Subclasses of cyclic AMP-specific phosphodiesterase in left ventricular muscle and their involvement in regulating myocardial contractility

Abstract

Ventricular muscle contains a low K(m), cyclic AMP-specific form of phosphodiesterase (PDE III), which is believed to represent the site of action for several of new cardiotonic agents including imazodan (CI-914), amrinone, cilostamide, and enoximone. However, species differences in the inotropic response to these agents have raised questions about the relationship between PDE III inhibition and cardiotonic activity. The present study demonstrates that these differences can be accounted for by the presence of two subclasses of PDE III in ventricular muscle and variations in the intracellular localization of these two enzymes. For these experiments, PDE III was initially isolated from canine, guinea pig, and rat left ventricular muscle. The results demonstrate that canine left ventricular muscle contains two functional subclasses of PDE III: an imazodan-sensitive form, which is membrane bound, and an imazodan-insensitive form, which is soluble. Although only weakly inhibited by imazodan, this latter enzyme is potently inhibited by the selective PDE III inhibitors, Ro 20-1724 and rolipram. Guinea pig ventricular muscle also contains the imazodan-sensitive subclass of PDE III. Unlike canine left ventricle, however, this enzyme is soluble in the guinea pig. No membrane-bound subclass of PDE III apparently represents a mixture of the imazodan-sensitive and imazodan-insensitive subclasses of PDE III. Measurements of in vivo contractility in these three species showed that imazodan exerts a potent positive inotropic effect only in the dog, in which the imazodan-sensitive subclass of PDE III is membrane bound. In addition, a strong correlation was observed between in vitro inhibition of the membrane-bound, imazodan-sensitive PDE III from canine ventricular muscle and the in vivo positive inotropic response to imazodan, amrinone, and related cardiotonic agents in the dog. Inhibitors of the imazodan-insensitive subclass of PDE III did not exert any pronounced inotropic effect in the dog. These results demonstrate that functional subclasses of PDE III exist in ventricular muscle and suggest that species differences in the positive inotropic response to imazodan and related cardiotonics can be attributed to the proper intracellular localization of the imazodan-sensitive subclass of PDE III.