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Functional Study of the Excitatory α‐Adrenoceptor in Guinea Pig Ileum, in Relation to the Hypothesis of an Atypical Receptor

According to functional studies, the al,-adrenoceptor subtype has a high affinity for phentolamine, WB-4101,5-methyl-urapidil and (+)niguldipine, and is not inactivated by the irreversible antagonist chloroethylclonidine (CEC). On the contrary, the a,,-adrenoceptor subtype is blocked by CEC, but has a lower affinity for the other antagonists.' It has been previously suggested that there are several populations of excitatory a-adrenoceptors in guinea pig terminal ileum? In contrast, it has been recently advanced that the guinea pig ileum has a single population of atypical excitatory a,-adrenoceptors, because of the lack of inhibitory effect of CEC and low affinity for 5-methylurapidl and WB-4101, tested against phenylephrine.' Our objective was to verify if concentration-response curves for epinephrine (Epi), norepinephrine (NE), and phenylephrine (Ph) are shifted by the antagonists prazosin (Pz), indoramin (In), piperoxane (Pip) and W-4101 according to the expectations for an interaction with a single population of adrenoceptors.