The three-finger toxin MTα is a selective α2B-adrenoceptor antagonist

Muscarinic toxins (MTs) are three-finger folded peptides isolated from mamba snake venoms. In this report we describe a selective antagonistic interaction of MTα with the human α2B-adrenoceptor. In a functional assay, measuring the α2B-adrenoceptor-induced increase in intracellular [Ca2+], we found that both venomous MTα and synthetic MTα inhibited the response in a concentration-dependent way. MTα did not affect the responses of α2A-, α2C-, α1A- or α1B-adrenoceptors. To further explore the binding of MTα to the α2B-adrenoceptor, we performed ligand binding experiments on Sf9 cell homogenates with [3H]RX821002 as reporter ligand. MTα bound to the receptor rather slowly requiring about 60 min to reach equilibrium. In equilibrium binding experiments, MTα displaced the radioligand with an IC50 of 3.2 nM, but was not able to displace all bound radioligand. Using a saturation binding protocol, we found that MTα suppressed the maximum binding without any greater impact on the affinity of the radioligand, indicating a non-competitive mode of inhibition. The toxin bound reversibly to α2B-adrenoceptor, but extensive washing was needed for full recovery of binding sites at high toxin concentrations. Surprisingly, MTα did not affect [3H]-N-methylscopolamine binding to the muscarinic receptor subtypes at concentrations found to fully block α2B-adrenoceptors, showing that the toxin is a more potent antagonist for the α2B-adrenoceptor than for muscarinic receptors. These findings should open up new views in terms of selective adrenoceptor drug design as well as in elucidation of α2-adrenoceptor physiology.