Selective inhibition of cyclic AMP phosphodiesterase from various human tissues by milrinone, a potent cardiac bipyridine.

We observed the effects of milrinone, an inotropic agent prescribed to treat congestive heart failure, on cyclic nucleotide messenger systems in various human tissues in vivo. Cyclic nucleotide phosphodiesterases (PDEs) from the human heart were separated into three isoforms, FI, FII and FIII, by DEAE-cellulose chromatography. Milrinone proved to be a potent and selective inhibitor of human cardiac FIII PDE, a "low Km" enzyme for cyclic AMP (cAMP-PDE). The IC50 value for the inhibition of FIII PDE was 0.42 microM, while those of FI and FII PDEs, "high Km" enzymes, were 38 and 19 microM, respectively. Kinetic studies showed that milrinone inhibited the activity of FIII PDE, competitively with respect to cAMP, and the Ki was 0.15 microM. Milrinone in doses to 100 microM had no effect on human cardiac cAMP-dependent protein kinase and adenylate cyclase. The activity of cAMP-PDEs from human platelets and the aorta, as well as that from heart, were potently inhibited by milrinone, with much the same IC50 values. Cyclic AMP-PDEs from human kidney, liver and lung were not readily inhibited by milrinone, and the IC50 values of cAMP-PDEs from these tissues were about 7-30-fold higher than that from heart. On the other hand, papaverine had a relatively lesser selectivity for any of the cAMP-PDEs. All these results suggest that milrinone exerts inotropic effects by inhibiting cAMP-PDE selectively in the human heart tissues and that this compound can be used to evaluate different forms of cAMP-PDEs present in human tissues.