Characterization of β-adrenoceptors mediating relaxation of the guinea-pig ileum

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Abstract
Relaxation responses to sympathomimetic amines were recorded in potassium-contracted segments of guinea-pig ileum. Experiments were performed in the presence of phentolamine (5 times 10−6 m) to eliminate β-adrenoceptor-mediated effects. Metanephrine (10−5 m) and desmethylimipramine (5 times 10−7 m) were also present to prevent extraneuronal and neuronal uptake respectively. A potency order (-)-isoprenaline > (-)-noradrenaline > (-)-adrenaline was established, indicating a β1-adrenoceptor involvement for this relaxation. The potency of salbutamol (β2-selective) relative to isoprenaline in the ileum compared closely with its relative potency in isolated cardiac tissues (β1) but differed significantly from the value in lung parenchymal strips and vas deferens (β2). The pA2 values for antagonism of selective agonists (-)-noradrenaline (β1-selective) and fenoterol (β2-selective) by practolol (β1-selective) were identical, indicating a single β1-adrenoceptor population. The pA2 values for antagonism of these agonists by ICI 118,551 (β2-selective) were also identical and compatible with a β1-adrenoceptor population. Relaxation of the guinea-pig ileum is therefore mediated via a homogeneous population of β1-adrenoceptors.