Vol. 290, Issue 2, 893-900, August 1999

Characterization of Prejunctional and Postjunctional Muscarinic Receptors of the Ascending Reflex Contraction in Rat Ileum¹

M. Kurjak, D. Sattler, V. Schusdziarra and H. D. Allescher

Departments of Internal Medicine II (M.K., V.S., H.D.A.) and Gynecology (D.S.), Technical University of Munich, Munich, Germany

The ascending reflex contraction of the small intestine involves predominantly cholinergic neurotransmission. The orally projecting neural excitatory pathway of the myenteric reflex was studied in an in vitro model of rat ileal segments. The contractile response elicited by aboral field stimulation was significantly inhibited by a range of muscarinic receptor antagonists. Methoctramine and tripitramine (both M₂ selective, pIC₅₀ = 9.3 and 8.8, respectively), darifenacin and hexahydrosiladifenidol (both M₃ selective, pIC₅₀ = 7.3 and 7.7, respectively), and pirenzepine (M₁ selective, pIC₅₀ = 7.0). In radioligand binding experiments on synaptosomal and smooth muscle plasma membrane fractions, we examined whether prejunctional or postjunctional muscarinic receptors exist that could potentially contribute to the reflex contraction. In the synaptosomal fraction, the muscarinic ligand [³H]N-methylscopolamine labeled a homogeneous population of receptors (Hill coefficient = 1) with a K_d value of 0.31 ± 0.09 nM and a B_max value of 185 ± 16.6 fmol/mg protein. The ratio of potency of subtype-selective muscarinic receptor antagonists in competition studies was tripitramine (pK_i = 8.7 ± 0.3) > 1/6 × methoctramine (pK_i = 7.9 ± 0.02) > 1/25 × darifenacin (pK_i = 7.3 ± 0.2) > 1/316 × hexahydrosiladifenidol (pK_i = 6.2 ± 0.1) > 1/2511 × pirenzepine (pK_i = 5.3 ± 0.1). In the smooth muscle plasma membrane fraction, the K_d value was 0.29 ± 0.05 nM and the B_max value was 770 ± 29 fmol/mg. The competition studies revealed a similar ratio of potency of the respective antagonists. These data suggest that muscarinic M₂ receptors, located at prejunctional and postjunctional sites, are predominantly involved in the ascending reflex contraction.