Significance of platelet-activating factor in mesenteric ischemia-reperfusion

Superior mesenteric artery occlusion models shock-induced gut ischemia-reperfusion

INTRODUCTION

Superior mesenteric artery occlusion (SMAO) is a simple and reproducible model of shock-induced gut ischemia/reperfusion, but some argue that it is not clinically relevant. The purpose of the current study was to compare SMAO to a standard model of controlled hemorrhage (CH) and uncontrolled hemorrhage (UH).

METHODS

Rats had femoral lines and a jejunal mucosal laser Doppler placed followed by SMAO (60 min of ischemia, no resuscitation), controlled hemorrhage (40 mm Hg for 60 min, 2:1 resuscitation shed blood and lactated Ringers), or uncontrolled hemorrhage (liver injury, 3:1 resuscitation with lactated Ringers). Base deficit, lactate, and jejunal mucosal flow (as a percentage of baseline) were recorded during ischemia and for 120 min after reperfusion. Jejunal tissue was harvested for morphological evaluation. Comparison among groups was by analysis of variance (ANOVA), and significance was set at P < 0.05.

RESULTS

Mucosal blood flow was similar among groups at the onset of reperfusion (CH, 16.9 +/- 5.0% versus UH, 10.9 +/- 3.1% versus SMAO, 13.9 +/- 6.2%) and during the initial period of reperfusion. By 120 min, however, flow in CH (75.4 +/- 2.5%) was significantly higher that in either UH (36.4 +/- 13.1%) or SMAO (31.7 +/- 8.4%). Histological injury was less with CH, while base deficit was significantly higher in CH at the onset of reperfusion (-24 +/- 2 versus UH, -10 +/- 3 and SMAO, -6 +/- 3 mM/L) but comparable by the end (CH, -17 +/- 4 versus UH, -16 +/- 3 and SMAO, -17 +/- 2 mM/L).

CONCLUSIONS

SMAO is a clinically relevant model of shock-induced gut ischemia/reperfusion.

Beneficial effects of intraluminal nitroglycerin in intestinal ischemia-reperfusion injury in rats

BACKGROUND

We examined the effects of intraluminal nitroglycerin (NTG) on various physiologic functions and intestinal pathology in intestinal ischemia-reperfusion (IR) injury in rats.

MATERIAL AND METHODS

Intraluminal NTG (0.15-3.75 mg/kg) was administered at different doses and stages during the experimental disease, and intestinal permeability and histology, bile flow, and systemic hemodynamics were measured.

RESULTS

Prophylactic intraluminal NTG treatment at 0.75 mg/kg, but not at 0.15 mg/kg, significantly attenuated the deleterious changes in intestinal barrier function and mucosal injury caused by IR. However, administration of NTG after ischemia was not effective, even up to 3.75 mg/kg. In vitro intestinal NTG metabolism was significantly decreased following intestinal ischemia. Intraluminal NTG at 0.75 mg/kg significantly attenuated the reduction in bile flow that accompanied IR. Reperfusion induced a precipitous and sustained decrease in mean arterial pressure, which was blunted by intraluminal NTG.

CONCLUSIONS

Intraluminal NTG produced several beneficial local and systemic effects in a rat model of intestinal IR. In this disease model, 0.75 mg/kg intraluminal NTG did not exacerbate, but rather reduced, the hypotensive effects induced by IR.

Comparative protection against rat intestinal reperfusion injury by a new inhibitor of sPLA2, COX-1 and COX-2 selective inhibitors, and an LTC4 receptor antagonist

