Reversal of isoflurane-induced depression of myocardial contraction by nitroxyl via myofilament sensitization to Ca ²⁺

Isoflurane (ISO) is known to depress cardiac contraction. Here, we hypothesized that decreasing myofilament Ca ²⁺ responsiveness is central to ISO-induced reduction in cardiac force development. Moreover, we also tested whether the nitroxyl (HNO) donor 1-nitrosocyclohexyl acetate (NCA), acting as a myofilament Ca ²⁺ sensitizer, restores force in the presence of ISO. Trabeculae from the right ventricles of LBN/F1 rats were superfused with Krebs-Henseleit solution at room temperature, and force and intracellular Ca ²⁺ ([Ca ²⁺] _i) were measured. Steady-state activations were achieved by stimulating the muscles at 10 Hz in the presence of ryanodine. The same muscles were chemically skinned with 1% Triton X-100, and the force- Ca ²⁺ relation measurements were repeated. ISO depressed force in a dose-dependent manner without significantly altering [Ca ²⁺] _i. At 1.5%, force was reduced over 50%, whereas [Ca ²⁺] _iremained unaffected. At 3%, contraction was decreased by ~75% with [Ca ²⁺] _i reduced by only 15%. During steady-state activation, 1.5% ISO depressed maximal Ca ²⁺-activated force (F _max) and increased the [Ca ²⁺] _i required for 50% activation (Ca ₅₀) without affecting the Hill coefficient. After skinning, the same muscles showed similar decreases in F _max and increases in Ca ₅₀ in the presence of ISO. NCA restored force in the presence of ISO without affecting [Ca ²⁺] _i. These results show that 1) ISO depresses cardiac force development by decreasing myofilament Ca ²⁺ responsiveness, and 2) myofilament Ca ²⁺sensitization by NCA can effectively restore force development without further increases in [Ca ²⁺] _i. The present findings have potential translational value because of the efficiency and efficacy of HNO on ISO-induced myocardial contractile dysfunction.