α1B-Adrenoceptor Mechanisms in Rabbit Iris Dilator

Abstract

Rabbit isolated iris dilator strips were contracted by norepinephrine, an α1A- and α1B-nonselective agonist, but not by methoxamine, an α1A-selective agonist. The concentration-response curve for norepinephrine was considerably inhibited by chloroethylclonidine. The pA2 values for WB4101 and 5-methylurapidil were 8.16 + 0.09 and 7.84 + 0.08 (means ± S.E. of 8–12 experiments), respectively, and significantly smaller than the values reported in the rat renal artery and thoracic aorta, and rabbit bronchus, where the α1A-subtype is predominant. These results suggest that the rabbit iris dilator contains primarily the α1B-subtype. Clonidine and tizanidine did not contract the rabbit iris dilator but shifted the curve for norepinephrine in a parallel manner, suggesting that they interact with the aiB-subtype and act as competitive antagonists in this muscle. Methoxamine (up to 10−3 M) had no effect on the contractile response to norepinephrine, suggesting that methoxamine does not interact with the α1B-subtype.