(1) A new group IIa sPLA2 inhibitor was compared with selective inhibitors of COX-1, COX-2 and an LTC4 antagonist for effects on local and remote tissue injuries following ischaemia and reperfusion (I/R) of the small intestine in rats. (2) In an acute model of ischaemia (30 min) and reperfusion (150 min) injury in the absence of inhibitors, there was significant intestinal haemorrhage, oedema and mucosal damage, neutropenia, elevated serum levels of aspartate aminotransferase (AST) and hypotension. (3) Preischaemic treatment with the inhibitor of sPLA2 (Group IIa), at 5 mg kg-1 i.v. or 10 mg kg-1 p.o. significantly inhibited I/R-induced neutropenia, the elevation of serum levels of AST, intestinal oedema and hypotension. (4) Pretreatment with the COX-2 inhibitor celebrex (10 mg kg-1 i.v.) and the LTC4 antagonist zafirlukast (1 mg kg-1 i.v.) also showed marked improvement with I/R-induced AST, oedema and neutropenia. Hypotension was only reduced by the LTC4 antagonist. The COX-1 inhibitor flunixin (1 mg kg-1 i.v.) did not effect improvement in the markers of tissue injury. (5) Histological examination of rat I/R injury showed that all of the drugs offered some protection to the mucosal layer damage compared to no drug treatment. Given i.v., the sPLA2 inhibitor was more effective than either the COX-1 or COX-2 inhibitors in preventing rat I/R injury. (6) These results indicate that a potent new inhibitor of sPLA2 (group IIa) protects the rat small intestine from I/R injury after oral or intravenous administration. COX-2 and LTC4 inhibitors also showed some beneficial effects against intestinal I/R injury. Our study suggests that sPLA2 (Group IIa) may have a pathogenic role in intestinal I/R in rats.

Intestinal and hemodynamic impairment following mesenteric ischemia/reperfusion

BACKGROUND

Clinical intestinal ischemia/reperfusion (I/R) injury results in local and systemic dysfunction. A rat model of transient mesenteric occlusion has been used to study this phenomenon. However, a systematic analysis of the rat model with respect to intestinal permeability and hemodynamics has not been carried out.

MATERIALS AND METHODS

In anesthetized rats, the superior mesenteric artery was occluded for 60 min, followed by reperfusion for 4 h. Intestinal impairment was evaluated via histological examination and by measuring ex vivo apparent permeability coefficients (Papp) of mannitol (0.18 kDa), inulin (5 kDa), and dextran (70 kDa). Hemodynamic effects of intestinal I/R were determined by monitoring mean arterial pressure (MAP) and heart rate (HR) via a catheter placed in the femoral artery.

RESULTS

The animal model was associated with increased ex vivo Papp for mannitol and inulin. Although I/R injury was accompanied by significant histological disruption, there was no observable alteration in dextran permeability, suggesting that the loss in normal barrier function was limited to low-molecular-weight compounds. Hemodynamic measurements indicated that reperfusion induced a precipitous and sustained fall in MAP. HR values fell sharply following reperfusion but gradually increased and eventually "overshot" to values greater than baseline.

CONCLUSIONS

Our findings demonstrate the selective loss of barrier function of the small bowel following intestinal I/R. Furthermore, these results also illustrate the importance of selecting appropriate permeability markers for the evaluation of intestinal damage. In light of the significant hemodynamic disruption accompanying the animal model, our investigation also points toward the need for developing therapeutic strategies that mitigate the local and systemic effects of intestinal I/R injury.

BB-882 is a potent antagonist of the haemodynamic changes induced by platelet-activating factor in pigs

The effects of bolus doses (0.2-2 micrograms) of platelet-activating factor (PAF) on systemic and pulmonary arterial pressures were tested in five juvenile pigs, and were demonstrated to have a biphasic effect. There was an initial drop in the mean systemic arterial pressure within seconds, followed by an increase within 1-2 min., whereas mean pulmonary arterial pressure only rose. The response was dose-related and did not include tachyphylaxis. The pressures returned to baseline within 10 min. After confirming these results a novel specific PAF receptor antagonist (BB-882) was given as a 1 mg/kg bolus followed by a continuous infusion of 2 mg/kg/hr, and a third dose response curve was repeated with ten-fold higher PAF doses (2-20 micrograms). BB-882 effectively counteracted these effects. Six pigs were given a continuous infusion of BB-882 33 mg/kg/hr for 5 hr and were compared with another group of five pigs given vehicle only. In this high dose BB-882 did not affect the intravascular pressures. These results indicate that BB-882 is a potent PAF receptor antagonist in juvenile pigs.

