Implications for Muscle Fatigue; Effects of Decreased pH on Muscle Activity

M. Dettmer
Marquette University
Mentor: Dr. Robert Fitts

Muscle fatigue can be defined as the inability to maintain a maximum power output.  Power is calculated from the values of muscle force and shortening velocity.  It has been well documented that at 15°, low pH is implicated in the process of muscle fatigue.  Low pH causes a significant decrease in force under maximal activation (pCa=4.5), probably through direct effect on cross-bridge kinetics.  However, some studies conducted at 30° indicate that pH may not have a similar effect on muscle activity under physiological conditions. 

Bundles of muscle fibers were isolated from rat muscles (slow - soleus, fast - gastrocnemius) and stored in skinning solution, which permeabilizes the surface membrane and then exposed to Brig solution.  Individual fibers (2 mm in length) were extracted from the bundles, placed in a stainless steel chamber containing relaxing solution (pCa 9.0, pH 7.0) and suspended between a movable lever arm and a force transducer.  Sarcomere length was set at 2.5 ?m.  Fiber velocity was determined via the slack test, in which a maximally activated fiber is shortened a prescribed distance, and the time necessary for the fiber to redevelop tension is recorded.  The slack procedure is repeated at 4 or 5 slack distances and the slack distance is plotted versus the time required to take up the slack.  Fiber velocity is determined from the slope of the line.  Fibers were maximally activated and then force clamped at various (12-15) submaximal loads and fiber shortening and velocity of shortening measured.  The percent of peak force was plotted against velocity to generate the Force-Velocity curve.  Fibers were activated and allowed to develop steady tension at six to eight different pCa values. pCa values were selected to obtain 3 to 4 values above and below pCa50 (pCa producing one half of peak force).  The data were plotted pCa versus log(Pr/1-Pr) as a Hill plot analysis where Pr = P/Po and P = force per cross sectional area and Po is peak force per cross sectional area at pCa = 4.5.  The data was also plotted as pCa vs. percentage of peak force.  Following functional measurements, all fibers were typed as type I or fast type II by gel electrophoresis analysis of myosin heavy chain. 

The focus of the experiments was the pCa-force relationship in the slow type I fiber.  Under decreased pH, the pCa-force curve shifts to the right, indicating that more Ca2+ is required to activate a fiber to given percentage of peak tension.  Cell Ca2+ required as activation threshold and half maximal activation levels are both significantly increased (i.e. lower pCa) under pH 6.2 conditions (Table 1).

This effect, when considered in conjunction with the decrease of peak force, shows that low pH may play a more significant role in muscle fatigue than previously thought.  Future studies must be conducted in order to demonstrate these effects at 30°, in order to prove that low pH is in fact a significant contributor to muscle fatigue under physiological conditions. 
 

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