Short-term acetaminophen consumption enhances the exercise-induced increase in Achilles peritendinous IL-6 in humans

Acetaminophen influences musculoskeletal signaling but not adaptations to endurance exercise training

Acetaminophen (ACE) is a widely used analgesic and antipyretic drug with various applications, from pain relief to fever reduction. Recent studies have reported equivocal effects of habitual ACE intake on exercise performance, muscle growth, and risks to bone health. Thus, this study aimed to assess the impact of a 6-week, low-dose ACE regimen on muscle and bone adaptations in exercising and non-exercising rats. Nine-week-old Wistar rats (n = 40) were randomized to an exercise or control (no exercise) condition with ACE or without (placebo). For the exercise condition, rats ran 5 days per week for 6 weeks at a 5% incline for 2 min at 15 cm/s, 2 min at 20 cm/s, and 26 min at 25 cm/s. A human equivalent dose of ACE was administered (379 mg/kg body weight) in drinking water and adjusted each week based on body weight. Food, water intake, and body weight were measured daily. At the beginning of week 6, animals in the exercise group completed a maximal treadmill test. At the end of week 6, rats were euthanized, and muscle cross-sectional area (CSA), fiber type, and signaling pathways were measured. Additionally, three-point bending and microcomputer tomography were measured in the femur. Follow-up experiments in human primary muscle cells were used to explore supra-physiological effects of ACE. Data were analyzed using a two-way ANOVA for treatment (ACE or placebo) and condition (exercise or non-exercise) for all animal outcomes. Data for cell culture experiments were analyzed via ANOVA. If omnibus significance was found in either ANOVA, a post hoc analysis was completed, and a Tukey's adjustment was used. ACE did not alter body weight, water intake, food intake, or treadmill performance (p > .05). There was a treatment-by-condition effect for Young's Modulus where placebo exercise was significantly lower than placebo control (p < .05). There was no treatment by condition effects for microCT measures, muscle CSA, fiber type, or mRNA expression. Phosphorylated-AMPK was significantly increased with exercise (p < .05) and this was attenuated with ACE treatment. Furthermore, phospho-4EBP1 was depressed in the exercise group compared to the control (p < .05) and increased in the ACE control and ACE exercise group compared to placebo exercise (p < .05). A low dose of ACE did not influence chronic musculoskeletal adaptations in exercising rodents but acutely attenuated AMPK phosphorylation and 4EBP1 dephosphorylation post-exercise.

Directing iPSC differentiation into iTenocytes using combined scleraxis overexpression and cyclic loading

Regenerative therapies for tendon are falling behind other tissues due to the lack of an appropriate and potent cell therapeutic candidate. This study aimed to induce tenogenesis using stable Scleraxis (Scx) overexpression in combination with uniaxial mechanical stretch of iPSC-derived mesenchymal stromal-like cells (iMSCs). Scx is the single direct molecular regulator of tendon differentiation known to date. Bone marrow-derived (BM-)MSCs were used as reference. Scx overexpression alone resulted in significantly higher upregulation of tenogenic markers in iMSCs compared to BM-MSCs. Mechanoregulation is known to be a central element guiding tendon development and healing. Mechanical stimulation combined with Scx overexpression resulted in morphometric and cytoskeleton-related changes, upregulation of early and late tendon markers, and increased extracellular matrix deposition and alignment, and tenomodulin perinuclear localization in iMSCs. Our findings suggest that these cells can be differentiated into tenocytes and might be a better candidate for tendon cell therapy applications than BM-MSCs.

Impact of essential amino acid intake, resistance exercise, and aging on the concentration of Achilles peritendinous amino acids and procollagen Iα1 in humans

