Doxorubicin: an antagonist of muscarinic receptors in guinea pig heart.

While studying the mechanisms of doxorubicin-induced cardiotoxicity, we observed that doxorubicin inhibited the negative inotropic effect of acetylcholine in isolated heart muscle preparations. We therefore examined the effects of doxorubicin on muscarinic acetylcholine receptors. In left atrial muscle preparations isolated from guinea pig heart and stimulated at 2 Hz at 30 degrees C, doxorubicin caused a parallel right-ward shift of the dose-response curves for the negative inotropic effects of acetylcholine. The inhibitory action was reversed by an additional incubation in the absence of doxorubicin. Doxorubicin reversed the carbachol-induced inhibition of developed tension: a high concentration of doxorubicin brought the force back to its original strength. Doxorubicin inhibited specific [3H]quinuclidinyl benzilate (QNB) binding to membrane preparations obtained from ventricular muscle of guinea pig hearts. The pA2 value for doxorubicin obtained in the inotropic study corresponded to the IC50 value for doxorubicin observed in the [3H]QNB binding assay. These results indicate that doxorubicin acts as a weak competitive antagonist on muscarinic acetylcholine receptors.