Cardiotonic action of plumbagin on guinea-pig papillary muscle

Plumbagin, an active principle from Plumbaginaceous plants, produced a triphasic inotropic response (first positive, second negative, and third positive phases) in guinea-pig papillary muscle. The inotropic potency of plumbagin at the first positive phase was pD2 6.40, and the methylation of 5-hydroxy group of plumbagin reduced the potency. The triphasic pattern of inotropism of plumbagin was unaffected by reserpine or propranolol treatments. Plumbagin did not produce the positive inotropic effects under anoxic conditions or in the presence of mitochondrial oxidative phosphorylation inhibitors such as 2,4-dinitrophenol and dicumarol.