Blockade by WB 4101 of alpha-adrenoceptors in the rat vas deferens and guinea-pig taenia caeci.

The effectiveness of WB 4101, a recently described alpha-adrenoceptor antagonist, in blocking an excitatory and two inhibitory responses to alpha-receptor activation was studied. One of the inhibitory responses was the reduction by the selective alpha-agonist amidephrine of carbachol contractures of isolated guinea-pig taenia caeci. WB 4101 antagonised this inhibition with a Schild plot slope of 0.99 and a pA2 of 8.9. The same pA2 value was obtained for blockade of the contractile effect of amidephrine and noradrenaline on the rat vas deferens. WB 4101 was, however, several hundred times less active in antagonising the inhibitory effect of clonidine on the twitch response of the vas deferens to field stimulation. Incidental observations were that the twitch was increased by low concentrations of amidephrine, and by relatively high concentrations of WB 4101. Because of its potency and postsynaptic selectivity, WB 4101 should be useful for adrenoceptor classification.