Platelet activating factor inhibition reduces lung injury after cardiopulmonary bypass

Because cardiopulmonary bypass (CPB) produces a diffuse inflammatory reaction that may injure multiple organs and complicate cardiac surgical procedures, we examined the use of a competitive inhibitor of platelet activating factor (SDZ HUL-412) in a porcine model of CPB as a means to ameliorate pulmonary injury after CPB. Thirteen pigs (35 to 40 kg) underwent CPB at 28 degrees C for 2 hours, followed by 2 hours of observation. Group I (n = 6) received SDZ HUL-412 (a quinolinium compound) intravenously (3 mg/kg loading dose and 2 mg.kg-1.h-1 continuous infusion) starting before sternotomy. Group II (n = 7) received a saline vehicle. Peak airway pressure, pulmonary arterial pressure, left atrial pressure, and arterial blood gases were measured and flow cytometry evaluated surface expression of adhesion molecule subunit CD18 on circulating neutrophils. Pulmonary function was significantly improved in group I. Fifteen minutes after CPB, dynamic lung compliance in group I was 91% +/- 12% of baseline versus 49% +/- 5.2% in group II (p = 0.06 by analysis of variance). After CPB, the arterial oxygen pressure was also significantly better in group I than in group II (425 +/- 61 versus 234 +/- 76 mm Hg) (p < 0.05). The rise in pulmonary vascular resistance after CPB was less in group I (p < 0.05) (323 +/- 55 to 553 +/- 106 dynes.s.cm-5) than in group II (531 +/- 177 to 884 +/- 419 dynes.s.cm-5) at the end of the observation period. CD18 up-regulation increased similarly in the two groups during CPB.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve and muscle blood flow during hindlimb ischemia and reperfusion in rats

Animal models of peripheral nerve ischemia have yielded variable results. The question of whether post-ischemia re-establishment of blood flow to the nerves augments injury has not been examined. To study this question, the ipsilateral common iliac and femoral arteries were occluded with arterial snares for 3 hours in rats; 14C-butanol tissue distribution was then used to measure blood flow in both sciatic and posterior tibial nerve trunks and in both biceps femoris muscles during occlusion and reperfusion. Clinical limb function was graded serially, with the undisturbed contralateral limb serving as the study control. Nerve blood flow was reduced throughout the ischemic period and was only 20% of the control value in the posterior tibial nerve. Muscle blood flow was unchanged. All rats had functional impairment, with an average limb function score of 7.5 (normal score < 2). During reperfusion, blood flow in the distal sciatic and posterior tibial nerves was approximately double that of control nerves at 2 hours, and muscle blood flow was also elevated. At 21 hours, tibial nerve blood flow was still twice that of the control nerve, but flows in the distal sciatic nerve and muscle were unchanged from control levels. Clinically, limb function improved progressively after reperfusion. It was concluded that nerve ischemia is attended by a relatively prolonged hyperemic flow response during reperfusion.

Modulation by nitric oxide of platelet-activating factor-induced albumin extravasation in the conscious rat

1. The objective of this study was to assess whether or not endogenous nitric oxide (NO) could mediate the hypotensive response to platelet-activating factor (PAF) and modulate PAF-induced microvascular albumin leakage in the conscious rat. 2. PAF (0.19 and 1.9 nmol kg-1, i.v.) evoked dose-dependent hypotension and significantly enhanced albumin extravasation in the large airways, pancreas, stomach and duodenum 15 min after its administration. Inhibition of NO synthesis by NG-nitro-L-arginine methyl ester (L-NAME, 0.125-2 mg kg-1, i.v.) produced marked dose-dependent increases in albumin accumulation (up to 290%) in large airways, liver, spleen, pancreas, kidney, stomach and duodenum as measured by the extravasation of Evans blue dye. L-NAME (2 mg kg-1) treatment markedly potentiated PAF (1.9 nmol kg-1)-induced albumin extravasation in these tissues, whereas it did not modify the hypotensive response to PAF. 3. Maintenance of mean arterial blood pressure at the level observed following 2 mg kg-1 L-NAME by infusion of noradrenaline (620-790 ng kg-1 min-1) neither affected significantly albumin extravasation nor potentiated the permeability effect of PAF in the vascular beds studied with the exception of large airways, where noradrenaline mimicked the effects of L-NAME. 4. These results indicate that inhibition of endogenous NO formation leads to an increase in albumin extravasation and to potentiation of the vascular permeability effect of PAF, whereas the hypotensive action of PAF seems to be independent of NO formation in the conscious rat. These data suggest an important role for NO in the regulation of albumin extravasation.