Respiratory muscles are skeletal muscles charged with the task of expanding and compressing the chest wall; the primary goal of these actions is to move gas in and out of the lungs to maintain arterial blood gas and pH homeostasis. At rest or during low intensity exercise, the work of breathing in healthy individuals is relatively small and respiratory muscles have no difficulty in maintaining this level of power output. However, prolonged high-intensity exercise (e.g., > 85% o2max) presents a significant challenge to respiratory muscle endurance. Under these circumstances, respiratory muscle fatigue can occur and this fatigue could result in dyspnea and contribute to impaired exercise tolerance (5). Furthermore, patients with chronic obstructive lung disease often exhibit inspiratory muscle weakness and/or reduced inspiratory muscle endurance. This is an important clinical issue because these patients are predisposed to pulmonary limitations during routine activities of daily living.
Because of the potential for respiratory muscle fatigue in both health and disease, interest in the plasticity of respiratory muscles in response to increased respiratory loads has increased during the past decade. Much of this research has focused upon the effects of regular exercise on the primary muscle of inspiration, the diaphragm. This review will highlight the cellular changes that occur in diaphragmatic bioenergetic and antioxidant capacity in response to endurance exercise training. We will limit our discussion to studies using a rat model because the vast majority of studies investigating diaphragmatic bioenergetic and antioxidant properties have used this animal model.
OVERVIEW OF DIAPHRAGMATIC STRUCTURE AND FUNCTION
The mammalian diaphragm is a unique skeletal muscle that is considered to be two muscles in one, based upon the anatomical and functional differences between the costal and crural regions (2) (Fig. 1 a). Compared with locomotor skeletal muscles, the diaphragm is distinctive because it is chronically active and maintains a relatively high work-to-rest ratio (i.e., duty cycle = ∼40%). Anatomically, the diaphragm is unique because it possesses a large central tendon connecting the costal and crural regions of the muscle. The fibers of the costal portion of the human diaphragm insert on the xiphoid process of the sternum and the upper margins of the lower six ribs. In contrast, the crural region of the diaphragm inserts on the anterior and lateral bodies of three vertebrae.
From a functional perspective, stimulation of the diaphragm results in a descent of the diaphragmatic dome and an expansion of the rib cage. The inspiratory action of the diaphragm on the rib cage has three separate components. The first action is due to the descent of the diaphragm dome, resulting in an increase in abdominal pressure; this rise in abdominal pressure is transmitted to the ribs and results in an expansion of the lower rib cage (Fig. 1 b). Secondly, diaphragmatic contraction reduces pleural pressure, which acts to pull the rib cage upward. The third component of the inspiratory action of the diaphragm is directly related to the attachment of costal diaphragm fibers to the lower ribs. In this case, contraction of the costal diaphragm results in a cranially oriented force that serves to lift the lower ribs with an outward rotation (Fig. 1 c). Because the crural diaphragm does not insert on the rib cage, this insertional component of diaphragmatic action on the rib cage is due to the costal diaphragm only. Hence, when selectively stimulated, the crural diaphragm has less inspiratory effect on the lower rib cage than does the costal portion (2).
EXERCISE-INDUCED CHANGES IN THE DIAPHRAGM
In the following sections, we discuss the current knowledge regarding exercise-induced changes in diaphragmatic bioenergetic and antioxidant capacity. Our primary focus will be on exercise-induced changes in diaphragmatic antioxidants (i.e., redox buffers). We begin with a brief overview of training-induced changes in muscle fiber bioenergetic capacity.
Exercise-Induced Changes in Diaphragmatic Bioenergetic Capacity
Before discussing training-induced changes in the diaphragm, it is noteworthy that the diaphragm is chronically active and is among the most aerobically adapted skeletal muscles. Indeed, diaphragmatic oxidative capacity and capillary density exceed the values measured in antigravity limb muscles and approach the values measured in the myocardium of most species. Hence, even in relatively inactive (i.e., untrained) mammals, the diaphragm has a relatively high oxidative capacity.
