OXOTREMORINE-M ACTIVATES SINGLE NICOTINIC ACETYLCHOLINE-RECEPTOR CHANNELS IN CULTURED XENOPUS MYOCYTES

Oxotremorine methiodide (oxotremorine-M) is the quaternary amine deriv ative of oxotremorine and is known to be a potent and oft-reported pur e, muscarinic receptor agonist. We report here, for the first time, th at oxotremorine-M also has strong nicotinic actions at the single chan nel level. Although previous reports have suggested that oxotremorine- M has mixed cholinergic properties, its nicotinic actions have only be en reported in systems which contain both muscarinic and nicotinic rec eptors, or in skeletal neuromuscular systems where the site of action of oxotremorine-M may have been ambiguous. We tested the possibility t hat oxotremorine-M is a nicotinic receptor agonist by examining the re sponses of single nicotinic acetylcholine receptors in primary culture s of myocytes from skeletal myotomes of Xenopus larvae. Myotomal myocy tes are known to express the nicotinic acetylcholine receptor and no e vidence exists that muscarinic receptors are expressed in these progen itors of the skeletal musculature. Furthermore, because we used aneura l myocyte cultures, the effects of oxotremorine-M cannot be attributed to action on presynaptic receptors. Using cell-attached patches, we c ompared the responses of the nicotinic acetylcholine receptors to sube ryldicholine and oxotremorine-M. Our results show that (1) both agonis ts activate the receptor channel in nanomolar concentrations; (2) the mean channel open-time is significantly smaller in oxotremorine-M; and (3) activation of the nicotinic acetylcholine receptor by oxotremorin e-M is accompanied by a large percentage of short openings and a high frequency of event flickering. We conclude that oxotremorine-M is a mi xed function agonist, showing partial blocking behavior, which effecti vely activates pure nicotinic acetylcholine receptors.