Alteration by lipopolysaccharide of the relationship between intracellular calcium levels and contraction in rat mesenteric artery

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.

Redox signaling and splicing dependent change in myosin phosphatase underlie early versus late changes in NO vasodilator reserve in a mouse LPS model of sepsis

Microcirculatory dysfunction may cause tissue malperfusion and progression to organ failure in the later stages of sepsis, but the role of smooth muscle contractile dysfunction is uncertain. Mice were given intraperitoneal LPS, and mesenteric arteries were harvested at 6-h intervals for analyses of gene expression and contractile function by wire myography. Contractile (myosin and actin) and regulatory [myosin light chain kinase and phosphatase subunits (Mypt1, CPI-17)] mRNAs and proteins were decreased in mesenteric arteries at 24 h concordant with reduced force generation to depolarization, Ca(2+), and phenylephrine. Vasodilator sensitivity to DEA/nitric oxide (NO) and cGMP under Ca(2+) clamp were increased at 24 h after LPS concordant with a switch to Mypt1 exon 24- splice variant coding for a leucine zipper (LZ) motif required for PKG-1α activation of myosin phosphatase. This was reproduced by smooth muscle-specific deletion of Mypt1 exon 24, causing a shift to the Mypt1 LZ+ isoform. These mice had significantly lower resting blood pressure than control mice but similar hypotensive responses to LPS. The vasodilator sensitivity of wild-type mice to DEA/NO, but not cGMP, was increased at 6 h after LPS. This was abrogated in mice with a redox dead version of PKG-1α (Cys42Ser). Enhanced vasorelaxation in early endotoxemia is mediated by redox signaling through PKG-1α but in later endotoxemia by myosin phosphatase isoform shifts enhancing sensitivity to NO/cGMP as well as smooth muscle atrophy. Muscle atrophy and modulation may be a novel target to suppress microcirculatory dysfunction; however, inactivation of inducible NO synthase, treatment with the IL-1 antagonist IL-1ra, or early activation of α-adrenergic signaling did not suppressed this response.

Calcium desensitisation in late polymicrobial sepsis is associated with loss of vasopressor sensitivity in a murine model

Background

Sepsis is characterised by diminished vasopressor responsiveness. Vasoconstriction depends upon a balance: Ca²⁺-dependent myosin light-chain kinase promotes and Ca²⁺-independent myosin light-chain phosphatase (MLCP) opposes vascular smooth muscle contraction. The enzyme Rho kinase (ROK) inhibits MLCP, favouring vasoconstriction. We tested the hypothesis that ROK-dependent MLCP inhibition was attenuated in late sepsis and associated with reduced contractile responses to certain vasopressor agents.

Methods

This is a prospective, controlled animal study. Sixteen-week-old C57/BL6 mice received laparotomy or laparotomy with caecal ligation and puncture (CLP). Antibiotics, fluids and analgesia were provided before sacrifice on day 5. Vasoconstriction of the femoral arteries to a range of stimuli was assessed using myography: (i) depolarisation with 87 mM K⁺ assessed voltage-gated Ca²⁺ channels (L-type, Ca_v1.2 Ca²⁺ channels (LTCC)), (ii) thromboxane A₂ receptor activation assessed the activation state of the LTCC and ROK/MLCP axis, (iii) direct PKC activation (phorbol-dibutyrate (PDBu), 5 μM) assessed the PKC/CPI-17 axis independent of Ca²⁺ entry and (iv) α₁-adrenoceptor stimulation with phenylephrine (10⁻⁸ to 10⁻⁴ M) and noradrenaline (10⁻⁸ to 10⁻⁴ M) assessed the sum of these pathways plus the role of the sarcoplasmic reticulum (SR). ROK-dependent MLCP activity was indexed by Western blot analysis of P[Thr855]MYPT. Parametric and non-parametric data were analysed using unpaired Student's t-tests and Mann-Whitney tests, respectively.

Results

ROK-dependent inhibition of MLCP activity was attenuated in both unstimulated (n = 6 to 7) and stimulated (n = 8 to 12) vessels from mice that had undergone CLP (p < 0.05). Vessels from CLP mice demonstrated reduced vasoconstriction to K⁺, thromboxane A₂ receptor activation and PKC activation (n = 8 to 13; p < 0.05). α₁-adrenergic responses were unchanged (n = 7 to 12).

Conclusions

In a murine model of sepsis, ROK-dependent inhibition of MLCP activity in vessels from septic mice was reduced. Responses to K⁺ depolarisation, thromboxane A₂ receptor activation and PKC activation were diminished in vitro whilst α₁-adrenergic responses remained intact. Inhibiting MLCP may present a novel therapeutic target to manage sepsis-induced vascular dysfunction.

