Rapid effect of LPS-activated macrophages on the reactivity of the rat mesenteric artery

This study was designed to evaluate the effects of lipopolysaccharide (LPS)-activated macrophages in the perfused mesenteric circulation of the rat. The mesenteric network of anesthetized rats was perfused in situ under constant flow conditions and the diameter and pressure of the mesenteric artery were continuously recorded. For the first 30 min the mesenteric network was perfused with an RPMI solution (control condition); thereafter it was perfused for 60 min with the same solution containing either (1) LPS (1 microgram/ml), (2) elicited macrophages (10(6) cells/ml), (3) LPS-activated macrophages or (4) supernatants derived from LPS-activated macrophages (SPN). The changes in arterial diameter induced by topical application of phenylephrine (PE, 10 mumol/l) were measured under control conditions and then 30 and 60 min after the onset of perfusions. The intravascular pressure was similarly increased (51 +/- 6%, p < 0.001) by the perfusion of activated macrophages or elicited macrophages but was not affected by perfusion of LPS or SPN. Despite the same level of transmural mesenteric pressure in rats perfused with activated and elicited macrophages, the mesenteric diameter was significantly larger with activated than with elicited macrophages (p < 0.05). Under control conditions, PE induced a marked decrease in arterial diameter from 495 +/- 15 to 265 +/- 13 microns (p < 0.001). Perfusion of LPS, elicited macrophages or SPN did not modify the vascular reactivity to PE. Perfusion of activated macrophages reduced the PE-induced contraction by 77 +/- 6% (p < 0.001). Perfusion of elicited macrophages with a nitric oxide (NO) donor (SIN-1, 10 mumol/l) reproduced the effect of LPS-activated macrophages while addition of an NO scavenger (oxyhemoglobin, 10 mumol/l) prevented the depression of the vascular reactivity to PE by activated macrophages. Finally, activated macrophages preincubated with an inhibitor of NO synthesis (NG-monomethyl-L-arginine); L-NMMA), and then perfused in RPMI solution without L-NMMA had no effect on the PE reactivity of the mesenteric artery suggesting that NO released by activated macrophages directly and rapidly inhibited the contractility of the mesenteric artery. The results of this study demonstrate the opposing effects of macrophages in the mesenteric circulation to increase microvascular resistance by a rheological effect while decreasing the reactivity of the mesenteric artery as a result of NO released by macrophages.