Prevention of endotoxin-induced sarcoplasmic reticulum calcium leak improves mitochondrial and myocardial dysfunction.

OBJECTIVE: Growing evidence suggests that mitochondria function is impaired in sepsis. Here, we tested the hypothesis that lipopolysaccharide would induce mitochondrial Ca2+ overload and oxygen utilization abnormalities as consequences of sarcoplasmic reticulum Ca2+ handling derangements that are typically observed in sepsis. As lipopolysaccharide-induced sarcoplasmic reticulum dysfunction was mainly characterized by reduced sarcoplasmic reticulum Ca2+ uptake and Ca2+ leak, we tested whether dantrolene, a sarco(endo)plasmic reticulum calcium ATPase leak inhibitor, would prevent mitochondrial and cardiac contractile dysfunction.
DESIGN: Randomized controlled trial.
SETTING: Experimental laboratory.
SUBJECTS: Male Sprague Dawley rats.
INTERVENTIONS: Sepsis was induced by injection of endotoxin lipopolysaccharide (10 mg/kg/intravenously). Assessment of contractile function and Ca2+ handling was performed 4 hr after lipopolysaccharide. The relative contribution of the different Ca2+ transporters to relaxation in intact cardiomyocytes was studied during successive electrically evoked twitches and caffeine stimulation. Sarcoplasmic reticulum vesicles and mitochondria from ventricles of rats treated or not with lipopolysaccharide were prepared to evaluate Ca2+ uptake-release and oxygen fluxes, respectively. Effects of dantrolene (10 mg/kg) treatment in rats were evaluated in sarcoplasmic reticulum vesicles, mitochondria, and isolated hearts.
MEASUREMENTS AND MAIN RESULTS: Lipopolysaccharide challenge elicited cardiac contractile dysfunction that was accompanied by severe derangements in sarcoplasmic reticulum function, i.e., reduced Ca2+ uptake and increased sarcoplasmic reticulum Ca2+ leak. Functional sarcoplasmic reticulum changes were associated with modification in the status of phospholamban phosphorylation whereas SERCA was unchanged. Rises in mitochondrial Ca2+ content observed in lipopolysaccharide-treated rats coincided with derangements in mitochondrial oxygen efficacy, i.e., reduced respiratory control ratio. Administration of dantrolene in lipopolysaccharide-treated rats prevented mitochondrial Ca2+ overload and mitochondrial oxygen utilization abnormalities. Moreover, dantrolene treatment in lipopolysaccharide rats improved heart mitochondrial redox state and myocardial dysfunction.
CONCLUSION: These experiments suggest that sarcoplasmic reticulum Ca2+ handling dysfunction is an early event during endotoxemia that could be responsible for, or contribute to, mitochondrial Ca2+ overload, metabolic failure, and cardiac dysfunction.