Inhibition of guinea pig aortic sarcolemmal Ca2+‐Mg2+ ATPase and cAMP phosphodiesterase activity by milrinone

Milrinone is a positive inotrope/vasodilator that inhibits cardiovascular low Km cAMP phosphodiesterase (PDE) and not Na+−K+ ATPase activity. To explore other possible mechanisms of action, we quantitated the effects of milrinone on Ca2+‐stimulated Mg2+ ATPase activity in guinea pig aortic smooth muscle plasma membranes. Milrinone inhibited Ca2+‐ stimulated activity, but not basal activity, in aortic microsomes. Maximum inhibition (70%) occurred at 1 μM, which coincided with the inflection of a parabolic dose‐response curve. In a sarcolemmal‐enriched (F1) aortic preparation, 1 μM cAMP, 1 μM Cl‐930 (another low Km cAMP PDE inhibitor), and 100 μM W‐7 (a calmodulin antagonist) all inhibited Ca2 ‐stimulated Mg2+ ATPase activity. This F1 preparation contained cAMP PDE activity which was inhibited by 1 μM milrinone (26%) and 1 μM Cl‐930 (40%) but not by 100 μM W‐7. The inhibition of F1 Ca2+ ‐Mg2+ ATPase activity by 1 μM milrinone could be diminished by increasing the concentration of CaCl2 in reaction mixtures. In sum, these studies show that milrinone can inhibit vascular sarcolemmal Ca2+ ‐stimulated Mg2+ ATPase activity. However, inhibition may be direct or direct or may be secondary to cAMP PDE inhibition in vascular sarcolemma, since inhibition also occurs with cAMP and another low‐Km cAMP PDE inhibitor, Cl‐930.