Autoregulation of blood flow drives early hypotension in a rat model of systemic inflammation induced by bacterial lipopolysaccharide.

Uncontrolled vasodilation is known to account for hypotension in the advanced stages of sepsis and other systemic inflammatory conditions, but the mechanisms of hypotension in earlier stages of such conditions are not clear. By monitoring hemodynamics with the highest temporal resolution in unanesthetized rats, in combination with ex-vivo assessment of vascular function, we found that early development of hypotension following injection of bacterial lipopolysaccharide is brought about by a fall in vascular resistance when arterioles are still fully responsive to vasoactive agents. This approach further uncovered that the early development of hypotension stabilized blood flow. We thus hypothesized that prioritization of the local mechanisms of blood flow regulation (tissue autoregulation) over the brain-driven mechanisms of pressure regulation (baroreflex) underscored the early development of hypotension in this model. Consistent with this hypothesis, an assessment of squared coherence and partial-directed coherence revealed that, at the onset of hypotension, the flow-pressure relationship was strengthened at frequencies (<0.2 Hz) known to be associated with autoregulation. The autoregulatory escape to phenylephrine-induced vasoconstriction, another proxy of autoregulation, was also strengthened in this phase. The competitive demand that drives prioritization of flow over pressure regulation could be edema-associated hypovolemia, as this became detectable at the onset of hypotension. Accordingly, blood transfusion aimed at preventing hypovolemia brought the autoregulation proxies back to normal and prevented the fall in vascular resistance. This novel hypothesis opens a new avenue of investigation into the mechanisms that can drive hypotension in systemic inflammation.

A leukotriene-dependent spleen-liver axis drives TNF production in systemic inflammation.

Production of the proinflammatory cytokine tumor necrosis factor (TNF) must be precisely regulated for effective host immunity without the induction of collateral tissue damage. Here, we showed that TNF production was driven by a spleen-liver axis in a rat model of systemic inflammation induced by bacterial lipopolysaccharide (LPS). Analysis of cytokine expression and secretion in combination with splenectomy and hepatectomy revealed that the spleen generated not only TNF but also factors that enhanced TNF production by the liver, the latter of which accounted for nearly half of the TNF secreted into the circulation. Using mass spectrometry-based lipidomics, we identified leukotriene B4 (LTB4) as a candidate blood-borne messenger in this spleen-liver axis. LTB4 was essential for spleen-liver communication in vivo, as well as for humoral signaling between splenic macrophages and Kupffer cells in vitro. LPS stimulated the splenic macrophages to secrete LTB4, which primed Kupffer cells to secrete more TNF in response to LPS in a manner dependent on LTB4 receptors. These findings provide a framework to understand how systemic inflammation can be regulated at the level of interorgan communication.

Elucidating the role of leptin in systemic inflammation: a study targeting physiological leptin levels in rats and their macrophages.

To elucidate the role of leptin in acute systemic inflammation, we investigated how its infusion at low, physiologically relevant doses affects the responses to bacterial lipopolysaccharide (LPS) in rats subjected to 24 h of food deprivation. Leptin was infused subcutaneously (0-20 µg·kg-1·h-1) or intracerebroventricularly (0-1 µg·kg-1·h-1). Using hypothermia and hypotension as biomarkers of systemic inflammation, we identified the phase extending from 90 to 240 min post-LPS as the most susceptible to modulation by leptin. In this phase, leptin suppressed the rise in plasma TNF-α and accelerated the recoveries from hypothermia and hypotension. Suppression of TNF-α was not accompanied by changes in other cytokines or prostaglandins. Leptin suppressed TNF-α when infused peripherally but not when infused into the brain. Importantly, the leptin dose that suppressed TNF-α corresponded to the lowest dose that limited food consumption; this dose elevated plasma leptin within the physiological range (to 5.9 ng/ml). We then conducted in vitro experiments to investigate whether an action of leptin on macrophages could parallel our in vivo observations. The results revealed that, when sensitized by food deprivation, LPS-stimulated peritoneal macrophages can be inhibited by leptin at concentrations that are lower than those reported to promote cytokine release. It is concluded that physiological levels of leptin do not exert a proinflammatory effect but rather an anti-inflammatory effect involving selective suppression of TNF-α via an action outside the brain. The mechanism of this effect might involve a previously unrecognized, suppressive action of leptin on macrophage subpopulations sensitized by food deprivation, but future studies are warranted.

Respiratory gas exchange as a new aid to monitor acidosis in endotoxemic rats: relationship to metabolic fuel substrates and thermometabolic responses.

