Beta-adrenergic receptors regulating vascular smooth muscle tone are only localized to the intraocular segment of the long posterior ciliary artery in bovine eye.

Beta-adrenergic drugs are important drugs in glaucoma treatment. Their exact mechanism of action is not yet fully understood but a decreased perfusion pressure in the ciliary body due to blockade of vasodilatory beta-adrenoceptors is thought to participate in the reduction of intraocular pressure. This study investigates the vasodilator action of beta adrenergic-agents in intra- and extraocular arteries from bovine and human eyes. Ring segments of retinal, choroidal, intraocular segment of long posterior ciliary artery, and segments of extraocular posterior ciliary arteries from bovine eyes and short posterior ciliary arteries from freshly enucleated human eyes were mounted on an isometric myograph and their reactivity to beta-adrenergic drugs were studied. Of all vessel types examined only the intraocular segment of long posterior ciliary artery precontracted with prostaglandin F2 alpha responded to 1-isoprenaline (1 nM-1 microM) with a graded concentration-dependent relaxation (32 +/- 7%, n = 10) and a pD2 of 7.5 +/- 0.2 concentrations of 1-isoprenaline greater than 1 microM induced contractions which were antagonized by a combined blockade of alpha1- and alpha2-adrenoceptors. Salbutamol (selective beta2-agonist) induced relaxations of similar magnitude as that of 1-isoprenaline but the sensitivity of the intraocular segment of long posterior ciliary arteries to salbutamol was 312 times less than that to 1-isoprenaline, pD2 7.2 +/- 0.2 and 4.4 +/- 0.1 (n = 6), respectively, whereas dobutamine (selective beta 1-agonist) had no effect between 1 nM and 10 microM. Betaxolol (selective beta 1-antagonist) and 1CI 115811 (selective beta 2-antagonist) competitively antagonized isoprenaline-induced relaxations giving pKB-values of 5.9 +/- 0.1 (n = 11) and 8.5 +/- 0.1 (n = 6), respectively. The slope of the Schild-plots were equal to unity. Human posterior ciliary arteries with spontaneous tone or tone induced by 10 microM PGF2 alpha did not react to isoprenaline (1 nM-10 microM). The bovine intraocular segment of long posterior ciliary arteries contracted to 1-noradrenaline without alpha-adrenoceptor blockade and addition of propranolol or cocaine did not change the concentration-response curve. 1-Phenylephrine and B-HT933 induced both potent and strong contractions indicating the presence of both alpha 1- and alpha 2-adrenoceptors in these arteries. Although beta-adrenoceptors have been identified in various parts of the ocular circulation by radio-ligand binding techniques our data show that beta-adrenoceptors linked to regulation of vascular tone are only found in intraocular branches of the intraocular segment of long posterior ciliary artery in bovine eyes.(ABSTRACT TRUNCATED AT 400 WORDS)