Effects of lipopolysaccharide on the neuronal control of mesenteric vascular tone in rats: mechanisms involved

The aim of the present study was to investigate the effects of lipopolysaccharide (LPS) on the contractile response induced by electrical field stimulation (EFS) in rat mesenteric segments, as well as the mechanisms involved. Effects of LPS incubation for 2 or 5 h were studied in mesenteric segments from male Wistar rats. Vasomotor responses to EFS, nitric oxide (NO) donor DEA-NO, and noradrenaline (NA) were studied. Phosphorylated neuronal NO synthase protein expression was analyzed, and NO, superoxide anion (O2·), and peroxynitrite releases were also determined. Lipopolysaccharide increased EFS-induced vasoconstriction at 2 h. This increase was lower after 5-h preincubation. N-nitro-L-arginine methyl ester increased vasoconstrictor response only in control segments. Vasodilator response to DEA-NO was increased by LPS after 5-h preincubation and was decreased by O2· scavenger tempol. Basal NO release was increased by LPS. Electrical field stimulation-induced NO release was reduced by LPS compared with control conditions. Lipopolysaccharide exposure increased both O2· and peroxynitrite release. Vasoconstriction to exogenous NA was markedly increased by LPS compared with control conditions after 2-h incubation and remained unchanged after 5-h incubation. Short-term exposure of rat mesenteric arteries to LPS produced a time-dependent enhanced contractile response to EFS. The early phase (2 h) was associated to a reduction in NO from neuronal NO synthase and an enhanced response to NA. After 5 h of LPS exposure, this enhancement was reduced, because of restoration of the adrenergic component and maintenance of the nitrergic reduction.

The Rho-A/Rho-kinase pathway is up-regulated but remains inhibited by cyclic guanosine monophosphate-dependent mechanisms during endotoxemia in small mesenteric arteries

OBJECTIVE

We investigated whether a reduced activity in the Rho-A/Rho-kinase pathway could be involved in the impaired vascular reactivity observed in septic shock.

DESIGN

Ex vivo animal study.

SETTING

University research laboratory.

SUBJECTS

Male Wistar rats.

INTERVENTIONS

Rats received an intraperitoneal injection of lipopolysaccharide (LPS, 10 mg/kg) either 6 or 24 hours before the onset of our experiments. The effects of Y-27632 (a Rho-kinase inhibitor) were assessed in first-order mesenteric rings taken from these animals using wire myograph. The expression of Rho-A, Rho-kinases I and II, and the total and phosphorylated myosin phosphatase targeting subunit 1 (MYPT1) were assessed by Western blotting.

MEASUREMENTS AND MAIN RESULTS

The EC50 to Y-27632 was reduced from 2.10 microM (1.22-3.66 microM) (control) to 0.21 microM (0.09-0.44 microM), and 9.54 (0.82-110.30) nM in LPS-treated groups 6 and 24 hours, respectively. The increased potency of Y-27632 was partially reversed by endothelium removal at both 6 and 24 hours. Incubation of Nomega-nitro-l-arginine methyl ester hydrochloride or 1400W (a nonselective and an inducible nitric oxide synthase inhibitor, respectively) normalized the responses to Y-27632 seen 6 hours after LPS. However, 1400W had no effect, whereas Nomega-nitro-l-arginine methyl ester hydrochloride caused a partial reduction in the enhanced potency of Y-27632 found 24 hours after LPS. The soluble guanylate cyclase inhibitor oxadiazolo[4,3-alpha]quinoxalin-1-one was able to bring the Y-27632 response back to normal both 6 and 24 hours after LPS. Rho-A, Rho-kinase I, Rho-kinase II, and MYPT1 were increased in mesenteric arteries from endotoxemic rats, but the phosphorylated MYPT1 was significantly reduced. However, incubation with oxadiazolo[4,3-alpha]quinoxalin-1-one circumvented the inhibition of MYPT1 phosphorylation found in preparations from LPS-treated animals.

CONCLUSIONS

Our findings revealed an impaired Rho-A/Rho-kinase-mediated phosphorylation of MYPT1 in vessels from endotoxemic animals in a cyclic guanosine monophosphate-dependent manner, suggesting that changes in mechanisms involved in calcium sensitization play a pivotal role in cardiovascular changes observed in septic shock.

Voltage-independent calcium channels mediate lipopolysaccharide-induced hyporeactivity to endothelin-1 in the rat aorta

