Studies on the Mechanism of the Cardiotonic Activity of MDL 19205

MDL 19205, 4-ethyl-1,3-dihydro-5-(4-pyridinyl-carbonyl)-2H-imidazol-2-one, is a new drug with cardiotonic properties. Its effects on several biochemical systems considered to be important in myocardial contraction were investigated. Cyclic nucleotide phosphodiesterases (PDEs) from dog hearts were separated into three isoenzymes, F I, F II, and F III, and effect of the drug on these enzymes was tested. MDL 19205 inhibited F III PDE specifically and produced little or no inhibition of F I and F II PDEs. The IC50 for inhibition of F III PDE was 8.6 microM when 0.5 microM cyclic AMP (cAMP) was used, whereas no more than 10% inhibition of F I and 18% of F II PDEs occurred at drug concentrations up to 200 microM when 1 microM cAMP was used. Concentrations of MDL 19205 up to 100 microM had no effect on Ca2+-adenosine triphosphatase (ATPase) or Ca2+ uptake by dog cardiac sarcoplasmic reticulum. At 100 microM, the drug produced a weak (18%) inhibition of Na+,K+-ATPase. It is suggested that inhibition of F III PDE may be the primary mechanism by which MDL 19205 produces its cardiotonic effect. Inhibition of Na+,K+-ATPase may also be involved at very high concentrations of this drug.