Septic cardiomyopathy in rat LPS-induced endotoxemia: relative contribution of cellular diastolic Ca²⁺ removal pathways, myofibrillar biomechanics properties and action of the cardiotonic drug levosimendan

Cardiac dysfunction is a common complication in sepsis and is characterized by forward pump failure. Hallmarks of septic cardiomyopathy are decreased myofibrillar contractility and reduced Ca²⁺ sensitivity but it is still not clear whether reduced pump efficiency is predominantly a diastolic impairment. Moreover, a comprehensive picture of upstream Ca²⁺ handling mechanisms and downstream myosin biomechanical parameters is still missing. Ca²⁺-sensitizing agents in sepsis may be promising but mechanistic insights for drugs like levosimendan are scarce. Here, we used an endotoxemic LPS rat model to study mechanisms of sepsis on in vivo hemodynamics, multicellular myofibrillar Ca²⁺ sensitivity, in vitro cellular Ca²⁺ homeostasis and subcellular actomyosin interaction with intracardiac catheters, force transducers, confocal Fluo-4 Ca²⁺ recordings in paced cardiomyocytes, and in vitro motility assay, respectively. Left ventricular ejection fraction and myofibrillar Ca²⁺ sensitivity were depressed in LPS animals but restored by levosimendan. Diastolic Ca²⁺ transient kinetics was slowed down by LPS but ameliorated by levosimendan. Selectively blocking intracellular and sarcolemmal Ca²⁺ extrusion pathways revealed minor contribution of sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) to Ca²⁺ transient diastole in LPS-evoked sepsis but rather depressed Na⁺/Ca²⁺ exchanger and plasmalemmal Ca²⁺ ATPase. This was mostly compensated by levosimendan. Actin sliding velocities were depressed in myosin heart extracts from LPS rats. We conclude that endotoxemia specifically impairs sarcolemmal diastolic Ca²⁺ extrusion pathways resulting in intracellular diastolic Ca²⁺ overload. Levosimendan, apart from stabilizing Ca²⁺-troponin C complexes, potently improves cellular Ca²⁺ extrusion in the septic heart.