Muscarinic receptor heterogeneity in guinea pig intestinal smooth muscle: binding studies with AF-DX 116
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Updated: Oct 18, 2026

Quantifying Agonist Activity at G Protein-coupled Receptors
Published on: December 26, 2011
E Giraldo1, E Monferini, H Ladinsky
1Department of Biochemistry, Istituto De Angeli, Milan, Italy.
Abstract:
Muscarinic receptor subtypes in longitudinal and circular smooth muscles of the guinea pig ileum were characterized with the use of the cardioselective antagonist AF-DX 116 in binding competition experiments against 0.3 nM [3H] N-methylscopolamine [( 3H]NMS). This compound recognized a heterogeneous receptor population in both smooth muscles, revealing the existence of different percentages of the cardiac (KD = 92-110 nM) and the glandular (KD = 1150-2541 nM) muscarinic receptor subtypes. These results, together with the low potency displayed by AF-DX 116 to inhibit the agonist-stimulated smooth muscle contraction and salivary secretion allow the suggestion that the glandular muscarinic receptor subtype, showing a low affinity for AF-DX 116, is involved in smooth muscle contraction.
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