Responses to norepinephrine resistant to inhibition by alpha and beta adrenoceptor antagonists.

The mode of action of clonidine and norepinephrine (NE) has been investigated in the isolated transmurally stimulated guinea-pig ileum preparation using rauwolscine, idazoxan and benextramine as antagonists. Both clonidine and NE produced concentration-dependent inhibition of the cholinergically induced twitch response and were antagonized by rauwolscine and idazoxan, but only clonidine was antagonized in a truly competitive fashion. Benextramine, an irreversible alpha adrenoceptor antagonist, in a concentration of 10(-5) M, inhibited the effect of clonidine completely but only partially antagonized the inhibitory action of NE. The antagonism of NE by rauwolscine and benextramine was most pronounced after reserpine pretreatment and blockade of neuronal and extraneuronal uptake. Under these conditions, the concentration-effect curve to NE remaining after treatment with benextramine showed an IC50 of about 3 X 10(-7) M and an intrinsic activity of 0.6. This curve was resistant to further inhibition by clonidine (10(-5) M), phentolamine (3 X 10(-6) M), rauwolscine (3 X 10(-6) M), prazosin (10(-6) M) and propranolol (1.2 X 10(-5) M). Thus, alpha and beta adrenergic receptors do not appear to be involved. It is postulated that NE-induced inhibition of the guinea-pig ileum twitch response is mediated by two distinct sites: one is the classical alpha-2 adrenoceptor (the site of action of clonidine and the alpha adrenoceptor antagonists) whereas the other is a site seemingly unrelated to the alpha and beta subtypes.