From the preceding discussion, it is clear that a rapid accumulation of hydrogen ions during intense exercise can negatively influence muscular performance. Therefore, it is important that the body has control systems capable of regulating acid-base status to prevent drastic decreases or increases in pH. One of the most important means of regulating hydrogen ion concentrations in body fluids is by the aid of buffers. A buffer resists pH change by removing hydrogen ions when the hydrogen ion concentration increases, and releasing hydrogen ions when the hydrogen ion concentration falls.
Buffers often consist of a weak acid and its associated base (called a conjugate base). The ability of individual buffers to resist pH change is dependent upon two factors. First, individual buffers differ in their intrinsic physiochemical ability to act as buffers. Simply stated, some buffers are better than others. A second factor influencing buffering capacity is the concentration of the buffer present (22, 39). The greater the concentration of a particular buffer, the more effective the buffer can be in preventing pH change.
The first line of defense in protecting against exercise-induced decreases in pH resides within the muscle fiber itself. Indeed, muscle fibers can protect against the accumulation of hydrogen ions and a decrease in cellular pH in two different ways. First, muscle fibers contain numerous classes of chemical buffers that can eliminate hydrogen ions. Second, the muscle fiber membrane (sarcolemma) contains two major types of hydrogen ion transporters that carry hydrogen ions from inside the muscle fiber into the interstitial space. Let’s discuss each of these buffering systems in more detail.
Four major classes of intracellular chemical buffer systems exist in the cytosol of muscle fibers: (1) bicarbonate, (2) phosphates, (3) cellular proteins, and (4) histidine-dipeptides (primarily carnosine) (1, 25, 39). The presence of bicarbonate in skeletal muscle fibers is a useful buffer during exercise (4, 16). Further, several phosphate-containing compounds also serve as intracellular buffers in skeletal muscle fibers, and phosphate buffers are of particular importance at the beginning of exercise (22). Numerous cellular proteins contain the amino acid histidine, which possesses an ionizable group that can accept (i.e., buffer) hydrogen ions. This combination of a hydrogen ion with this cellular protein results in the formation of a weak acid, which protects against a decrease in cellular pH. Finally, muscle fibers also contain several histidine-dipeptides (dipeptides are two linked amino acids) that are capable of buffering hydrogen ions. One of the major histidine-dipeptides found in skeletal muscle is carnosine, and growing evidence indicates that carnosine is an important buffer in muscle fibers. Collectively, these four intracellular chemical buffer systems work as unit to provide protection against decreases in muscle pH during intense exercise. A summary of these buffer systems is provided in Table 11.2.
Chemical Acid-Base Buffer Systems
||Download (.pdf) TABLE 11.2
Chemical Acid-Base Buffer Systems
Converts strong acid into weak acid
Converts strong acid into weak acid
Proteins containing histidine groups
Primary component is carnosine
Muscle pH homeostasis is also regulated by the transport of hydrogen ions from muscle fibers into the interstitial space; these hydrogen ions are then buffered by extracellular fluid and blood buffer systems. Two primary transporters that move hydrogen ions across the sarcolemma are the sodium-hydrogen exchanger (NHE) and monocarboxlate transporters (MCTs). The NHE moves sodium ions into the muscle and hydrogen ions out of the muscle into the interstitial space (Fig. 11.4). Specifically, this transporter moves one hydrogen ion out the cell in exchange for one sodium ion. The second hydrogen ion transporter is the MCT. Human skeletal muscles contain two different MCTs that are labeled as MCT1 and MCT4. Both of these molecules mediate a one-to-one co-transport of lactate and hydrogen ions out of the muscle fiber. In other words, these MCTs carry one lactate molecule and one hydrogen ion across the sarcolemma. Research reveals that these transporters are important in regulating muscle pH during high-intensity exercise.
Illustration of the two important hydrogen ion (H+) transporters in skeletal muscle fibers. The sodium-hydrogen exchanger (NHE) moves one sodium (Na+) molecule into the fiber in exchange for transporting one H+ outward. The monocarboxlate transporters (MCTs) co-transport one lactate molecule and one H+ out of the muscle fiber.