Recent studies have shown that consuming amino acid-rich compounds improves tendon collagen content and biomechanical properties. Yet, it is unclear if the consumption of amino acids alters local (peritendinous) amino acid concentrations. If aging or exercise influence local amino acid concentrations in conjunction with an amino acid bolus is also not known. We conducted two studies. In Study 1, young women (n = 7, 25 ± 2 years) completed two identical resistance training sessions with either essential amino acid (EAA) or placebo consumption. In Study 2, an EAA bolus identical to Study 1 was given to younger (n = 7; 27 ± 1 year) and older adults (n = 6; 68 ± 2 years). Microdialysis was used to determine Achilles peritendinous amino acid and pro-collagen Iα1 (a marker of collagen synthesis) concentrations. In Study 1, amino acid consumption increased peritendinous concentrations of all EAA except histidine (p < 0.05). In Study 2, the peritendinous concentration of EAAs except for methionine, histidine, and lysine (p > 0.05) increased with time (p < 0.05). Further, the concentrations of most measured amino acids were greater in older adults (p < 0.05). Pro-collagen Iα1 concentration (p > 0.05) was unaffected by exercise, EAA, or aging (p > 0.05). Our findings demonstrate the following: (1) when not combined with exercise, an oral EAA bolus leads to only modest increases in Achilles peritendinous amino acid concentrations; (2) when combined with resistance exercise, EAA consumption resulted in greater peritendinous amino acid concentrations compared to no exercise; (3) the basal concentrations of most amino acids were greater in older adults, and (4) neither the EAA bolus nor exercise altered peritendinous pro-collagen concentrations.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Profiling classical neuropsychiatric biomarkers across biological fluids and following continuous lumbar puncture: A guide to sample type and time

Identification of putative biomarkers for neuropsychiatric disorders has produced a diverse list of analytes involved in inflammation, hypothalamic–pituitary–adrenal axis (HPA) regulation, growth factor and metabolic pathways. However, translation of these findings to accurate and robust assays has been stalled, affecting objective diagnoses, tracking relapse/remission, and prediction/monitoring of drug affect. Two important factors to control are the sample matrix (e.g. serum, plasma, saliva, or cerebrospinal fluid) and time of sample collection. Additionally, sample collection procedures may affect analyte level. In this study, a panel of 14 core neuropsychiatric biomarkers was measured in serum, plasma, saliva, and cerebrospinal fluid (CSF), all collected from 8 healthy volunteers at the same time. In a second cohort of 7 healthy volunteers, 6 analytes were measured in serum and CSF collected at 13 timepoints over a 24-h period after catheter placement. We found that many of the analytes were quantifiable in all sample types examined, but often at quite different concentrations and without correlation between the sample types. After catheter placement, a diurnal pattern was observed for cortisol and interleukin-6 in serum, and transient spikes were observed in interleukin-1β. In CSF, a chronic elevation of several cytokines was observed instead, perhaps due to the continuous sampling procedure. These findings enable more informed decision-making around sample type and collection time, which can be implemented in future biomarker studies.

Clinicaltrial.gov identifiers

NCT02933762, NCT02475148.

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Diurnal pattern for cortisol, interleukin (IL)-6 and transient spikes for IL-1β were observed

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Chronic elevation of cytokines observed may be due to continuous sampling procedure

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Informed decision-making around sample types and collection time can be implemented

Reparative and Maladaptive Inflammation in Tendon Healing

Tendon injuries are common and debilitating, with non-regenerative healing often resulting in chronic disease. While there has been considerable progress in identifying the cellular and molecular regulators of tendon healing, the role of inflammation in tendon healing is less well understood. While inflammation underlies chronic tendinopathy, it also aids debris clearance and signals tissue repair. Here, we highlight recent findings in this area, focusing on the cells and cytokines involved in reparative inflammation. We also discuss findings from other model systems when research in tendon is minimal, and explore recent studies in the treatment of human tendinopathy to glean further insights into the immunobiology of tendon healing.

Acute-Onset Achilles Tendon Pain and Swelling Treated with an Amniotic Fluid-Derived Allograft: A Case Study

BACKGROUND

Tendinopathies are common musculoskeletal disorders that often develop because of chronic loading and failed healing. Tendinopathy related to systemic inflammation has been less extensively examined. Furthermore, although the use of biological agents to treat tendinopathies continues to gain popularity, the use of amniotic fluid-derived allografts in outpatient settings to resolve tendinopathies requires further evaluation.

