Erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), a potential inhibitor of adenosine deaminase (ADA), was tested as an inhibitor of the soluble cyclic nucleotide phosphodiesterase (PDE) isoenzymes from pig and human myocardium. Four soluble PDE activities were resolved from human papillary muscle extracts using anion exchange chromatography (DEAE Sepharose CL-6B). These activities were designated PDE I-IV according to the nomenclature of Beavo. PDE I was stimulated by Ca(2+)-calmodulin and PDE II by cGMP (1 microM). PDE III was inhibited by cGMP (1 microM) as well as SK&F 94120, and PDE IV by both rolipram and Ro 20-1724. Enzyme kinetics and inhibition constants were similar with the PDE isoenzymes from pig heart. However, porcine myocardium lacked Ca(2+)-calmodulin-stimulated soluble PDE I activity. The present data reveal that EHNA exerted a concentration-dependent inhibition of the cGMP-stimulated PDE II (cGs-PDE) (IC50: 0.8 microM (human), 2 microM (pig)) but did not inhibit the other PDE isoenzymes (IC50 > 100 microM). These findings indicate that EHNA is a potent and, as far as cytosolic PDEs are concerned, selective inhibitor of cGMP-stimulated PDEs. The compound may lend itself for the rational design of other isozyme selective PDE II inhibitors and for examining the specific biological functions of cGs-PDEs. EHNA may be used in systems in which inhibition of ADA is of no concern. Conversely, dual inhibition of both ADA and cGs-PDE by EHNA may cause accumulation of two inhibitory metabolites, adenosine and cGMP, which may act in synergy to mediate diverse pharmacological responses, including antiviral, antitumour and antiarrhythmic effects.