Endotoxin-induced mesenteric microvascular changes involve iNOS-derived nitric oxide: Results from a study using iNOS knock out mice

The administration of endotoxin alters intestinal blood flow, increases nit ric oxide (NO) production, and induces gut barrier dysfunction. Thus, we in vestigated the hypothesis that microvascular reactivity and permeability of the mesenteric vascular bed are altered to a lesser degree in iNOS knock o ut (iNOS -/-) mice than their wild-type (iNOS +/+) litter mates after an en dotoxin challenge. To test this hypothesis, we compared the microvascular r esponse of iNOS knockout (iNOS -/-) mice after a topical or systemic endoto xin challenge against that of their wild-type litter mates (iNOS +/+). Intr avital microscopy was used to measure arteriolar diameter and postcapillary venular permeability in the mouse ileum. Both parameters were determined b y computer-assisted image analysis. Diameter was measured in A(1), A(2), an d A(3) arterioles (1, 2, 3 = rank of deployment). Changes in microvascular permeability were measured from changes in interstitial fluorescence caused by extravasation of fluorescein isothiocyanate (FITC)-dextran 150 (molecul ar weight = 150 kDa) and expressed as changes in integrated optical intensi ty (101). In the first series of experiments, endotoxin (100 mu g/mL) was a pplied topically to the ileal segment. In the second series, endotoxin (10 mg/kg) was administered intraperitoneally (i.p.). Administration of topical or i.p.. endotoxin caused vasoconstriction and was associated with an earl y increase in permeability in both iNOS +/+ and -/- mice, although over tim e the responses of the iNOS -/- and iNOS +/+ mice diverged. Twenty minutes after topical endotoxin, the increase in permeability in iNOS -/- mice had reached a plateau whereas it continued to increase in the iNOS +/+ mice, su ch that at 80 min post-topical endotoxin, 101 was 27 +/- 7 in iNOS -/- vs. 39 +/- 5 in iNOS +/+ (P < 0.05). A similar permeability response was observ ed after i.p.. endotoxin, where the increase in post-capillary venular perm eability was greater in the iNOS +/+ mice (P < 0.05). Both iNOS -/- and iNO S +/+ mice had a similar transient vasoconstrictive response after topical endotoxin challenge (reduction in A2 arteriolar diameters by -17 +/- 4% vs. -24 +/- 7%), with return to baseline values by 60-80 min post-endotoxin ch allenge. The iNOS +/+ but not the iNOS -/- mice manifested a secondary vaso dilatory response that persisted throughout the experimental period. The ar teriolar vasoreactive response of the iNOS -/- and iNOS +/+ mice to i.p.. e ndotoxin was similar to that of topical endotoxin, but of a lesser magnitud e. In conclusion, the similarity in effects between topical and systemic en dotoxin indicates that endotoxin causes microvascular dysfunction in the gu t by directly on the microcirculation. In addition, our data suggest that N O production by iNOS is involved in the microvascular alterations associate d with gut barrier dysfunction.