METHODS

The focus of this case report is a 25-year-old man who presented for a second opinion, having been diagnosed with Haglund deformity and Achilles tendinopathy. At the time of presentation, he complained of 10 of 10 pain to the right Achilles tendon. He was treating the injury conservatively with intermittent use of a controlled ankle motion boot and working with physiotherapy for approximately 5 months before presentation. Diagnostic ultrasound along with magnetic resonance imaging indicated distal thickening of the Achilles tendon, substantial fluid and edema in the Kager fat pad, and retrocalcaneal erosions with bursitis. Conservative management did not resolve the symptoms. As an alternative to surgery, the patient elected to undergo an Achilles tendon injection of an amniotic fluid-derived allograft. Before and after the initial injection, a microdialysis catheter was inserted into the Achilles peritendinous space to sample local levels of extracellular matrix enzymes and growth factors important for tendon remodeling. The patient received considerable relief with the initial injection, but did not return to full strength. Over the subsequent 8 weeks, the patient was followed closely and was able to return to daily activities with minimal pain. He was not able to return to a more active lifestyle without further Achilles pain, so a second amniotic fluid-derived allograft injection was performed 8 weeks after the initial injection.

RESULTS

Injection of the initial allograft resulted in significant improvement, but not complete resolution of pain and swelling. Microdialysis findings suggested a reduction in peritendinous levels of the cytokine interlukin-6 in addition to changes in extracellular matrix regulatory enzymes. After 8 weeks of additional conservative therapy and a second injection, no further improvement in pain was noted.

CONCLUSIONS

Based on the clinical improvement of symptoms in this individual and the changes seen with microdialysis methodology, the authors find the use of amniotic fluid-derived allograft injection for treatment of Achilles pain in this patient to be a viable treatment. Additional comorbidities of systemic inflammatory polyarthritis and possible seronegative disease were addressed after rheumatology consultation with a variety of medications that provided the patient additional relief of his symptoms. The patient ultimately moved and was lost to further follow-up.

Effectiveness of Amniotic Fluid Injection in the Treatment of Trigger Finger: A Pilot Study

Purpose

To assess the efficacy and safety of amniotic fluid therapy injections in patients with mild to moderate trigger finger.

Methods

All participants received 1 mL of amniotic fluid injected into the tendon sheath of the affected tendon. Pretreatment and posttreatment data were collected for triggering frequency, Disabilities of the Arm, Shoulder, and Hand (DASH) questionnaire scores, and numerical pain rating scale scores.

Results

Of 111 digits from 96 patients, 51% experienced clinically notable improvement and did not receive an alternative treatment. Average length of follow-up was 11 months. From baseline to end of follow-up, average pain score (0–10) decreased from 5.19 to 1.19 (P < .001), median triggering per day decreased from 5 to 0 (P < .001), and median DASH score (1–100) decreased from 20 to 6.03 (P < .001). There was a 50% success rate in patients with diabetes and a 52.6% success rate in digits diagnosed with concomitant Dupuytren contracture in the same hand.

Conclusions

Amniotic fluid therapy injections may offer a biologic alternative for conservative treatment of trigger finger, particularly for patients with diabetes. Decreased pain, decreased triggering, and improved DASH scores offer preliminary evidence supporting the use of amniotic injections for stenosing tenosynovitis.

Type of study/level of evidence

Therapeutic IV.

Cytokines in tendon disease: A Systematic Review

OBJECTIVES

Emerging evidence indicates that tendon disease is an active process with inflammation that is critical to disease onset and progression. However, the key cytokines responsible for driving and sustaining inflammation have not been identified.

METHODS

We performed a systematic review of the literature using MEDLINE (U.S. National Library of Medicine, Bethesda, Maryland) in March 2017. Studies reporting the expression of interleukins (ILs), tumour necrosis factor alpha (TNF-α) and interferon gamma in diseased human tendon tissues, and animal models of tendon injury or exercise in comparison with healthy control tissues were included.

RESULTS

IL-1β, IL-6, IL-10, and TNF-α are the cytokines that have been most frequently investigated. In clinical samples of tendinopathy and tendon tears, the expression of TNF-α tended not to change but IL-6 increased in tears. Healthy human tendons showed increased IL-6 expression after exercise; however, IL-10 remained unchanged. Animal tendon injury models showed that IL-1β, IL-6, and TNF-α tend to increase from the early phase of tendon healing. In animal exercise studies, IL-1β expression showed a tendency to increase at the early stage after exercise, but IL-10 expression remained unchanged with exercise.

CONCLUSIONS

This review highlights the roles of IL-1β, IL-6, IL-10, and TNF-α in the development of tendon disease, during tendon healing, and in response to exercise. However, there is evidence accumulating that suggests that other cytokines are also contributing to tendon inflammatory processes. Further work with hypothesis-free methods is warranted in order to identify the key cytokines, with subsequent mechanistic and interaction studies to elucidate their roles in tendon disease development.Cite this article: W. Morita, S. G. Dakin, S. J. B. Snelling, A. J. Carr. Cytokines in tendon disease: A Systematic Review. Bone Joint Res 2017;6:656-664. DOI: 10.1302/2046-3758.612.BJR-2017-0112.R1.

