Thermodynamic limitation for Ca²⁺handling contributes to decreased contractile reserve in rat hearts

Abstract

The free energy release from ATP hydrolysis (‖ΔG ∼p ‖) is decreased by inhibiting the creatine kinase (CK) reaction, which may limit the thermodynamic driving force for the sarcoplasmic reticulum (SR) Ca 2+ pumps and thereby cause a decrease in contractile reserve. To determine whether a decrease in ‖ΔG ∼p ‖ results in decreased contractile reserve by impairing Ca 2+ handling, we measured left ventricular pressure and cytosolic Ca 2+ concentration ([Ca 2+ ] c ; by indo 1 fluorescence) in isolated perfused rat hearts, with >95% inhibition of CK with 90 μmol iodoacetamide. Iodoacetamide did not directly alter SR Ca 2+ -ATPase activity, baseline left ventricular developed pressure, or baseline [Ca 2+ ] c . When perfusate Ca 2+ concentration was increased from 1.2 to 3.3 mM, LV developed pressure increased from 67 ± 6 to 119 ± 8 mmHg in control hearts ( P < 0.05) but did not significantly increase in CK-inhibited hearts. Similarly, the amplitude of the [Ca 2+ ] c transient increased from 548 ± 54 to 852 ± 140 nM in control hearts ( P < 0.05) but did not significantly increase in CK-inhibited hearts. We conclude that decreased ‖ΔG ∼p ‖ limits intracellular Ca 2+ handling and thereby limits contractile reserve.