Angiogenesis may be induced in skeletal muscle by metabolic factors, or mechanical factors including overload. In the latter case it is thought to be initiated by muscle stretch, rather than the more usual increase in blood flow (Egginton et al. 1998). We therefore sought to regulate the degree of angiogenesis by varying the degree of overload, and determined the extent of neovascular formation. Male Sprague-Dawley rats underwent unilateral surgery (2 % Fluothane in oxygen), randomised with respect to side, and treated post-operatively with analgesics and antibiotics (Temgesic, Duplocillin), in accordance with the Animals (Scientific Procedures) Act, 1986. In one group the m. tibialis anterior (TA) was extirpated under aseptic conditions, and animals allowed to recover for 4, 7 or 14 days (n = 3, 6 and 5, respectively) before the m. extensor digitorum longus was sampled. In a second group the distal tendon of the TA was sectioned and EDL samples taken at the same time points (n = 4, 7 and 14, respectively). A third group of unoperated animals acted as controls (n = 8). All animals were humanely killed. Serial 8 µm cryostat sections were immunolabelled for proliferating cell nuclear antigen (PCNA) located at the position of capillaries, identified by histochemical staining for alkaline phosphatase by the indoxyl tetrazolium method.

The transient increase in density of PCNA positive capillaries following extirpation is similar to that described elsewhere (Rivilis et al. 2002), with the continued increase in total capillary density reflecting the translation of cell turnover into new vessels. A similar pattern was observed for labelling indices of both capillaries and muscle fibres, supporting the contention that overload-induced angiogenesis is primarily a mechanical response. The reduced and delayed response following tenotomy is consistent with a smaller degree of overload when the agonist muscle was maintained in situ.