Voltage Dependence of the Apparent Affinity for External Na+ of the Backward-Running Sodium Pump
| Autor: | R.F. Rakowski, Paul De Weer, David C. Gadsby |
|---|---|
| Rok vydání: | 2001 |
| Předmět: |
Patch-Clamp Techniques
Physiology Sodium-Potassium-Exchanging ATPase Sodium Potassium Radioisotopes Analytical chemistry Biological Transport Active chemistry.chemical_element Models Biological Ouabain Membrane Potentials Ion 03 medical and health sciences 0302 clinical medicine Electric field medicine Animals active transport Enzyme Inhibitors Digitoxigenin 030304 developmental biology Membrane potential 0303 health sciences Valence (chemistry) Chemistry Decapodiformes modeling Axons Kinetics Membrane Na K-ATPase electrogenicity Original Article 030217 neurology & neurosurgery medicine.drug |
| Zdroj: | The Journal of General Physiology |
| ISSN: | 1540-7748 0022-1295 |
| DOI: | 10.1085/jgp.117.4.315 |
| Popis: | The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was -1.5 +/- 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is -2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100-103). This model predicts valence z = nlambda, where n (1.33 +/- 0.05) is the Hill coefficient of Na binding, and lambda (0.61 +/- 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely. |
| Databáze: | OpenAIRE |
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