Dexamethasone inhibits endotoxin-induced changes in calcium and contractility in rat isolated papillary muscle

This study investigates whether endotoxin-induced contractile dysfunction is associated with a defect in the modulation of calcium homeostasis and the potential mechanisms involved. Treatment of rats in vivo with endotoxin significantly decreased the magnitude of contractile transients in electrically stimulated left ventricular papillary muscle isolated after an equilibration period of 6 hours. Although no significant difference was found in the peak intracellular calcium concentration ([Ca2+]i) between the endotoxin-treated and control groups, resting [Ca2+]iwas significantly elevated in the endotoxin-treated group, producing a smaller Ca2+transient (basal-peak difference) in this group. Pretreatment of rats with dexamethasone prevented the endotoxin-induced decrease in peak tension and inhibited the elevation in resting [Ca2+]i, with a resultant maintenance of Ca2+transient magnitude. Similar observations were made during stimulation of the muscles by the β-adrenoceptor agonist, isoprenaline. These results show that endotoxin-induced reduction of cardiac contractile performance is mediated, at least in part, by elevating resting [Ca2+]i, and a glucocorticoid protected from these negative effects. While endotoxin reduces the magnitude of the Ca2+transient it does not alter peak [Ca2+]iavailability. Further investigation is required to determine whether endotoxin decreases contractile performance by reducing the sensitivity of cardiac myofilaments to calcium.