Impact of acetaminophen consumption and resistance exercise on extracellular matrix gene expression in human skeletal muscle

Acetaminophen (APAP) given during chronic exercise reduces skeletal muscle collagen and cross-linking in rats. We propose that the effect of APAP on muscle extracellular matrix (ECM) may, in part, be mediated by dysregulation of the balance between matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs). The purpose of this study was to evaluate the impact of APAP consumption during acute resistance exercise (RE) on several regulators of the ECM in human skeletal muscle. In a double-blinded, placebo-controlled, randomized crossover design, recreationally active men (n = 8, 25 ± 2 yr) performed two trials of knee extension. Placebo (PLA) or APAP (1,000 mg/6 h) was given for 24 h before and immediately following RE. Vastus lateralis biopsies were taken at baseline and 1 and 3 h post-RE. Quantitative RT-PCR was used to determine differences in mRNA expression. MMP-2, type I collagen, and type III collagen mRNA expression was not altered by exercise or APAP (P > 0.05). When compared with PLA, TIMP-1 expression was lower at 1 h post-RE during APAP conditions but greater than PLA at 3 h post-RE (P < 0.05). MMP-9 expression and protein levels were elevated at 3 h post-RE independent of treatment (P < 0.05). Lysyl oxidase expression was greater at 3 h post-RE during APAP consumption (P < 0.05) compared with PLA. MMP-2 and TIMP-1 protein was not altered by RE or APAP (P > 0.05). Phosphorylation of ERK1/2 and p38-MAPK increased (P < 0.05) with RE but was not influenced by APAP. Our findings do not support our hypothesis and suggest that short-term APAP consumption before RE has a small impact on the measured ECM molecules in human skeletal muscle following acute RE.

The influence of chronic IL-6 exposure, in vivo, on rat Achilles tendon extracellular matrix

When compared to placebo, acetaminophen (APAP) reduces tendon stiffness and collagen cross-linking. APAP also enhances the exercise-induced increase in peritendinous levels of IL-6. Elevated levels of IL-6 are associated with tendinopathy, thus we hypothesized that chronic, elevated peritendinous IL-6 would alter tendon extracellular matrix (ECM). IL-6 (∼3000pgml^-1) was injected (3dwk^-1 for 8-wks) into the Achilles peritendinous region of male Wistar rats (n=16) with the opposite leg serving as a sham. Fractional synthesis rates (FSR) were determined using deuterium oxide. Collagen (hydroxyproline) and hydroxylysl pyridinoline (HP) cross-linking were analyzed by HPLC. ECM and IL-6 related genes were evaluated using qRT-PCR. Relative to sham, collagen (Col) 1a1 but not Col3a1 expression was suppressed (47%) in tendons exposed to IL-6 (p<0.05). Lysyl oxidase (LOX) and MMP-1 expression were also reduced (37%) in IL-6 treated tendons (p<0.05). Relative to sham the expression of MMP-2, -3, -9, and TIMP-1 were not altered by IL-6 treatment (p>0.05). Interleukin-6 receptor subunit beta precursor (IL6st) was lower (16%) in IL-6 treated tendons when compared to sham (p<0.05). Suppressor of cytokine signaling 3 (Socs3), signal transducer and activator of transcription 3 (STAT3), and protein inhibitor of activated STAT 1 (Pias1) were not altered by IL-6 exposure (p>0.05). Neither collagen nor cross-linking content were altered by IL-6 (p>0.05). Additionally, IL-6 treatment did not alter tendon FSR. Chronic treatment with physiologically relevant levels of IL-6 suppresses expression of Col1a1 and LOX while also altering expression of select MMPs but does not alter Achilles tendon collagen synthesis.

