Cirazoline, an alpha 2-adrenoceptor antagonist in guinea-pig ileum

Prejunctional effects of cirazoline have been investigated in guinea pig ileum. At high concentrations cirazoline has been shown to have antimuscarinic activity, pA2 5.25 +/- 0.29. At concentrations below those producing blockade of acetylcholine, cirazoline blocks the prejunctional alpha 2-adrenoceptor activity of clonidine, pA2 6.81 +/- 0.22, and alpha-methylnoradrenaline. Results are discussed in the light of controversial evidence for the activity of cirazoline on alpha-adrenoceptors.