Benzoquinoid tyrosine kinase inhibitors are potent blockers of cardiac muscarinic receptor function.

The whole-cell mode of the patch-clamp technique was used to study the effect of methyl-2,5-dihydroxycinnamate (MDC), a specific protein tyrosine kinase inhibitor, on the K+ currents induced by muscarinic cholinergic agonists in atrial myocytes. Extracellular MDC abolished muscarinic K+ currents irreversibly, with an apparent inactivation constant Kinact of 1.3 microM. Binding studies using purified cardiac sarcolemma indicated that MDC disrupts functional interactions between muscarinic receptors and G proteins with an IC50 of 0.7 microM but does not change significantly the distribution of muscarinic binding sites between forms with low and high affinity for agonists. The effects of MDC on muscarinic receptors appear to be unrelated to changes in tyrosine phosphorylation, because (i) the binding experiments were performed in the total absence of phosphorylating nucleotides; (ii) lavendustin-A, a tyrosine kinase inhibitor that is active in vitro but not in vivo, presumably because it does not cross plasma membranes, inhibited the muscarinic K+ currents of atrial cells similarly to MDC; and (iii) vanadate, a well known inhibitor of phosphotyrosine phosphatases that potentiates the effects of tyrosine phosphorylation, did not affect K+ currents when applied extracellularly or into the cytosol of atrial myocytes. The effects of MDC and lavendustin-A were abolished by reducing agents and were mimicked by hydroquinone (or p-benzoquinone), indicating that the common quinol moiety is involved in the antimuscarinic activity of the tyrosine kinase inhibitors. It is suggested that these compounds inhibit muscarinic receptor function through oxidation to the quinone form, followed by covalent reaction with a nucleophilic group in the receptor molecule.