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The Na+-K+ pump current was studied in smooth muscle cells from mesenteric resistance arteries of guinea-pigs by the use of the perforated patch-clamp technique in the presence of blockers for various ion channels and exchangers.
When the Na+ concentration in the pipette solution ([Na+]i) was 50 mM, an increase in the extracellular K+ concentration ([K+]o) from 0 to 10 mM caused an outward current. Both the removal of K+ from the bath solution and the application of 10 µM ouabain abolished this current. Thus, this K+-induced and ouabain-sensitive current was considered to be the Na+-K+ pump current.
The amplitude of the Na+-K+ pump current increased as the membrane potential was made more positive until around 0 mV, while the amplitude saturated at more positive potentials than 0 mV.
An increase in [K+]o or [Na+]i amplified the Na+-K+ pump current. For [K+]o, the binding constant (Kd) was 1·6 ± 0·3 mM and the Hill coefficient (nH) was 1·1 ± 0·2 (n = 6). For [Na+]i, Kd was 22 ± 5 mM and nH was 1·7 ± 0·5 (n = 4-19).
The presence of various monovalent cations other than Na+ in the bath solution also evoked the Na+-K+ pump current. The order of potency was K+ 
Rb+ > Cs+ >> Li+.
Ouabain inhibited the Na+-K+ pump current in a dose-dependent manner with a Kd of 0·35 ± 0·03 µM and an nH of 1·2 ± 0·1 (n = 6-8).
The Na+-K+ pump current increased as temperature increased. The temperature coefficient (Q10; 26-36 °C) was 1·87 (n = 9).
In summary the present study characterized for the first time the Na+-K+ pump current in vascular smooth muscle cells by the use of the voltage-clamp method. The use of this method should provide essential information for Na+,K+-ATPase-mediated changes in the cell functions of vascular smooth muscle cells.
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