Methoctramine effects on nicotinic acetylcholine receptor kinetics

Aim: Methoctramine shares common elements with other polyamine-containing compounds (hydrophobic heads inserted on a polyamine backbone) which arę potent inhibitors of cation-selective ionotropic receptors. Our aim was to characterize methoctramine binding site on nAChR using single-channel technique. Methods: Experiments were performed in cell-attached configuration of the patch clamp technique on murine skeletal muscle i28 cells differentiated in vitro for 2-6 days. Results: At high concentration (100 [iM) methoctramine completely abolish the activity of AChR induced by 100 nM ACh. At lower concentration (3 JiM) methoctramine, decreased of the open time of nAChR from 5,4 ± 0,4 ms (»=8, control conditions) to 1,7 + 0,5 ms («=5). The side of methoctramine application, outside of the celi mem¬branę, indicates an extracellular site for inhibition. However, after 15 min of registration in hyperpolarized potentials, the inhibitory effect of methoctramine was enhanced. Since the structure of the methoctramine prevents its deep penetration into the membrane, the local-ization of methoctramine site is uncertain. Either the access to the methoctramine site depends on membranę potential or it is localized within the lipid phase on the membranę surface. Conclusion: Methoctramine binding site/sites share the common properties of the open-channel blockers and of noncompetitive inhibitors increasing desensitization ratę of AChRs. Like other open-channel blockers methoc¬tramine decreases the receptor open time and induces, in a yoltage-dependent way, the burst activity. Like other inhibitors methoctramine decreases the probability of receptor opening. However unlikely other inhibitors methoctramine action changes during experiment. Future experiments arę needed to explain if methoctramine mech-anism is time- or potential-dependent.