During the past 20 yrs, numerous studies have investigated the effects of endurance exercise training on diaphragmatic bioenergetic capacity. These studies indicate that endurance exercise training does not alter diaphragmatic glycolytic capacity. In contrast, exercise training promotes an increase in the oxidative capacity of Type I and IIa fibers (6). However, unlike locomotor skeletal muscles, the magnitude of the training-induced increase in oxidative capacity in the costal diaphragm does not follow the typical dose-response pattern of adaptation. For example, although high-intensity exercise is superior to low-intensity exercise in elevating oxidative capacity in the costal diaphragm after 12 wk of training (30 min·d−1), the dose-response effect of exercise on the costal diaphragm is eliminated when the daily duration of exercise is increased beyond 30 min·d−1 (Fig. 2). Indeed, in the rat, the maximal training-induced changes in costal diaphragm oxidative capacity can be achieved during low intensity exercise if the exercise duration is 60 min·d−1 or more (7).
Why does the costal diaphragm differ from locomotor skeletal muscles in the adaptation to exercise at varying exercise intensities and durations? The most plausible explanation is that although the recruitment of costal diaphragm fibers increases when going from rest to low-intensity exercise, this relationship is not maintained when an animal proceeds from low-to-high intensities of exercise. A more detailed explanation for this possibility is as follows. In achieving the ventilatory needs during exercise, the nervous system can select from a large repertoire of respiratory muscle motor units that vary widely in their ability to resist fatigue and generate force (11). In this regard, although Type IIb fibers can generate a higher power output than Type I or IIa fibers, Type IIb fibers, when activated for extended periods of time, fatigue more rapidly than Type I or IIa fibers. Hence, because of their propensity toward fatigue, Type IIb fibers are not suitable for long-term ventilatory tasks and therefore it seems plausible that these fibers are only recruited during short-duration nonventilatory behaviors (e.g., expulsive behaviors such as coughing). Importantly, Sieck and Fournier (11) have provided direct evidence to support this concept and have demonstrated that Type IIb fibers are not recruited during normal ventilatory behaviors. Failure to recruit Type IIb fibers in the costal diaphragm during exercise is logical and may serve as a protective device to prevent fatigue in this important inspiratory muscle.
If the costal diaphragm reaches a plateau in motor unit recruitment before meeting the ventilatory demands during intense exercise, further increases in ventilation can only be achieved by recruiting other inspiratory muscles. Research from our laboratory suggests that both the parasternal intercostals and the crural diaphragm are recruited to maintain high ventilatory demands (7).
As discussed previously, the costal and crural diaphragm differ in their action on the chest wall. Current evidence indicates that these distinct regions of the diaphragm also differ in their response to exercise training (7). Indeed, in contrast to the costal diaphragm, it appears that only moderate- and high-intensity exercise of long duration (i.e., 90 min·d−1) results in an elevation in crural diaphragm oxidative capacity (Fig. 3). This observation indicates that high-intensity exercise is required to recruit a large number of motor units within the crural diaphragm. The fact that high-intensity exercise is required to recruit motor units in the crural diaphragm is consistent with the notion that at some moderate intensity of exercise, a plateau in motor unit recruitment is reached in the costal diaphragm and accessory inspiratory muscles are then recruited to assist in the ventilatory effort. Again, the physiological wisdom of this type of recruitment pattern is that this activation scheme does not engage Type IIb fibers within the costal diaphragm during normal ventilatory efforts and therefore serves as a protective strategy against muscle fatigue.
