Phosphorylation-dependent alteration in myofilament Ca2+ sensitivity but normal mitochondrial function in septic heart

The subcellular mechanisms responsible for myocardial depression during sep sis remain unclear. Recent data suggest a role for impaired energy generati on and utilization, resulting in altered contractile function. Here, we stu died the energetic and mechanical properties of skinned fibers isolated fro m rabbit ventricle in a nonlethal but hypotensive model of endotoxemia. Thi rty-six hours after (lipopolysaccharide (LPS) injection tin the presence of altered myocardial contractility), mitochondrial respiration, coupling bet ween oxidation and phosphorylation, and creatine kinase function were simil ar in preparations from endotoxemic: (LPS) and control animals. The maximal Ca2+-activated force was similar in LPS and control preparations. However, the Ca2+ concentration corresponding to half-maximal force (pCa(50) where pCa = -log(10)[Ca2+]) was 5.55 +/- 0.01 (n = 11) in LPS fibers versus 5.61 +/- 0.01 (n = 10) in control fibers (p < 0.01). Both protein kinase A (PKA) and alkaline phosphatase treatment led to the disappearance in the differe nce between control and LPS pCa(50) values. Incubation of control fibers wi th the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) did not ch ange the Ca2+ sensitivity after subsequent skinning, whereas isoproterenol decreased pCa(50) from 5.62 +/- 0.01 to 5.55 +/- 0.01 (p < 0.01). These dat a suggest that during sepsis, cardiac mitochondrial and creatine kinase sys tems remain unaltered, whereas protein phosphorylation decreases myofibrill ar Ca2+ sensitivity and may contribute to the depression of cardiac contrac tility.