| Summary: | The rate of gas secretion into the swimbladder of the Atlantic cod Gadus morhua was studied with X-ray measurements of swimbladder size. A direct correlation was found between the area of the swimbladder in the X-ray photographs and the volume of the swimbladder. Removal of the swimbladder gas stimulates the fish to produce new gas, and the mean rate of refilling of the swimbladder in the cod was found to be 1.5 ml kg-1 h-1 during the first 24 h. The influence of drugs and of vagotomy on the rate of filling was measured. Gas secretion was completely inhibited by vagotomy and partly inhibited by injections of atropine or porcine vasoactive intestinal polypeptide (VIP). Substance P and VIP both increased the rate of secretion in atropinized fish, while 5-hydroxytryptamine (5-HT) and noradrenaline did not affect the rate of filling in either control or atropinized fish. Carbachol, VIP and substance P did not induce secretion in vagotomized fish, indicating that certain other fibres in the vagus might be necessary for gas secretion. It is concluded that a VIP-like and a substance-P-like peptide might be involved in the control of the secretion of gas in the cod swimbladder, VIP by a neuronal mechanism and substance P by a neuronal and/or humoral mechanism.
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