Effects of timolol and betaxolol on choroidal blood flow in the rabbit

This study evaluated the effects of the topical pl-adrenergic antagonist be taxolol and the non-selective beta-adrenergic antagonist timolol on the cho roidal pressure-flow relationship. Pentobarbital-anesthetized rabbits were instrumented with hydraulic occluders on the aorta and inferior vena cava t o control MAP, an ear artery cannula to measure mean arterial pressure (MAP ), and two vitreous cannulas to control and measure intraocular pressure (I OP). Choroidal blood flow was measured by laser Doppler flowmetry with the fiber-optic probe tip positioned over the posterior pole. Choroidal pressur e-flow curves were obtained before and 30 min after topical application of 0.1 mi of betaxolol (Betoptic, 0.5%, n = 10), timolol (Timoptic, 0.5%, n = 10) or saline (n = 8) by varying the MAP without controlling the IOP and by raising IOP while holding the MAP constant at 70 mmHg. The IOP was signifi cantly reduced by betaxolol and timolol but not by saline. MAP was also sli ghtly, but significantly, reduced after betaxolol but not after timolol or saline. However, the systemic hypotensive response to isoproterenol (8 mu g /kg, i.v.) was blunted after betaxolol and timolol indicating appreciable s ystemic absorption of both drugs. Timolol, but not betaxolol or saline, cau sed a significant, small increase in baseline choroidal vascular resistance . Timolol also attenuated the TOP response to MAP; however, none of the tre atments had a significant effect on the choroidal pressure-flow relationshi p. We conclude that both drugs reach the systemic circulation after topical application, but neither betaxolol nor timolol alter the choroidal respons e to acute changes in perfusion pressure. (C) 1998 Academic Press.