Muscarinic receptor heterogeneity in guinea pig intestinal smooth muscle: binding studies with AF-DX 116

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 heterogenous receptor population in both smooth muscles, revealing the existence of different percentages of the cardiac (K D = 92 − 110 nM) and the glandular (K D = 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.