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TLR4 Modulates Myokine Gene Transcription Following in Vivo Electrical Muscle Stimulation to Fatigue: 1678 Board #23 May 28, 3

Vascular endothelial growth factor (VEGF) is traditionally considered important for skeletal muscle angiogenesis. VEGF is released from vascular endothelium as well as the muscle cells in response to exercise. The mechanism and the physiological role of VEGF secreted from the muscle cells remain unclear. However, as VEGF is also considered very important for the regulation of vascular permeability, it is possible that metabolic stress may trigger muscle VEGF release. PURPOSE: To study the role of metabolic stress induced by glycogen-depleting exercise on skeletal muscle VEGF expression. METHODS: Fifteen males (age 27.0±0.8; VO2max 66.0±1.2 ml•kg-1•min-1) carried out 4h of cycling exercise supplied with H2O only followed by 4h of recovery with either carbohydrate (CHO) (n=8) or H2O (n=7) supplementation. Hereafter both groups received CHO. Muscle biopsies were collected pre and post as well as 4 and 24 h following exercise. Muscle glycogen was investigated along with expression levels of mRNA (real time RT-PCR) and protein (immunohistochemistry) of VEGF, VEGF receptor 2 (VEGFR-2) and the molecular chaperones heat shock proteins 27 and 70 (HSP27, HSP70). RESULTS: Double labelling of PAS and myosin heavy chain II (MHC II) or HSP70 in muscle sections demonstrated a pronounced depletion of glycogen in type I fibers and concurrent increase of HSP70 also in type I fibers post and 4h after exercise. Baseline mRNA levels of VEGF, VEGFR-2 and HSP70 correlated positively with percentage of type I fibers (p<0.05). Further, significant increases in mRNA were detected in both CHO and H2O for VEGF, VEGFR-2, and HSP70 (CHO only) post (2.2-3.7 fold, p<0.05) and after 4h of recovery (1.5-2.5 fold, p<0.05). VEGF and HSP70 protein expression increased post (~35; ~200 %, respectively, p< 0.05) and after 4h of recovery (~60; ~150 %, respectively, p< 0.05) primarily in type I fibers and around nuclei, while VEGFR-2 increased after 4h of recovery (40 %, p<0.05). Immunohistochemistry and electron microscopy revealed co-localization of VEGF and HSP70 after exercise. Generally, all data returned to pre-exercise levels by 24h irrespective of treatment. CONCLUSIONS: Muscle glycogen depletion induced by prolonged exercise leads to up-regulation as well as co-localization of HSP70 and VEGF primarily in type I fibers, thus suggesting that VEGF released from muscle is involved in the maintenance of muscle metabolic homeostasis in response to metabolic stress.