Effects of Elevated Potassium and Mild Acidification on Skeletal Muscle Contractility and Fatigue in Xenopus Laevis

Abstract

Skeletal muscle fatigue is typically thought to involve accumulated lactic acid, increased Pi concentration, decreased muscle glycogen, and/or increased levels of oxygen free radicals, but a clear cause of muscle fatigue has remained elusive. Additionally, it is unclear if these factors affect type I (slow oxidative) and type II (A: fast oxidative, B: fast glycolytic) fibers equally. Recent evidence suggests that increasing extracellular K+ concentration reduces the contractile function of isolated rat soleus muscle, while moderately elevated H+ levels provide a protective effect on contractile force. To study the combined effects of mild acidification with elevated K+ on muscle contraction and fatigue, we stimulated isolated Xenopus laevis gastrocnemius muscles to complete exhaustion following 30 minutes of pre-incubation with one of the following solutions: normal amphibian Ringer’s (pH = 7.45), amphibian Ringer’s with elevated (4mM) K+, amphibian Ringer’s with decreased pH (20mM L-lactic acid), and amphibian Ringer’s with both elevated K+ and H+. Following pre-incubation in their respective solutions, muscles were mounted on a force transducer and stimulated with supramaximal stimuli at 60 Hz until completely fatigued. Force of contraction, muscle kinetics (latency, contraction time, and ½-relaxation time), and time to exhaustion were measured.