Substance P in subdivisions of the sciatic nerve, and in red and white skeletal muscles.

Substance P (SP) has been determined by radioimmunoassay in the sciatic nerve and its fibular, tibial and sural branches; and in a red, slow-twitch and a white, fast-twitch muscle supplied by the sciatic nerve. The mixed sciatic nerve contained 25 ng SP per g tissue wet wt., while the sural branch, which is supplying mainly skin, and thus is rich in sensory fibres, had a significantly higher content (49 ng/g). The mixed fibular and tibial branches contained approximately the same amount as the sciatic nerve proper. SP was also found in the skeletal muscles. The red m. soleus had a significantly higher content (0.61 ng/g) than the white m. extensor digitorum longus (0.22 ng/g). A dorsal root lesion which leads to degeneration of sensory fibres, reduced the SP level more than 90% both in nerves and muscles. Ventral root section, which leads to degeneration of somato-motor fibres and terminals, had, on the other hand, no effect. We conclude that sensory neurons are the only source of importance for SP in the sciatic nerve and in skeletal muscles. Based on the above findings, the possibility that SP may function as a mediator of an axon reflex in skeletal muscle is discussed.