Exercise and Diaphragmatic Antioxidant Capacity
Overview of muscle antioxidants
Muscular exercise results in an increased production of radicals and other forms of reactive oxygen species (ROS) (reviewed in (9)). Although the site of ROS production in muscle continues to be debated, mitochondrial production of ROS is a likely source for many of the exercise-induced ROS. Other potential venues for ROS production during contractile activity include xanthine oxidase and prostanoid pathways (4). Regardless of the source of ROS, production of these reactive species during exercise is significant because ROS can contribute to redox disturbances in skeletal muscle leading to fatigue, oxidative injury, and altered expression of redox-sensitive genes (3,4,10). Therefore, given that exercise can greatly increase ROS production and upset muscle redox balance, it is not surprising that skeletal muscle contains “redox buffers” to minimize exercise-induced oxidative damage. Specifically, two classes of endogenous antioxidants (i.e., enzymatic and nonenzymatic) work as a complex team to decrease the potentially harmful effects of ROS in cells. The principal strategies to prevent ROS-induced oxidative stress include conversion of ROS into less active molecules (i.e., scavenging) and prevention of the transformation of less active ROS into more damaging forms (e.g., prevention of the transformation of hydrogen peroxide (H2O2) into hydroxyl radicals).
Primary antioxidant enzymes in skeletal muscle cells include superoxide dismutase (SOD), glutathione peroxidase (GPX), and catalase (CAT) (3). Each of these enzymes reduces a specific ROS resulting in a less reactive molecule. Table 1 summarizes the cellular location and primary antioxidant function of these enzymes.
Cellular defense against superoxide radicals is provided by two different isoforms of SOD. Both isoforms dismutate superoxide anions with similar efficiency and form H2O2 but differ in cellular location and the metal cofactor bound to the enzyme (3). The Cu-ZnSOD isoform is primarily located in the cytosol whereas the MnSOD isoform is located in the mitochondria (3). The ratio of MnSOD activity to Cu-ZnSOD activity varies across muscle fiber types due to differences in mitochondrial volumes between fiber types. Specifically, fibers with relatively large mitochondrial volumes (i.e., Type I fibers) have high MnSOD/Cu-ZnSOD activity ratios compared with fibers with a relatively low mitochondrial volume (i.e., Type IIb fibers).
GPX is an enzyme that is located both in the cytosol and mitochondria and is responsible for reducing H2O2 and organic hydroperoxides. This enzyme uses glutathione (GSH) as the electron donor and reduces a wide variety of hydroperoxides ranging from H2O2 to complex organic hydroperoxides (3). This characteristic makes GPX an important cellular protector against ROS-mediated damage to lipids and other molecules sensitive to oxidative injury.
CAT is widely distributed in the cell as high concentrations are found in both peroxisomes and mitochondria. As an antioxidant enzyme, CAT is responsible for the breakdown of H2O2 to form water and oxygen. Although there is overlap between the function of CAT and GPX, the two enzymes differ widely in their affinity for H2O2 as a substrate with GPX (Km = 1 micromole) possessing a 1000-fold greater affinity for H2O2 compared with CAT (Km = 1 mmol). Therefore, when cellular levels of H2O2 are low, GPX is more active in removing H2O2 from the cell (3).
In addition to the aforementioned antioxidant enzymes, numerous nonenzymatic antioxidants exist in cells. Important nonenzymatic defenses include vitamin E, vitamin C, GSH, carotenoids, bilirubin, alpha-lipoic acid, and ubiquinone. In reference to exercise training and diaphragmatic antioxidants, the only nonenzymatic antioxidant that has received experimental attention is GSH. A summary of the exercise training-induced changes in diaphragmatic antioxidant capacity follows.
Exercise training-induced increases in diaphragmatic antioxidant capacity
It is clear that long-term (weeks-to-months) exercise training elevates total SOD activity by 10–30% in both the costal and crural diaphragm. This exercise-induced change in total SOD activity is due primarily to elevated MnSOD activity. Although long-term training does not promote an increase in diaphragmatic CAT activity, training does augment GPX activity by 10–30% in both the costal and crural diaphragm (reviewed in (8)). Further, exercise training enhances diaphragmatic levels of GSH by 25–45% above untrained control animals (12) (Fig. 4). Collectively, these changes improve the diaphragm’s ability to scavenge ROS and thus minimize exercise-induced oxidative stress. Specifically, the training-induced increase in diaphragmatic SOD activity should improve the cell’s ability to scavenge superoxide radicals whereas the elevation in GPX activity and GSH concentration could enhance the capacity to remove a variety of ROS including hydroxyl radicals, H2O2, and organic hydroperoxides (Fig. 5).
