Energy-wasteful total Ca²⁺handling underlies increased O₂cost of contractility in canine stunned heart

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.