Mechanisms of acute vasodilator response to bacterial lipopolysaccharide in the rat coronary microcirculation

Abstract

1. In this study the mechanisms of the acute vasodilator action of bacterial lipopolysaccharide (LPS) were investigated in the rat Langendorff perfused heart.

2. Infusion of LPS (5 μg ml⁻¹) caused a rapid and sustained fall in coronary perfusion pressure (PP) of 59±4 mmHg (n=12) and a biphasic increase in NO levels determined in the coronary effluent by chemiluminescent detection. Both the fall in PP and the increase in NO release were completely abolished (n=3) by pretreatment of hearts with the NO synthase inhibitor L-NAME (50 μM).

3. LPS-induced vasodilatation was markedly attenuated to 5±4 mmHg (n=3) by pretreatment of hearts with the B₂ kinin receptor antagonist Hoe-140 (100 nM).

4. Vasodilator responses to LPS were also blocked by brief pretreatment with mepacrine (0.5 μM, n=3) or nordihydroguaiaretic acid (0.1 μM, n=4) and markedly attenuated by WEB 2086 (3 μM, n=4).

5. Thirty minutes pretreatment of hearts with dexamethasone (1 nM), but not progesterone (1 μM), significantly modified responses to LPS. The action of dexamethasone was time-dependent, having no effect when applied either simultaneously with or pre-perfused for 5 min before the administration of LPS but inhibiting the response to LPS by 91±1% (n=4) when pre-perfused for 15 min. The inhibition caused by dexamethasone was blocked by 15 min pretreatment with the glucocorticoid receptor antagonist RU-486 (100 nM) or by 2 min pre-perfusion of a 1 : 200 dilution of LCPS1, a selective anti-lipocortin 1 (LC1) neutralizing antibody.

6. Treatment with the protein synthesis inhibitor, cycloheximide (10 μM, for 15 min) selectively blunted LPS-induced vasodilatation, reducing the latter to 3±5 mmHg (n=3), while having no effect on vasodilator responses to either bradykinin or sodium nitroprusside.

7. These results indicate that LPS-induced vasodilatation in the rat heart is dependent on activation of kinin B₂ receptors and synthesis of NO. In addition, phospholipase A₂ (PLA₂) is activated by LPS resulting in the release of platelet-activating factor (PAF) and lipoxygenase but not cyclo-oxygenase products. These effects are dependent on de novo synthesis of an intermediate protein which remains to be identified.

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