Voltage dependence of the apparent affinity for external Na(+) of the backward-running sodium pump.
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.
De Weer
P
P
Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA. deweer@mail.med.upenn.edu
Gadsby
D C
DC
Rakowski
R F
RF
D000818:Animals; D001369:Axons / Q000201:enzymology*; D001693:Biological Transport, Active / Q000502:physiology; D049832:Decapodiformes; D004073:Digitoxigenin / Q000031:analogs & derivatives* / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D010042:Ouabain / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011197:Potassium Radioisotopes / Q000493:pharmacokinetics; D012964:Sodium / Q000493:pharmacokinetics*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D004791:Enzyme Inhibitors; D011197:Potassium Radioisotopes; D004073:Digitoxigenin; C017227:20,22-dihydrodigitoxigenin; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase