Abstract
The effects of the new cardiotonic compound, DPI 201-106, on muscarinic responses and muscarinic receptor binding were studied in the guinea pig heart. DPI 201-106 exerted a pronounced anticholinergic action in isolated auricles and a moderate one in papillary muscles, which resembled the pattern of a functional antagonism. However, in competition binding experiments, DPI 201-106 inhibited binding of the specific muscarinic antagonist [ 3H]NMS with equal potency in atrial and ventricular homogenates (apparent K I=0.7 μmol/l in atria and 1.2 μmol/l in ventricles). At higher concentations (>3 μ mol/l), DPI 201-106 slowed the dissociation of [ 3H]NMS from cardiac muscarinic receptors, indicating that DPI 201-106 affects in addition a site allosteric to the muscarinic receptor. It is concluded that DPI 201-106 is able to inhibit cholinergic responses in the heart, not only by a functional antagonism but also by direct interaction with muscarinic receptors.
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