Effects of amrinone and isoproterenol on mechanoenergetics of blood-perfused rabbit heart.

To compare the effects of amrinone (AMR) and isoproterenol (Iso) on left ventricular contractility and energetics, we assessed Emax (ventricular contractility index) and the relation between oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA, a measure of left ventricular total mechanical energy) in isolated cross-circulated (blood-perfused) rabbit hearts during infusion of AMR or Iso in either a constant-flow (CF) or constant-pressure (CP) perfusion mode. Both Emax and the VO2 intercept of the linear VO2-PVA relation increased significantly during AMRCP (increase in Emax 15% and increase in VO2 intercept 11%), ISOCF (49 and 43%), and ISOCP (55 and 54%) but not during AMRCF. However, neither drug changed the slope of the VO2-PVA relation (reciprocal of contractile efficiency) in either perfusion mode. Furthermore, with both drugs the relation between increases in Emax and the VO2 intercept fell on a single regression line (r = 0.92). We conclude that 1) although the mechanism of action and inotropic potency of the two drugs differ, their effects on cardiac energetic cost are essentially the same, i.e., both drugs increase the nonmechanical oxygen cost in proportion to the increase in contractility without changing contractile efficiency, and 2) a significant portion of the inotropic effect of AMR in the whole ventricle is likely due to increased coronary blood flow, i.e., Gregg's phenomenon.