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Skeletal Muscle Fatigue: Lactic Acid or Something Else? Ian Stowe University of Florida, PET 5936 November 22, 2013 Abstract The study of skeletal muscle fatigue during exercise is a complicated endeavor. There are many physiological changes that occur during exercise and it is difficult to isolate specific variables. For many years acidosis has been targeted as the primary source of muscle fatigue; specifically acidosis due to lactic acid. Recent studies, however, challenge this notion and have demonstrated that acidosis at physiological temperatures does not have as large of an impact on muscle fatigue as previously believed. Alternatively, researchers have investigated the role that inorganic phosphate (Pi) and potassium ions (K+) play in skeletal muscle fatigue. The results suggest that each of these factors contribute more greatly to muscle fatigue than does lactic acid. Furthermore, lactic acid has been shown to actually decrease the rate of muscle fatigue by maintaining muscle force-producing capabilities. Introduction In trying to determine whether or not lactic acid causes skeletal muscle fatigue, it is imperative to first define fatigue and discuss the underlying physiological mechanisms. Skeletal muscle fatigue can be defined as the temporary decline in muscle forceproducing capabilities associated with an acute exercise bout. For the purposes of the current study we will not examine persistent fatigue arising from chronic exercise and overtraining. Such a broad definition does not actually tell us much about fatigue or its underlying causes. However, it does provide an excellent segue into detailed studies and experiments aimed at uncovering the phenomenon that is skeletal muscle fatigue. While vague, there are some concepts that can be extrapolated from this definition. For example, we can eliminate disease as being the cause of skeletal muscle fatigue because we know that fatigue is a fleeting condition. An inhibition of force-producing capabilities due to an illness would be a symptom and not the source of skeletal muscle fatigue. To examine fatigue, we must examine the entire skeletal muscle force-producing process from the onset of excitation thru individual cross-bridge contraction. During volitional muscle contraction, the initial urge to move originates in higher brain centers in subcortical and cortical regions. The movement blueprint then undergoes a series of revision processes as it is passed between various brain regions. Eventually, the motor cortex “approves” and passes the final plan down to the spinal cord, which puts on the “finishing touches” and relays the plan to the motor units as an action potential or series of action potentials. An action potential is an electrical signal that travels through a neuron or muscle cell in the form of a wave of depolarization and subsequent repolarization. This impulse is passed on to the next cell via the release of neurotransmitters, which react with protein channels, allow for the rapid movement of ions between inter- and intracellular areas, and incite an action potential in the new cell. When an action potential reaches the myocyte, acetylcholine is released and activates an action potential in the muscle fiber, ending excitation events. Excitation-contraction (E-C) coupling, or the events that bridge the gap between excitation and contraction, commence when the action potential traverses the cell membrane and down T-tubules triggering the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm. The calcium binds to troponin and E-C coupling concludes. Contraction begins when calcium-bound troponin causes tropomyosin to move exposing myosin-binding sites on the actin. When sufficient energy is present, in the form of ATP, myosin heads can continually bind to, pull, and release from actin. This is referred to as cross-bridge cycling and it causes the actin protein complex to slide past myosin contributing to the subsequent shortening of the sarcomere and, in turn, the entire muscle; a contraction. When neural activation ceases, calcium is taken back up by the sarcoplasmic reticulum via calcium-handling proteins such as parvalbumin and calsequestrin, tropomyosin shifts back to its original position and contraction is terminated. Acidosis The manifestation of any physiological condition that interrupts or retards any part of the excitation-contraction cascade over the course of an exercise bout contributes to muscle fatigue. Muscle acidosis has long been considered the major source of skeletal muscle fatigue during exercise.2,7 An acid is a molecule that when dissolved in water releases hydrogen ions (H+). In theory, acids can cause damage to myofibrillar proteins as well as metabolic enzymes resulting in fatigue due to inhibited energy production and decreased functioning of contractile machinery. Lactic acid, specifically, is regarded as the primary culprit. Lactic acid dissociates quickly in the blood effectively forming hydrogen ions and lactate. So, when we question lactate’s contribution to fatigue we are actually referring to its formation via lactic acid dissociation and not lactate itself. The exponential accumulation of lactate that begins at the lactate threshold is quite noticeable. Because lactate accumulation is a major symptom of the lactate threshold and the lactate threshold is associated with rapid muscle fatigue, a correlational effect between acidosis and fatigue is evident.3,15 This has led many people to believe that lactic acid is the source of the large majority of the physiological impediments that occur at this flexion point. However, other molecules also tend to accumulate during exercise and may actually contribute to muscle fatigue to a greater extent than lactic acid. Inorganic phosphate (Pi) and extracellular potassium ions (K+) are just two examples of molecules that may in fact have such an effect. Pi appears to disrupt cross-bridge cycling and calcium sensitivity while K+ affects potassium-sodium dependent activation processes.8,11,24 Many studies on lactic acid acidosis and muscle fatigue have been performed using in vitro skinned muscle fiber testing. In these studies, experimenters measured force production and maximal shortening velocity as indicators of muscle fatigue. This form of testing has shown to be highly temperature-dependent and has skewed our understanding for years. At lower temperatures, artificially induced acidification caused a greater decline in force production than at higher temperatures. One study demonstrated significantly diminished force production in response to acidification at 10˚c and yet another study demonstrated similar findings at 12˚c.17,23 The same experimenters demonstrated a 20% decrease in muscle maximum shortening velocity at 12˚c.23 One may be inclined to quickly attribute muscle fatigue to lactate acidification based on this data. However, there is a distinctly important difference between that which is statistically significant and that which is physiologically significant. As we know, 10-12˚c is far