Alternating Carrier Models and the Energy Conservation Laws

Lapointe, Jean‐Yves; Sasseville, Louis J.; Longpré, Jean-Philippe

doi:10.1016/j.bpj.2009.07.063

Cited by 7 publications

(13 citation statements)

References 5 publications

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“…The hypothesis that Li ϩ interacts with the GABA transporter and under some conditions substitutes for a Na ϩ ion during Na ϩ / GABA cotransport is strongly supported by the above results as well as previous studies (19,27,45). To provide a more direct test of this hypothesis, 1 we performed 22 Na uptake experiments under voltage-clamp conditions and correlated the molar uptake of Na ϩ ions [n Naϩ to net charge translocated (Q t ) during GABA-cotransport]. From these experiments we obtained the Na ϩ -to-Q t coupling ratio, CR Qt Na , at different cation conditions.…”

Section: Resultssupporting

confidence: 63%

“…From these experiments we obtained the Na ϩ -to-Q t coupling ratio, CR Qt Na , at different cation conditions. Experiments were conducted under three different superperfusion conditions: 20Na80Ch, 20Na80Li, and 5Na95Li; experiments with 5Na95Ch were excluded as both Q t and 22 Na uptake were considered to be too close to, or below detection limits. For individual oocytes, the time of exposure was chosen in the range 5-10 min and constant holding potential, to obtain a broad range of CR Qt Na to allow linear regression analysis.…”

Section: Resultsmentioning

confidence: 99%

“…For individual oocytes, the time of exposure was chosen in the range 5-10 min and constant holding potential, to obtain a broad range of CR Qt Na to allow linear regression analysis. A representative current trace obtained during 22 Na superfusion is shown in Fig. 4A.…”

Section: Resultsmentioning

confidence: 99%

“…This series of experiments was conducted as described above, but with [ 3 H]GABA instead of 22 Na as the tracer in the uptake solution. As the charge-GABA ratio for Na ϩ -only conditions was previously established to be two charges per GABA, independent of membrane voltage, e.g., Refs.…”

Section: Resultsmentioning

confidence: 99%

“…Combined radiolabeled 22 Na-and [ 3 H]GABA uptake and TEVC. The TEVC instrumentation comprised an OC-725C oocyte clamp (Warner Instruments) interfaced to a data acquisition system (Digidata 1322A, Molecular Devices) controlled by pClamp8 (Molecular Devices).…”

Section: Methodsmentioning

confidence: 99%

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Using lithium to probe sequential cation interactions with GAT1

Meinild

Forster

2012

American Journal of Physiology-Cell Physiology

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Li(+) interacts with the Na(+)/Cl(-)-dependent GABA transporter, GAT1, under two conditions: in the absence of Na(+) it induces a voltage-dependent leak current; in the presence of Na(+) and GABA, Li(+) stimulates GABA-induced steady-state currents. The amino acids directly involved in the interaction with the Na(+) and Li(+) ions at the so-called "Na2" binding site have been identified, but how Li(+) affects the kinetics of GABA cotransport has not been fully explored. We expressed GAT1 in Xenopus oocytes and applied the two-electrode voltage clamp and (22)Na uptake assays to determine coupling ratios and steady-state and presteady-state kinetics under experimental conditions in which extracellular Na(+) was partially substituted by Li(+). Three novel findings are: 1) Li(+) reduced the coupling ratio between Na(+) and net charge translocated during GABA cotransport; 2) Li(+) increased the apparent Na(+) affinity without changing its voltage dependence; 3) Li(+) altered the voltage dependence of presteady-state relaxations in the absence of GABA. We propose an ordered binding scheme for cotransport in which either a Na(+) or Li(+) ion can bind at the putative first cation binding site (Na2). This is followed by the cooperative binding of the second Na(+) ion at the second cation binding site (Na1) and then binding of GABA. With Li(+) bound to Na2, the second Na(+) ion binds more readily GAT1, and despite a lower apparent GABA affinity, the translocation rate of the fully loaded carrier is not reduced. Numerical simulations using a nonrapid equilibrium model fully recapitulated our experimental findings.

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Section: Resultssupporting

confidence: 63%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Using lithium to probe sequential cation interactions with GAT1

Meinild

Forster

2012

American Journal of Physiology-Cell Physiology

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Reassessment of Models of Facilitated Transport and Cotransport

Naftalin

2010

J Membrane Biol

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Most membrane transport models are determinate, requiring the transported ligand(s) to bind initially to a vacant site, which undergoes translation and releases ligand to the alternate side. The carrier reverts to its initial position to complete the net transport cycle. Ligand affinity may change during translation, but this must be compensated by an equivalent energy change(s) within the transport cycle. However, any asymmetric cyclic equilibrium deduced on this basis is thermodynamically fallacious. Determinate cotransport models imply lossless stoichiometric relationships between the complexed cotransported ligands. Independent ligand leakage apart from the mobile cotransport complex must occur outside the canonical cotransport pathway. In contrast, stochastic transport models assume independent ligand diffusion through a variably occluded channel(s) containing binding sites where ligands may undergo bimolecular exchanges. Energy dissipation is intrinsic to all stochastic transport models and occurs within the primary transport pathway. Frictional interactions within a shared path generate flow coupling between ligands. The primary driving forces causing transmembrane ligand flows are their electrochemical potential differences between the external solutions. Demonstrations that ligand exchanges in CLC and neurotransmitter transporters can be multimodal, encompassing both "channel"-like high and "transporter"-like lower conductance states and have independently regulated import and export exchange fluxes are major challenges to determinate models but are explicable by transient widening of a close-encounter region within the channel, leading to decreased coupling and enhanced efflux.

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A critique of the alternating access transporter model of uniport glucose transport

Naftalin

2018

Biophys Rep

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Alternating Carrier Models and the Energy Conservation Laws

Cited by 7 publications

References 5 publications

Using lithium to probe sequential cation interactions with GAT1

Using lithium to probe sequential cation interactions with GAT1

Reassessment of Models of Facilitated Transport and Cotransport

A critique of the alternating access transporter model of uniport glucose transport

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