Bilaterally increased VEGF-levels in muscles during experimental unilateral callus distraction

Angiogenesis is essential for wound healing and proliferative processes such as bone formation and repair. Since increased expression of the vascular endothelial growth factor (VEGF) stimulates bone formation, it can be hypothesized that surgical procedures leading to a systemic increase of VEGF for instance during wound healing, influence enchondral ossification processes and might be responsible for observed growth phenomena during callus distraction. To study the mechanisms of angiogenesis in soft tissue during unilateral callus distraction, lengthening of the right tibia was performed in 12 beagles. After osteotomy, application of a ring fixator and after five latency days, distraction was started for 25 days. A control group of four additional beagles underwent no surgical procedure. Subsequent to the distraction period (Group A), muscle samples from six beagles were taken from the distracted side (ds) and the contralateral non-distracted side (n-ds), six beagles underwent an additional consolidation period of 25 days (Group B). Samples were analyzed for VEGF, VEGFR-1 and VEGFR-2 mRNA expression using real-time PCR and protein expression using Western Blot analysis. Muscles from both extremities showed significantly increased expression of VEGF and its cognate receptors VEGFR-1/2. Expression decreased significantly after the consolidation period, whereby the level at the non-distracted side decreased more than the level at the distracted side. Interestingly VEGF and VEGFR-1 levels at the non-distracted side were significantly higher than at the distracted side. In contrast VEGFR-2, the receptor that mediates endothelial cell proliferation, showed higher levels at the distracted than at the non-distracted side. These findings indicate that callus distraction results not only in locally increased expression of VEGF and its receptors, but leads also to increased VEGF and VEGFR-1/2 levels at distant sides and might therefore be responsible for the observed growth phenomena during callus distraction.