Chronic training also has been shown to increase skeletal muscle stiffness in some models (10). There is a strong positive correlation between skeletal muscle stiffness and collagen content (10), suggesting that the loss of collagen observed in APAP-treated trained rats, in conjunction with loss of cross-links, may have led to a reduction in skeletal muscle stiffness. Such a hypothesis requires further study, including measures of tissue mechanical properties. In addition, work is needed to determine if similar changes in collagen and cross-linking are seen in humans chronically consuming APAP or other COX-inhibiting analgesics. Collectively, these findings demonstrate that APAP, and possibly other analgesic medications, have strong effects on connective tissue ECM, which are not limited to tendon and are tissue specific. Also intriguing is the fact that, in contrast to cross-linking, which seems to be reduced by APAP in skeletal muscle and tendon regardless of exercise, the effect of APAP on skeletal muscle collagen is dependent on exercise training. The tissue-specific effect of APAP on collagen could be caused by differences in tissue concentration of APAP noted above. The lower concentration of drug realized in the tendon may not be sufficient to alter MMP activity or collagen synthesis. A similar logic may explain the lack of change in patellar tendon properties in humans chronically consuming ibuprofen (6). Further investigation of the mechanisms contributing to the effects of exercise and APAP, and other COX inhibitors, on skeletal muscle collagen formation is warranted, especially given the strong impact of collagen on skeletal muscle function.
Few studies have evaluated detailed mechanisms contributing to the effects of various analgesics on tendon or skeletal muscle ECM. Some insight can be gained by considering the established mechanism of action of analgesic medications. As discussed above, the target of traditional NSAID is known to be COX, the rate-limiting enzyme in the production of prostaglandins. Many common NSAID are nonspecific inhibitors of COX-1 and -2, whereas recent work suggests that APAP is a potent and specific COX-2 inhibitor (13). Although the rationale for blocking prostaglandins to reduce pain is sound, COX inhibitors provide a general inhibition of COX and thus the production of any downstream prostaglandin likely is blunted. One of the most studied prostaglandins in tendon and fibroblasts is PGE2. Although the exact role of PGE2 in regulating ECM remodeling is far from clear, tendon production of PGE2 is increased with exercise or cell stretching (1,17) whereas basal and exercise-induced increases in tendon and skeletal muscle PGE2 are blunted with exposure to COX inhibitors (9,17,25). In nontendon models, PGE2 has been shown to modulate the production of matrix-degrading MMP and their inhibitors TIMP. Specifically, PGE2 may downregulate MMP production while enhancing TIMP production ((12) Fig. 2). More importantly, nonselective (ibuprofen) and COX-2-selective (rofecoxib) inhibition leads to excessive production of MMP in tendon cell culture (28) and other models (12). Although additional mechanistic studies are needed, I hypothesize that the effect of APAP and other COX inhibitors on ECM may be mediated by an excessive upregulation of collagen-degrading MMP caused by inhibition of COX and PGE2 production (Fig. 2).
In addition to the possible impact of COX inhibition on MMP, some analgesic medications may blunt collagen synthesis, but limited data are equivocal. In humans, administration of indomethacin during acute bouts of exercise has been shown to reduce markers (procollagen I intact N-terminal propeptide (PINP)) of tendon collagen synthesis (9) and decrease tendon blood flow (17). In contrast, ibuprofen consumption in humans did not alter patellar tendon collagen fractional synthesis rates before or after an acute bout of kicking exercise (21). The lack of an effect of ibuprofen on tendon collagen synthesis is supported by the work of Trappe and colleagues (6) in which ibuprofen did not alter tendon CSA. In addition, chronic APAP consumption in rats did not alter total collagen content in the Achilles tendon (8), thus, it seems that the acute effects of COX inhibition on tendon collagen synthesis may not translate into chronic effects on tendon collagen content. In contrast, as already mentioned, exercise had a strong impact on skeletal muscle collagen formation in rats, an effect that was negated in animals consuming APAP (7). More specific studies evaluating the impact of APAP and other COX inhibitors on collagen synthesis are needed, especially with controls for exercise bout and drug-type used.
