HP-228, A NOVEL SYNTHETIC PEPTIDE, INHIBITS THE INDUCTION OF NITRIC-OXIDE SYNTHASE IN-VIVO BUT NOT IN-VITRO

alpha-Melanocyte stimulating hormone has been shown to prevent endotox in shock. A heptapeptide analog (HP-228) has recently been synthesized and shown to be an even more potent protective agent. Because the hyp otensive and toxic actions of lipopolysaccharide (LPS) appear to invol ve the induction of type II nitric oxide synthase (iNOS), we have exam ined the actions of HP-228 on nitric oxide production using an endotox emia model in conscious rats given E. coli LPS (5 mg/kg i.v.) and moni tored for 6 h. A group of rats received HP-228 (30 mu g/kg) 30 min bef ore LPS. Using nitro L-arginine methyl ester-sensitive cGMP production as an estimate of nitric oxide synthase activity in aortic segments, ex vivo, we determined that LPS increases iNOS activity and that HP-22 8 pretreatment markedly reduces this response. Additionally, the rate of conversion of (3)[H]-arginine to (3)[H]-citrulline was significantl y reduced in lung homogenates from HP-228-treated rats. HP-228 did not alter the activity of the constitutive nitric oxide synthase in aorti c rings or in cerebella. In isolated rat aortic smooth muscle cells, L PS or interleukin-1 beta caused prominent rises in nitric oxide genera ted by iNOS. HP-228 did not antagonize the effect of these inducing ag ents. However, in these cells, plasma obtained from rats 1 h after adm inistration of HP-228 prevented the induction of iNOS by both LPS and interleukin-1 beta. In conclusion, HP-228 prevents the in vivo inducti on of nitric oxide synthase by LPS. It does not appear to directly pre vent these actions of LPS and interleukin-1 beta, which suggests that a metabolite of HP-228 and/or secondary mediator (s) may be involved.