In vitro studies of the effects of beta-adrenergic drugs on retinal and posterior ciliary microarteries

The small-vessel myograph allows for precise measurements of physiopharmacologic responses of the ocular microarteries under controlled conditions. Studies using the myograph have shown that beta-adrenergic agonists are unable to induce significant relaxation in retinal and posterior ciliary microarteries, indicating that these microarteries have very few or no functional beta-adrenoceptors. Thus, beta-blockers would not be expected to have important adverse vasoconstrictory effects in the posterior part of the eye that are caused by their beta-adrenoceptor binding capacities. On the contrary, some beta-blockers, such as propranolol (the standard beta-blocker in pharmacology) and betaxolol (a beta-blocker used in ophthalmology), have vasorelaxant effects, probably a result of their Ca2+ channel-blocking activity. This activity shows no stereospecificity. Betaxolol could thus act as a vasodilator in glaucoma patients, on the condition that it penetrates in the posterior part of the eye after topical application. If so, it could also induce vasodilatation in circumstances of vascular endothelium injury, because this effect is endothelium-independent.