The detection and properties of electrogenic Na+-K+ transport in the squid axon membrane
Kishimoto, U.; Inoue, I.; Tsutsui, I.; Ohkawa, T. (1995). The detection and properties of electrogenic Na+-K+ transport in the squid axon membrane, in: Abbott, N.J. et al. (Ed.) Cephalopod neurobiology: neuroscience studies in squid, octopus and cuttlefish. pp. 52-70
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| Authors | | Top |
- Kishimoto, U.
- Inoue, I.
- Tsutsui, I.
- Ohkawa, T.
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| Abstract |
The ouabain-sensitive current (ip) in the squid giant axon (Sepioteuthis lessoniana) was extracted under varied external Na+ and K+ concentrations. All the ip-V curves can be fitted with a reaction kinetic scheme that assumes a single electrogenic step in the reaction cycle. Two rate constants (k12 and k21) in the electrogenic step, two rate constants (r21 and r12) in the electro neutral step, and the amount of charge carried during each cycle are determined with a lumped two-state model. The charge carried in the electrogenic step (translocation of 3 Na + across the membrane) is +1. Not only the occlusion and deocclusion of Na+ or K+ but also the translocation of 2 K+ is independent of the membrane potential. Only two rate constants in the electrogenic step, i.e., K12 (=kO12 exp(zFV/2RT)) and k21 (=kO21 exp(-zFV/2RT)), are assumed to be highly voltage-dependent. All the rate constants (kO12, kO21 r21, r12) change with external Na+ and K+ concentrations in systematic ways. Decrease of external Na+ concentration causes a great increase of r12, a marked decrease of kO21 and a slight decrease of kO12; decrease of external K+ concentration causes a great increase of r21, increase of kO21, and a slight decrease of kO12. Affinity of the enzyme for the ligand increases markedly when the ligand concentration decreases. The standard free energy for the cycle remains unchanged at - 33 kJ/mol, which corresponds to that of ATP-hydrolysis. The reversal potential (Ep) of the pump is highly dependent on the external concentrations of Na+ and K+, that is, Ep shifts toward hyperpolarization almost instantly with decreasing external [Na]o. If [Na]o is lower than 50 mM, the sodium pump behaves as a constant current source in the physiological voltage range. On the other hand, Ep shifts toward depolarization with decreasing external [K]o. If [K]o is lower than 1 mM, the pump current cannot be outward at the resting potential, but is inward. |
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