Changes in acetylcholine and noradrenaline sensitivity of chick smooth muscle wholly innervated by sympathetic nerve during development.

Developmental changes in sensitivity of the isolated expansor sedumdariorum muscle of posthatching chicks to noradrenaline (NA), Acetylcholine (ACh) and some other drugs were investigated. This muscle responded to both NA and ACh in early life. The sensitivity to ACh decreased progressively with increasing age and disappeared on day 40 after hatching, however, a corresponding elevation of cholinesterase activity was not observed. The sensitivity to NA remained at the same level during the period of 2-60 days after hatching. The Contractile action of ACh on this muscle was not affected by d-tubocurarine, hexamethonium or phentolamine, but was completely abolished by atropine. These results suggest that there are at least two kinds of responsive sites on the expansor secumdariorum muscle in the new-born chick and that the sites sensitive to ACh degenerate progressively during the developmental processes. The cholinergic sensitive sites of this muscle in the new-born chick may be muscarinic.