This study introduces the respiratory exchange ratio (RER; the ratio of whole-body CO2 production to O2 consumption) as an aid to monitor metabolic acidosis during the early phase of endotoxic shock in unanesthetized, freely moving rats. Two serotypes of lipopolysaccharide (lipopolysaccharide [LPS] O55:B5 and O127:B8) were tested at shock-inducing doses (0.5-2 mg/kg). Phasic rises in RER were observed consistently across LPS serotypes and doses. The RER rise often exceeded the ceiling of the quotient for oxidative metabolism, and was mirrored by depletion of arterial bicarbonate and decreases in pH It occurred independently of ventilatory adjustments. These data indicate that the rise in RER results from a nonmetabolic CO2 load produced via an acid-induced equilibrium shift in the bicarbonate buffer. Having validated this new experimental aid, we asked whether acidosis was interconnected with the metabolic and thermal responses that accompany endotoxic shock in unanesthetized rats. Contrary to this hypothesis, however, acidosis persisted regardless of whether the ambient temperature favored or prevented downregulation of mitochondrial oxidation and regulated hypothermia. We then asked whether the substrate that fuels aerobic metabolism could be a relevant factor in LPS-induced acidosis. Food deprivation was employed to divert metabolism away from glucose oxidation and toward fatty acid oxidation. Interestingly, this intervention attenuated the RER response to LPS by 58%, without suppressing other key aspects of systemic inflammation. We conclude that acid production in unanesthetized rats with endotoxic shock results from a phasic activation of glycolysis, which occurs independently of physiological changes in mitochondrial oxidation and body temperature.

Riparin A, a compound from Aniba riparia, attenuate the inflammatory response by modulation of neutrophil migration.

Inflammation is a local tissue response to attacks characterized by vascular and cellular events, including intense oxidative stress. Riparin A, a compound obtained from Aniba riparia, has been shown to have antioxidant activity and cytotoxicity in vitro. This study was aimed at evaluating the anti-inflammatory effect of riparin A against acute inflammation. The results of our evaluations in various experimental models indicated that riparin A reduced paw edema induced by carrageenan, compound 48/80, histamine, and serotonin. Furthermore, it decreased leukocyte and neutrophil counts, myeloperoxidase activity, thiobarbituric acid reactive substance (TBARS) levels, and cytokine (tumor necrosis factor-α and interleukin-1ß) levels increased by carrageenan-induced peritonitis, and reversed glutathione levels. Riparin A also reduced carrageenan-induced adhesion and rolling of leukocytes on epithelial cells and did not produce gastric-damage as compared with indomethacin. In conclusion, the data show that riparin A reduces inflammatory response by inhibiting vascular and cellular events, modulating neutrophil migration, inhibiting proinflammatory cytokine production, and reducing oxidative stress.

Carvacryl acetate, a derivative of carvacrol, reduces nociceptive and inflammatory response in mice.

AIMS: The present study aimed to investigate the potential anti-inflammatory and anti-nociceptive effects of carvacryl acetate, a derivative of carvacrol, in mice. MAIN METHODS: The anti-inflammatory activity was evaluated using various phlogistic agents that induce paw edema, peritonitis model, myeloperoxidase (MPO) activity, pro and anti-inflammatory cytokine levels. Evaluation of antinociceptive activity was conducted through acetic acid-induced writhing, hot plate test, formalin test, capsaicin and glutamate tests, as well as evaluation of motor performance on rotarod test. KEY FINDINGS: Pretreatment of mice with carvacryl acetate (75 mg/kg) significantly reduced carrageenan-induced paw edema (P<0.05) when compared to vehicle-treated group. Likewise, carvacryl acetate (75 mg/kg) strongly inhibited edema induced by histamine, serotonin, prostaglandin E2 and compound 48/80. In the peritonitis model, carvacryl acetate significantly decreased total and differential leukocyte counts, and reduced levels of myeloperoxidase and interleukin-1 beta (IL-1ß) in the peritoneal exudate. The levels of IL-10, an anti-inflammatory cytokine, were enhanced by carvacryl acetate. Pretreatment with carvacryl acetate also decreased the number of acetic acid-induced writhing, increased the latency time of the animals on the hot plate and decreased paw licking time in the formalin, capsaicin and glutamate tests. The pretreatment with naloxone did not reverse the carvacryl acetate-mediated nociceptive effect. SIGNIFICANCE: In conclusion, the current study demonstrated that carvacryl acetate exhibited anti-inflammatory activity in mice by reducing inflammatory mediators, neutrophil migration and cytokine concentration, and anti-nociceptive activity due to the involvement of capsaicin and glutamate pathways.