The effect of echothiophate on the biphasic response of rabbit ocular pressure to dipivefrin.

A time-course study was performed on the intraocular pressure response of pigmented rabbit eyes. Dipivefrin administration produced in initial hypertensive phase lasting less than two hours followed by a prolonged hypotensive phase. Echothiophate iodide therapy produced a more pronounced and prolonged hypertensive response; there was no hypotensive phase. Administration of echothiophate plus dipivefrin resulted in a hypertensive phase similar to that from echothiophate alone; as previously reported, this combination was not followed by a hypotensive phase. The alpha-blocker phentolamine mesylate prevented the echothiophate-induced hypertension. When dipivefrin was administered with echothiophate plus phentolamine, there was an immediate hypotensive effect. It was concluded that the hypertensive effect of echothiophate in pigmented rabbit eyes may mask the hypotensive action of dipivefrin. This, rather than an echothiophate-induced inhibition of esterases, may explain why combination therapy with these drugs seemed ineffective.