Effects of topical beta-blockers on the diameter of the isolated porcine short posterior ciliary artery

PURPOSE. Based on diameter measurements on the short posterior ciliary arte ry, this study was intended to determine the direct pharmacologic effect of beta-blockers; to determine the differences among a selective beta-blocker betaxolol, a beta-blocker with intrinsic sympathetic activity befunolol, a nd a nonselective beta-blocker timolol; and to find experimental evidence f or the indirect hemodynamic effect of beta-blockers. METHODS. A Segment of isolated porcine shea posterior ciliary artery was ca nnulated at both ends and mounted in a pressurized vessel chamber. Vessel d iameter was measured as a function of beta-blocker concentration and as a f unction of change in transmural pressure. RESULTS. In the absence of flow, the mean effective doses (ED50) were 0.8 /- 0.3 mM, 1.0 +/- 0.3 mM, and 11.6 +/- 6.6 mM (SEM) for betaxolol, befunol ol, and timolol, respectively. In the presence of flow, vessel diameter inc reased with an increase of transmural pressure. The mean relative diameter increased 4.2% +/- 1.0% (SEM) at a transmural pressure step from 30 mm Hg t o GO mm Hg. This increase was not significantly dependent on the presence o f any of the beta-bIockers. CONCLUSIONS. Only at concentrations far exceeding their expected plasma con centrations, betaxolol, befunolol and timolol increased the diameter of the isolated porcine short posterior ciliary artery, as a result of their dire ct pharmacologic effect. Only the difference between the vasodilatory poten cy of the selective and the nonselective beta-blocker was significant: ED50 of betaxolol was 15 times smaller than ED50 of timolol. There was a positi ve correlation between the diameter of the isolated porcine short posterior artery (when used as a model for an intraocular artery) and the transmural pressure, which corroborates the indirect hemodynamic effect of beta-block ers. It is speculated that instillation of topical beta-blockers into the c onjunctival sac may increase the perfusion of the optic nerve head by an in direct hemodynamic mechanism, but not by a direct pharmacologic mechanism.