Anti-muscarinic actions of mitoxantrone in isolated heart muscles of guinea pigs

A hypotheses that mitoxantrone is a competitive antagonist at muscarinic ch olinergic receptors was examined in guinea-pig hearts. In isolated left atr ial muscle preparations, electrically paced at 2 Hz, the muscarinic agonist , carbachol, caused a concentration-dependent decrease in developed tension . Mitoxantrone caused a parallel right-ward shift of the concentration-resp onse curve for carbachol. Schild plots for the effect of mitoxantrone on th e carbachol concentration-response relationship were linear with a slope of 0.88 which was not significantly different from the unity. The right-ward shift of the carbachol concentration-response relationship by mitoxantrone significantly reversed after an additional incubation with a mitoxantrone-f ree solution, although the reversal was incomplete after a 2-h incubation i n the mitoxantrone-free solution. Mitoxantrone caused a concentration-depen dent displacement of specific [H-3]quinuclidinyl benzilate binding to membr ane preparations obtained from ventricular muscles of guinea-pig hearts. Th ese results indicate that mitoxantrone acts as a competitive antagonist for the muscarinic receptors. (C) 2000 Elsevier Science B.V. All rights reserv ed.