Alpha2-adrenergic and VIP receptors in rabbit ciliary processes interact

Abstract

The interaction between alpha₂-adrenergic and VIP receptors has been studied by examining inhibition of VIP-stimulated cyclic AMP production by adrenergic agonists in intact, excised rabbit ciliary processes. Epinephrine, norepinephrine, isoproterenol, dopamine, and the specific alpha₂-adrenergic agonists clonidine and p-aminoclonidine exhibit dose-dependent inhibition of VIP-stimulated cyclic AMP production. I₅₀s, clonidine (0.05 μM) = p-aminoclonidine (0.05 μM) = epinephrine (0.1 μM) < norepinephrine (2.0 μM) < isoproterenol (15 μM) = dopamine (15 μM), are consistent with the characteristic binding affinities of these adrenergic agonists for alpha₂-adrenergic receptors. Inhibition of VIP-stimulated cyclic AMP production by clonidine, epinephrine, isoproterenol, and dopamine is blocked by yohimbine but not by prazosin. These data establish the alpha₂-adrenergic specificity of the inhibitory effects observed. We have previously shown that beta₂-adrenergic receptor-mediated stimulation of cyclic AMP production in rabbit ciliary processes is also inhibited by postjunctional alpha₂-adrenergic receptors. These studies support the idea that the catecholamines may regulate aqueous humor formation by inhibiting stimulation of cyclic AMP production via postjunctional alpha₂-adrenergic receptors in ciliary processes.