A comparison between ATP and bradykinin as possible mediators of the responses of smooth muscle to non-adrenergic non-cholinergic nerves.

The effects of ATP, bradykinin (BK) and electrical stimulation of intramural non-adrenergic non-cholinergic nerves (NS) were compared in four smooth preparations. In the guinea-pig taenia caeci and rat duodenum, ATP (10(-7)-5 x 10(-5) M) and BK (5 x 10(-10)-10(-7) M) closely mimicked the response to NS. The relaxations to BK, but not to ATP or NS, were inhibited by carboxypeptidase B (3-15 U/ml) and apamin (10(-8)-5 x 10(-8) M) prevented the relaxations to all three stimuli. BK contracted the guinea-pig distal colon whereas ATP and NS caused inhibition. In the guinea-pig bladder, ATP and NS induced rapid phasic contractions whereas BK caused tonic contractions. In the latter two preparations, incubation with indomethacin failed to reveal any BK relaxation. In view of its failure to mimic the nerve-mediated response in two of the tissues, and of its selective inhibition by carboxypeptidase B in the other two, BK is less likely to be the transmitter in non-adrenergic non-cholinergic nerves supplying smooth muscle.