Dual effects of PACAP on guinea pig gallbladder muscle via PACAP-preferring and VIP/PACAP-preferring receptors.

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Abstract
The aims of this study were to determine the effect and mechanism of action of pituitary adenylate cyclase-activating peptide (PACAP) on gallbladder muscle. Guinea pig gallbladder muscle strips were studied isometrically. In noncontracted muscle strips, PACAP-27 and PACAP-38 caused dose-dependent contractions, whereas vasoactive intestinal peptide (VIP) caused dose-dependent relaxation. PACAP-27 contractions were resistant to tetrodotoxin, atropine, and the substance P receptor antagonist [D-Arg1,D-Trp7,9,Leu11]substance P (Spantide) but were inhibited by the selective PACAP receptor antagonist PACAP-(6-38) and slightly increased with the VIP receptor antagonist [4-chloro-D-Phe6,Leu17]VIP. In cholecystokinin-precontracted muscle strips, both VIP and PACAP caused relaxations. This relaxant effect of PACAP-27 was inhibited by PACAP-(6-38) and [4-chloro-D-Phe6,Leu17]VIP, but not by tetrodotoxin. These studies suggest that PACAP has dual excitatory and inhibitory effects on guinea pig gallbladder muscle. The contractile effect of PACAP is a direct action on muscle through PACAP-preferring receptors. The relaxant effect of PACAP is seen in precontracted muscle strips and mediated through VIP/ PACAP-preferring receptors.

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