Skeletal Myocyte-specific VEGF Gene Deletion In Adult Mice Leads To Impaired Exercise Capacity Without Evidence Of Capillary Regression Or Altered In Vitro Muscle Contraction

Abstract

Vascular endothelial growth factor (VEGF) is involved in controlling skeletal muscle capillarity, an essential component of exercise capacity. PURPOSE: Determine the mechanisms by which skeletal myocyte VEGF maintains exercise capacity. METHODS: Inducible, skeletal myocyte-specific VEGF gene-deleted mice were generated by crossing VEGFLoxP mice with HSA-CreERT2 mice (skmVEGF-/-) in which cre recombinase is under control of a human skeletal actin (HSA) sequence and can be conditionally induced with tamoxifen. VEGF levels were measured in several organs and in the gastrocnemius muscle by ELISA, 2 and 7 weeks after tamoxifen delivery to adult (skmVEGF-/-) and control (VEGF+/+) mice. Exercise capacity was evaluated by recording the maximum running speed and time to exhaustion. Contractile function in isolated EDL and soleus muscles was assessed by peak force, time to fatigue and the force-frequency relationship. RESULTS: SkmVEGF-/- mice showed 95% and 92% reductions in skeletal muscle VEGF protein 2 and 7 weeks after tamoxifen delivery, respectively, with no reduction in other organs. Two-weeks after tamoxifen, maximum running speed (-10%) and endurance capacity (-47%) were reduced in skmVEGF-/- mice compared to controls. Seven weeks post-tamoxifen, maximum running speed remained low in skmVEGF-/- mice, with a non-significant trend towards lower endurance capacity. Body and muscle weights, muscle cross-sectional areas, peak force or time to fatigue and capillary to fiber ratios were not different between skmVEGF-/- mice and VEGF+/+ at either time point. Analysis of skeletal muscle force-frequency relationships also did not reveal any difference at 2 weeks in the EDL or soleus, though by 7 weeks a significant right shift in the force-frequency curve was observed in the EDL. CONCLUSION: Mice with VEGF-deficient skeletal myocytes exhibit a limitation to exercise that does not stem from an inability of the muscle to contract or the number of capillaries that supply muscle fibers.