BK1 and BK2 bradykinin receptors in the rat duodenum smooth muscle

1 The dual action of bradykinin (relaxation and contraction) on the rat duodenum was investigated by studying its effect on adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) levels in cultured duodenal smooth muscle cells, and the effects of apamin on the isolated muscle responses to agonists and antagonists of BK1 and BK2 receptors. 2 No change was observed in the cyclic AMP content of cultured cells incubated with up to 100 nm bradykinin. 3 Apamin (100–500 nm) inhibited the relaxant component and enhanced the contractile component of the responses to bradykinin and to the BK2‐specific analogue [Thi5,8,d‐Phe7]‐bradykinin. 4 Apamin (100–500 nm) did not affect the contractile response of stretched duodenum preparations to the BK1‐specific agonist des‐Arg9‐bradykinin. 5 The BK2 antagonist, [d‐Arg0Hyp3Thi5,8,d‐Phe7]‐bradykinin, at a concentration which completely inhibited the relaxant response to bradykinin and to [Thi5,8,d‐Phe7]‐bradykinin, also prevented the contraction in response to either agonist in the presence of apamin. 6 Our results demonstrate two populations of bradykinin receptors in rat duodenum: a BK2 subtype responsible for the biphasic response of the non‐stretched duodenum, and a BK1 subtype responsible for the contractile effect on the stretched tissue.