Interactions of vasoactive intestinal polypeptide and cholecystokinin octapeptide on the control of gallbladder contraction.

The gallbladder is supplied by three types of vagal fibers: cholinergic, cholecystokinin (CCK)-ergic, and vasoactive intestinal polypeptide (VIP)-ergic. Most previous studies on the interaction of VIP and CCK on gallbladder contraction have been in vitro. In this study we evaluate this interaction more fully in vivo using six dogs with chronic bile fistula. Dose-response curves were constructed to CCK-8 alone and to VIP alone. The effect of graded doses of VIP on maximal response to CCK-8 was also studied. CCK-8 caused the expected dose-related stimulation of gallbladder contraction, while graded doses of VIP had the opposite effect. VIP also caused a dose-related inhibition of the maximal response to CCK-8, decreasing bilirubin output from 39 +/- 8 to 15 +/- 3 mg/hr (p less than 0.05). The corollary to these findings is that gallbladder tone and contraction is regulated by the interplay of stimulatory cholinergic-CCK-ergic and inhibitory VIP-ergic fibers. Further, a plausible explanation for the variable effects of vagotomy on gallbladder contraction is that variable proportions of these opposing fibers are cut.