Sarcoplasmic Reticulum Ca2+Pump Blockade Decreases O2Use of Unloaded Contracting Rat Heart Slices: Thapsigargin and Cyclopiazonic Acid

We previously established a new measuring method of the myocardial O2consumption of mechanically unloaded rat left-ventricular slices. O2consumption of unstimulated myocardium corresponds to basal metabolism. We have found O2consumption of stimulated myocardium to include basal metabolism and O2consumption for Ca2+handling in the excitation–contraction coupling, but not for crossbridge cycling. Thus, O2consumption for the excitation–contraction coupling is obtained by subtracting basal metabolism from O2consumption of the stimulated myocardium. We have shown that O2consumption for the excitation–contraction coupling corresponds to 40% of basal metabolism. The purpose of the present study was to analyse the component of myocardial O2consumption for the excitation–contraction coupling by this method. Blockade of the sarcoplasmic reticulum Ca2+pump by thapsigargin (0.1–1μmol/l), or by cycloplazonic acid (10μmol/l), significantly reduced O2consumption for the excitation–contraction coupling by 40 or 70% of the respective controls. Neither thapsigargin nor cyclopliazonic acid reduced basal metabolism O2consumption. The magnitude of free shortening of the unloaded myocardial slices, quantified by slice surface area reduction, was small (about 1.5%) because of the lack of external preload. Thapsigargin (1μmol/l) and cycloplazonic acid (10μmol/l) markedly attenuated the already reduced free shortening. 2,3-butanedione monoxime (5 mmol/l) also largely suppressed the free shortening, although this agent did not alter the O2consumption of either unstimulated or stimulated myocardium. Some residual cross-bridge cycling may occur without detectable O2consumption. Our present energetic results revealed that the O2consumption of myocardial slices for the Ca2+handling in the excitation–contraction coupling was mainly used for the sarcoplasmic reticulum Ca2+pump.