Differences in imidazoline and phenylethylamine alpha-adrenergic agonists: comparison of binding affinity and phosphoinositide response.

The imidazoline class of compounds, reported to be partial agonists at alpha 1 adrenoceptors, were compared with phenylethylamines for their ability to displace the binding of [3H]prazosin and to stimulate hydrolysis of phosphoinositides in the cerebral cortex of the rat. Both classes of alpha adrenoceptor compounds exhibited two sites of interaction with binding sites for [3H]prazosin in 30 mM Tris buffer. In a Na+ containing ionic buffer, the competition by phenylethylamines for [3H]prazosin sites shifted to a one-site best-fit, while imidazolines retained their two-site best-fit. Phenylethylamines stimulated hydrolysis of phosphoinositides in a dose-dependent manner, with ED50 values that correlated with Kd values from competition curves. In contrast, imidazolines were not potent or efficacious at stimulating hydrolysis of phosphoinositides and the binding affinities did not correlate with the ED50 values. The alpha 1 adrenoceptor antagonist, prazosin potently inhibited phenylethylamine, but not imidazoline-stimulated hydrolysis of phosphoinositides. Dose-response curves to the imidazoline, oxymetazoline, in the presence and absence of maximally stimulating concentrations of norepinephrine, indicated that oxymetazoline caused a dose-dependent inhibition of norepinephrine-stimulated hydrolysis of phosphoinositide. The inhibition of norepinephrine-stimulated hydrolysis of phosphoinositides was evident up to 100 microM, at which point oxymetazoline elicited hydrolysis of phosphoinositides through a non-alpha 1 adrenoceptor-mediated mechanism. These data indicate that imidazolines act primarily as antagonists at the alpha 1 adrenoceptor, coupled to hydrolysis of phosphoinositide and stimulate the hydrolysis of phosphoinositide through a non-alpha 1 adrenoceptor mechanism.