ALTERATIONS IN INOTROPY, NITRIC-OXIDE AND CYCLIC-GMP SYNTHESIS, PROTEIN-PHOSPHORYLATION AND ADP-RIBOSYLATION IN THE ENDOTOXIN-TREATED RAT MYOCARDIUM AND CARDIOMYOCYTES

To evaluate the effects of the in vivo endotoxin treatment of the rat on (1) the contractile responses in the subsequently isolated papillar y muscle to adrenergic and cholinergic agonists and (2) the biochemica l parameters (cyclic GMP, nitric oxide synthesis, protein phosphorylat ion and ADP-ribosyslation) in the subsequently isolated cardiomyocytes . Following the in vivo endotoxin treatment (4 mg/kg i.p., 18 h), cont ractile responses to increasing amounts of isoprenaline or to increasi ng amounts of oxotremorine in the presence of a fixed amount of isopre naline were determined in isolated papillary strips. Activities of nit ric oxide synthase, guanylyl cyclase, as well as phosphorylation of ph ospholamban and troponin-inhibitory subunit, and pertussis toxin-catal yzed and endogenous ADP-ribosylations were determined in the intact ca rdiomyocytes and subcellular fractions. The increase in the force of c ontraction by isoprenaline was reduced, while its inhibition by oxotre morine was greater in the endotoxin-treated papillary strips. The acti vities of both nitric oxide synthase, primarily of the inducible form of the enzyme, and cytosolic guanylyl cyclase were higher while the ph osphorylations of both phospholamban and troponin-inhibitory subunit w ere of lesser magnitude in the cardiomyocytes following the in vivo en dotoxin treatment. Pertussis toxin-catalyzed ADP-ribosylation of the 4 1 kDa polypeptide, which is the alpha subunit of Gi, was also decrease d. The results of the present study support the postulate that alterat ions in both the cyclic AMP and cyclic GMP signalling cascade contribu te to the myocardial dysfunction caused by endotoxin and cytokines.