Differential Effects of α -Adrenoceptor Agonists on Human Retinal Microvessel Diameter

Abstract

The effects of locally administered brimonidine, clonidine, and p-aminoclonidine on microvessel caliber were compared in human retinal tissues grafted into the hamster cheek pouch. Clonidine and p-aminoclonidine, but not brimonidine, potently constricted human retinal microvessels over a broad concentration range. All three agonists elicited significant vasoconstriction in naive hamster cheek pouch microvasculature. The α₂-adrenoceptor antagonist, rauwolscine, inhibited p-aminoclonidine-induced constriction in naive hamster cheek pouch microvessels, but not p-aminoclonidine-induced effects in retinal grafts. Selective α₁-adrenoceptor agonists evoked vasoconstriction in retinal grafts only at relatively high concentrations. These differential effects on the retinal microvasculature could not be readily explained solely on the basis of α₁- or α₂-adrenoceptor involvement. Clonidine, p-aminoclonidine and brimonidine are also imidazoline derivatives that interact with putative non-adrenergic imidazoline-sensitive binding sites, the so-called I₁-imidazoline binding site subtype implicated by some investigators in mediation of peripheral vasoconstriction. As with p-aminoclonidine, the potent vasoconstriction in human retinal microvasculature elicited by moxonidine, an α-adrenergic agonist that has also been reported to exhibit selectivity for putative I₁-imidazoline binding sites, was not inhibited by the selective α-adrenoceptor antagonist, rauwolscine, nor by idazoxan, an antagonist characterized as having substantial activity at putative I₂-imidazoline binding sites. These data suggest the possible involvement of an unconventional non-adrenergic imidazoline-sensitive pathway in regulation of microvascular responses in the inner retina, and that drug activity mediated via such an imidazoline-sensitive component could potentially evoke deleterious effects in the retinal microvasculature.