The relaxant action of betaxolol on isolated bovine retinal microarteries

We investigated the possible effects on retinal blood flow of betaxolol, a beta blocker which is used as antiglaucomatous medication. Ring segments of retinal microarteries were isolated from bovine eyes and mounted in an organ bath for measurement of contractile force. The effects of betaxolol were studied during K(+)-, stretch-, and serotonin-induced contractions, to enable comparison to the effects of the standard beta blocker propranolol and the Ca2+ channel blocker verapamil. Betaxolol relaxed K(+)-induced contractions in a dose-dependent manner. The drug relaxed the phasic part of the K(+)-induced contractions more than the tonic part. Betaxolol had no effect on stretch-induced contractions. The tonic part of the serotonin-induced contractions showed a trend to decrease in response to betaxolol. These highly specific effects of betaxolol resembled the effects of propranolol. Since betaxolol has significantly less membrane stabilizing activity than has propranolol, this activity is not responsible for the relaxant action of beta blockers on retinal microarteries. The action of both beta blockers resembled the action of the Ca2+ channel blocker verapamil.