Alpha 1B-adrenoceptor mechanisms in rabbit iris dilator.
Rabbit isolated iris dilator strips were contracted by norepinephrine, an alpha 1A- and alpha 1B-nonselective agonist, but not by methoxamine, an alpha 1A-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 alpha 1A-subtype is predominant. These results suggest that the rabbit iris dilator contains primarily the alpha 1B-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 alpha 1B-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 alpha 1B-subtype.
Takayanagi
I
I
Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Chiba, Japan.
Shiraishi
K
K
Kokubu
N
N
D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D007498:Iris / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D011680:Pupil / Q000502:physiology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*
D011942:Receptors, Adrenergic, alpha; D008729:Methoxamine; D003000:Clonidine; D009638:Norepinephrine