DOWN-REGULATION OF NITROVASODILATOR-INDUCED CYCLIC-GMP ACCUMULATION IN CELLS EXPOSED TO ENDOTOXIN OR INTERLEUKIN-1-BETA

1 Induction of nitric oxide synthase (iNOS) results in overproduction of nitric oxide (NO), which may be a principal cause of the massive va sodilatation and hypotension observed in septic shock. Since NO-induce d vasorelaxation is mediated via the soluble isoform of guanylate cycl ase (sGC), the regulation of sGC activity during shock is of obvious i mportance, but yet poorly understood. The aim of the present study was to investigate the activation of sGC by sodium nitroprusside (SNP) be fore and after exposure of rat aortic smooth muscle cells to endotoxin (LPS) or interleukin-1 beta (IL-1 beta). 2 Exposure of rat aortic smo oth muscle cells to SNP (10 mu M) elicited up to 200 fold increases in cyclic GMP. This effect was attenuated by 30-70% in IL-1 beta- or LPS -pretreated cells, in a pretreatment time- and IL-1 beta- or LPS-conce ntration-dependent manner. When, however, cells were exposed to IL-1 b eta or LPS and then stimulated with the particulate guanylate cyclase activator, atriopeptin II, no reduction in cyclic GMP accumulation was observed. 3 Pretreatment of rats with LPS (5 mg kg(-1), i.v.) for 6 h led to a decrease in aortic ring SNP-induced cyclic GMP accumulation. 4 The IL-1 beta-induced reduction in SNP-stimulated cyclic GMP accumu lation in cultured cells was dependent on NO production, as arginine d epletion abolished the downregulation of cyclic GMP accumulation in re sponse to SNP. 5 Reverse-transcriptase-polymerase chain reaction analy sis revealed that the ratio of steady state mRNA for the alpha(1) subu nit of sGC to glyceraldehyde phosphate dehydrogenase was decreased in LPS- or IL-1 beta-treated cells, as compared to vehicle-treated cells. 6 Protein levels of the al sGC subunit remained unaltered upon exposu re to LPS or IL-1 beta, suggesting that the early decreased cyclic GMP accumulation in IL-1 beta- or LPS-pretreated cells was probably due t o reduced sGC activation. Thus, the observed decreased responsiveness of sGC to NO stimulation following cytokine or LPS challenge may repre sent an important homeostatic mechanism to offset the extensive vasodi latation seen in sepsis.