title: Voltage dependence of current through the Na,K-exchange pump of Rana oocytes.
abstract: We have studied current (IStr) through the Na,K pump in amphibian oocytes under conditions designed to minimize parallel undesired currents. Specifically, IStr was measured as the strophanthidin-sensitive current in the presence of Ba2+, Cd2+ and gluconate (in place of external Cl-). In addition, IStr was studied only after the difference currents from successive applications and washouts of strophanthidin (Str) were reproducible. The dose-response relationship to Str in four oocytes displayed a mean K0.5 of 0.4 microM, with 2-5 microM producing 84-93% pump block. From baseline data with 12 Na(+)-preloaded oocytes, voltage clamped in the range [-170, +50 mV] with and without 2-5 microM Str, the average IStr depended directly on Vm up to a plateau at 0 mV with interpolated zero current at -165 mV. In three oocytes, lowering the external [Na+] markedly decreased the voltage sensitivity of Ip, while producing only a small change in the maximal outward IStr. In contrast, decreasing the external [K+] from 25 to 2.5 mM reduced IStr at 0 mV without substantially affecting its voltage dependence. At K+ concentrations of less than 1 mM, both the absolute value of IStr at 0 mV and the slope conductance were reduced. In eight oocytes, the activation of the averaged IStr by [K+]0 over the voltage interval [-30, +30 mV] was well fit by the Hill equation, with K' = 1.7 +/- 0.4 mM and nH (the minimum number of K+ binding sites) = 1.7 +/- 0.4. The results unequivocally establish that the cardiotonic-sensitive current of Rana oocytes displays only a positive slope conductance for [K+]0 greater than 1 mM. There is therefore no need to postulate more than one voltage-sensitive step in the cycling of the Na, K pump under physiologic conditions. The effects of varying external Na+ and K+ are consistent with results obtained in other tissues and may reflect an ion-well effect.
authors.affiliation: Graduate Group of Bioengineering, University of Pennsylvania, Philadelphia 19104.
authors.forename: M M
authors.initials: MM
authors.lastname: Wu
authors.forename: M M
authors.initials: MM
authors.lastname: Civan
mesh_terms: D000818:Animals; D001464:Barium / Q000378:metabolism; D002104:Cadmium / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D004553:Electric Conductivity / Q000187:drug effects / Q000502:physiology; D005942:Gluconates / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008566:Membranes / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism; D011894:Rana pipiens; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D013327:Strophanthidin / Q000494:pharmacology
chemical_list: D002712:Chlorides; D005942:Gluconates; D015221:Potassium Channels; D002104:Cadmium; D001464:Barium; D013327:Strophanthidin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C030691:gluconic acid; D011188:Potassium