The roles of intracellular calcium concentration ([Ca(2+)](i)) and Ca(2+) sensitization in lipopolysaccharide (LPS)-induced vascular smooth muscle (VSM) hyporesponsiveness are incompletely understood. To investigate these roles, contraction responses to endothelin-1 (ET-1) and 80 mM KCl; relaxation responses to nifedipine; the expression levels of mRNAs of ET-1 and its receptors (ET(A) or ET(B)); the expression levels of protein kinase C (PKC) and phosphorylation of Rho kinase (ROKalpha), CPI-17, and myosin phosphatase target subunit-1 (MYPT1); and changes in aortic VSM cell [Ca(2+)](i) were measured in LPS-treated aortic rings from male Wistar rats (250-300 g). LPS (10 mug/ml, 20 h) decreased contraction induced by ET-1 (0.3-100 nM) or 80 mM KCl. LPS-induced hypocontractility was not observed in the absence of external Ca(2+), but LPS-treated aorta remained hypocontractile on subsequent stepwise restoration of extracellular Ca(2+) (0.01-10 mM). Vascular relaxation to nifedipine; mRNA expression levels of ET-1, ET(A), or ET(B); protein expression levels of PKC; and phosphorylation levels of ROKalpha, CPI-17, and MYPT1 were not affected by LPS. In isolated aortic VSM cells, ET-1 caused a transient initial increase in [Ca(2+)](i), followed by a maintained tonic increase in [Ca(2+)](i), which was decreased by LPS pretreatment and was dependent on external Ca(2+). Subsequent restoration of extracellular Ca(2+) increased [Ca(2+)](i), but this increase was lower in the LPS-treated group. This difference in response to extracellular Ca(2+) addition was not affected by diltiazem, but was abolished by SKF-96365. Therefore, LPS induces hyporeactivity to ET-1 in rat aorta that depends on external Ca(2+) influx through non-voltage-operated Ca(2+) channels, but not on ET-1 receptor expression or Ca(2+) sensitization.

Protection against endotoxic shock as a consequence of reduced nitrosative stress in MLCK210-null mice

This study investigated the consequences of deletion of the long isoform of myosin light chain kinase (MLCK210) on the cardiovascular changes induced by the bacterial endotoxin lipopolysaccharide (LPS) and cecal ligation puncture using MLCK210-/- mice. Here, we provide evidence that deletion of MLCK210 enhanced survival after intraperitoneal injection of LPS or cecal ligation puncture. LPS-induced vascular hyporeactivity to vasoconstrictor agents was completely prevented in aorta from MLCK210-/- mice. This was associated with a decreased up-regulation of nuclear facor-kappaB expression and activity, inducible nitric-oxide synthase, and level of oxidative stress in the vascular media. Furthermore, LPS-induced increase of nitric oxide production in the circulation and tissues (including heart, liver, and lung) that was correlated with an increased expression of inducible nitric-oxide synthase was also reduced in MLCK210-/- mice. These data demonstrate a role for MLCK210 in endotoxin shock injury associated with oxidative and nitrosative stresses and vascular hyporeactivity.

Effects of dexamethasone andl-canavanine on the intracellular calcium-contraction relation of the rat tail artery during septic shock

The intracellular mechanism by which sepsis lowers vascular reactivity and the subsequent reversal by dexamethasone or nitric oxide synthase (NOS) inhibitors remain unclear. We measured the sensitivity of contraction of the rat tail artery to intracellular Ca2+in a model of polymicrobial septic shock. At 22 h after cecal ligation and puncture (CLP), rats were treated with an anti-inflammatory glucocorticoid (dexamethasone, 1 mg/kg ip), an inducible NOS inhibitor (l-canavanine, 100 mg/kg ip), or saline. At 24 h after CLP, endothelium-denuded, perfused segments of tail artery were loaded with the intracellular Ca2+-sensitive dye fura 2 in vitro. Intracellular Ca2+concentration and perfusion pressure were measured simultaneously. The rightward shift of the perfusion pressure-intracellular Ca2+mobilization curve after norepinephrine stimulation subsequent to CLP indicates decreased intracellular Ca2+sensitivity of contraction. The relation was restored by dexamethasone (which also restored in vivo blood pressure and flow), but not by l-canavanine (which restored perfusion pressure by further mobilization of intracellular Ca2+). We conclude that CLP lowers vasomotion by lowering intracellular Ca2+sensitivity, which can be restored with glucocorticoid treatment. The involvement of inducible NOS does not solely account for the sepsis-induced reduction in Ca2+sensitivity of contraction.

Upregulation of Rho-kinase (ROCK-2) expression and enhanced contraction to endothelin-1 in the mesenteric artery from lipopolysaccharide-treated rats

