Identification of β-adrenergic receptor subtypes mediating relaxation in isolated equine ileum

Objective-To identify β-adrenergic receptor subtypes in ileum smooth muscle of the horse. Sample Population-Isolated strips of equine longitudinal ileum smooth muscle and membrane preparations from smooth muscle of the intestinal wall. Procedure-Functional assays and radioligand binding assays. Results-Relaxation of ileum longitudinal smooth muscle proved to be mainly caused by stimulation of β-atypical and β2-adrenergic receptors. Binding studies on cell membranes indicated that the total β-adrenergic receptors population consists of 54% β-atypical, 34% β2-, and 12% β1-subtypes. Conclusions-The data suggest that sympathetic relaxation of equine ileum smooth muscle depends mainly on β-atypical receptor subtypes activation, with a minor contribution by β2-subtypes. Clinical Relevance-The important role of β-atypical adrenergic receptor subtypes in the relaxation of equine ileum suggests possible clinical use of selective β-atypical receptor agonists to control intestinal disturbances.