RESUMO
We have isolated a new muscarinic protein (MT-Mlá) from the venom of the Brazilian coral snake Micrurus lemniscatus.The MT-Mlá was able to displace the [3H]QNB binding in the hippocampus of rats. The bindingcurve in competition experiments with MT-Mlá was indicative of two types of [3H]QNB-binding site with pKivalues of 9.08±0.67 and 6.17±0.19, n=4, suggesting that various muscarinic acetylcholine receptor(mAChR) subtypes may be the target proteins of MT-Mlá. The MT-Mlá and the M1 antagonist pirenzepinecaused a dose-dependent block on total [3H]inositol phosphate accumulation induced by carbachol. TheIC50 values for MT-Mlá and pirenzepine were, respectively, 33.1 and 2.26 nM. Taken together, these studies indicate that the MT-Mlá has antagonist effect on mAChRs in rat hippocampus.The results of the present study show, for the first time, that mAChRs function is drasticallyaffected by MT-Mlá since it not only has affinity for mAChRs but also has the ability to inhibit mAChRs.