Energy for Myocardial Ca²⁺ handling per beat increases with heart rate in excised cross-circulated canine heart

Objective: Although tachycardia is well known to increase cardiac oxygen consumption (Vo₂) per min, the relationship between Vo₂ for excitation-contraction (E-C) coupling per beat and heart rate change over its full working range still remains controversial. Methods: To elucidate this relationship, we varied heart rate over a reasonably wide range (60-180 beat/min) and studied the relationship between left ventricular (LV) Emax (load-independent contractility index), PVA (pressure-volume area)-independent Vo₂, and basal metabolic Vo₂ in nine excised, cross-circulated canine hearts. Results: PVA-independent Vo₂ per min significantly increased linearly with increasing heart rate while Emax remained unchanged. Basal metabolic Vo₂per min was measured under KCl arrest. E-C coupling Vo₂ per min obtained by subtracting the constant basal metabolic Vo₂ from the PVA-independent Vo₂ also significantly increased linearly with increasing heart rate. However, PVA-independent Vo₂ per beat significantly decreased with increasing heart rate. In contrast, E-C coupling Vo₂ per beat, as well as that normalized to Emax, slightly but significantly increased with increasing heart rate. Conclusion; The E-C coupling energy for myocardial Ca²⁺ handling increases with heart rate despite constant contractility in the left ventricle of the excised cross-circulated canine heart.