Effects of bacterial lipopolysaccharide (Escherichia coli serotype, 055:B5, 20 mg kg(-1), i.p., for 6 h) and a Rho-kinase inhibitor, (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide dihydrochloride monohydrate, Y-27632 (10(-9)-10(-5) M) were investigated on the contractile responses of the rat mesenteric artery to phenylephrine (10(-9)-3 x 10(-5) M), angiotensin-2 (10(-10)-10(-6) M) and endothelin-1 (10(-10)-10(-7) M). Moreover, alteration in the level of Rho-kinase (ROCK-2) expression was examined in the superior mesenteric artery obtained from saline- and lipopolysaccharide-treated rats by Western blotting. Endotoxemic rat mesenteric rings exhibited no different contractions to phenylephrine and angiotensin-2 but augmented contractile activity to endothelin-1. In the mesenteric artery obtained from the endotoxemic rats, acetylcholine-induced vasorelaxation did not differ; pD2 value for acetylcholine was 7.85+/-0.12 in the endotoxemic rings; however, it was 7.81+/-0.15 in the control rings (P>0.05). Y-27632 induced relaxation, which was the same in the control arteries as in endotoxemic ones when contracting agent was phenylephrine. However, when endothelin-1 was used to precontract the rings, Y-27632 produced enhanced relaxation in endotoxemic vessels. pD2 values for Y-27632 were, respectively, 7.69+/-0.12 and 8.20+/-0.10 in control and endotoxemic rings precontracted by endothelin-1 (10(-8) M) (P<0.01). Moreover, Y-27632 (10(-5) M) suppressed the contraction induced by angiotensin-2 (10(-10)-10(-6) M). Western blot analysis revealed that Rho-kinase was upregulated significantly in the mesenteric artery obtained from the rats treated with LPS for 6 h. In addition, serum NO2-/NO3- level, which was detected by Griess method, was 10.0+/-1.4 microM in endotoxemic rats; however, it was 6.6+/-0.5 microM in control (P<0.05). Taken together, these results show that the expression of the contractile protein Rho-kinase could be upregulated in endotoxemic mesenteric artery and this upregulation may be coincided with an enhanced contraction to endothelin-1 but not phenylephrine and angiotensin-2.

In vivo vascular effects of genistein on a rat model of septic shock induced by lipopolysaccharide

The aim of this study was to investigate the effects of in vivo administration of genistein on rat cardiovascular abnormalities induced by lipopolysaccharide (LPS). Four hours after injection, LPS (10 mg/kg) caused a stable fall in mean arterial pressure (13%) accompanied by ex vivo vascular hyporeactivity to noradrenaline (NA) and relaxation to l-arginine (L-arg), which were inhibited by previous incubation with l-NAME. Endotoxin also caused impairment of aortic relaxant response to acetylcholine, increase nitrite and malonaldehyde plasma levels by 8.6-fold and 2-fold, respectively, and induced aortic expression of inducible nitric-oxide synthase (iNOS) and nitrotyrosine protein. Genistein (1 mg/kg) and daidzein (1 mg/kg) reduced contractile response to NA in vascular tissue, but only genistein was able to inhibit hyporesponsiveness to NA, relaxation to l-arg, increase in nitrite plasma levels, and iNOS expression produced by endotoxin. Moreover, genistein restored impaired aortic relaxation to acetylcholine, lipid peroxidation, and suppressed long-term hypotension. In conclusion, genistein administrated in vivo prevents hypotension and vascular alterations induced by LPS. These protective effects are mediated by both its antioxidant properties and the inhibition of nitric oxide overproduction from de novo synthesis of iNOS due to its tyrosine kinase inhibitor effect.

Extracellular L-arginine is required for optimal NO synthesis by eNOS and iNOS in the rat mesenteric artery wall

1. The formation of NO from endothelial nitric oxide synthase (eNOS) in rat superior mesenteric artery rings was dependent on extracellular L-arginine, and was optimal at a concentration of L-arginine close to the plasma level (carbachol-stimulated NO: control 15.7+/-0.9, L-arginine 100 micro M 22.8+/-1.3 nM). 2. Enhancement of NO output by L-arginine was stereospecific, required the cationic amino-acid transporter and was dependent on caveolin. 3. Induction of inducible nitric oxide synthase (iNOS) impaired the stimulated NO synthesis from eNOS (100 nM carbachol-stimulated NO: control 5.7+/-0.6, iNOS 0.3+/-0.3 nM). 4. The interaction between iNOS and eNOS was reversed by the superoxide scavenger MnTMPyP. Impairment of eNOS by iNOS was also prevented by L-arginine 100 micro M administered simultaneously with carbachol, but not by L-arginine administered during incubation with lipopolysaccharide. 5. These data provide functional evidence that supplementing L-arginine from the extracellular medium optimises the formation of NO from eNOS and suggests that the impairment of eNOS by iNOS is caused by excess formation of superoxide by NO synthase, which can be prevented by L-arginine. These results provide an explanation for the observations that extracellular L-arginine can enhance endothelium function only when the endothelium is impaired or when iNOS has been induced.