from being physiologically normal. As temperatures approach 37˚c (body temperature) these adverse effects are abated. Acidification at 32˚c, for example, caused a force production reduction of less than 10% while not significantly reducing maximal shortening velocity.23 Various experiments conducted at near physiologic temperatures that decreased normal pH by .4.5 units exhibited extremely similar findings, suggesting that acidosis and, by extension, lactic acid is not the primary source of skeletal muscle fatigue in the working muscle.13,17 Furthermore, some studies have even indicated an association between lactic acid accumulation and improved force production.18 Inorganic Phosphate (Pi) Recent studies have demonstrated that some of the effects once attributed to lactic acid can actually be ascribed to Pi.6,10,24 Like lactate, Pi levels are positively associated with exercise intensity. Pi is formed when ATP-stimulated myosin heads release ADP and Pi after ATP is hydrolyzed by myosin ATPase. Pi is also formed as a direct product of the phosphagen, or ATP-phosphocreatine (PC), system. We know that when Pi is present in muscle fibers that myosin heads and actin form a relatively weak bond. Only after Pi is removed is a strong bond established. Theoretically, increased levels of Pi inhibit cross-bridge force production in this manner. Unfortunately, it has proven very difficult to isolate the effects of increased Pi on force production because the introduction of Pi alters various aspects of metabolic functioning. However, one study measured the effects of Pi by reducing, as opposed to infusing, Pi into muscle cells and was successful at increasing cross-bridge force-producing capabilities.19 This indicates a direct relationship between increased Pi and skeletal muscle fatigue. Needless to say, the data is equivocal and further research in this area would be hugely beneficial. Two ubiquitous consequences of augmented Pi quantities include decreased myofilament calcium-sensitivity and inhibition of calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR).10,14,24 One study in particular demonstrated the deleterious reactive effects of Pi on calcium. In this study, skinned rat muscle fibers were mixed in solutions of varying quantities of Pi and various calcium-related processes were adversely affected.10 A decrease in calcium sensitivity was observed as indicated by a significant decrease in maximum fiber tension of both fast and slow-twitch muscle fiber types. Phosphate’s adverse effects on muscle tension were demonstrated in other studies as well.5 A decrease is cross-bridge force production was also observed. This study also investigated the mechanisms responsible for inhibited calcium release from the SR. It has been suggested that Pi leaks into the SR via ATP-sensitive anion channels.1 Once it enters, Pi reacts with calcium stores and forms CaPi and eventually undergoes precipitation forming CaPi precipitate.12 Precipitation refers to the process of a substance becoming separated from a solution following the introduction of a reagent and subsequent reaction. The term precipitation is derived from the visual resemblance of the chemical reaction to rain. When a reagent is added to a solution and precipitation occurs the separated substance appears to “rain” down through the solution to the bottom of the container. Calcium ions in the form of CaPi precipitate are not functional for the purposes of activating muscle contraction via troponin binding. In addition, this reaction reduces free Ca2+ availability in the SR thus inhibiting calcium release and, subsequently, calcium-dependent muscle contraction. To combat this decrease in calcium availability, the SR undergoes prolonged Ca2+ uptake, which runs counter to the calcium release that is necessary for muscle contraction.21 Therefore, this calcium “loading” by the SR is associated with skeletal muscle fatigue.10 Potassium Ions (K+) The final possible mechanism of muscle fatigue that we will examine is the exercise-induced rise in extracellular potassium ions (K+). The increase in extracellular K+ observed during exercise parallels increases in exercise intensity.20 As we know, K+, sodium ions (Na+), and other various ions play a pivotal role in neural signaling and communication. As mentioned previously, neural communication is achieved thru a series of action potentials. The rapid depolarization and polarization that compose an action potential are the products of the movement of the aforementioned ions across the plasma membrane via specific ion channels. When excess K+ ions exit the cell, the resting membrane potential is altered such that the cell becomes hyperpolarized. In other words, the cell shifts to a state that is further from threshold thereby reducing the speed of neural activation.4,16 This in turn retards muscle contraction and contributes to fatigue over time. Amazingly, studies have demonstrated that lactic acid can actually enhance muscle contractility previously inhibited by extracellular K+. In one particular study, experimenters first successfully demonstrated the stifling effects of extracellular K+ on in vitro slow and fast-twitch rat muscles.11 They demonstrated, using a force transducer, that increased extracellular K+ does in fact contribute to decreased force production. When lactic acid was introduced, force production improved significantly. For example, in a rat soleus muscle exhibiting a 25% decrease in force production due to K+ exposure, the addition of lactic acid almost entirely restored tetanic force.11 The mechanism for such an occurrence is not entirely certain. In the working muscle, lactic acid is formed when pyruvate accepts two hydrogen ions but lactic acid dissociation involves the release of just a single H+ to form lactate, which actually produces a net removal of harmful ions from the cell and blood. Theoretically, the formation of lactate can act as an acid buffer by preventing a rapid decline in pH.22 Here we see that lactic acid formation is not the cause of muscle fatigue but is actually critical in retarding the fatigue process. Conclusion Lactic acid, and by extension lactate, is not the physiological miscreant as once imagined. It appears as if early studies and popular belief have accused lactic acid of causing muscle fatigue because of the very strong correlation between increased lactate accumulation and muscle fatigue. This has given rise to the term “lactate threshold”, or the exercise intensity at which lactate begins to accumulate and muscle fatigue begins to manifest. However, many physiological changes occur at this flexion point and it is difficult to determine isolated effects. More and more studies are emerging that relieve lactate of its poor reputation, suggesting that at physiological temperatures acidosis does not play a large role in muscle fatigue. Furthermore, research has uncovered other suspects such as Pi and extracellular K+. The data suggests that these chemicals actually contribute greatly to skeletal muscle fatigue. 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