Cardiac inotropy, lusitropy, and Ca²⁺ handling with major metabolic substrates in rat heart

Fatty acid (FA)-dependent oxidation is the predominant process for energy supply in normal heart. Impaired FA metabolism and metabolic insufficiency underlie the failing of the myocardium. So far, FA metabolism in normal cardiac physiology and heart failure remains undetermined. Here, we evaluate the mechanisms of FA and major metabolic substrates (termed NF) on the contraction, relaxation, and Ca²⁺ handling in rat left ventricular (LV) myocytes. Our results showed that NF significantly increased myocyte contraction and facilitated relaxation. Moreover, NF increased the amplitudes of diastolic and systolic Ca²⁺ transients ([Ca²⁺]_i), abbreviated time constant of [Ca²⁺]_i decay (tau), and prolonged the peak duration of [Ca²⁺]_i. Whole-cell patch-clamp experiments revealed that NF increased Ca²⁺ influx via L-type Ca²⁺ channels (LTCC, I_Ca-integral) and prolonged the action potential duration (APD). Further analysis revealed that NF shifted the relaxation phase of sarcomere lengthening vs. [Ca²⁺]_i trajectory to the right and increased [Ca²⁺]_i for 50 % of sarcomere relengthening (EC₅₀), suggesting myofilament Ca²⁺ desensitization. Butanedione monoxime (BDM), a myosin ATPase inhibitor that reduces myofilament Ca²⁺ sensitivity, abolished the NF-induced enhancement of [Ca²⁺]_i amplitude and the tau of [Ca²⁺]_i decay, indicating the association of myofilament Ca²⁺ desensitization with the changes in [Ca²⁺]_i profile in NF. NF reduced intracellular pH ([pH_i]). Increasing [pH]_i buffer capacity with HCO₃/CO₂ attenuated Δ [pH]_i and reversed myofilament Ca²⁺ desensitization and Ca²⁺ handling in NF. Collectively, greater Ca²⁺ influx through LTCCs and myofilament Ca²⁺ desensitization, via reducing [pH]_i, are likely responsible for the positive inotropic and lusitropic effects of NF. Computer simulation recapitulated the effects of NF.