Influence of Muscle Fiber Type and Exercise Training on Intracellular Buffer Capacity
As discussed in the previous section, muscle fibers can protect against the accumulation of hydrogen ions and a decrease in cellular pH in two different ways: (1) intracellular buffers and (2) hydrogen ion transporters that move hydrogen ions from inside the muscle fiber into the interstitial space. Studies reveal that, compared to slow (i.e., type I) muscle fibers, the intracellular buffering capacity is higher in fast (i.e., type II) muscle fibers (1). Obviously, this higher buffer capacity in fast fibers is advantageous for performance during high-intensity exercise because fast muscle fibers produce high levels of both lactate and hydrogen ions during intense exercise.
Several studies show that high-intensity exercise training improves muscle buffer capacity in both untrained and trained individuals (8, 15). The precise mechanism(s) to explain exercise training-induced improvements in muscle buffer capacity remains a topic of debate. Nonetheless, training has been shown to increase the intracellular content of both carnosine and hydrogen ion transporters (i.e., MCT) in muscle fibers (8, 15). Therefore, it seems likely that a training-induced improvement in muscle buffer capacity is due to increases in both carnosine and hydrogen ion transporters in skeletal muscle.
The blood contains three principal buffer systems (4, 12, 16, 25): (1) proteins, (2) hemoglobin, and (3) bicarbonate. Blood proteins act as buffers in the extracellular compartment. Like intracellular proteins, these blood proteins contain ionizable groups that are weak acids and therefore act as buffers. However, because blood proteins are found in small quantities, their usefulness as buffers during heavy exercise is limited.
In contrast, hemoglobin is a particularly important protein buffer and is a major blood buffer during resting conditions. In fact, hemoglobin has approximately six times the buffering capacity of plasma proteins due to its high concentration (12, 25). Also contributing to the effectiveness of hemoglobin as a buffer is the fact that deoxygenated hemoglobin is a better buffer than oxygenated hemoglobin. As a result, after hemoglobin becomes deoxygenated in the capillaries, it is better able to bind to hydrogen ions formed when CO2 enters the blood from the tissues. Thus, hemoglobin helps to minimize pH changes caused by loading of CO2 into the blood (12).
The bicarbonate buffer system is probably the most important buffer system in the body (4, 25). This fact has been exploited by some investigators who have demonstrated that an increase in blood bicarbonate concentration (ingestion of bicarbonate) results in an improvement in performance in some types of exercise (4, 7, 20, 26) (see The Winning Edge 11.1).
THE WINNING EDGE 11.1
Exercise Physiology Applied to Sports
Nutritional Supplements to Buffer Exercise-Induced Acid-Base Disturbances and Improve Performance
Because intramuscular acidosis is associated with muscle fatigue, numerous studies have explored nutritional supplements to increase buffering capacity in hopes of improving athletic performance during high-intensity exercise. Indeed, it appears that supplements including sodium bicarbonate, sodium citrate, and beta-alanine have the potential to improve buffering capacity and enhance exercise performance during, high-intensity exercise. Let’s discuss these supplement strategies to improve muscle buffering capacity in more detail.
Sodium bicarbonate. Bicarbonate is a buffer that plays an important role in maintaining both extracellular and intracellular pH, despite its inability to freely cross the muscle membrane (i.e., sarcolemma). Although controversy exists (2), many studies conclude that performance during high-intensity exercise is improved when athletes ingest sodium bicarbonate prior to exercise (6, 7, 26, 30–32, 34, 37, 40). Specifically, results from numerous studies reveal that boosting the blood-buffering capacity by ingestion of sodium bicarbonate increases time to exhaustion during high-intensity exercise (e.g., 80% to 120% max). For example, a recent survey of the scientific literature reveals that sodium bicarbonate is effective in improving a 60-second “all out” exercise bout by approximately 2% (5). Further, laboratory studies employing repeated bouts of high-intensity exercise (i.e., >100% max) have reported that ingestion of sodium bicarbonate prior to exercise can enhance performance by more than 8% (18). In addition to these laboratory studies, evidence exists that sodium bicarbonate is also beneficial to sport performance in activities where the metabolic demands are primarily anaerobic, such as judo, swimming, and water polo (18).
