2,3-BUTANEDIONE MONOXIME INCREASES CONTRACTILE EFFICIENCY IN THE RABBIT VENTRICLE

The efficiency of chemomechanical energy transduction (contractile efficiency) of the left ventricle (LV) has been calculated from the linear correlation of the systolic pressure-volume area (PVA) of the LV and its O2 consumption (VO2). Thus far, a wide range of acute interventions, including adrenergic agents, Ca2+-sensitizing drugs, and Ca2+ channel blockers have not altered contractile efficiency. In contrast, hyperthyroidism has been reported to decrease contractile efficiency, an effect attributed at the cross-bridge level to an increase in the V1/V3 myosin isoenzyme ratio. We hypothesized that an acute intervention which directly alters cross-bridge cycling would also change contractile efficiency. Accordingly, 2,3-butanedione monoxime (BDM), a negative inotropic agent that is thought to directly inhibit cross-bridge formation, was administered to seven excised, red blood cell-perfused, isovolumically beating rabbit LVs. At 3-4 mM perfusate concentration, BDM resulted in the following reversible mechanical and energetic effects compared with control. Contractility, assessed by the slope (E(max)) of the end-systolic pressure-volume relation, decreased by 11% (196.6 +/- 25.9 vs. 222 +/- 28 mmHg/ml). Both the time to end systole (T(max)) and relaxation half time (T1/2) decreased. The slope of the VO2-PVA relation decreased by 20% (1.55 +/-0.44 x 10(-5) vs. 1.95 +/- 0.52 x 10(-5) ml O2 . mmHg-1 . ml-1), equivalent to an increase in contractile efficiency from 36.5 +/-10.4 to 46.4 +/- 14.4%, while the O2 costs of the mechanically unloaded LV decreased by 12% (0.0258 +/- 0.0060 vs. 0.0292 +/-0.0064 ml O2 . beat-1 . 100 g-1). Finally, BDM also produced coronary vasodilation. These results indicate that BDM has the unique property of decreasing contractility while increasing contractile efficiency.