The effects of betaxolol on isolated rat arteries and the modes of action were investigated. Betaxolol (10
-5 - 10
-3M) relaxed the 80 mM K
+-induced contraction of aortic strips concentration-dependently. The 50% inhibitory concentration of betaxolol in the K
+-induced contraction was 3 times higher than that of papaverine and about 3 times lower than that of bunitrolol. The relaxations by betaxolol were also demonstrated in renal, mesenteric and femoral arteries. Betaxolol (3 × 10
-6M - 10
-4M) produced rightward parallel shifts of the concentration-response curves for Ca 2+ in the K
+-depolarized aortic strips. On the other hand, betaxolol produced downward shifts as well as rightward shifts of the concentration-response curves for norepinephrine, 5-HT and angiotensin II. In K
+-depolarized aortic strips, the cytosolic Ca
2+ concentration measured with a fluorescent indicator, fura-2, was decreased by betaxolol (10
-4M) almost concomitantly with the loss of tention. An elevation of external Ca
2+ from 2.5 mM to 10 mM restored both the cytosolic Ca
2+ concentration and tention. The relaxations of arteries induced by betaxolol were not influenced by glybenclamide, methylene blue, indomethacin or removal of the endothelium. These results suggest that betaxolol possesses a direct vasodilating action, and the action may be due to the inhibition of Ca
2+ influx across the cell membrane.
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