Ocular hypotensive action of labetalol

Labetalol has a unique and profound effect on the IOP of rabbits, in contrast to pure beta-adrenergic blockers which have little or no effect in this animal. Its action cannot be satisfactorily explained by simple alpha and beta-adrenergic blocking activity. In the concentrations used, it failed to block the action of a beta-adrenergic agonist (isoproterenol), the action of an alpha-adrenegic agonist (norepinephrine), and its own hypotensive action was neither blocked nor potentiated by the alpha-adrenergic blocker phenoxybenzamine. However, the beta blocker, timolol, did reduce the action of labetalol. These observations and the reduced response to labetalol after cervical sympathectomy, suggest that labetalol's ocular hypotensive effect may occur through some mechanism other than alpha or beta blockade.