Abstract
α-Adrenoceptor blocking effects of the cardioselective β-blocker celiprolol were tested. 1. Celiprolol antagonized the contractions induced by UK-14304, but not those by phenylephrine, in isolated rat tail arteries with a pA2 value of 4.95. 2. In the isolated rat vas deferens, twitch contractions elicited by the transmural electrical stimulation (TS) were almost blocked by TTX (10-7 M). Clonidine inhibited the TTX-sensitive contraction in a concentration-dependent manner. Celiprolol produced little effect on the inhibitory effects of clonidine. The concentration-inhibition curve of clonidine was shifted to the right by yohimbine (10-8M), but not by prazosin (10-8M). 3. Although celiprolol slightly increased the 3H-efflux to TS from the (3H)-norepinephrine-loaded rat vas deferens, the increment was not significant. Yohimbine (10-7 M) significantly increased the 3H-efflux. TTX (10-7M) and bretylium (3 × 10-5 M) blocked the 3H-efflux. 4. In the spinal rat treated with propranolol (3 mg/kg, i.v.) and prazosin (0.1 mg/kg, i.v.), celiprolol (30 and 100 mg/kg, i.v.) and yohimbine (0.1 ?? 1 mg/kg, i.v.) inhibited the pressor response to clonidine. 5. These results indicate that celiprolol may have a weak α2-blocking effect which was more effective on postsynaptic α2-receptors than presynaptic ones.
