The biphasic intraocular pressure response of rabbits to epinephrine

A study has been made of the pupillary and intraocular pressure responses of conscious rabbits to daily topical applications of submaximal doses of epinephrine. On the first day, epinephrine caused rapid pupil dilation which preceded a prolonged -ecrease of intraocular pressure. On the second and subsequent days, the application of the same dose of epinephrine increased the duration of the pupillary response and caused a biphasic pressure response in all treated eyes; an initial increase of intraocular pressure lasting two to four hours followed by decrease of intraocular pressure below the initial value which lasted for more than twenty-four hours. The beta-receptor antagonist, propranolol, and the alpha-receptor antagonist, phenoxybenzamine, caused small and large reductions, respectively, in the hypertensive response to epinephrine. Phenoxybenzamine, but not propranolol, also inhibited the pupil dilation and the hypotensive response to epinephrine. Topical administration of phenoxybenzamine strongly inhibited the hypertensive response to epinephrine but left unaffected the pupillary response.