Phosphodiesterase Isozyme Inhibition, Activation of the cAMP System, and Positive Inotropy Mediated by Milrinone in Isolated Guinea Pig Cardiac Muscle

The purpose of the present study was to examine the interrelationships among phosphodiesterase (PDE) isozyme inhibition, cAMP formation, activation of cAMP-dependent protein kinase (cAPK), and positive inotropy in isolated guinea pig cardiac muscle mediated by the cardiotonic/vasodilator agent, milrinone. Milrinone was a potent and selective inhibitor of the “low Km” cAMP PDE isozyme (peak III) isolated by diethylaminoethyl ether cellulose chromatography, with IC50 values of 0.7 μM for peak III PDE and 100 μM for peak I PDE. In isolated papillary muscles frozen at peak inotropic responses, positive and significant correlations were evident between isometric force development as a function of cAMP content (r = 0.72, p < 0.05) or cAPK activity ratio, an index of activation of cAPK (r = 0.79, p < 0.001), for concentrations of milrinone from 0.1-1000 μM. Similar correlations were evident in muscles frozen at peak inotropic responses for the β-adrenoreceptor agonist isoproterenol (r = 0.96, p < 0.001; r = 0.98, p < 0.001, respectively), but not for ouabain or Bay K-8644. The temporal sequence of these events was also quantitated for concentrations of milrinone (100 μM) and isoproterenol (3 nM) that produced approximately a 100% increase in isometric force. Whereas early time intervals of force development (30 s, 1 min, isoproterenol; 30 s, milrinone) were not accompanied by significant increases in either cAMP content or cAPK activity ratio, peak increases in force development for both isoproterenol (2 min) and milrinone (1 min) were related to peak increases in cAPK activity ratios. In summary, these results show that significant increases in cAMP content or cAPK activation are correlated with positive inotropy in isolated guinea pig papillary muscles with milrinone. These correlations occur at concentrations of milrinone that inhibit cardiac PDE isozymes and are similar to the known cAMP-dependent cardiostimulant isoproterenol. These data support the hypothesis that selective PDE isozyme inhibition is a mechanism by which milrinone effects positive inotropy.