title: Voltage dependence of Na translocation by the Na/K pump.
abstract: During each complete reaction cycle, the Na/K pump transports three Na ions out across the cell membrane and two K ions in. The resulting net extrusion of positive charge generates outward membrane current but, until now, it was unclear how that net charge movement occurs. Reasonable possibilities included a single positive charge moving outwards during Na translocation; or a single negative charge moving inwards during K translocation; or either positive or negative charges moving during both translocation steps, but in unequal quantities. Any step that involves net charge movement through the membrane must have voltage-dependent transition rates. Here we report measurements of transient, voltage-dependent, displacement currents generated by the pump when its normal Na/K transport cycle has been interrupted by removal of external K and it is thus constrained to carry out Na/Na exchange. The quantity and voltage sensitivity of the charge moved during these transient currents suggests that Na translocation includes a voltage-dependent transition involving movement of one positive charge across the membrane. This single step can thus fully account for the electrogenic nature of Na/K exchange. The result provides important new insight into the molecular mechanism of active cation transport.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials; D012508:Sarcolemma / Q000502:physiology; D012964:Sodium / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013327:Strophanthidin / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D013327:Strophanthidin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase