Activation of the Na-K pump by intracellular Na in rat slow- and fast-twitch muscle.
Experiments were performed on isolated rat soleus (slow-twitch) and extensor digitorum longus (EDL) (fast-twitch) muscle of 4-week-old rats. In soleus muscle, electrical simulation at 2 Hz for 5 min increased the ouabain-suppressible 86Rb+ uptake by 138%, without significant changes in intracellular Na+ content or Na+/K+ ratio. In EDL muscle, the ouabain-suppressible 86Rb+ uptake was stimulated by only 58%, whereas intracellular Na+ content and Na+/K+ ratio were increased by around 70%. Na(+)-loading of the muscles by exposure to K(+)-free or K(+)-Ca(2-)-Mg(2+)-free buffer stimulated the ouabain-suppressible 86Rb+ uptake in the two muscles to roughly the same extent, but in EDL muscle this was associated with a more than twofold larger increase in Na+/K+ ratio. When the Na+ influx was increased by exposure to veratridine similar results were obtained. Graded variation in intracellular Na+ content was achieved by exposure to monensin. In soleus muscle, a 25% increase in intracellular Na+/K+ ratio resulted in a doubling of the ouabain-suppressible 86Rb+ uptake, whereas a doubling of the Na(+)-K+ transport rate in EDL muscle required a 140% increase in Na+/K+ ratio. The results indicate that in soleus muscle the Na+/K+ pump is much more sensitive to changes in intracellular Na+ content than in EDL muscle. This might explain the larger activation of the Na(+)-K+ pump in slow-twitch muscle during electrical stimulation and might be of significance for the activation of the Na(+)-K+ pump in vivo during work.
Everts
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Institute of Physiology, University of Aarhus, Denmark.
Clausen
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D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D004558:Electric Stimulation; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008985:Monensin / Q000494:pharmacology; D009132:Muscles / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012414:Rubidium Radioisotopes; D012964:Sodium / Q000494:pharmacology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects*; D014701:Veratridine / Q000494:pharmacology
D012414:Rubidium Radioisotopes; D010042:Ouabain; D014701:Veratridine; D000255:Adenosine Triphosphate; D008985:Monensin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase