Molecular and cellular mechanisms of sepsis-induced myocardial dysfunction

We hypothesized that a progressive decline in myocardial performance would correlate with the upregulation of apoptosis following polymicrobial sepsis in the rat. We observed that prolonged durations of sepsis result in decreased mean arterial pressure (7-days post sepsis), left ventricular contractility dysfunction, alteration in clinical biomarkers [total cardiac troponin I (cTnI) and C Reactive Protein (CRP) and endothelin-1 (ET-1)], and 10, 33 and 42% mortality in the 1-, 3- and 7-day sepsis groups, respectively. Sepsis produced a significant upregulation in the expression of myocardial TRADD, cytosolic active caspase-3, Bax/Bcl2 ratio, and mitochondrial release of cytochrome C in the 3- and 7-day post-sepsis groups. We observed a progressive increase in the number of TUNEL-positive nuclei, cytosolic caspase-3 activation and co-localization of PARP in the nuclei at 1, 3 and 7 days post-sepsis. These data suggest that a sepsis-induced decline in myocardial performance correlates with the induction of myocardial apoptosis. Since sepsis causes increased sympathetic stimulation, we observed that myocardial syntaxin1A mRNA and protein expression also significantly increased at 1 day post-CI (cecal inoculum) compared to the sham group. Our results suggested that during severe sepsis (CLP model), both the upregulation of syntaxin1A and the downregulation of NET contribute to increased concentrations of NE during early and late stages of sepsis. In vitro studies revealed that prolonged exposure of norepinephrine (NE) produces an increase in active caspase-3, which in turn contributes to sepsis-induced adult rat ventricular myocyte (ARVM) contractile dysfunction. NE produced a blunted contractile response in septic ARVMs because the effect of NE on the PS of the sham ARVMs was more pronounced compared to the septic ARVMs. NE in the presence of QVD-OPH (a broad-spectrum caspase inhibitor) ameliorated the sepsis-induced decrease in PS at 18h but not at 1h, suggesting that caspase-inhibition maintained the positive inotropic response of NE. The transfection of ARVMs using caspase-3 siRNA not only blocked the sepsis-induced upregulation of caspase-3, it increased PS following NE treatment as well as ameliorated the blunted contractile response of NE and cleavage of contractile proteins (alpha-actin, tropomyosin and MLC-1) in the septic ARVMs.