It appears that sodium bicarbonate improves physical performance by increasing the extracellular buffering capacity, which, in turn, increases the transport of hydrogen ions out of the muscle fibers (38). This would reduce the interference of hydrogen ions on muscle ATP production and/or the contractile process itself.
In deciding whether to use sodium bicarbonate prior to a sporting event, an athlete should understand the risks associated with this decision. Ingestion of sodium bicarbonate in the doses required to improve blood-buffering capacity can cause gastrointestinal problems, including diarrhea and vomiting (7, 37).
Sodium Citrate. Similar to sodium bicarbonate, sodium citrate is another agent capable of increasing extracellular buffering capacity (18). The question of whether ingestion of sodium citrate can improve exercise performance during high-intensity exercise remains controversial because experimental results are often inconsistent. Nonetheless, a review of the research literature suggests that although low doses of sodium citrate does not improve performance, ingestion of high doses of sodium citrate (i.e., >0.5 grams/kilogram body weight) improves performance during high-intensity cycling exercise lasting 120 to 240 seconds (18).
Unfortunately, similar to sodium bicarbonate, ingestion of high doses of sodium citrate can produce undesired side effects such as nausea, gastrointestinal discomfort, and headaches. Therefore, before deciding whether to use sodium citrate prior to competition, athletes should consider the negative side effects associated with the use of sodium citrate.
Beta-alanine. Recent evidence suggests that supplementation with beta-alanine can play a beneficial role in protecting against exercise-induced acidosis and improve performance during short, high-intensity exercise (39). Beta-alanine is a non-essential amino acid produced in the liver, gut, and kidney. However, fasting blood levels of beta-alanine are low indicating that endogenous synthesis of this amino is limited.
The link between beta-alanine and protection against acidosis is linked to the fact that beta-alanine is an important precursor for the synthesis of carnosine in skeletal muscle. As discussed in the text, carnosine is a small molecule (dipeptide) found in the cytoplasm of excitable cells (i.e., neurons, skeletal and cardiac muscle fibers) (18). Carnosine has several important physiological functions including the ability to buffer hydrogen ions and protect against exercise-induced decreases in cellular pH (18).
The availability of beta-alanine is the rate limiting factor for carnosine synthesis in muscle fibers. However, supplementation (2 to 3 grams/day) with beta-alanine for >2 weeks results in a 60% to 80% increase in muscle carnosine levels. Importantly, this increase in muscle carnosine levels is associated with a 3% to 5% increase in muscle buffering capacity (18). Theoretically, this increase in intracellular buffering capacity could translate into improvements in performance during high-intensity exercise. In this regard, growing evidence suggests that beta-alanine supplementation improves high-intensity exercise performance in both running and cycling events lasting 1 to 4 minutes (18). Interestingly, some of these studies have recorded performance improvements of 12% to 14% (18).
The only known side effect of beta-alanine supplementation is paraesthesia (tingling of the skin); this sensation begins within 20 minutes after ingestion and lasts up to 60 minutes (18). Although harmless, paraesthesia is unpleasant and, several investigations have reported that paraesthesia can be avoided by staggering dosing throughout the day (18).
Final words of caution on use of “supplement buffers” to improve exercise performance. Regardless of the type of buffer ingested, extremely large doses of any buffer can result in severe alkalosis and pose negative health consequences. Another important consideration in the use of any ergogenic aid is the legality of the drug. In regard to the use of acid-base buffers, some sports regulatory agencies have banned the use of sodium buffers during competition. See Sahlin (2014) and Junior et al. (2015) in Suggested Readings for detailed information about the possible ergogenic effects of sodium bicarbonate, sodium citrate, and beta-alanine.
