Exercise acutely increases the concentrations of metabolites and hormones such as growth hormone (GH) and, to a lesser extent, insulin-like growth factor 1 (IGF-1); however, the biological function of this response is unclear. Pharmacological administration of these hormones stimulates collagen synthesis in muscle and tendon; however, whether the post-exercise biochemical milieu has a similar action is unknown. Treating engineered ligaments with serum obtained from young healthy men after exercise resulted in more collagen and improved tensile strength over those treated with serum from resting men. Further, we show that the increase in collagen induced by post-exercise serum (i) is not reproduced by treatment with recombinant GH or IGF-1, and (ii) is associated with the activation of PI3 kinase/mTORC1 and ERK1/2 signalling. Exercise stimulates a dramatic change in the concentration of circulating hormones, such as growth hormone (GH), but the biological functions of this response are unclear. Pharmacological GH administration stimulates collagen synthesis; however, whether the post-exercise systemic milieu has a similar action is unknown. We aimed to determine whether the collagen content and tensile strength of tissue-engineered ligaments is enhanced by serum obtained post-exercise. Primary cells from a human anterior cruciate ligament (ACL) were used to engineer ligament constructs in vitro. Blood obtained from 12 healthy young men 15 min after resistance exercise contained GH concentrations that were ∼7-fold greater than resting serum (P <0.001), whereas IGF-1 was not elevated at this time point (P = 0.21 vs. rest). Ligament constructs were treated for 7 days with medium supplemented with serum obtained at rest (RestTx) or 15 min post-exercise (ExTx), before tensile testing and collagen content analysis. Compared with RestTx, ExTx enhanced collagen content (+19%; 181 ± 33 vs. 215 ± 40 μg per construct P = 0.001) and ligament mechanical properties - maximal tensile load (+17%, P = 0.03 vs. RestTx) and ultimate tensile strength (+10%, P = 0.15 vs. RestTx). In a separate set of engineered ligaments, recombinant IGF-1, but not GH, enhanced collagen content and mechanics. Bioassays in 2D culture revealed that acute treatment with post-exercise serum activated mTORC1 and ERK1/2. In conclusion, the post-exercise biochemical milieu, but not recombinant GH, enhances collagen content and tensile strength of engineered ligaments, in association with mTORC1 and ERK1/2 activation.
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