Protection against endotoxemia-induced contractile dysfunction in mice with cardiac-specific expression of slow skeletal troponin I

Gram negative endotoxemia is associated with an intrinsic impairment of cardiomyocyte contraction, in part due to a reduction in myofilament Ca2+ responsiveness. Endotoxemic rat hearts show increased cardiac troponin I (cTnI) phosphorylation at serines 23 and 24, residues required for the protein kinase A (PKA)-dependent reduction of myofilament Ca2+ sensitivity after beta-adrenoceptor stimulation. To investigate the functional significance of increased TnI phosphorylation in endotoxemia, we studied the contractile effects of systemic bacterial lipopolysaccharide (LPS) treatment in transgenic mice (TG) with cardiac-specific replacement of cTnI by slow skeletal TnI (ssTnI, which lacks the PKA phosphorylation sites) and matched nontransgenic littermates (NTG) on a CD1 background. In wild-type CD1 mice treated with LPS (6 mg/kg ip), after 16-18 h there was a significant reduction in the maximum rates of left ventricular pressure development and pressure decline in isolated Langendorff-perfused hearts compared with saline-treated controls and a decrease in isolated myocyte unloaded sarcomere shortening from 6.1 +/- 0.2 to 3.9 +/- 0.2% (1 Hz, 32 degrees C, P<0.05). Similarly, in NTG myocytes, endotoxemia reduced myocyte shortening by 42% from 6.7 0.2 to 3.9 +/- 0.1% (P<0.05) with no change in intracellular Ca2+ transients. However, in the TG group, LPS reduced myocyte shortening by only 13% from 7.5 +/- 0.2 to 6.5 +/- 0.2% (P<0.05). LPS treatment significantly reduced the positive inotropic effect of isoproterenol in NTG myocytes but not in TG myocytes, even though isoproterenol-induced increases in Ca2+ transient amplitude were similar in both groups. Only LPS-treated NTG hearts showed a significant increase in cTnI phosphorylation. Investigation of the sarcomere shortening-Ca2+ relationship in Triton-skinned cardiomyocytes revealed a significant reduction in myofilament Ca2+ sensitivity after LPS treatment in NTG myocytes, an effect that was substantially attenuated in TG myocytes. In conclusion, the replacement of cTnI with ssTnI in the heart provides significant protection against endotoxemia-induced cardiac contractile dysfunction, most probably by preserving myofilament Ca2+ responsiveness due to prevention of phosphorylation of TnI at PKA-sensitive sites.