Cyclic AMP induces a relaxation response in the bullfrog Rana catesbeiana, but nitric oxide does not.

Cholecystokinin octapeptide (CCK), acetylcholine (ACh) and ceruletide have been shown to produce contraction in bullfrog (Rana catesbeiana) gallbladder strips. Agents capable of relaxing the bullfrog gallbladder are less numerous. Calcitonin gene-related peptide reduced the amount of both CCK- and ACh-induced tension in bullfrog gallbladder strips. The purpose of this study was to determine whether vasoactive intestinal peptide (VIP), nitric oxide (NO) and the second messengers cyclic GMP or cyclic AMP had any effect on gallbladder motility in the bullfrog. In vitro tension studies using l-NG-nitro-arginine methyl ester, Methylene Blue, sodium nitroprusside and N2,2'-O-dibutyryl guanosine 3',5'-cyclic monophosphate suggested that nitric oxide did not modulate gallbladder motility in the bullfrog gallbladder. Histochemical staining for NADPH diaphorase (nitric oxide synthase) failed to demonstrate nerve fibers containing nitric oxide synthase in the bullfrog gallbladder. In vitro studies demonstrated that VIP had no effect on CCK-induced tension. However, in vitro studies using either 8-bromoadenosine 3',5'-cyclic monophosphate or forskolin demonstrated that both agents relaxed strips precontracted with CCK. The results of this study suggested that, while neither NO nor VIP had a role in modulating bullfrog gallbladder motility, cyclic AMP was capable of modulating bullfrog gallbladder motility.