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

Postischemic myocardial stunning halved left ventricular contractility [end-systolic maximum elastance (E(max))] and doubled the O₂ cost of E(max) in excised cross-circulated canine heart. We hypothesized that this increased O₂ cost derived from energy-wasteful myocardial Ca²⁺ handling consisting of a decreased internal Ca²⁺ recirculation, some futile Ca²⁺ cycling, and a depressed Ca²⁺ reactivity of E(max). We first calculated the internal Ca²⁺ 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 O₂ cost and analyzed total Ca²⁺ handling using our recently developed integrative method. We found a decreased total Ca²⁺ transport and a considerable shift of the relation between futile Ca²⁺ cycling and Ca²⁺ reactivity in an energy-wasteful direction in the stunned heart. These changes in total Ca²⁺ handling reasonably account for the doubled O₂ cost of E(max) in stunning, supporting the hypothesis.