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