Leukocyte adherence inhibits adenosine-dependent venular control of arteriolar diameter and nitric oxide

In vivo visualization of nitric oxide and interactions among platelets, leukocytes, and endothelium following hemorrhagic shock and reperfusion

OBJECTIVES

We examined changes in nitric oxide (NO) distribution in the mesenteric microcirculation after hemorrhagic shock and reperfusion (H/R), and correlated NO production to leukocyte and platelet behavior.

MATERIALS AND METHODS

The behavior of leukocytes and platelets in mesenteric venules was observed by intravital microscopy at 0.5 and 24 h after H/R in male Wistar rats. Transvascular leakage of fluorescein isothiocyanate-labeled albumin was assessed by epi-illumination. The NO-sensitive dye, 4,5-diaminofluorescein diacetate, was used for imaging NO release.

RESULTS

H/R significantly increased vascular albumin leakage and adhesion of leukocytes and platelets (P < 0.05). In H/R 0.5 h rats, NO production in the venular endothelium declined. However, NO production was elevated in H/R 24 h rats in mast cells (P < 0.05). Leukocyte adherence, platelet adherence, and venular permeability were attenuated by iNOS inhibition.

CONCLUSION

Mesenteric endothelial cell dysfunction after H/R 0.5 h is associated with reduced NO, whereas after H/R 24 h is related to increase NO in mast cells.

Losartan and ozagrel reverse retinal arteriolar constriction in non-obese diabetic mice

OBJECTIVE

Reductions in retinal blood flow are observed early in diabetes. Venules may influence arteriolar constriction and flow; therefore, we hypothesized that diabetes would induce the constriction of arterioles that are in close proximity to venules, with the constriction mediated by thromboxane and angiotensin II.

METHODS

Using nonobese diabetic (NOD) mice, retinal measurements were performed three weeks following the age at which glucose levels exceeded 200 mg/dL, with accompanying experiments on age-matched normoglycemic NOD mice. The measurements included retinal arteriolar diameters and red blood cell velocities and were repeated following an injection of the thromboxane synthase inhibitor, ozagrel. Mice were subdivided into equal groups and given drinking water with or without the angiotensin II receptor antagonist, losartan.

RESULTS

Retinal arterioles were constricted in hyperglycemic mice, with a significant reduction in flow. However, not all arterioles were equally affected; the vasoconstriction was limited to arterioles that were in closer proximity to venules. The arteriolar vasoconstriction (mean arteriolar diameters = 51 +/- 1 vs. 61 +/- 1 microm in controls; p < 0.01) was eliminated by both ozagrel (61 +/- 2 microm) and losartan (63 +/- 2 microm).

CONCLUSIONS

Venule-dependent arteriolar vasoconstriction in NOD mice is mediated by thromboxane and/or angiotensin II.

Mediators of CD18/P-selectin-dependent constriction of venule-paired arterioles in hypercholesterolemia

This study addresses the role of venule-derived mediators in the arteriolar constriction that accompanies hypercholesterolemia. Constriction was assessed by measuring the tone of small arterioles closely paired with venules in the mesentery of normal cholesterol rats (NC), high cholesterol rats (HC), HC rats injected with antibodies against CD18 and P-selectin (HC/mAbs), HC rats treated with the thromboxane synthase inhibitor, ozagrel (HC/ozagrel), and HC rats pretreated with anti-platelet serum (HC/APS). Venule-paired arterioles in the untreated HC group demonstrated enhanced tone compared with arterioles in the NC group, while no difference was found between unpaired arterioles of the two groups. Perivascular nitric oxide (NO) concentrations were found to be significantly decreased in venule-paired arterioles of HC rats (238+/-14 nM) compared with those of NC rats (426+/-42 nM). The injection of anti-adhesion antibodies successfully attenuated the enhanced arteriolar tone and venular leukocyte adherence in the HC group, and tended to increase levels of NO in venule-paired arterioles by 33% (to 326+/-19 nM; still lower than that of the NC group). Ozagrel and platelet depletion attenuated the enhanced arteriolar tone by 53% and 33%, respectively, without affecting NO concentrations. These findings indicate that the mechanism of blood cell-dependent arteriolar constriction during hypercholesterolemia may be dependent on thromboxane, a decrease in NO, and the proximity of the arterioles to postcapillary venules.

P-selectin-mediated adhesion impairs endothelium-dependent arteriolar dilation in hypercholesterolemic mice

Hypercholesterolemia is associated with an attenuation of endothelium-dependent dilation in arterioles and an increase in leukocyte and platelet adhesion in venules. The proximity of closely paired arterioles and venules is thought to facilitate heat and mass transport between the two and could be involved in transport of inflammatory and/or vasoactive mediators from venule to arteriole. In the current study, we tested the hypothesis that the impaired arteriolar dilation associated with hypercholesterolemia might be dependent on P-selectin-dependent blood cell adhesion in the closely paired venules. Leukocyte and platelet recruitment in venules and the endothelium-dependent response to bradykinin in second-order arterioles were observed in the mouse intestinal submucosa using intravital microscopy. Four weeks of a high-cholesterol diet decreased bradykinin-induced arteriolar dilation more dramatically in closely paired arterioles than in distantly paired arterioles. The dysfunctional arteriolar dilation of closely paired arterioles in hypercholesterolemic mice was significantly improved when the experiments were repeated in P-selectin-deficient mice (given the high-cholesterol diet) or in hypercholesterolemic mice injected with a P-selectin monoclonal antibody. A similar improvement in dilation of closely paired arterioles was attained in hypercholesterolemic mice given the superoxide dismutase mimetic Tempol. These findings indicate that hypercholesterolemia-induced increases in venular leukocyte and platelet adhesion might contribute to the impaired endothelium-dependent dilation of closely paired arterioles via a mechanism that is distance limited and dependent on P-selectin and superoxide.