EFFECTS OF REDUCED ELECTROCHEMICAL NA-MUSCLE - ROLE OF THE NA+-K+ PUMP( GRADIENT ON CONTRACTILITY IN SKELETAL)

Continued excitation of skeletal muscle may induce a combination of a low extracellular Na+ concentration ([Na+](o)) and a high extracellula r K+ concentration ([K+](o)) in the T-tubular lumen, which may contrib ute to fatigue. Here, we examine the role of the Na+-K+ pump in the ma intenance of contractility in isolated rat soleus muscles when the Na, K+ gradients have been altered. When [Na+](o) is lowered to 25 mM by substituting Na+ with choline, tetanic force is decreased to 30% of t he control level after 60 min. Subsequent stimulation of the Na+-K+ pu mp with insulin or catecholamines induces a decrease in [Na+](i) and h yperpolarization. This is associated with a force recovery to 80-90% o f the control level which can be abolished by ouabain. This force reco very depends on hyperpolarization and is correlated to the decrease in [Na+](i) (r = 0.93; P<0.001). The inhibitory effect of a low [Na+](o) on force development is considerably potentiated by increasing [K+](o ). Again, stimulation of the Na+-K+ pump leads to rapid force recovery . The Na+-K+ pump has a large potential for rapid compensation of the excitation-induced rundown of Na+, K+ gradients and contributes, via i ts electrogenic effect, to the membrane potential. We conclude that th ese actions of the Na+-K+ pump are essential for the maintenance of ex citability and contractile force.