The contractile effect of bombesin on the rat isolated urinary bladder

We have investigated and compared the myotropic effects of bombesin (BB) and carbachol (C) in the rat isolated urinary bladder. BB (0.5 x 10(-9) to 0.5 x 10(-5) M) and C (2.7 x 10(-8) to 5.4 x 10(-5) M) were found to produce dose-dependent increases of the basal tone of the rat detrusor muscle. The maximal contraction produced by C was about 4 times greater than that elicited by BB or substance P (SP). However, the threshold concentrations of BB and SP required to stimulate the detrusor muscle were much lower than those of C. The pD2 (-log ED50) values of BB, SP and C were respectively 7.63, 7.05 and 5.8. The tissues exposed to BB relaxed more slowly after washout than those challenged with C or SP. The contractile effects of medium range concentrations of BB were not affected by pretreating the tissues with tetrodotoxin, atropine, antihistaminics, indomethacin, alpha- and beta-adrenergic blockers, methysergide or 8-leucine-angiotensin II. Tissues desensitized with high concentrations of bradykinin maintained their sensitivity to BB. The result suggest that the contractile effect of BB on the rat isolated urinary bladder is likely to be the result of a direct effect on the smooth muscle cells.