Effects of cod bradykinin and its analogs on vascular and intestinal smooth muscle of the Atlantic cod, Gadus morhua

The effects of [Arg⁰,Trp⁵,Leu⁸]-BK (cod [Arg⁰]BK) on vascular preparations from branches of the cod celiac artery and on longitudinal smooth muscle preparations from the cod intestine were investigated. Cod [Arg⁰]BK (3 × 10^-8 M) caused a relaxation of the celiac artery precontracted with adrenaline. The relaxation was abolished by the cyclooxygenase inhibitor indomethacin, suggesting that the effect is mediated through the release of prostaglandins, but there was no evidence for the involvement of leukotrienes or nitric oxide in the response. In the intestinal preparations, cod [Arg⁰]BK produced concentration-dependent contractions (pD₂ = 8.28 ± 0.16). Experiments with N-terminally and C-terminally truncated analogs and with alanine-substituted analogs of cod [Arg⁰]BK demonstrate that the central amino acid Gly⁴ and the C-terminal amino acids Leu⁸ and Arg⁹ are the most important in determining the conformation of the peptide that interacts with the receptor. The results indicate that the ligand binding properties of the cod BK receptor are considerably different from the receptor present in trout tissues and may resemble those of the mammalian B₂ receptor more closely.