Rat Supraspinatus Tendon Responds Acutely and Chronically to Exercise

Tendon can beneficially adapt to exercise, but it is unknown how acute responses following a single bout of exercise lead to chronic, beneficial adaptations. The objective of this study was to identify the acute responses and chronic adaptations of rat

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hypothesized that chronic exercise would result in superior tendon mechanical properties, and acute exercise would result in increases in matrix metalloproteinase (MMP) activity.

Consistent with our hypothesis, tendon mechanical properties increased after 2 and 8 weeks of chronic exercise. In contrast, 24 hours after a single bout of exercise, tendon showed trends toward reduced mechanical properties. This mild decrease in tendon mechanical properties acutely may initiate the chronic, beneficial adaptations seen in this study. We detected more significant mechanical changes after 2 weeks of exercise than after 8 weeks, which suggests that this adaptive process begins soon after initiating a new exercise routine. This finding supports a previous gene screening study we

performed that found more matrix turnover-related genes altered at an early time point than a later time point.42

Exercise did not impact tendon cellularity or organization, suggesting that unlike tendinopathic conditions, the exercised tendon remained healthy. Cells became rounder with exercise, which could be due to altered cell type or metabolic activity, which was not measured in this study.

In contrast to our hypothesis, generic MMP activity decreased following a single bout of exercise and after 8 weeks of chronic exercise. This finding suggests that exercise induces a net anabolic instead of catabolic response, which may be one distinguishing factor between optimal tissue use and under or overuse.

In summary, this study used a previously validated rat model of supraspinatus exercise43 to investigate the temporal acute and chronic responses of tendon. Mild, acute decreases in MMP activity and tendon mechanical properties following a single bout of exercise led to enhanced tendon mechanical adaptations with repeated bouts of exercise.

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These characterizations of non-injurious exercise will help future studies distinguish deleterious responses of tendon to load and investigate the biologic mediators of tendon adaptations or maladaptations to exercise, overuse, and disuse.

C. Ibuprofen Differentially Affects Supraspinatus Muscle and Tendon Adaptations to Exercise in a Rat Model

Ibuprofen is one of the most used drugs by adults, particularly athletes, who commonly use ibuprofen prophylactically or as “treatment.” Ibuprofen is an inhibitor of cyclooxygenases-1 and -2 (COX-1, COX-2), which are components of the arachidonic acid inflammatory cascade. Previous studies suggest that early inflammation is important for successful tissue healing following acute injury, and ibuprofen can be detrimental to this healing; however, the effects of ibuprofen on tissue adaptations to exercise is

equivocal with some studies finding detrimental effects and others finding no effect. The objective of this study was to determine the effects of ibuprofen on the adaptations of supraspinatus tendon and muscle in a rat model of exercise. We hypothesized that administration of ibuprofen would abolish the beneficial adaptations found with exercise but have no effect on sedentary muscle and tendon properties.

In contrast to our hypothesis, results suggest that chronic intake of ibuprofen at pharmacologic doses does not detrimentally impact supraspinatus tendon mechanical adaptations to exercise. Following chronic administration of ibuprofen, tendon

mechanical properties were not impaired and in some instances increased. Administration of ibuprofen combined with a single bout of exercise brought tendon cross-sectional area and modulus closer to baseline properties, suggesting altered recovery. Whether this recovery is accelerated or delayed should be investigated with additional time points in

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future studies. Tendon organization was not impacted by ibuprofen, suggesting that the tissue remained healthy. Similarly, the number of centrally nucleated muscle fibers remained less than 1%, suggesting the muscle fiber were not undergoing regenerative processes due to injury.

In contrast, chronic administration of ibuprofen decreased average muscle fiber cross-sectional area, and a few muscle fiber type-specific changes were detected as well. Taken together, these findings suggest that the arachidonic acid cascade is not a primary mechanism responsible for inducing tendon adaptations to non-injurious exercise; however, the arachidonic acid cascade appears to be important for muscle growth. This study adds to the growing body of literature on the effects of nonsteroidal anti-

inflammatory drugs on musculoskeletal tissues and suggests that athletes should consider the risks and benefits before taking ibuprofen.

D. Doxycycline Improves Sedentary, but not Exercised, Supraspinatus Tendon and Muscle in a Rat Model

Matrix metalloproteinases (MMPs) are enzymes that degrade extracellular matrix proteins. MMP inhibition has been proposed as a method to improve skeletal muscle and tendon healing and some studies have shown improved healing following acute injury with administration of MMP inhibitors; however, the roles of MMPs and therefore the effects of MMP inhibition on muscle and tendon adaptations to exercise are unknown. The objective of this study was to investigate the effects of doxycycline as a broad- spectrum MMP inhibitor on sedentary and exercised supraspinatus tendon and muscle in a rat model of non-injurious exercise. We hypothesized that doxycycline would abolish

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the beneficial adaptations found with exercise but have no effect on sedentary muscle and tendon properties.

In contrast to our hypothesis, doxycycline significantly improved sedentary supraspinatus tendon mechanical properties and tendon collagen organization without adversely affecting the muscle; however, when administered to animals undergoing an exercise protocol, doxycycline did not produce these same beneficial effects. Combined with exercise, doxycycline increased tendon modulus but decreased tendon stiffness and maximum load. In addition, the average muscle fiber cross-sectional area was reduced in exercised, but not sedentary, animals administered doxycycline.

These results support previous studies that have shown increased MMP activity with stress-deprived tendons and that administration of MMP inhibitors can prevent the associated loss of tendon mechanical properties. The MMP activity assay in Chapter 2 also revealed decreased MMP activity in exercised compared to sedentary tendons. These results suggest that in a sedentary state, tendon MMP activity is heightened, which leads to matrix degradation; administration of doxycycline as an MMP inhibitor can reduce the matrix degradation in sedentary tendons, leading to enhanced tendon mechanical

properties. Furthermore, MMP activity may be important for muscle adaptations to exercise, as supported by the reduced muscle fiber cross-sectional area with

administration of doxycycline. Taken together, MMP inhibition with doxycycline may be a useful treatment option for sedentary tissues but is not advantageous when combined with non-injurious activity. This study examines the roles of MMP inhibition in sedentary and exercised muscle and tendon, and future studies can explore the effects of

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