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

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Abstract
1 α1 but not α2-adrenoceptors mediate contractions of the cat nictitating membrane. 2 The contractions of this tissue evoked by α1-adrenoceptor agonists, but not those evoked by angiotensin II, are potentiated by pre-dosing with α2-adrenoceptor agonists. 3 This potentiation is reversed by the α2-adrenoceptor antagonist, WY 2639. 4 Pressor responses evoked by α1-adrenoceptor agonists or angiotensin II were not affected by α2-adrenoceptor agonists. 5 Contractions of the nictitating membrane evoked by noradrenaline were reduced by pretreatment with WY 2639. 6 These results suggest that in some tissues the role of α2- adrenoceptors may be to modulate responses to α1-adrenoceptors, rather than to evoke a discrete response themselves.