Voltage dependence of current through the Na,K-exchange pump ofRana oocytes |
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Authors: | Michael M Wu Mortimer M Civan |
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Institution: | (1) Graduate Group of Bioengineering, University of Pennsylvania, 19104 Philadelphia, Pennsylvania;(2) Department of Physiology, University of Pennsylvania, 19104 Philadelphia, Pennsylvania;(3) Department of Medicines, University of Pennsylvania, 19104 Philadelphia, Pennsylvania |
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Abstract: | Summary We have studied current (I
Str) through the Na, K pump in amphibian oocytes under conditions designed to minimize parallel undesired currents. Specifically,I
Str was measured as the strophanthidin-sensitive current in the presence of Ba2–, Cd2+ and gluconate (in place of external Cl–). In addition,I
Str 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 meanK
0.5 of 0.4 m, with 2–5 m 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 m Str, the averageI
Str depended directly onV
m
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 ofI
p
, while producing only a small change in the maximal outwardI
Str. In contrast, decreasing the external K+] from 25 to 2.5mm reducedI
Str at 0 mV without substantially affecting its voltage dependence. At K+ concentrations of 1mm, both the absolute value ofI
Str at 0 mV and the slope conductance were reduced. In eight oocytes, the activation of the averagedI
Str by K+]
o
over the voltage interval –30, +30 mV] was well fit by the Hill equation, with K =1.7±0.4mm andnH (the minimum number of K+ binding sites) =1.7±0.4. The results unequivocally establish that the cardiotonic-sensitive current ofRana oocytes displays only a positive slope conductance for K+]
o
>1mm. 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. |
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Keywords: | amphibian oocyte Na K-exchange pump strophanthidin sodium potassium voltage sensitivity |
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