The induction and detection in vitro of iNOS in the porcine basilar artery

The expression of iNOS in vascular tissues has an adverse effect on vascular responses to vasoconstrictors and NO-mediated vasodilators. The development of a simple method for detecting the iNOS expression by functional means would be extremely useful. Here we describe a method for inducing iNOS in the porcine basilar artery followed by the detection of iNOS protein by immunocytochemical means and the characterisation of functional responses to U46619 and L-arginine. Porcine basilar arteries were treated with LPS (1, 10 and 100 microg/ml) for between 5 and 18 h at 37 degrees C. Inducible NOS protein was expressed in a concentration-dependent manner in the endothelial and smooth muscle cells after 5 h and persisted for 18 h. Vessels treated with LPS showed a time-dependent reduction in contractile function in response to U46619 (10 nM) reaching significance at the 18-h time point. Moreover, a similar time-dependent increase in the vasodilator response to exogenously applied L-arginine (30 microM) was observed at both 5- and 18-h time points. These effects of LPS at the 18-h time point were prevented by the incubation of vessels with dexamethasone (100 microM) in addition to LPS. The vasodilator response to L-arginine was prevented with the incubation with and in the presence of the inhibitor of inducible NOS, 1400W (10 microM) in addition to LPS. These results show that iNOS protein can be expressed in porcine cerebral arteries and that the iNOS is functional. The assessment of contractile function and responses to L-arginine using single concentrations is a rapid and effective method for establishing whether functional iNOS is present in porcine cerebral arteries.

Heterogeneity of endothelium-dependent vasorelaxation in conductance and resistance arteries from Lyon normotensive and hypertensive rats

OBJECTIVES

The nature of endothelial factors in response to acetylcholine (ACh) was investigated in conductance and resistance arteries from Lyon normotensive (LN) and Lyon hypertensive (LH) rats. Differences in endothelial function between the two strains were evaluated.

METHODS AND DESIGN

Relaxations to ACh were studied in the aorta and small mesenteric arteries (SMA). The relative contribution of nitric oxide (NO), prostanoids and endothelial-derived hyperpolarizing factor (EDHF) was assessed using appropriate inhibitors. Western blot of endothelial NO synthase was achieved. The membrane potential of smooth muscle cells was assessed using microelectrodes.

RESULTS

In LN rats, endothelium-dependent relaxation to ACh involved exclusively NO in the aorta, whereas both NO and EDHF were implicated in SMA. In the latter, relaxation was almost entirely prevented by blockade of either the NO or EDHF pathway, although ACh was still able to produce hyperpolarization in the presence of NO synthase and cyclooxygenase inhibitors. In LH rats, relaxation to ACh was unchanged in SMA but moderately depressed in the aorta, despite unchanged endothelial NO synthase protein expression and sensitivity to NO. In addition, indomethacin, but not a selective cyclooxygenase-2 inhibitor, significantly reduced ACh relaxations in the aorta from LH rats but not from LN rats.

CONCLUSIONS

These results document differential endothelial function in a conductance and in resistance arteries from LN rats and LH rats. They show that simultaneous participation of NO and EDHF is required to promote relaxation in SMA from both strains, whereas NO alone accounts for relaxation in aorta from LN rats. In LH rats, aortic relaxation induced by ACh is slightly decreased despite the involvement of vasodilator products from cyclooxygenase-1.

Effects of in vivo lipopolysaccharide infusion on vasoconstrictor function of rat isolated mesentery, kidney, and aorta

Continuous infusion of lipopolysaccharide (LPS) into conscious rats elicits regionally selective cardiovascular disturbances. The aim of the present study was to assess contractile function in different vascular preparations (renal, mesenteric, and thoracic aorta) taken from rats infused with LPS for 2 or 24 h. Sustained responses to continuous infusion of methoxamine but not to KCl were reduced in the aorta (at 2 and 24 h LPS) and mesentery (at 24 h LPS) but not in the renal vascular bed. In contrast, transient responses to bolus doses of methoxamine were unchanged in the mesentery. In Ca2+-imaging experiments with fura-2, challenge with a single concentration of methoxamine (10 microM, which showed an impaired contractile response at 24 h LPS) induced a rise in intracellular Ca2+ in the mesenteric artery that was not different from the control. Furthermore, in the aorta, the contractile response to caffeine was attenuated only in the 2 h LPS group. These results show that there is regional heterogeneity in in vitro vascular responsiveness in preparations taken from LPS-infused rats. Thus, in mesenteric beds and aortae, but not renal beds, there is hypocontractility to methoxamine that is not due to a generalized inability of the smooth muscle to contract, which is evident with sustained but not transient application of agonist (mesentery) and which, in late endotoxemia (24 h LPS), does not appear to involve abnormalities in Ca2+ mobilization or entry.

