Actions of imidazolines on bovine ciliary artery

A segment of bovine ciliary artery exhibits nonlinear biphasic contractile responses to agonists. The maximal responses of oxymetazoline (OMZ) and norepinephrine (NE) were 55% and 45%, respectively (KCl = 100%). The relative potency comparison based on the ED50 of the first component of the dose-response curve was OMZ; 1 > NE; 1/7 > phenylephrine; 1/72. At equipotent doses of NE and OMZ, the t1/2 duration of response of the latter agonist was 43 times greater than that of the former and, after 3 hrs, responses were 95% reproducible. Naphazoline, tetrahydrozoline and para-aminoclonidine did not produce significant contraction of the blood vessel but did antagonize the action of the agonists. As compared to that of NE, naphazoline was selective in blocking responses of OMZ. Phentolamine blocking action of NE was greater than that of OMZ. Nifedipine, 100 nM, reduced the second component of OMZ response. Although the vascular action of NE was blocked by prazosin, the contractile action of OMZ was totally resistant to block by 1 microM of prazosin. These results indicate that ciliary artery contains a subtype of alpha-adrenoceptors (alpha 11) with low sensitivity to prazosin. The tissue also contains a unique OMZ sensitive site or imidazoline receptors which exhibit low affinity to alpha-adrenoceptor blockers.