Effects of gastrointestinal hormones on vascular smooth muscle

The effects of the gastrointestinal hormones secretin and cholecystokinin (CCK) were investigated on rat portal-vein mechanical activity. Doses of 0.03-0.60 units/ml Boots secretin; 0.18-0.60 units/ml pure, natural secretin; and 0.12-0.60 units/ml CCK elicited dose-dependent decreases in amplitude and simultaneous increases in frequency of spontaneous contractions that were unaffected by alpha-or beta-adrenergic or cholinergic blocdade. These effects were mimicked by isoproterenol, theophylline, and cyclic AMP, and all agents tested were also effective in reducing K-or norepinephrine-induced tension. Additionally, subthreshold doses of CCK and secretin elicited responses when given in combination, and the effects of the hormomes at all doses were potentiated by theophyline, indicating an interaction berween these hormones on rat portal vein thatmay involve cyclic AMP. It is suggested that part of the mechanism of hormone-induced mesenteric vasodilatation may involve a direct relaxing effect of the hormones on vascularsmooth muscle.