Interaction of adrenergic antagonists with prostaglandin E2 and tetrahydrocannabinol in the eye

Both alpha- and beta-adrenergic antagonists have been utilized in an atempt to discern the site of action of prostaglandin (PG) and tetrahydrocannabinol (THC) in the eye. Both alpha- and beta-adrenergic antagonists (alpha-antagonists, phentolamine and phenoxybenzamine; beta-antagonists, propranolol and sotalol) cuased a dose-dependent reduction in intraocular pressure and blood pressure and increased total outflow facility. The results are consistent with the concept that both alpha- and beta-adrenergic receptors are present in the anterior uvea and that vasomotor tone is essential to the maintenance of normal intraocular pressure. No antagonist reduced the PG-induced elevation of intraocular pressure unless the blood pressure was severely lowered. All antagonists inhibit the normal PG-induced increase in total outflow facility, indicating that these agents protect the blood-aqueous barrier from breakdown without altering the vasodilatory response to PG. All antagonists reduced the fall in intraocular pressure produced by THC by approximately 50 per cent, except for sotalol which completely abolished the intraocular pressure fall. Only the alpha-adrenergic antagonists prevented the THC-induced increase in total outflow facility. The results indicate that true outflow facility may well be regulated exclusively by alpha-receptors. The data are consistent with the effect of THC being primarily a vasodilation of the efferent blood vessels of the anterior uvea. The partial inhibition by alpha-adrenergic antagonists may also suggest a lesser role of THC on the afferent vessels.