Nitric oxide produced via neuronal NOS may impair vasodilatation in septicrat skeletal muscle

Impaired vascular responsiveness in sepsis may lead to maldistribution of b lood flow in organs. We hypothesized that increased production of nitric ox ide (NO) via inducible nitric oxide synthase (iNOS) mediates the impaired d ilation to ACh in sepsis. Using a 24-h cecal ligation and perforation (CLP) model of sepsis, we measured changes in arteriolar diameter and in red blo od cell velocity (V-RBC) in a capillary fed by the arteriole, following app lication of ACh to terminal arterioles of rat hindlimb muscle. Sepsis atten uated both ACh-stimulated dilation and V-RBC increase. In control rats, art eriolar pretreatment with the NO donors S-nitroso-N-acetylpenicillamine or sodium nitroprusside reduced diameter and V-RBC responses to a level that m imicked sepsis. In septic rats, arteriolar pretreatment with the "selectite " iNOS blockers aminoguanidine (AG) or S-methylisothiourea sulfate (SMT) re stored the responses to the control level. The putative neuronal NOS (nNOS) inhibitor 7-nitroindazole also restored the response toward control. At 24 -h post-CLP, muscles showed no reduction of endothelial NOS (eNOS), elevati on of nNOS, and, surprisingly, no induction of iNOS protein; calcium-depend ent constitutive NOS (eNOS+nNOS) enzyme activity was increased whereas calc ium-independent iNOS activity was negligible. We conclude that 1) AG and SM T inhibit nNOS activity in septic skeletal muscle, 2) NO could impair vasod ilative responses in control and septic rats, and 3) the source of increase d endogenous NO in septic muscle is likely upregulated nNOS rather than iNO S. Thus agents released from the blood vessel milieu (e.g., NO produced by skeletal muscle nNOS) could affect vascular responsiveness.