Mechanism of gallbladder relaxation in the cat: role of norepinephrine.

We investigated the mechanisms of neurally mediated relaxation of cat gallbladder muscle. Muscle strips from the gallbladder corpus placed in the muscle bath with oxygenated Krebs' solution developed spontaneous active tension. Tension was measured with isometric force transducers, and muscle relaxation was expressed as percent decrease of active basal tension. Electrical field stimulation (EFS) evoked a tetrodotoxin-sensitive and hexamethonium-insensitive frequency-dependent relaxation with a maximal relaxation at 20 Hz. Gallbladder muscle strips also relaxed in response to increasing concentrations of vasoactive intestinal peptide (VIP), isoproterenol and, after pretreatment with phentolamine, norepinephrine. Nitric oxide synthase inhibitors N omega-nitro-L-arginine and N omega-nitro-L-arginine methyl ester at a concentration of 100 microM, which blocked EFS-induced relaxation in the lower esophageal sphincter, had no significant effect on EFS-induced gallbladder muscle relaxation. The VIP antagonists VIP10-28 and [4Cl-D-Phe6,Leu17]VIP at a concentration of 10 microM that blocked exogenous VIP-induced gallbladder relaxation also had no effect on the relaxation caused by EFS. In contrast, either propranolol or guanethidine at concentrations of > or = 1 microM significantly reduced EFS-evoked gallbladder relaxation (P < .01, analysis of variance). It is concluded that norepinephrine utilizing beta adrenergic receptors mediates EFS-stimulating postganglionic intramural neurons in the cat gallbladder.