Energy-wasteful total Ca2+ handling underlies increased O2 cost of contractility in canine stunned heart

Departments of 1 Physiology II, 2 Internal Medicine II, 3 Cardiovascular Medicine, and 4 Anesthesiology and Resuscitology, Okayama University Medical School, Okayama 700-8558; and 5 Department of Anesthesiology and Resuscitology, Faculty of Medicine, Kagoshima University, Kagoshima 890-8520, Japan Postischemic myocardial stunning halved left ventricular contractility [end-systolic maximum elastance ( E max )] and doubled the O 2 cost of E max in excised cross-circulated canine heart. We hypothesized that this increased O 2 cost derived from energy-wasteful myocardial Ca 2+ handling consisting of a decreased internal Ca 2+ recirculation, some futile Ca 2+ cycling, and a depressed Ca 2+ reactivity of E max . We first calculated the internal Ca 2+ recirculation fraction (RF) from the exponential decay component of postextrasystolic potentiation. Stunning significantly accelerated the decay and decreased RF from 0.63 to 0.43 on average. We then combined the decreased RF with the halved E max and its doubled O 2 cost and analyzed total Ca 2+ handling using our recently developed integrative method. We found a decreased total Ca 2+ transport and a considerable shift of the relation between futile Ca 2+ cycling and Ca 2+ reactivity in an energy-wasteful direction in the stunned heart. These changes in total Ca 2+ handling reasonably account for the doubled O 2 cost of E max in stunning, supporting the hypothesis. stunning; end-systolic maximum elastance; mechanoenergetics; postextrasystolic potentiation