The contractile effect of bombesin, gastrin releasing peptide and various fragments in the rat stomach strip

Abstract

The contractile activity of bombesin (BB) and related peptides including the gastrin releasing peptide (GRP) was evaluated in the rat stomach strip. BB and GRP were found to elicit concentration-dependent contractile effects in concentrations varying between 5.0 × 10 −10 and 5.0 × 10 −7 M. EC 50 values of BB and GRP were 6.5 × 10 −9 and 10 −8 M, respectively. The contractile effect of BB (1.5 × 10 −8 M) was not modified by a mixture of antagonists containing atropine (1.7 × 10 −6 M), mepyramine (1.2 × 10 −6 M), methysergide (1.4 × 10 −6 M), phentolamine (1.8 × 10 −6 M), propranolol (1.9 × 10 −6 M) and indomethacin (7.0 × 10 −6 M), nor by the nerve paralyzing drug tetrodotoxin (1.6 × 10 −6 M). These results suggest the existence of BB receptors in the rat stomach strip. The results also suggest that the contractile effects of BB in this tissue result from a direct effect on the smooth muscle cell. The data derived from our structure-activity study indicate that deletion of the N-terminal sequence pGlu 1-Gln 2-Arg 3-Leu 4-Gly 5- from the BB molecule causes practically no loss of affinity and intrinsic activity. Further shortening of BB gives rise to a gradual reduction of both parameters. Residual contractile activity could still be observed with the tetra- and pentapeptide BB-(11–14) and BB-(10–14) at 10 −5 M. On the other hand, deletion of only two (Leu 13-Met 14) or three (His 12-Leu 13-Met 14) amino acids from the C-terminus gave BB fragments with low affinities (BB-(1–12) and BB-(1–11)) and, in the case of BB-(1–11), a reduced intrinsic activity. GRP-(14–27) and acetyl-GRP-(20–27) were found to be either as potent (GRP-(14–27)), or slightly more potent (Ac-GRP-(20–27)) than BB itself. GRP and its two fragments exhibited the same intrinsic activity as BB. These results underline the essential contribution of the chemical groups scattered in the C-terminal region of BB and GRP for their myotropic effects in the rat stomach strip. The sequence His 12-Leu 13-Met 14 seems to be critical to provide full activation of BB receptors in the rat stomach strip since BB-(1–11) exhibited only 50% of the intrinsic activity of BB.