Effects of nitric oxide synthase inhibition on microvascular reactivity inseptic mice

Persistent vasodilation refractory to vasopressor agents is characteristic of septic shock. Induction of nitric oxide synthase (NOS) by sepsis-induced cytokines within the vasculature is one of the primary mediators of this r efractory vasodilation. To evaluate the mechanism of vasodilation in sepsis , we used in vivo videomicroscopy to measure microvascular vasoconstrictive responses to topical suffusion of norepinephrine in mice made septic by ce cal ligation and puncture, and contrasted the effects of topical superfusio n of the nonselective NOS inhibitor N-G-methyl-L-arginine (L-NMMA) and the selective inducible NOS (iNOS) inhibitor S-methyl-isothiourea (SMT). Mice w ith sepsis were less sensitive to the vasoconstrictive effects of norepinep hrine than controls (EC50, the concentration that produces half-maximal res ponse 2.0 +/- 0.6 x 10(-6) M vs. 7.9 +/- 2.2 x 10(-8) M, P = 0.01). Selecti ve inhibition of inducible INOS with topical SMT (100 mu M) markedly increa sed catecholamine reactivity in mice with sepsis but did not affect reactiv ity in controls (P = 0.0007 for sepsis, P = 0.24 for controls). Nonselectiv e NOS inhibition with topical L-NMMA produced a similar increase in catecho lamine reactivity in mice with sepsis but not controls (P = 0.001 for sepsi s, P = 0.56 for controls). When excess (1 mM) L-arginine, the substrate for NOS, was added to the superfusion buffer along with both SMT and L-NMMA, a rteriolar responsiveness to norepinephrine was decreased to the original va lues. These experiments demonstrate that iNOS inhibition is as effective as nonselective NOS inhibition in reversing decreased catecholamine reactivit y in sepsis. This suggests a crucial role for microvascular activation of i NOS in the pathophysiology of hypotension and decreased vasopressor respons iveness in sepsis.