Load Independence of Temperature-Dependent Ca²⁺ Recirculation Fraction in Canine Heart

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Intramyocardial Ca²⁺ recirculation fraction (RF) critically determines the economy of excitation-contraction coupling. RF is obtainable from the exponential decay of the postextrasystolic potentiation of left ventricular (LV) contractility. We have shown that RF remains unchanged despite increasing LV volume (LVV) at normothermia, but decreases with increasing temperature at a constant LVV. However, it remains unknown whether the temperature-dependent RF was not due to the simultaneously changed peak LV pressure (LVP) at a constant LVV. We hypothesized that this temperature-dependent RF would be independent of the simultaneous change in LVP. We used nine excised, cross-circulated canine hearts and allowed their LVs to contract isovolumically. During stable regular beats at 500 msec intervals, we inserted an extrasystolic beat at 360 msec interval followed by the postextrasystolic beats (PESs) at 500 msec intervals. We equalized the temperature-dependent peak LVPs of the regular beats at 36°C and 38°C to the peak LVP level of the stable regular beat at 33°C by adjusting LVV. We fitted the same equation: nEmax = a exp[−(i − 1)/τ_e] + b exp[−(i − 1)/τ_s]cos[π(i − 1)] + 1, used before to the normalized Emax (maximum elastance) values of PESi (i = 1–6) relative to the regular beat Emax. RF given by exp(−1/τ_e) decreased by 19% to 38°C from 33°C. The temperature coefficient (Q₁₀) of 1/RF was significantly greater than 1.3. The present results indicated a similar temperature dependence of RF and its Q₁₀ to those we observed previously without equalizing peak LVP. Thus, the temperature-dependent RF is independent of ventricular loading conditions.