Energy expenditure by Ba2+ contracture in rat ventricular slices derives from cross-bridge cycling

To clarify the energy-expenditure mechanism during Ba2+ contracture of mech anically unloaded rat left ventricular (LV) slices, we measured myocardial O-2 consumption ((V) over dot o(2)) of quiescent slices in Ca2+-free Tyrode solution and (V) over dot o(2) during Ba2+ contracture by substituting Ca2 + with Ba2+. We then investigated the effects of cyclopiazonic acid (CPA) a nd 2,3-butanedione monoxime (BDM) on the Ba2+ contracture (V) over dot o(2) . The Ca2+-free (V) over dot o(2) corresponds to that of basal metabolism ( 2.32 +/- 0.53 ml O-2.min(-1).100 g LV-1). Ba2+ increased the (V) over dot o (2) in a dose-dependent manner (from 0.3 to 3.0 mmol/l) from 110 to 150% of basal metabolic (V) over dot o(2). Blockade of the sarcoplasmic reticulum (SR) Ca2+ pump by CPA (10 mu mol/l) did not at all decrease the Ba2+-activa ted (V) over dot o(2). BDM (5 mmol/l), which specifically. inhibits cross-b ridge cycling, reduced the Ba2+-activated (V) over dot o(2) almost to basal metabolic (V) over dot o(2). These energetic results revealed that the Ba2 +-activated (V) over dot o(2) was used for the cross-bridge cycling but not for the Ca2+ handling by the SR Ca2+ pump.