NA- IMPLICATION FOR THE MECHANISM OF FATIGUE( AND K+ EFFECT ON CONTRACTILITY OF FROG SARTORIUS MUSCLE )

Although a decrease in extracellular Na+ and an increase in K+ concent ration are believed to contribute to the decrease in force during fati gue, the force of unfatigued muscle decreases only with quite large ch anges in Na+ and K+ concentration. The objective of this study was to determine whether concomitant and smaller changes in Na+ and K+ concen tration have greater effects on muscle contractility than individual c hanges. At 3 mM K+, a large decrease in Na+ from 120 to 60 mM had no e ffect on the twitch force, while the tetanic force decreased by 31.2%. At 120 mM Na+, an increase in K+ from 3 to 9 mM potentiated the twitc h force by 41.1%, had no effect on the tetanic force at 7 mM, and decr eased the tetanic force by 40.4% at 9 mM; both the twitch force and te tanic force were completely abolished at 11 mM K+. The potentiation of the twitch force between 3 and 9 mM K+ was less at 60, 80, and 100 mM than at 120 mM Na+. A reduction in Na+ concentration also reduced the K+ concentration at which the twitch force and tetanic force decrease d and were completely abolished. It is shown that the combined effects of Na+ and K+ on the twitch and tetanic contractions were greater tha n the sum of their individual effects. Furthermore, it is proposed tha t neither Na+ nor K+ alone can be considered as an important factor in the decrease in force during fatigue, whereas together they are impor tant for the tetanic contraction, but not for the twitch contraction.