Ocular hypotension in the rabbit. Receptor mechanisms of pirbuterol and nylidrin

Pirbuterol and nylidrin, both purported sympathomimetic amines, reduced intraocular pressure (IOP) when given topically (50 microliter, 0.1%) to albino rabbits. Pirbuterol increased the cyclic-AMP concentration in aqueous humor by a factor of 3.25, while nylidrin had no effect on aqueous cyclic-AMP nor on adenylate cyclase activity of iris-ciliary body membranes assayed in vitro. Studies of the receptor affinity of pirbuterol, timolol and nylidrin were carried out on iris-ciliary body membranes by competition binding with radioactive ligands. Four ligands were used that appear to label separate subpopulations of adrenergic receptors; dihydroalprenolol (beta-receptors), WB-4101 (alpha 1-receptors) prazosin (alpha 1-receptor subpopulation) and yohimbine (alpha 2-receptors). Pirbuterol and timolol showed exclusive selectivity for beta-receptors with high affinities (Kd 12.6 and 0.48 nM, respectively) compared with other adrenergic receptor populations in iris-ciliary body. Nylidrin had high affinities for beta-receptors (Kd 22 nM) and for the subpopulation of alpha 1-receptors labelled by prazosin (Kd 6.5 nM), but showed 100-fold lower affinity and complex binding characteristics to the two other classes of alpha-adrenergic receptor sites labelled by WB-4101 and yohimbine, respectively. The results show that pirbuterol and timolol are highly beta-receptor selective and that hypotensive responses to these drugs are not mediated by the other classes of alpha-adrenergic receptor determined in this study. However, the hypotensive response to nylidrin may be related to its prazosin-like (alpha 1-receptor) antagonist properties with additional activity at beta-receptors.