Elevated Potassium and Possible Protective Effects of Mild Acidification on Muscle Contractility in X. laevis Gastrocnemius Muscles Luke Jensen, Debora Christensen (Mentor) Dept of Biology, Drake University INTRODUCTION  Muscle activity beyond the aerobic threshold causes an accumulation of lactic acid and a loss of intracellular K + to the extracellular space.  Increasing concentration of extracellular K + reduces contractile function in isolated rat soleus muscle.  Intracellular acidification reduces maximal force and velocity of isolated skeletal muscle cells.  Moderately elevated extracellular H + provides a protective effect on contractile force when paired with increased extracellular K + in rat soleus muscles. METHODS 4 test groups were evaluated to determine the effects of extracellular K + and pH on muscle fatigue (n=3/treatment). Solutions used were: normal Amphibian Ringer’s solution (pH 7.45) Amphibian Ringer’s solution with 10 mM K + (pH 7.45) Amphibian Ringer’s solution with lactic acid (pH 6.85) Amphibian Ringer’s solution with 10 mM K + and lactic acid (pH 6.85) Amphibian Ringer’s solution consists of 111 mM NaCl, 3.35 mM KCl, 2.7 mM CaCl 2, 2.38 mM NaHCO 3, 5.5 mM D -glucose. Gastrocnemius muscles isolated with their sciatic nerve from pithed frogs were incubated at room temperature for 30 minutes in their respective solutions (Figure 1). Gastocnemius muscles were mounted on a force transducer and stimulated through the sciatic nerve at a supramaximal stimulus and a frequency of 1 Hz with an ADInstruments PowerLab. Each muscle was continuously bathed with their respective solution to prevent the muscle and nerve from drying out (Figure 2). RESULTS STUDY OBJECTIVES Our objective in this study was to determine the separate and combined effects of lowered pH and increased K + on: Time to complete muscular fatigue in frog (Xenopus laevis) gastocnemius muscle Force of muscle contraction DISCUSSION The time to exhaustion showed large differences between treatment groups, but this was not statistically significant. The lack of significance is likely due to insufficient sample size. Acidosis has been implicated in inhibiting actin-myosin interactions and increased K + decreases the excitability of membrane of muscle fibers during muscle fatigue. Overgaard, Hojfeldt and Nielsen (2010) found mild acidification to have a protective effect on dynamic muscle contractions on rat soleus muscles in the presence of elevated K +. The results of this study do not show the same protective effect to be true for time to total muscular fatigue in frog gastrocnemius muscles. There was no significant effect in the reduction of force of contraction at any time point when compared to normal Amphibian Ringer’s solution. Future studies should increase sample size to see if the differences in time to exhaustion between treatment groups is an artifact of small sample size. ACKNOWLEDGEMENTS I would like to thank Dr. Debora Christensen for her guidance and long hours of work on this project as well as Dr. Charisse Buising for providing the IV drip equipment. Figure 2. Muscles were mounted on an MLT-500 force transducer. The sciatic nerve was placed over the stimulator and an IV drip bathed the tissues. Figure 1. Muscles were preincubated for 30 minutes in their respective solutions prior to testing RESULTS (CONT’D) Figure 3. Average hours to exhaustion for each treatment group (mean + std dev). Lactic acid, elevated K +, and lactic acid with elevated K + decreased the time to fatigue compared to normal Amphibian Ringer’s by 42.3%, 67.8%, and 83.5%, respectively. Despite this, none of the treatment groups showed a statistically significant difference from the control group (all p-values > 0.05). Figure 4. Average force of contraction for the first 10 minutes of testing for each treatment group. The general trends are the same for all groups and there were no statistically significant deviations from the normal control (all p-values > 0.05).