Synergistic effect of vascular endothelial growth factor gene inactivation in endothelial cells and skeletal myofibres on muscle enzyme activity, capillary supply and endurance exercise in mice

Abstract

New findings: What is the central question of this study? Does vascular endothelial growth factor (VEGF) expressed by both endothelial cells and skeletal myofibres maintain the number of skeletal muscle capillaries and regulate endurance exercise? What is the main finding and its importance? VEGF expressed by both endothelial cells and skeletal myofibres is not essential for maintaining capillary number but does contribute to exercise performance.

Abstract: Many chronic diseases lead to exercise intolerance, with loss of skeletal muscle capillaries. While many muscle cell types (myofibres, satellite cells, endothelial cells, macrophages and fibroblasts) express vascular endothelial growth factor (VEGF), most muscle VEGF is stored in myofibre vesicles which can release VEGF to signal VEGF receptor-expressing cells. VEGF gene ablation in myofibres or endothelial cells alone does not cause capillary regression. We hypothesized that simultaneously deleting the endothelial cell (EC) and skeletal myofibre (Skm) VEGF gene would cause capillary regression and impair exercise performance. This was tested in adult mice by simultaneous conditional deletion of the VEGF gene (Skm/EC-VEGF^-/- mice) through the use of VEGFLoxP, HSA-Cre-ER^T2 and PDGFb-iCre-ER^T2 transgenes. These double-deletion mice were compared to three control groups - WT, EC VEGF gene deletion alone and myofibre VEGF gene deletion alone. Three weeks after initiating gene deletion, Skm/EC-VEGF^-/- mice, but not SkmVEGF^-/- or EC-VEGF^-/- mice, reached exhaustion 40 min sooner than WT mice in treadmill tests (P = 0.002). WT, SkmVEGF^-/- and EC-VEGF^-/- , but not Skm/EC-VEGF^-/- , mice gained weight over the 3 weeks. Capillary density, fibre area and capillary: fibre ratio in soleus, plantaris, gastrocnemius and cardiac papillary muscle were similar across the groups. Phosphofructokinase and pyruvate dehydrogenase activities increased only in Skm/EC-VEGF^-/- mice. These data suggest that deletion of the VEGF gene simultaneously in endothelial cells and myofibres, while reducing treadmill endurance and despite compensatory augmentation of glycolysis, is not required for muscle capillary maintenance. Reduced endurance remains unexplained, but may possibly be related to a role for VEGF in controlling perfusion of contracting muscle.

Keywords: angiogenesis; exercise; peripheral vascular disease.