Abstract
Acute heart failure patients require inotropic support to improve haemodynamic function and cardiac output. Current inotropes such as dobutamine, increase contractile force by enhancing cytosolic Ca2+ cycling, consequently promoting arrhythmias, prompting a demand for inotropes with little effect on intracellular Ca2+ flux. Low-dose carbon monoxide (CO) induces inotropic effects in perfused hearts. Although this mechanism remains unclear, CO can interact with the myofilament protein troponin C. Using our CO-releasing prodrug, oCOm-21, we proposed that this inotropic effect may result from a CO-mediated increase in myofilament Ca2+ sensitivity.
Male Sprague Dawley rat left ventricular (LV) cardiomyocytes were skinned and attached to a force-transducer. Force was measured as a function of pCa (-log [Ca2+]) in the range of pCa 9.0-4.5 under four conditions: vehicle, oCOm-21 (3 or 10μM), and the drug control BP-21 (9 cells/group). The Ca2+ concentration at which 50% of maximal force is produced (pCa50), was used to measure Ca2+ sensitivity. Results were analysed using a one-way ANOVA with a Bonferroni post-hoc test. Additionally, haemodynamic function was examined in Langendorff-perfused rat hearts treated with vehicle or 10μM oCOm-21 (n=5 hearts/treatment). Subsequent skinned cardiomyocytes were examined for changes in Ca2+ sensitivity. Haemodynamic responses over time were analysed by a two-way ANOVA with a Bonferroni post-hoc test while the Ca2+ sensitivity results were analysed by an unpaired Students t-test.
oCOm-21 (10μM) (but not BP-21) significantly increased pCa50 compared to vehicle, respectively (pCa50 5.52 vs. 5.47 vs. 5.44; P < 0.05). oCOm-21 significantly increased both LV-developed pressure and Ca2+ sensitivity compared to vehicle, respectively (103 mmHg vs. 72 mmHg; P < 0.01; pCa50 5.50 vs. 5.47; P < 0.05).
These results support the hypothesis that oCOm-21-derived CO directly increases myofilament Ca2+ sensitivity. Importantly, the increased Ca2+ sensitivity was conserved in a pre-treated heart. Further analyses will examine if CO impacts cytosolic Ca2+ cycling.