Elevation of basal intracellular calcium as a central element in the activation of brain macrophages (microglia): suppression of receptor-evoked calcium signaling and control of release function

Microglia-brain macrophages are immune-competent cells of the CNS and respond to pathologic events. Using bacterial lipopolysaccharide (LPS) as a tool to activate cultured mouse microglia, we studied alterations in the intracellular calcium concentration ([Ca 2+]i) and in the receptor-evoked generation of transient calcium signals. LPS treatment led to a chronic elevation of basal [Ca 2+]i along with a suppression of evoked calcium signaling, as indicated by reduced [Ca 2+]i transients during stimulation with UTP and complement factor 5a. Presence of the calcium chelator BAPTA prevented the activation-associated changes in [Ca 2+]i and restored much of the signaling efficacy. We also evaluated downstream consequences of a basal [Ca 2+]i lifting during microglial activation and found BAPTA to strongly attenuate the LPS-induced release of nitric oxide (NO) and certain cytokines and chemokines. Furthermore, microglial treatment with ionomycin, an ionophore elevating basal [Ca 2+]i, mimicked the activation-induced calcium signal suppression but failed to induce release activity on its own. Our findings suggest that chronic elevation of basal [Ca 2+]i attenuates receptor-triggered calcium signaling. Moreover, increased [Ca 2+]i is required, but by itself is not sufficient, for release of NO and certain cytokines and chemokines. Elevation of basal [Ca 2+]i could thus prove a central element in the regulation of executive functions in activated microglia.

Elevation of basal intracellular calcium as a central element in the activation of brain macrophages (microglia): suppression of receptor-evoked calcium signaling and control of release function

Microglia-brain macrophages are immune-competent cells of the CNS and respond to pathologic events. Using bacterial lipopolysaccharide (LPS) as a tool to activate cultured mouse microglia, we studied alterations in the intracellular calcium concentration ([Ca 2+]i) and in the receptor-evoked generation of transient calcium signals. LPS treatment led to a chronic elevation of basal [Ca 2+]i along with a suppression of evoked calcium signaling, as indicated by reduced [Ca 2+]i transients during stimulation with UTP and complement factor 5a. Presence of the calcium chelator BAPTA prevented the activation-associated changes in [Ca 2+]i and restored much of the signaling efficacy. We also evaluated downstream consequences of a basal [Ca 2+]i lifting during microglial activation and found BAPTA to strongly attenuate the LPS-induced release of nitric oxide (NO) and certain cytokines and chemokines. Furthermore, microglial treatment with ionomycin, an ionophore elevating basal [Ca 2+]i, mimicked the activation-induced calcium signal suppression but failed to induce release activity on its own. Our findings suggest that chronic elevation of basal [Ca 2+]i attenuates receptor-triggered calcium signaling. Moreover, increased [Ca 2+]i is required, but by itself is not sufficient, for release of NO and certain cytokines and chemokines. Elevation of basal [Ca 2+]i could thus prove a central element in the regulation of executive functions in activated microglia.

Hypertension alters the participation of contractile prostanoids and superoxide anions in lipopolysaccharide effects on small mesenteric arteries

The involvement of cyclooxygenase-2 (COX-2)-derived products and superoxide anion in the effect of lipopolysaccharide in noradrenaline (NA)-induced contraction was investigated in small mesenteric arteries (SMA) from normotensive, Wistar Kyoto (WKY), and spontaneously hypertensive (SHR) rats. In WKY, lipopolysaccharide (10 microg/ml, 1 and 5 h) only inhibited the NA response (0.1-30 microM) in the presence of dexamethasone (1 microM), indomethacin (10 microM), the selective COX-2 inhibitor, NS 398 (10 microM), and the TXA(2)/PGH(2) receptor antagonist, SQ 29,548 (10 microM) but not of superoxide dismutase (SOD, 100 U/ml). In SHR, lipopolysaccharide inhibited the NA response by itself; this inhibition was potentiated by dexamethasone, indomethacin, NS 398, SQ 29,548 and SOD. The effect of lipopolysaccharide plus indomethacin, NS 398 or SQ 29,548 was higher in SMA from WKY than SHR only after 1 h lipopolysaccharide incubation. N(G)-nitro-L-arginine methyl ester (100 microM) and endothelium removal abolished the indomethacin-induced potentiatory effect of lipopolysaccharide in both strains. Endothelium removal also abolished the SOD potentiatory effect in SMA from SHR. Lipopolysaccharide increases COX-2 expression to a similar level in both strains and iNOS expression in a greater extent in SHR; these increases were reduced by dexamethasone. These results indicate: 1) lipopolysaccharide induces the endothelial production of contractile prostanoids from COX-2 in SMA, probably to compensate the increase in NO from iNOS; 2) the production of prostanoids in the presence of lipopolysaccharide seems to be greater in normotensive than hypertensive rats only after lipopolysaccharide short incubation times; 3) endothelial production of O(2)(.-) contributes to counteract depression of NA contraction caused by lipopolysaccharide only in SHR.

