Reduced arteriolar conducted vasoconstriction in septic mouse cremaster muscle is mediated by nNOS-derived NO

Objective: Increased nitric oxide (NO) production in sepsis precipitates microcirculatory dysfunction. We aimed (i) to determine if NO is the key water-soluble factor in the recently discovered sepsis-induced deficit in arteriolar conducted vasoconstriction, (ii) to identify which nitric oxide synthase (NOS) isoforms account for this deficit, and (iii) to examine the potential role of connexin37 (Cx37, a hypothesized signaling target of NO) in arteriolar conduction. Methods: Using intravital microscopy and the cecal ligation and perforation 24-h model of sepsis, arterioles in the cremaster muscle of male C57BL/6 wild-type (WT), iNOS^-/-, eNOS^-/-, nNOS^-/- and Cx37^-/- mice were locally stimulated with KCl to initiate conducted vasoconstriction. We used the ratio of conducted constriction (500 μm upstream) to local constriction as an index of conduction (CR₅₀₀). NOS enzymatic activity and protein expression were determined in control and septic cremaster muscles. Results: Sepsis reduced CR₅₀₀ in WT mice [from 0.77 ± 0.05 to 0.20 ± 0.02 (means ± SE) independent of the site of stimulation along the arteriole], in iNOS^-/- and eNOS^-/- mice, but not in nNOS ^-/- mice. The nNOS inhibitor 7-nitroindazole or NO scavenger HbO ₂ restored CR₅₀₀ in septic WT mice, but blockade of soluble guanylate cyclase had no effect. Sepsis increased cNOS (eNOS + nNOS) activity in WT mice (from 340 ± 40 to 490 ± 30 pmol/mg/h) and in eNOS^-/-, but not in nNOS^-/- mice (iNOS activity was negligible in all mice). Sepsis did not alter nNOS protein expression in WT mice. CR₅₀₀ in non-septic Cx37^-/- mice (0.15 ± 0.1) was similar to that observed in septic WT mice. Conclusion: Increased nNOS activity and the resultant increased NO production in the septic mouse cremaster muscle are the key factors responsible for the deficit in conducted vasoconstriction along the arteriole. Deletion of Cx37 results in reduced CR₅₀₀, which is consistent with the hypothesis that Cx37 in the arteriole could be a target of NO signaling.