Prevention of guanine nucleotide-induced reductions in muscarinic agonist binding to rabbit ileal submucosal membranes by lidamidine and tetracaine

Summary

The effects of low concentrations (< = 1 μmM) of the anti-diarrhoeal drug lidamidine and sub-anaesthetic concentrations (< = 1 μM) of tetracaine on the binding of the muscarinic antagonist [³H] N-methylscopolamine ([³H]NMS) and the high affinity muscarinic agonist, [³H] oxotremorine-M ([³H]oxo-M) to broken cell preparations of rabbit ileal submucosal membranes were investigated. High affinity [3H]NMS binding (K _D = 0.79 ± 0.05 nM, B _max = 1.75 ± 0.13 pmoles · mg⁻¹) was unaffected by either lidamidine or tetracaine. Inhibition of NMS binding by the muscarinic agonist carbachol was reduced by the nonhydrolyzable analogue of GTP, GppNHp. This effect of GppNHp was partially prevented by lidamidine. Analysis of equilibrium binding of the muscarinic agonist [³H] oxotremorine-M revealed that the binding of oxotremorine-M could be best described by the presence of the least two populations of sites with affinities of 0.9 ± 0.2 and 27.7 ± 0.9 nM respectively. High affinity [³H] oxotremorine-M binding was markedly reduced by GppNHp, GDPβS and fluoride (5 mM). Neither lidamidine, nor tetracaine had any effect on the binding of oxotremorine-M when added alone. However, lidamidine and tetracaine prevented GppNHp, GDPβS and fluoride-induced reductions in oxotremorine-M binding. The present findings are consistent with an allosteric interaction between lidamidine or tetracaine and GppNHp-induced reductions in oxotremorine-M binding to submucosal muscarinic receptors. These effects are discussed in relation to the observed action of lidamidine in potentiating presynaptic inhibition of acetylcholine release by muscarinic agonists.