IN-VITRO ENDOTOXIN EXPOSURE INDUCES CONTRACTILE DYSFUNCTION IN ADULT-RAT CARDIAC MYOCYTES

In vivo endotoxin treatment causes a nitric oxide-mediated hypocontrac tility in cardiac myocytes. The objective of this study was to assess whether in vitro endotoxin exposure confers similar contractile defect s in adult rat cardiac cells. We found that incubation of cardiac myoc ytes for 6 h with 10-100 ng/ml endotoxin resulted in progressive time- and protein synthesis-dependent decreases in electrically stimulated twitch magnitudes and increased contraction and relaxation times. Seru m was not required for the endotoxin-induced hypocontractility. The en dotoxin-induced defect in contractility was reversed over time, since myocytes continuously incubated with endotoxin for 24 h exhibited norm al contractility; in contrast, control cells incubated for 18 h were s uppressed by a subsequent 6-h exposure to endotoxin. Nitric oxide synt hase activity was increased after a 6-h endotoxin treatment as evidenc ed by a dose-dependent enhanced conversion of [H-3]arginine to [H-3]ci trulline and by elevated guanosine 3',5'-cyclic monophosphate levels. Superfusion of endotoxin-incubated cells with N-omega-nitro-L-arginine methyl ester restored contractile function, whereas superfusion with L-arginine reimposed abnormal contractility. Naive myocytes superfused with 8-bromoguanosine 3',5'-cyclic monophosphate expressed contractil e defects similar to those induced by endotoxin. These findings demons trate that endotoxin has direct negative effects on cardiac cell contr actile function and that induction of NO synthase activity is a primar y intracellular mediator of the diminished contractility.