Sex-based difference in Achilles peritendinous levels of matrix metalloproteinases and growth factors after acute resistance exercise

Several recent investigations have demonstrated that the ability of various tendons to alter structural and functional properties in response to exercise are muted in women compared with men. We hypothesize that this disparity between men and women may be due to a reduced tendon production of key mediators of tendon extracellular matrix (ECM) remodeling in response to mechanical loading, e.g., exercise. Using microdialysis before and after an acute bout of resistance exercise, we evaluated Achilles peritendinous levels of insulin-like growth factor-1 (IGF-1) and interleukin-6 (IL-6), which have both been shown to increase tendon collagen synthesis. Additionally, the matrix remodeling enzymes matrix metalloproteinase-2 (MMP-2), MMP-9, and tissue inhibitor of metalloproteinase-1 (TIMP-1) were also evaluated. IGF-1 levels were elevated ( P < 0.05) to a similar extent in men and women after 3 h of exercise but remained elevated at 4 h in only women. IL-6 levels were ~4-fold greater after exercise in both men and women ( P < 0.05). MMP-2 levels increased to a similar extent (~2-3-fold) in men and women ( P < 0.05). In contrast, MMP-9 increased with exercise but only in men ( P < 0.05). Last, TIMP-1 levels also increased ( P < 0.05) with exercise in men and women but the increase was more prolonged in women. In conclusion, we observed modest sex differences in tendon release of MMP-9, TIMP-1, and IGF-1 after acute resistance exercise. If such differences persist throughout a chronic exercise training, they may contribute to the reduced ability of women to adapt to exercise compared with men.

NEW & NOTEWORTHY In this investigation we utilized microdialysis of the peritendinous Achilles to evaluate potential differences between men and women in tendon production of key regulators of extracellular matrix remodeling. We demonstrate that a modest sex-specific difference exists in peritendinous levels of several key extracellular matrix modulators after an acute bout of resistance exercise.

Impact of TGF-β inhibition during acute exercise on Achilles tendon extracellular matrix

The purpose of this study was to evaluate the role of TGF-β₁ in regulating tendon extracellular matrix after acute exercise. Wistar rats exercised (n = 15) on a treadmill for four consecutive days (60 min/day) or maintained normal cage activity. After each exercise bout, the peritendinous space of each Achilles tendon was injected with a TGF-β₁ receptor inhibitor or sham. Independent of group, tendons injected with inhibitor exhibited ~50% lower Smad 3 (Ser^423/425) (P < 0.05) and 2.5-fold greater ERK1/2 phosphorylation (P < 0.05) when compared with sham (P < 0.05). Injection of the inhibitor did not alter collagen content in either group (P > 0.05). In exercised rats, hydroxylyslpyridinoline content and collagen III expression were lower (P < 0.05) in tendons injected with inhibitor when compared with sham. In nonexercised rats, collagen I and lysyl oxidase (LOX) expression was lower (P < 0.05) in tendons injected with inhibitor when compared with sham. Decorin expression was not altered by inhibitor in either group (P > 0.05). On the basis of evaluation of hematoxylin and eosin (H&E) stained cross sections, cell numbers were not altered by inhibitor treatment in either group (P > 0.05). Evaluation of H&E-stained sections revealed no effect of inhibitor on collagen fibril morphology. In contrast, scores for regional variation in cellularity decreased in exercised rats (P < 0.05). No differences in fiber arrangement, structure, and nuclei form were noted in either group (P > 0.05). Our findings suggest that TGF-β₁ signaling is necessary for the regulation of tendon cross-link formation, as well as collagen and LOX gene transcription in an exercise-dependent manner.

Effects of maternal medication use on NGF and IL-6 levels in human breast milk

BACKGROUND

The objective of this study was to evaluate the effect of maternal medications on nerve growth factor (NGF) and interleukin-6 (IL-6) levels in human breast milk (HBM).

METHODS

A total of 30 samples of HBM were collected after consent from consecutively born term newborns. NGF and IL-6 concentrations were analyzed using ELISA assays from R&D Systems. The HBM samples were centrifuged, and the clear portion of the HBM after discarding the fat was analyzed and cytokine data were expressed as NGFC or IL-6C. Ten samples of HBM, which were not centrifuged, were also used in ELISA assays and cytokine data were expressed as NGFF or IL-6F.

RESULTS

After exposure to NSAIDs (7636 ± 9610, mean ± SD, pg/mL), the NGFC levels in HBM were significantly higher as compared to those who were exposed to narcotics (522 ± 1000) (p = 0.008). NGFC and IL-6C levels positively correlated with each other in HBM (R = 0.194 p < 0.0001). NGFC levels (360 ± 237) were significantly lower than NGFF levels (888 ± 751) (p < 0.0001). IL-6F was higher than IL-6C levels without statistical significance.

