Exercise and angiogenic growth factors in human skeletal muscle

Abstract

Long-term electrical stimulation and endurance exercise increase the amount of capillaries in skeletal muscle. VEGF-A is a well-characterized stimulatory angiogenic growth factor and has shown to play an important role in angiogenesis in pathological conditions in humans and in physiological conditions in animal models. A close relationship has recently been observed between VEGF-A and another group of endothelial specific growth factors, angiopoietins, during development and during angiogenesis in adult animal tissue. In situations with increased expression of Ang-2 and high availability of VEGF-A a favourable impact exists on the angiogenic process through Ang-2 induced endothelial destabilization, which facilitates the effects of VEGF-A on endothelial activation.

The general aim of this thesis was to study the influence of exercise on angiogenic growth factors. The levels and regulation of VEGF-A its splice variants and receptors as well as Ang-1 and Ang-2 and their receptors were studied. The specific aims were to study 1) effects of single and repetitive bouts of endurance exercise on the expression of angiogenic growth factors in skeletal muscle, 2) acute effects of a single bout of endurance exercise on the exchange of angiogenic growth factors over the working leg, and 3) effects of restricted leg blood flow, and thereby reduced oxygen delivery to the working leg on the expression of angiogenic growth factors in skeletal muscle.

The main hypotheses were that endurance exercise increases the expression of angiogenesis g rowth factors and their receptors and that this response is enhanced by reduced oxygen delivery to the exercising muscles in a fashion that facilitates the angiogenic process. A single bout of endurance exercise increased the expression of VEGF-A mRNA, but not of FGF-2 mRNA in human skeletal muscle, as measured in biopsy samples from vastus lateralis.

Metabolic perturbation, induced by exercise with restricted leg blood flow, enhanced the VEGF-A response. Also the expression of the VEGF receptors, VEGFR-1 and VEG17R-2 increased at the mRNA level in response to a single bout of exercise. The recognized splice variants in skeletal muscle were VEGF-A,21, VEGF-A165 and VEGF-A189, which all increased in response to a single bout of exercise. However, the relative distribution of VEGFA splice-variant expression changed after exercise; initially with a relative increased expression of VEGF-A165, followed by a later relative increase of VEGF-A189. There was no uptake of VEGF-A protein into exercising skeletal muscle from the circulation, which indicates that the blood stream is not a source for the observed increase in skeletal muscle VEGF-A protein after exercise.

Repetitive bouts of endurance exercise with restricted blood flow increased a) basal VEGF-A protein levels, but not basal VEGF-A mRNA, which increases only temporarily in association to each single bout of exercise, b) the basal level of VEGFR-2 mRNA, but not of VEGFR- 1, which suggests a selective contribution of the VEGF-A receptors in exercise-induced angiogenesis c) basal level of Ang-2 protein and the mRNA ratio Ang-2 to Ang-1. Several components of the HIF-1 pathway were activated in response to acute changes in oxygen demand in human skeletal muscle. However, the present data also shows a relationship between the exercise-induced increase in venous lactate concentration and VEGF-A mRNA expression, which indicates that energy metabolites may be involved in the regulation of exercise-induced VEGF-A expression.

The results presented in this thesis support that oxygen sensitive pathways may be relevant for adaptation to physical activity in human skeletal muscle. The present data also shows that the exercise-induced increase in VEGF-A is more complex than activation of HIF-1 alone. Finally, it is suggested that the concurrent changes observed in the VEGF-A and angiopoietin systems may have exerted a permissive effect on angiogenesis.