Wine polyphenols modulate calcium handling in rat aorta: involvement of nitric oxide pathway

The effects of short-term oral administration of red wine polyphenolic compounds (RWPCs) on blood pressure and vascular reactivity were investigated in rats. The consequence of RWPCs treatment on agonist-induced contractility of rat aorta with respect to Ca2+ handling was assessed, by examining both intracellular Ca2+ store and extracellular Ca2+ influx components of the response. Rats were treated daily for 7 days by intragastric administration of either 5% glucose, or RWPCs (20 mg/kg) [from two different sources, i.e. Provinols (SFD, Vallont Pont d'Arc, France) and RWPC1 (INRA, Montpellier, France)]. Administration of these compounds produced a decrease in systolic blood pressure. The consequence of RWPCs treatment on vascular smooth muscle was investigated in rat aorta without endothelium exposed to noradrenaline. In Ca(2+)-free medium, RWPC1 but not Provinols treatment induced an increase in noradrenaline-induced contraction. After depletion of intracellular Ca2+ stores by noradrenaline in Ca(2+)-free medium, addition of CaCl2 in the continuous presence of agonist induced an increase in contraction, which was not significantly different between control, Provinols- and RWPC-treated rats. The presence of an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin, significantly reduced noradrenaline-induced contraction in Ca(2+)-free medium in RWPCs-treated aorta, as compared to that of control. Interestingly, the inhibitory effect of thapsigargin on the response linked to the release of Ca2+ from internal stores in RWPCs-treated vessels was completely prevented in the presence of NO-synthase inhibitor, L-nitro arginine methyl ester, the inhibitor of guanylyl cyclase, oxadiazolo-quinoxaline or the protein kinase G inhibitor, 8-Bromoguanosine-3'-5-cyclic mono-phosphorothioate, Rp isomer. These results suggest that short-term administration of RWPCs in rats induced subtle alteration of thapsigargin-sensitive component of agonist-induced contraction in rat aorta linked to Ca2+ release from intracellular store. Calcium release from intracellular stores sensitive to thapsigargin was implicated in this mechanism. The prevention of the inhibitory effect of thapsigargin by the inhibitors of NO/cyclic guanosine monophosphate pathway after RWPCs treatment highlights the role of NO in this phenomenon.

Endotoxin-induced vascular hyporesponsiveness in rat aorta: in vitro effect of aminoguanidine

The current study was designed to evaluate the endotoxin-induced alterations of the mechanisms involved in Ca(2+)handling within the rat thoracic aorta and further to examine whether in vitro inhibition of inducible nitric oxide synthase (iNOS) by aminoguanidine would account for this effect or not. Endothelium denuded aortic rings from rats injected with lipopolysaccharide (LPS) (5 mg kg(-1), i.p. 18 h prior to functional studies) or saline were mounted in isolated organ baths. Various experimental conditions were studied on paired rings of the same animal which were incubated in the presence or absence of aminoguanidine (100 microM). Phenylephrine contractility in Ca(2+)-containing buffer or in Ca(2+)-free buffer, contractions induced by K(+)depolarization and CaCl(2)in depolarized muscle and by caffeine exposure were significantly decreased in LPS-treated rings and were reversed by aminoguanidine exposure. Aminoguanidine also improved the contractions recorded while switching the Ca(2+)-free buffer to Ca(2+)-containing buffer. We conclude that endotoxin induces a generalized contractile defect in vascular smooth muscle including impairment in the influx of extracellular Ca(2+)and release of Ca(2+)from intracellular stores. An increase in iNOS activation leading to excessive nitric oxide synthesis, possibly non-endothelial in origin, may account for this defect.

Sepsis decreases the spontaneous and agonist-induced contractile activities in the rat portal vein

BACKGROUND/AIMS

The portal vein has spontaneous and agonist-induced contractile activities and whether sepsis alters these two types of contractile activities is unknown.

METHODS

To study the effect of sepsis on the spontaneous contractile activity and the contractile responses to norepinephrine (NE), angiotensin II (AT(II)), and neurokinin B (NKB) in the rat portal vein (RPV), we performed a cecal ligature and puncture (CLP) 24 h before RPV isolation.

RESULTS

CLP decreased the spontaneous activity and induced hyporesponsiveness to AT(II) and NKB. The vascular failure was correlated to the severity of sepsis. In contrast, the reactivity to NE was not altered. Although inducible NO synthase was detected in RPV isolated from CLP rats, NO synthase inhibitors did not restore either the responsiveness to AT(II) and NKB or the spontaneous activity. Additionally, hyporesponsiveness to AT(II) and NKB was not modified by indomethacin.

CONCLUSIONS

CLP decreases the spontaneous activity of the RPV as well as the contractile responses to AT(II) and NKB. The vascular failure is correlated to the severity of sepsis. The reactivity to NE is not altered in this model. Neither NO nor prostaglandins are responsible for the vascular abnormalities observed during CLP.

