Voltage dependence of current through the Na,K-exchange pump of Rana oocytes

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.