Expression of collagen and related growth factors in rat tendon and skeletal muscle in response to specific contraction types

Abstract

Acute exercise induces collagen synthesis in both tendon and muscle, indicating an adaptive response in the connective tissue of the muscle-tendon unit. However, the mechanisms of this adaptation, potentially involving collagen-inducing growth factors (such as transforming growth factor-β-1 (TGF-β-1)), as well as enzymes related to collagen processing, are not clear. Furthermore, possible differential effects of specific contraction types on collagen regulation have not been investigated. Female Sprague-Dawley rats were subjected to 4 days of concentric, eccentric or isometric training (n = 7-9 per group) of the medial gastrocnemius, by stimulation of the sciatic nerve. RNA was extracted from medial gastrocnemius and Achilles tendon tissue 24 h after the last training bout, and mRNA levels for collagens I and III, TGF-β-1, connective tissue growth factor (CTGF), lysyl oxidase (LOX), metalloproteinases (MMP-2 and -9) and their inhibitors (TIMP-1 and 2) were measured by Northern blotting and/or real-time PCR. In tendon, expression of TGF-β-1 and collagens I and III (but not CTGF) increased in response to all types of training. Similarly, enzymes/factors involved in collagen processing were induced in tendon, especially LOX (up to 37-fold), which could indicate a loading-induced increase in cross-linking of tendon collagen. In skeletal muscle, a similar regulation of gene expression was observed, but in contrast to the tendon response, the effect of eccentric training was significantly greater than the effect of concentric training on the expression of several transcripts. In conclusion, the study supports an involvement of TGF-β-1 in loading-induced collagen synthesis in the muscle-tendon unit and importantly, it indicates that muscle tissue is more sensitive than tendon to the specific mechanical stimulus.

Endurance exercise activates matrix metalloproteinases in human skeletal muscle

“…As was observed for MMP-9, restricted blood flow to the exercising muscle had no effect on MMP-2 mRNA level and activity, or on MMP-14 and TIMP-1 mRNA levels. The expression of MMPs in skeletal muscle endothelial cells is greatly affected by mechanical stretch (22,23,27) and MMP-2 expression in muscle tendon is increased by mechanical loading (15,20). These results are consistent with our findings, in that the workload and thus presumably the mechanical load was identical under the two exercise conditions.…”

“…This is consistent with differences in the cis-element composition of the MMP-9, MMP-2, and MMP-14 promoters (33). TIMP-1 mRNA increased alongside the increase in MMP-2 and MMP-14, which is consistent with a previous report on exercise in the rat (15). Thus these studies collectively suggest a concurrent initiation of both activation and inhibition of the MMP-family by exercise, and they encourage further exploration of whether type and mode of exercise influence these processes differently.…”

“…These observations are novel for human skeletal muscle, but they are supported by studies on exercise and the human tendon and by electrical stimulation of an animal model (14,15,20). The absolute number of transcripts of MMP-9 was much lower than that of MMP-2, but it increased after a single exercise bout.…”

“…A recent study suggested that changes in skeletal muscle MMP-9 mRNA are associated with an inflammatory response to the biopsy procedure (15). To test this, we measured MMP-9 mRNA in muscle biopsies of eight subjects taken from the same leg at 2-h intervals in the absence of exercise.…”

Age-related changes in structure and extracellular matrix protein expression levels in rat tendons

“…LOX is very sensitive to mechanical loading and is often used as a marker of the ECM response to mechanical stimulation. For example, mechanical loading of tendons increased expression of LOX by~37-folds while Col1a1 expression was up-regulated only~5-fold (Heinemeier et al 2007). Our expectation was that an age-associated decrease in physical activity would be associated with decreased LOX expression in tendons of old rats, but we detected no noticeable changes in LOX mRNA expression and increased LOX protein expression in tendons of old when compared with young rats.…”