BINDING AND FUNCTIONAL-CHARACTERIZATION OF THE CARDIOSELECTIVE MUSCARINIC ANTAGONIST METHOCTRAMINE

  • 1 March 1988
    • journal article
    • research article
    • Vol. 244  (3) , 1016-1020
Abstract
The antimuscarinic properties of the newly synthesized polymethylene tetramine derivative, methoctramine, were investigated in binding and functional assays. Methoctramine displaced the specific binding of [3H]-N-methylscopolamine ([3H]NMS) and [3H]pirenzepine from membranes of rat tissues with the following order of affinities: heart = cerebellum > cortex > submandibular glands, the ratio of the affinities of the compound for the heart and the glands amounting to about 130. Computer fits of binding curves generated in cardiac and cortical membranes were compatible with an interaction at one binding site, whereas those in submandibular glands and cerebellum had slopes significantly lower than 1. Experiments performed in cardiac membranes to investigate the effect of methoctramine on the dissociation kinetics of [3H]-NMS showed that concentrations of compound up to 1 .mu.M did not affect the dissociation of [3H]-NMS elicited by an excess of NMS. At greater concentrations (10-100 .mu.M), methoctramine dose dependently inhibited [3H]-NMS dissociation, thus revealing an allosteric interaction. In in vitro functional assays, methoctramine displayed more than 100 times greater affinity for the muscarinic receptors mediating negative inotropic and chronotropic effects in guinea pig atria than for those responsible for tracheal contraction. Similarly, the compound was a more potent antagonist of the bradycardial response to bethanechol that of the bladder tonus increase, saliva secretion and hypotension induced by the muscarinic agonist in anesthetized cats. Finally, in the pithed rat, methoctramine preferentially inhibited cardiac M2 (vagal bradycardia) over ganglionic M1 (McN-A-343-induced hypertension) responses. The evidence appears to characterize methoctramine as being the most selective M2 muscarinic antagonist described to date. Moreover, the compound interacts both competitively and allosterically with cardiac muscarinic receptors.