Neurotensin and cholecystokinin contract gallbladder circular muscle in chickens

The contractile effects of neurotensin (NT) and cholecystokinin octapeptide (CCK-8) on isolated circular smooth muscle strips of chicken gallbladder were investigated. The NT (0.25-300 nM) produced concentration-dependent contractions on smooth muscle with an EC₅₀ of 8.5 nM (95% confidence limits = 5.3-13.6 nM). In comparison, CCK-8 produced concentration-dependent contractions with an EC₅₀ of 13 nM (95% confidence limits of 9-20 nM). There were no statistical differences in contractile responses when comparing NT and CCK-8 at equimolar concentrations. The NT appears to act directly on smooth muscle tissue in the chicken; the contractile responses were not blocked by 10 μM atropine or tetrodotoxin.A portion of the activity is mediated by extracellular calcium as 100 nM nifedipine inhibited 30% of peptide-induced muscle tension. The NT receptor (NTR) type 1 antagonist SR 48692 (0.1 μM) did not significantly reduce NT potency. The contractile effects of CCK-8 remained unaltered in tissues pretreated with atropine, TTX, or nifedipine. The CCK-A antagonist lorglumide, at a concentration of 1 μM, reduced the contractile potency of CCK-8 by one-half. Avian receptors for NT and CCK may differ pharmacologically from their mammalian counterparts, but their contractile actions on the gallbladder resulting in increased biliary output by flow are further evidence of their role in the postprandial regulation of lipid digestion in chickens.