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Alpha 1B-adrenoceptor mechanisms in rabbit iris dilator
I Takayanagi1, K Shiraishi, N Kokubu
1Department of Chemical Pharmacology, Toho University School of Pharmaceutical Sciences, Chiba, Japan.
Japanese Journal of Pharmacology
|July 1, 1992
Summary
The rabbit iris dilator primarily contains the alpha 1B-subtype receptor, as shown by its contraction to norepinephrine and inhibition by specific antagonists. This finding differentiates it from other tissues where the alpha 1A-subtype predominates.
Area of Science:
- Pharmacology
- Adrenergic Receptor Research
Background:
- Alpha-1 adrenergic receptors (α1) are crucial for smooth muscle contraction.
- Subtypes α1A, α1B, and α1D mediate distinct physiological responses.
- The specific subtype distribution in the iris dilator muscle is not fully elucidated.
Purpose of the Study:
- To determine the predominant alpha-1 adrenergic receptor subtype in the rabbit iris dilator muscle.
- To characterize the functional role of α1 receptor subtypes in iris dilator contraction.
Main Methods:
- Isolated rabbit iris dilator strips were used for in vitro contractile studies.
- Concentration-response curves for agonists and antagonists were generated.
- Pharmacological profiling using selective and non-selective α1 adrenergic agents.
Main Results:
- Norepinephrine (nonselective α1 agonist) induced contraction, while methoxamine (α1A-selective agonist) did not.
- Chloroethylclonidine significantly inhibited norepinephrine-induced contractions.
- pA2 values for WB4101 and 5-methylurapidil were lower in the iris dilator compared to tissues rich in α1A subtypes.
- Clonidine and tizanidine acted as competitive antagonists at the α1B-subtype.
Conclusions:
- The rabbit iris dilator muscle predominantly expresses the alpha-1B adrenergic receptor subtype.
- This subtype mediates the contractile response to norepinephrine in this tissue.
- The findings contribute to understanding α1 receptor heterogeneity and function in ocular tissues.