α2‐Adrenoceptor agonists potentiate responses mediated by α1‐adrenoceptors in the cat nictitating membrane

Alpha 1 but not alpha 2-adrenoceptors mediate contractions of the cat nictitating membrane. The contractions of this tissue evoked by alpha 1-adrenoceptor agonists, but not those evoked by angiotensin II, are potentiated by pre-dosing with alpha 2-adrenoceptor agonists. This potentiation is reversed by the alpha 2-adrenoceptor antagonist, WY 26392. Pressor responses evoked by alpha 1-adrenoceptor agonists or angiotensin II were not affected by alpha 2-adrenoceptor agonists. Contractions of the nictitating membrane evoked by noradrenaline were reduced by pretreatment with WY 26392. These results suggest that in some tissues the role of alpha 2-adrenoceptors may be to modulate responses to alpha 1-adrenoceptors, rather than to evoke a discrete response themselves.