NOS-1-derived NO is an essential triggering signal for the development of systemic inflammatory responses

Nitric oxide (NO) produced by the NO synthase type 2 (NOS-2) is known to have a prominent role in the course of the inflammatory response but less is known concerning the role of NO derived from the constitutive NOS isoforms. We have examined the role of NO derived from NOS-1 in the initiation of the systemic inflammatory response using sepsis models. Injection of LPS in rats induced an early hypotension, NOS-2 expression, increased lung myeloperoxidase activity and increased NO metabolite (NOx) levels in the skeletal muscle. Pre-treatment with 7-nitroindazol (7-NI) prevented all these changes, but its administration after LPS injection was ineffective. Septic (cecal ligation and puncture method, CLP) rats exhibited signs of organ failure, hyporesponsiveness to vasoconstrictors and 75% mortality over 3 days after surgery. Pre-treatment with 7-NI prevented or significantly reduced these alterations. Injection of 7-NI after sepsis onset was without effect. Wild type mice injected with LPS exhibited increased plasma NOx, NOS-2 and COX-2 expression and 80% mortality. NOS-1(-/-) mice injected with LPS exhibited smaller increase in plasma NOx, no NOS-2 and COX-2 expression and reduced mortality. Injection of an NO donor in CLP rats pre-treated with 7-NI or in NOS-1(-/-) mice returned the mortality rate to those of CLP in rats and LPS in mice. Our results demonstrate that NOS-1-derived NO acts as a signaling element and it is essential for the initiation of systemic inflammation as demonstrated by the reduction of the inflammatory response and mortality by both pharmacological inhibition and genetic deletion of NOS-1.