Excitation‐ and β2‐agonist‐induced activation of the Na+−K+ pump in rat soleus muscle

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Abstract
In rat skeletal muscle, Na+–K+ pump activity increases dramatically in response to excitation (up to 20-fold) or β2-agonists (2-fold), leading to a reduction in intracellular Na+. This study examines the time course of these effects and whether they are due to an increased affinity of the Na+–K+ pump for intracellular Na+. Isolated rat soleus muscles were incubated at 30 oC in Krebs-Ringer bicarbonate buffer. The effects of direct electrical stimulation on 86Rb+ uptake rate and intracellular Na+ concentration ([Na+]i) were characterized in the subsequent recovery phase. [Na+]i was varied using monensin or buffers with low Na+. In the [Na+]i range 21–69 mm, both the β2-agonist salbutamol and electrical stimulation produced a left shift of the curves relating 86Rb+ uptake rate to [Na+]i. In the first 10 s after 1 or 10 s pulse trains of 60 Hz, [Na+]i showed no increase, but 86Rb+ uptake rate increased by 22 and 86 %, respectively. Muscles excited in Na+-free Li+-substituted buffer and subsequently allowed to rest in standard buffer also showed a significant increase in 86Rb+ uptake rate and decrease in [Na+]i. Na+ loading induced by monensin or electroporation also stimulated 86Rb+ uptake rate but, contrary to excitation, increased [Na+]i. The increase in the rate of 86Rb+ uptake elicited by electrical stimulation was abolished by ouabain, but not by bumetanide. The results indicate that excitation (like salbutamol) induces a rapid increase in the affinity of the Na+–K+ pump for intracellular Na+. This leads to a Na+–K+ pump activation that does not require Na+ influx, but possibly the generation of action potentials. This improves restoration of the Na+–K+ homeostasis during work and optimizes excitability and contractile performance of the working muscle.

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