Although in theory these training-induced changes in diaphragmatic antioxidant capacity protect against exercise-induced oxidative stress, does experimental evidence confirm this postulate? The answer to this question is yes; recent evidence clearly reveals that exercise-induced changes in diaphragmatic antioxidant capacity protect against contraction-induced oxidative injury in the diaphragm (12,13) (Fig. 6). Specifically, compared with untrained controls, endurance exercise training is associated with ∼35% less lipid peroxidation in isolated diaphragms subjected to 60 min of rigorous in vitro contractile activity (12).
Time Course of Exercise-Induced Diaphragmatic Changes
Historically it has been believed that weeks-to-months of exercise training are required to bring about significant, training-induced changes in the diaphragm. However, new evidence reveals that short-term endurance exercise training (i.e., 60 min·d−1 at ∼70% o2max for 5 d) results in significant improvements in both oxidative and antioxidant capacity (13). These new findings reveal that training-induced changes in the diaphragm occur much more rapidly than previously believed. It is interesting that after only 5 d of training, diaphragm oxidative capacity and antioxidant capacity increased by ∼20% which is similar to the magnitude of training-induced increases after 8–10 wk of training (reviewed in (8)).
Does Exercise Training Improve Diaphragmatic Endurance?
Numerous factors are thought to contribute to muscle fatigue during prolonged contractile activity. These include depletion of muscle glycogen, high levels of lactate production, and ROS-mediated oxidation of lipids and proteins involved in excitation-contraction coupling (10). Hence, from a theoretical standpoint, the training-induced improvement in diaphragmatic oxidative and antioxidant capacity should lead to improved muscular endurance. Indeed, it is well known that an increase in muscle oxidative capacity enhances the tissue’s ability to metabolize fat and reduce the rate of both carbohydrate metabolism (i.e., less glycogen depletion) and lactate production (1). Furthermore, an improved antioxidant capacity would reduce contractile-induced oxidation of both lipids and proteins (12). Collectively, these training-induced changes should improve diaphragmatic performance during prolonged contractile activity. Experimental evidence supports this notion as diaphragmatic performance is improved after both short- (5 d) and long-duration (10 wk) exercise training (13,14).
SUMMARY AND CONCLUSIONS
The diaphragm is the primary inspiratory muscle in mammals. Similar to locomotor skeletal muscles, the diaphragm is metabolically plastic and responds to altered activity patterns. Indeed, endurance exercise training results in an increase in both the oxidative and antioxidant capacity in the costal and crural diaphragm. Interestingly, these training-induced changes occur rapidly as marked improvements in the oxidative and antioxidant capacity of the diaphragm can be achieved after as few as 5 consecutive days of exercise. Importantly, these exercise-induced changes are associated with reduced oxidative injury and improvement in diaphragmatic endurance (Fig. 7). Many research questions regarding diaphragmatic biochemical changes during increased and decreased activity remain unanswered. For example, an important area for future research is to investigate the cellular signals responsible for exercise-induced changes in diaphragmatic gene expression (Fig. 7). Further, limited information exists about the mechanisms responsible for the rapid diaphragmatic atrophy and contractile dysfunction associated with unloading the diaphragm during mechanical ventilation. There is clearly much more to be learned about this fascinating and important respiratory muscle.
The authors would like to thank Mary Dunnington for the artistic assistance in preparation of the figures included in this manuscript. This work was supported in part by grants from the American Lung Association-Florida and the National Institutes of Health (Heart, Lung, and Blood Institute).
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