Potentiation of the contractile response to acetylcholine in aortic strips by low concentrations of vascular contractile agonists.

1 The potentiating effect of a low concentration of various vascular contractile agonists on the response of rabbit aortic strips to acetylcholine was examined. 2 A marked parallel displacement to the left (potentiation; not additive effect) of the dose-response curve for acetylcholine was observed when 5-hydroxytryptamine (5-HT), noradrenaline, histamine, angiotensin II or KCl were added at a concentration that caused only slight contraction (0.2 to 0.3 g) in the strips. This leftward displacement (potentiation) of the dose-response curve by these agonists was reversed to the control response curve by the respective blocking agents for each agonist, although the blocking agents themselves did not shift the acetylcholine dose-response curve to the right. Atropine produced a rightward displacement of the dose-response curve for acetylcholine. 3 The addition of an extremely low concentration of 5-HT or KCl which did not cause contraction, also produced a potentiation. 4 The order of potency of the agonists in potentiating the acetylcholine-induced contraction was KCl > 5-HT greater than or equal to histamine greater than or equal to angiotensin II > noradrenaline. 5 The contractile response of aortic strips to carbamylcholine was also potentiated by these agonists. 6 The results suggest that the potentiation by these agonists was not mediated through muscarinic receptors but through the respective receptors for each agonist.