The bicarbonate buffer system involves carbonic acid (H2CO3), which undergoes the following dissociation reaction to form bicarbonate ():
The ability of bicarbonate and carbonic acid (H2CO3) to act as a buffer system is described mathematically by a relationship known as the Henderson-Hasselbalch equation:
where pKa is the dissociation constant for H2CO3 and has a constant value of 6.1. In short, the Henderson-Hasselbalch equation states that the pH of a weak acid solution is determined by the ratio of the concentration of base (i.e., bicarbonate, ) in solution to the concentration of acid (i.e., carbonic acid). The normal pH of arterial blood is 7.4, and the ratio of bicarbonate to carbonic acid is 20 to 1.
Let’s consider an example using the Henderson-Hasselbalch equation to calculate arterial blood pH. Normally the concentration of blood bicarbonate is 24 mEq/l and the concentration of carbonic acid is 1.2 mEq/l. Note that mEq/l is an abbreviation for milli-equivalent per liter, which is a measure of concentration. Therefore, the blood pH can be calculated as follows:
Although the traditional view of acid-base chemistry has considered the levels of bicarbonate and hydrogen ions as two primary determinants of pH, new ideas were advanced in the early 1980s that exposed the complexity of regulating pH balance in the body. These new concepts were launched by Peter Stewart and are introduced in A Look Back—Important People in Science.
A LOOK BACK—IMPORTANT PEOPLE IN SCIENCE
Peter Stewart Challenged Acid-Base Research by Proposing the Concept of “Strong Ion Difference”
Peter Stewart (1921–1993) was born and raised in Winnipeg, Canada. He earned both his Master’s and Ph.D. degrees from the University of Minnesota. During a long and productive research career, Dr. Stewart served on the faculty of the University of Illinois, Emory University, and Brown University.
In the 1970s, Dr. Stewart developed a strong interest in acid-base regulation and started to mathematically analyze the variables involved in controlling the pH of body fluids. In 1981, he challenged the traditional concepts of acid-base control by publishing his book titled How to Understand Acid-Base Balance—A Quantitative Acid-Base Primer for Biology and Medicine. This book quickly stirred both controversy and debate among researchers in acid-base balance. A brief synopsis of this controversy follows. Historically, it has been believed that the balance between the levels of hydrogen ions and bicarbonate ions determines the pH of body fluids. Dr. Stewart challenged this concept and argued that hydrogen ions and bicarbonate ions are not the independent variables that control acid-base. Instead, he suggested that these variables are dependent variables that are regulated by other factors, including the strong ion difference,* carbon dioxide levels, and the amount of weak and nonvolatile acids in body fluids. So, does this mean that calculating pH using the Henderson-Hasselbalch equation is of no value? The short answer to this question is no. However, Dr. Stewart’s work demonstrates that the factors that regulate acid-base balance in the body are probably more complex than originally believed.
Although some physiologists have accepted the science supporting Dr. Stewart’s proposal of how acid-base balance is maintained, criticism of his concept of strong ion difference remains. A concern with the strong ion balance concept of acid-base control is the complexity of the chemistry and mathematics behind this model. Moreover, in practice, it is often difficult to measure all the variables required to calculate pH using Dr. Stewart’s strong ion difference method. Therefore, in the foreseeable future, the traditional approach to acid-base balance seems certain to prevail.
The body maintains acid-base homeostasis by buffer-control systems. A buffer resists pH change by removing hydrogen ions when the pH declines and by releasing hydrogen ions when the pH increases.
The principal intracellular buffers are proteins, phosphates, bicarbonates, and histidine-dipeptides (i.e., carnosine).
Muscle fibers possess two important hydrogen ion transporters that carry hydrogen ions from inside the muscle fiber into the interstitial space: (1) sodium/hydrogen exchanger and (2) monocarboxlate transporters.
Primary extracellular buffers include bicarbonates, hemoglobin, and blood proteins.