To gain further insights into the mechanisms contributing to the effects of the COX-2 inhibitor APAP on tendon, our laboratory has used microdialysis to evaluate the effects of APAP consumption on tendon IL-6 production after exercise. Acute exercise results in a rapid increase in peritendinous IL-6 levels (11). IL-6 can stimulate tendon collagen synthesis (2) and is required for maintaining normal tendon size and function. Interestingly, we found (11) that APAP consumption enhanced the exercise-induced increase in peritendinous IL-6 (Fig. 6). A similar increase in the production of IL-6 also has been noted during administration of COX inhibitors in other models (12). Given the substantial augmentation of IL-6 release by APAP, it would seem likely that APAP enhanced collagen turnover after exercise. In contrast, as mentioned above, administration of a nonselective COX inhibitor (indomethacin) blunted PINP release in tendon (9). Direct comparisons between our work and that of Christensen et al. (9) are difficult because our measurement of IL-6 was completed only a few hours after exercise, whereas PINP in the work of Christensen et al. (9) was not evaluated until 24 h after exercise when IL-6 was likely no longer elevated. Interestingly, similar acute effects of IL-6 in response to COX inhibitor consumption have been noted in human skeletal muscle (18). Although acute COX inhibitor consumption seems to exacerbate the IL-6 response to exercise, chronic APAP consumption limited training-induced increases in skeletal muscle IL-6 mRNA expression (26), suggesting that the enhancement of IL-6 after exercise may be short-lived. Now that IL-6 has been identified as a potential mediator of APAP-induced effects on tendon, further chronic studies are needed to ascertain the long-term impact of COX inhibition on IL-6 levels while clarifying the effect of chronic elevations in IL-6 on tendon structure and function, especially in in vivo models. In addition, several next-generation anti-inflammatory medications, such as tocilizumab and anakinra, have been approved for use in humans. These new compounds are designed to antagonize cytokine receptors such as IL-6 and IL-1β. Given the vital role that cytokines such as IL-6 seem to play in normal ECM remodeling at rest and during exercise training, it seems likely that these receptor antagonists may interfere with normal connective adaptations to exercise training.
There are still several unresolved questions from the body of work described, which have significant clinical implications. First, the detailed mechanisms contributing to the drug-induced changes in tendon and skeletal muscle ECM remain largely unknown. Further defining the mechanisms contributing to the effect of analgesics may lead to new uses for these already important medications. Second, it is not clear why APAP, which was thought to act largely on the central nervous system (rather than peripheral tissues), would have such dramatic effects on collagen and cross-linking. The findings described in this article suggest that clinical practice may benefit from more detailed studies evaluating the extent to which APAP, and possibly other COX-inhibiting analgesics, impacts peripheral tissues. Last, further work is needed to resolve the apparent tissue-specific effects of some analgesics.
Although an analgesic-induced reduction in tissue stiffness could have negative implications for healthy individuals, a reduction in tissue collagen and stiffness could be beneficial in certain patient populations. Collagen cross-linking is elevated substantially in tendons of older adults, and our findings suggest that APAP may reduce cross-linking in tendon and skeletal muscle (6–8), which may be a healthy adaptation in older adults. Tissue stiffness also appears to be elevated in diabetic patients, thus APAP or other COX inhibitors may reduce tissue stiffness in these patients by blunting cross-link formation. Last, skeletal muscle fibrosis is common in several disorders, thus novel application of COX inhibitors to reduce collagen and tissue stiffness should be considered in various patient populations.
In summary, ample evidence suggests that APAP and possibly other COX inhibitors limit exercise-induced increases in skeletal muscle or tendon collagen and collagen cross-linking and reduced tendon stiffness. The clinical impact of analgesics on tendon and skeletal muscle connective tissue is unclear, but large reductions in tendon stiffness and changes in the ratio of collagen fibrils to cross-linking could impair physical function, especially because chronic consumption of these medications facilitates skeletal muscle hypertrophy (23). The use of analgesics during exercise training should be considered in the context of balancing pain reduction with performance adaptations and possible reductions in connective tissue stiffness.
I thank Todd Trappe, Ph.D., who started much of this work in skeletal muscle approximately 15 yr ago. Some of the work highlighted in this review was completed during my postdoctoral work with Dr. Trappe, which was funded by National Institutes of Health R01-AG-020532 to Dr. Trappe and a Post-Doctoral Initiative Award from the American Physiological Society to me. Much of the work highlighted in this review could not have been completed without the generous intramural support from Midwestern University. I would also like to extend my sincere appreciation to Broc Astill for generating Figure 1 and Jared Dickinson, Ph.D., for critical review of the manuscript.
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