Further evidence for the existence of two receptor sites for bradykinin responsible for the diphasic effect in the rat isolated duodenum.

Low doses of bradykinin (below 10 nM), as well as of K+ (below 10 mM) induced relaxation, whereas higher doses caused contraction of the rat duodenum. The relaxant responses induced by bradykinin and K+ were not affected by ouabain (1 microM), but pre-incubation with 5.9 mM K+ abolished the responses to that ion but not those to bradykinin. The contractile and relaxant components of the response to bradykinin (but not those to K+) increased with the time elapsed after mounting of the preparation, and this was due to stretching by the load of the recording system. Specific and reversible desensitization (tachyphylaxis) was observed with the contractile response (but not the relaxation) induced by bradykinin. Des-Arg9-bradykinin, an analogue specific for B1-receptors, was much less active than bradykinin, and elicited only a contractile response. Among four bradykinin potentiating peptides that were tested, potentiator C enhanced the relaxation only, whereas BPP5a and captopril potentiated only the contraction and BPP9a potentiated both types of response to bradykinin. Our results support the hypothesis that the relaxant and contractile components of the rat duodenum's response to bradykinin are due to actions at different receptor sites, which can be distinguished by their properties (desensitization) and their different apparent affinities for agonists and for potentiating peptides.