of block of an intrinsic leak, 41 analogous to the phlorizin-dependent block of the leak intrinsic to the Na + -coupled glucose transporter (SGLT1), first described by Umbach et al 17 The holding current level reached during PFA superfusion is not zero, but is given by the endogenous leak current of the oocyte (I Endog ), which at a membrane potential of -50 mV, is usually inward but its magnitude can vary widely both between and within batches of oocytes. In the absence of external Na + , PFA like P i , does not induce a change in holding current, which underscores the Na + -dependence for the electrogenic responses to P i and PFA.…”

Section: Introductionmentioning

confidence: 86%

“…5C), which was shown to be dependent on internal sugar. 17 Initially, we therefore adopted a similar strategy for simulations using the kinetic model for NaPi-IIa/b, by assuming a finite internal P i (reviewed in refs. 45 and 53).…”

Section: The Uncoupled Leak Of Slc34 Proteinsmentioning

confidence: 99%

“…Characterization is further complicated by the finding that the cation species contributing to the leak current can either be the same or different from the cation that preferentially drives cotransport. For example, the leak associated with the sodium glucose cotransporter (SGLT1) is mediated by Na + ions, 17,18 whereas for GAT1 from the same gene family, alkali metal cations such as K + and Li + , which do not drive transport, can nevertheless permeate. 2,5,[19][20][21] Several questions arise with respect to the identity and underlying mechanism of uncoupled currents:…”

Section: Introductionmentioning

confidence: 99%

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The leak mode of type II Na⁺-Pi cotransporters

Andrini¹,

Ghezzi²,

Murer³

et al. 2008

Channels

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Na + -coupled phosphate cotransporters of the SLC34 gene family catalyze the movement of inorganic phosphate (P i ) across epithelia by using the free energy of the downhill electrochemical Na + gradient across the luminal membrane. Electrogenic (NaPiIIa/b) and electroneutral (NaPi-IIc) isoforms prefer divalent P i and show strict Na + :P i stoichiometries of 3:1 and 2:1, respectively. For electrogenic cotransport, one charge is translocated per transport cycle. When NaPi-IIa or NaPi-IIb are expressed in Xenopus oocytes, application of the P i transport inhibitor phosphonoformic acid (PFA) blocks a leak current that is not detectable in the electroneutral isoform. In this review, we present the experimental evidence that this transport-independent leak originates from a Na + -dependent uniport carrier mode intrinsic to NaPi-IIa/b isoforms. Our findings, based on the characteristics of the PFA-inhibitable leak measured from wild type and mutant constructs, can be incorporated into an alternating access class model in which the leak and cotransport modes are mutually exclusive and share common kinetic partial reactions. IntroductionSecondary-active cotransporters catalyze uphill solute movement across membranes by using the free energy available from the electrochemical gradient of the driving substrate, usually a monovalent cation (Na + , H + or K + ). The transport cycles of many members of this class are electrogenic, whereby net charge transfer accompanies cotransport and the partial reactions that constitute the transport cycle show voltage dependent kinetics. We can investigate the kinetics of these carriers by electrophysiological techniques, as ideally the substrate-induced current is an indirect measure of the transport rate for a constant number of charges translocated per cycle. Uncoupled currents give rise to deviations from the tightly coupled electrogenicity and strict stoichiometry between driving and driven substrates that is expected of the ideal secondary-active carrier. They are also referred to as leak or slippage currents (reviewed in refs. 1 and 2) and are intimately associated with the heterologous expression of the carrier protein. Their co-existence alongside the coupled transport electrogenic activity has led to a reassessment of the traditional view of channels and carriers as unique molecular entities (reviewed in ref.3).Uncoupled currents are commonly sub-classified as transportdependent and transport-independent currents, depending on whether they are detectable in the absence or presence of substrate. Cotransporters can exhibit one or both types of uncoupled current with different contributing ions. Uncoupled currents appear to be ubiquitous among electrogenic carriers and they have been described for gene products of at least nine solute carrier families identified in the human genome 4 (Table 1). Within a given solute carrier family, the properties of the uncoupled currents can also vary considerably for different isoforms, measured under the same experimental conditions. For examp...

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Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

Cited by 110 publications

References 18 publications

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Kinetics of the Reverse Mode of the Na+/Glucose Cotransporter

The leak mode of type II Na⁺-Pi cotransporters

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Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

Cited by 110 publications

References 18 publications

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose

Kinetics of the Reverse Mode of the Na+/Glucose Cotransporter

The leak mode of type II Na+-Pi cotransporters

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The leak mode of type II Na⁺-Pi cotransporters