Lack of involvement of basic fibroblast growth factor (FGF-2) in capillary growth in skeletal muscles exposed to long-term contractile activity.
Fibroblast growth factors (FGFs) are involved in stimulation of angiogenesis in tumors and other pathological circumstances. Increased activity of normal skeletal muscles resulting from chronic electrical stimulation is a very potent stimulus for capillary growth but a relationship between the initiation of this angiogenesis and the involvement of autocrine growth factors has yet to be established. Although FGF expression has been reported in muscles stimulated for 3 weeks, capillary growth is underway significantly earlier, beginning around 3 days. The present experiments have therefore studied the possible involvement of basic fibroblast growth factor (FGF-2) in stimulated rat fast skeletal muscles prior to, and coincident with, capillary growth. Muscle contractions were induced via electrodes implanted in the vicinity of the peroneal nerve and maintained for 8h/day for 2, 4 or 7 days. Capillary/fiber ratio (C/F), based on staining of capillary endothelium for alkaline phosphatase, was not changed in either extensor digitorum longus (EDL) or tibialis anterior (TA) after 2 days stimulation, but increased in TA stimulated for 4 days and in both muscles after 7 days. The expression of mRNA for FGF-2, detected by ribonuclease protection assay, was decreased in all stimulated muscles compared with control or contralateral muscles; immunohistochemistry showed FGF-2 gene product in nerves and larger blood vessels but not in capillaries. There was no evidence from immunohistochemistry for up-regulation of receptors flg and bek for FGF-2. The presence of FGF-2, flg and bek in arterioles may indicate a possible role for FGF-2 in the regulation of blood flow since we have previously shown it to be a dilator of small arterioles. However, based on the lack of correlation between changes in capillary density and the expression of mRNA and protein for FGF-2 and its receptors, it is unlikely that it is directly linked with the initiation of angiogenesis resulting from chronic activity in skeletal muscles.