SARCOPLASMIC-RETICULUM CA2-2 USE OF UNLOADED CONTRACTING RAT-HEART SLICES - THAPSIGARGIN AND CYCLOPIAZONIC ACID (MC976630)( PUMP BLOCKADE DECREASES O)

We previously established a new measuring method of the myocardial O-2 consumption of mechanically unloaded rat left-ventricular slices. O-2 consumption of unstimulated myocardium corresponds to basal metabolis m. We have found O-2 consumption of stimulated myocardium to include b asal metabolism and O-2 consumption for Ca2+ handling in the excitatio n-contraction coupling, but not for crossbridge cycling. Thus, O-2 con sumption for the excitation-contraction coupling is obtained by subtra cting basal metabolism from O-2 consumption of the stimulated myocardi um. We have shown that O-2 consumption for the excitation-contraction coupling corresponds to 40% of basal metabolism. The purpose of the pr esent study was to analyse the component of myocardial O-2 consumption for the excitation-contraction coupling by this method. Blockade of t he sarcoplasmic reticulum Ca2+ pump by thapsigargin (0.1-1 mu mol/l), or by cycloplazonic acid (10 mu mol/l), significantly reduced O-2 cons umption for the excitation-contraction coupling by 40 or 70% of the re spective controls. Neither thapsigargin nor cyclopliazonic acid reduce d basal metabolism O-2 consumption. The magnitude of free shortening o f the unloaded myocardial slices, quantified by slice surface area red uction, was small (about 1.5%) because of the lack of external preload . Thapsigargin (1 mu mol/l) and cycloplazonic acid (10 mu mol/l) marke dly attenuated the already reduced free shortening. 2,3-butanedione mo noxime (5 mmol/l) also largely suppressed the free shortening, althoug h this agent did not alter the O-2 consumption of either unstimulated or stimulated myocardium. Some residual cross-bridge cycling may occur without detectable O-2 consumption. Our present energetic results rev ealed that the O-2 consumption of myocardial slices for the Ca2+ handl ing in the excitation-contraction coupling was mainly used for the sar coplasmic reticulum Ca2+ pump. (C) 1998 Academic Press Limited.