HP-228, a novel synthetic peptide, inhibits the induction of nitric oxide synthase in vivo but not in vitro

α-Melanocyte stimulating hormone has been shown to prevent endotoxin shock. A heptapeptide analog (HP-22B) has recently been synthesized and shown to be an even more potent protective agent. Because the hypotensive and toxic actions of lipopolysaccharide (LPS) appear to involve the induction of type II nitric oxide synthase (iNOS), we have examined the actions of HP-228 on nitric oxide production using an endotoxemia model in conscious rats given E. coli LPS (5 mg/kg i.v.) and monitored for 6 h. A group of rats received HP- 228 (30 μg/kg) 30 min before 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-22B pretreatment markedly reduces this response. Additionally, the rate of conversion of ³[H]-arginine to ³[H]-citrulline was significantly reduced in lung homogenates from HP-228-treated rats. HP-228 did not alter the activity of the constitutive nitric oxide synthase in aortic rings or in cerebella. In isolated rat aortic smooth muscle cells, LPS or interleukin- 1β caused prominent rises in nitric oxide generated by iNOS. HP-228 did not antagonize the effect of these inducing agents. However, in these cells, plasma obtained from rats 1 h after administration of HP-228 prevented the induction of iNOS by both LPS and interleukin-1β. In conclusion, HP-228 prevents the in vivo induction of nitric oxide synthase by LPS. It does not appear to directly prevent these actions of LPS and interleukin-1β, which suggests that a metabolite of HP-228 and/or secondary mediator (s) may be involved.