Changes in smooth muscle sensitivity to agonist drugs during development.

The receptors in the expansor secundariorum muscle of chicks were characterized pharmacologically and the changes in their response to nerve stimulation and agonist drugs determined during development. The muscle responded to noradrenergic nerve stimulation, noradrenaline and 5-hydroxytryptamine without any change in sensitivity during development. Expansor muscles from 15-day-old chicks were more sensitive to isoprenaline than muscles from older animals. The muscle from 15-day-old chicks responded to acetylcholine and histamine; the sensitivity to both drugs decreased progressively with increasing age of the chicks and disappeared by day 40 posthatching. The normal developmental decrease in response to acetylcholine and histamine were prevented by surgical denervation of the muscle; an intervention that also induced supersensitivity to noradrenaline greater than isoprenaline greater than 5-hydroxytryptamine. The muscle responded to potassium chloride without any change in sensitivity during development or following surgical denervation. These findings indicate that sympathetic nerves influence the responsiveness of the expansor secundariorum muscle to drugs, especially the development decrease in response to acetylcholine and histamine.