CONCLUSION

Further studies are warranted to elucidate effect of maternal medications on cytokine changes in HBM and effect of these cytokine changes on newborn gastrointestinal milieu.

Analgesic Drugs Alter Connective Tissue Remodeling and Mechanical Properties

Exercising individuals commonly consume analgesics, but these medications alter tendon and skeletal muscle connective tissue properties, possibly limiting a person from realizing the full benefits of exercise training. I detail the novel hypothesis that analgesic medications alter connective tissue structure and mechanical properties by modifying fibroblast production of growth factors and matrix enzymes, which are responsible for extracellular matrix remodeling.

Modulating exercise-induced hormesis: Does less equal more?

Hormesis encompasses the notion that low levels of stress stimulate or upregulate existing cellular and molecular pathways that improve the capacity of cells and organisms to withstand greater stress. This notion underlies much of what we know about how exercise conditions the body and induces long-term adaptations. During exercise, the body is exposed to various forms of stress, including thermal, metabolic, hypoxic, oxidative, and mechanical stress. These stressors activate biochemical messengers, which in turn activate various signaling pathways that regulate gene expression and adaptive responses. Historically, antioxidant supplements, nonsteroidal anti-inflammatory drugs, and cryotherapy have been favored to attenuate or counteract exercise-induced oxidative stress and inflammation. However, reactive oxygen species and inflammatory mediators are key signaling molecules in muscle, and such strategies may mitigate adaptations to exercise. Conversely, withholding dietary carbohydrate and restricting muscle blood flow during exercise may augment adaptations to exercise. In this review article, we combine, integrate, and apply knowledge about the fundamental mechanisms of exercise adaptation. We also critically evaluate the rationale for using interventions that target these mechanisms under the overarching concept of hormesis. There is currently insufficient evidence to establish whether these treatments exert dose-dependent effects on muscle adaptation. However, there appears to be some dissociation between the biochemical/molecular effects and functional/performance outcomes of some of these treatments. Although several of these treatments influence common kinases, transcription factors, and proteins, it remains to be determined if these interventions complement or negate each other, and whether such effects are strong enough to influence adaptations to exercise.

The effect of chronic treadmill exercise and acetaminophen on collagen and cross-linking in rat skeletal muscle and heart

The purpose of this study was to determine whether exercise and/or acetaminophen (APAP) alter collagen and cross-linking in the rat gastrocnemius muscle, soleus muscle, and heart. Male Wistar rats ( n = 50; 8 wk old) were divided into placebo (PLA) or APAP groups and sedentary (SED) or exercised (RUN) groups. APAP (200 mg/kg) was administered daily by oral gavage. Exercised groups ran on a treadmill 5 days/wk for 8 wk with progression to 60 min/day, 20 m/min, and 8° incline. Tissues were assayed for collagen (hydroxyproline) and hydroxylyslpyridinoline (HP) and lysylpyridinoline (LP) cross-links by HPLC. Collagen content (μg/mg dry weight) was greater in both the gastrocnemius (SED-PLA: 114 ± 16 vs. RUN-PLA: 244 ± 32; P < 0.001) and soleus (SED-PLA: 51 ± 7 vs. RUN-PLA: 99 ± 27; P = 0.005) of exercised animals. In contrast, collagen content was not significantly greater in exercised animals treated with APAP (SED-APAP: 113 ± 16 vs. RUN-APAP: 145 ± 21) and soleus (SED-APAP: 55 ± 8 vs. RUN-APAP: 57 ± 10). HP cross-linking (mmol/mol collagen) in the gastrocnemius (SED-PLA: 126 ± 28, RUN-PLA: 50 ± 7, SED-APAP: 41 ± 7, and RUN-APAP: 30 ± 4) and soleus muscles (SED-PLA: 547 ± 107, RUN-PLA: 318 ± 92, SED-APAP: 247 ± 64, and RUN-APAP: 120 ± 17) was lower in exercised rats compared with sedentary rats ( P < 0.05). Cross-linking was further reduced in animals treated with APAP ( P < 0.05). Neither heart collagen nor cross-linking was influenced by exercise or APAP ( P > 0.05). Our findings suggest that exercise and APAP have tissue-specific effects on muscle collagen. Given the widespread use of APAP as an analgesic and antipyretic, further work in humans is warranted.