Effects of Ca²⁺ and Epinephrine on Ca²⁺ Recirculation Fraction and Total Ca²⁺ Handling in Canine Left Ventricles

Abstract

We investigated the effects of intracoronary Ca²⁺ and epinephrine on the intracellular Ca²⁺ recirculation fraction (RF) and total Ca²⁺ handling in the left ventricle (LV) of the excised cross-circulated canine heart preparation. We analyzed LV postextrasystolic potentiation (PESP) following a spontaneous extrasystole that occurred sporadically under constant atrial pacing. All PESPs decayed in alternans and none decayed monotonically. We extracted an exponential decay component from the alternans PESP, determined its beat constant (τ_e), and calculated RF=exp(−1/τ_e). Increased intracoronary Ca²⁺ slightly increased τ_e and RF, but epinephrine did not change them, although both agents enhanced LV contractility 2-3 times. Neither Ca²⁺ nor epinephrine affected the sinusoidal decay of the alternans PESP. These results indicate that RF via the sarcoplasmic reticulum was slightly augmented by Ca²⁺, but not by epinephrine. We combined these RF data with LV Ca²⁺ handling O₂ consumption data and obtained 40-110 μmol/kg as the total amount of Ca²⁺ handled in one cardiac cycle in the control and enhanced contractile states. These results indicate that this new LV-level approach seems to better the understanding of the Ca²⁺ mass dynamics responsible for the mechanoenergetics enhanced by inotropic interventions.