Under conditions of insufficient vascular supply, compensatory neovascularisation is thought to be mediated by hypoxia-induced upregulation of vascular endothelial growth factor (VEGF). In skeletal muscles made chronically ischaemic by a precipitous reduction in blood supply, both VEGF protein and mRNA (in situ) were rapidly induced and marked capillary proliferation ensued (Couffinhal et al. 1998) but in other animal models of muscle ischaemia, VEGF protein was upregulated without capillary growth (Cherwek et al. 2000; Brown et al. 2002). This study investigated whether the time course of VEGF protein and mRNA expression is similarly co-ordinated in the latter situation.
One common iliac artery was ligated under Fluothane inhalation anaesthesia (2 % in O2) in thirteen male Sprague-Dawley rats. Animals were killed 1 (n = 3), 2 (n = 4) or 5 weeks (n = 6) later, together with three unoperated control rats, by pentobarbitone overdose I.P., and extensor digitorum longus muscles removed and frozen for analysis.
VEGF protein and mRNA are differentially altered during chronic muscle ischaemia, the former increasing while the latter decreases. Inhibition of VEGF mRNA expression has also been observed in cell cultures stressed by hypoxia (Stein et al. 1998). Whereas the increase in protein may arise through stabilisation of mRNA, it may also represent enhanced translation through internal ribosomal entry sites (IRES) in a cap-independent manner (Stein et al. 1998). These data indicate the variation in growth factor regulation in a realistic setting of progressive ischaemia but do not explain why capillary growth is so limited in this model.
This work is supported by the British Heart Foundation and the Rowbotham Bequest.