Adrenergic Innervation and Drug Responses of the Oval Sphincter in the Swimbladder of the God (Gadus morhua)1

The effects of drugs and of electrical stimulation of nerves on the circular and radial smooth muscles of the oval sphincter in the cod swimbladder have been studied. Opening of the oval seems to be caused by liberation of noradrenaline from vagal fibres in the muscularis mucosae. The noradrenaline relaxes the circular muscles by a beta effect, and contracts the radial muscles by an alpha effect. Acetylcholine contracts both circular and radial muscles, but no definite evidence for the existence of cholinergic fibres have been obtained. Closing of the oval may occur due to lack of adrenergic impulses. Adrenergic varicose nerve fibres have been observed running in both circular and radial muscle bundles, by use of a fluorescent histochemical method.