ABSTRACT-The effect of in vitro exposure of sarcolemmal membrane (SL) vesicles to Gram-negative endotoxin lipopolysaccharides (LPS) was studied. LPS decreased the Na,K-ATPase activity of SL vesicles; this effect was inhibited by hydroxyl radical (' OH) scavengers such as dimethylthiourea and dimethyl sulfoxide, but not by super oxide dismutase, a scavenger of superoxide anion radicals or by the hydrogen perox ide scavenger catalase. ESR spin-trapping with 5,5-dimethyl-l-pyrroline N-oxide veri fied the generation of *OH from LPS itself under the conditions used; *OH gener ated from LPS was not affected by deferoxamine, a powerful iron chelator. The Na,K-ATPase activity was reduced by an 'OH radical generating system consisting of dihydroxyfumarate and Fe 3+-ADP. Furthermore, exposure of SL vesicles to LPS caused an increase in malondialdehyde formation. It can be concluded that LPS dam ages cardiac SL by an oxygen free radical mechanism by the generation of 'OH, due to inhibition of Na,K-ATPase activity and peroxidation of lipids, and that the effect of LPS is not dependent on the presence of contaminating iron.Endotoxins are lipopolysaccharides located in the outer membrane of Gram-negative bac teria. Much of the morbidity and mortality associated with Gram-negative infections is thought to be due to release of endotoxin (1). Injection of microgram amounts of endotoxin into susceptible experimental animals produces a number of biological effects. These include multiple hematologic events such as comple ment activation, disseminated intravascular coagulation, and intestinal bleeding. Injection of larger doses of endotoxin frequently results in shock and death (2).A current hypothesis for the mechanism underlying the cardiovascular collapse of Gram-negative septic or endotoxin shock be gins with the observation of Jacob, et al. (3) that endotoxin within the vascular space acti vates complement and leads to the production of complement components; most importantly, C5a. C5a activates polymorphonuclear leuko cytes (4) which become adherent to endothe lial surfaces and other leukocytes and subse quently release oxygen free radicals, lysosomal hydrolases, and arachidonic acid derivatives. These oxygen free radicals (superoxide anion, Oz ; hydroxyl radical, *OH; singlet oxygen, 1O 7; and hydrogen peroxide, H202) are ca pable of extensive endothelial cell (5) and car diac cell damage (6-9) and could potentially produce the loss of contractility in vascular smooth muscle and cardiac muscle which is * To whom reprint requests should be addressed .

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