Effect of lipopolysaccharide treatment on neurogenic contraction and noradrenaline release in rat arteries

In the present study, contractile responses and [3H]-noradrenaline overflow evoked by electrical field stimulation were assessed, respectively, in the small mesenteric artery and in tail artery removed from rats pre-treated with either saline or lipopolysaccharide (LPS). In small mesenteric arteries, LPS treatment did not significantly modify the contractile responses elicited by electrical stimulation, in the absence or in the presence of L-arginine. However, in arteries removed from rats treated with LPS, L-arginine addition produced relaxation of vessels pre-contracted with noradrenaline. The amplification of neurogenic contraction by the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) was similar in arteries removed from saline and LPS-infused rats. In mesenteric arteries, LPS treatment suppressed the potentiation of the neurogenic responses by the alpha2-adrenoceptor antagonist, yohimbine and by the inhibitor of neuronal uptake of noradrenaline, cocaine. In rat tail artery exposed to L-arginine, LPS treatment produced an increase in [3H]-noradrenaline overflow evoked by electrical stimulation. Altogether, these data suggest that an enhanced noradrenaline release from sympathetic nerves, probably resulting from inhibition of the modulatory effect of both prejunctional alpha2-adrenoceptors and neuronal uptake mechanism, may play a role in the preservation of neurogenic response after LPS treatment despite evidence of the induction of NO synthase.

Characterization of endothelial factors involved in the vasodilatory effect of simvastatin in aorta and small mesenteric artery of the rat

1. Vascular effects of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, simvastatin, were studied in conductance (aorta) and resistance vessels (branch II or III of superior mesenteric artery, SMA) of the rat (12 - 14 weeks old). 2. Simvastatin produced relaxation of both aorta and SMA, with and without functional endothelium. These responses were inhibited by the product of HMG-CoA reductase, mevalonate (1 mmol l(-1)). 3. In vessels with functional endothelium, the NO-synthase inhibitor, L-N(G)-nitroarginine (L-NOARG, 30 micromol l(-1)), inhibited simvastatin-induced relaxation. In the presence of L-NOARG, relaxation to simvastatin was lower in vessels with endothelium than in endothelium-denuded arteries without L-NOARG. 4. The cyclo-oxygenase inhibitor, indomethacin (10 micromol l(-1)), abolished endothelium-dependent component of the response to simvastatin in both arteries. The combination of L-NOARG plus indomethacin did not produce further inhibition. The T(p) receptor antagonist, GR 32191B (3 micromol l(-1)), did not affect relaxation in aorta but it reduced response to low concentrations of simvastatin in SMA. However, the inhibitory effect of L-NOARG was less marked in the presence of GR 32191B in aorta but not in SMA. 5. The endothelium-dependent relaxation to simvastatin was inhibited by the superoxide dismutase (SOD, 100 u ml(-1)) or by the tyrosine kinase inhibitor, genistein (30 micromol l(-1)) in the two arteries. 6. The present study shows that simvastatin produces relaxation of conductance and small arteries through mevalonate-sensitive pathway. The endothelium-dependent relaxation to simvastatin involves both NO and vasodilator eicosanoids by a mechanism sensitive to SOD, and to genistein. Also, the results highlighted participation in the aorta of endothelial vasoconstrictor eicosanoids acting on the T(p) receptor after blockage of NO synthase only.

Vascular bed heterogeneity in age-related endothelial dysfunction with respect to NO and eicosanoids

1. Endothelial dysfunction has been described with ageing but the mechanisms responsible have not been clearly elucidated and might be different from one vessel to the other. This study assesses the relative contribution of endothelial nitric oxide (NO) and cyclo-oxygenase (COX) metabolites in relaxation to acetylcholine with ageing in the aorta and the small mesenteric artery of the rat. 2. In the aorta and branch II or III of superior mesenteric artery (SMA), endothelium-dependent relaxation to acetylcholine was not different between 12 - 14 (adult) and 32-week-old rats whereas it was reduced at 70 - 100 (old) weeks of age. 3. Despite an increased endothelial NO-synthase protein expression, the NO-synthase inhibitor, N(G)-nitro-L-arginine-sensitive component of relaxation decreased with ageing. 4. In old rats, exposure to the COX inhibitor, indomethacin, but not the selective COX-2 inhibitor, NS-398, potentiated response to acetylcholine. The thromboxane A(2)/prostaglandin H(2) receptor antagonist, GR 32191B enhanced relaxation to acetylcholine in aorta but it had no effect in SMA. Furthermore, acetylcholine increased thromboxane B(2) production (enzymeimmunoassay) in aorta but not in SMA. Finally, Western blot analysis showed enhanced expression of COX-1 and 2 in the two arteries with ageing. 5. These results suggest that the decrease in acetylcholine-induced relaxation with ageing involves reduced NO-mediated dilatation and increased generation of vasoconstrictor prostanoids most likely from COX-1. They also point out vascular bed heterogeneity related to the nature of prostanoids involved between the aorta (i.e., thromboxane A(2)) and the SMA (unidentified) arteries even though increased expression of COX occurs in both vessels.

Influence of hypertension on nitric oxide synthase expression and vascular effects of lipopolysaccharide in rat mesenteric arteries

1. Experiments were designed to investigate the effects of the inducible nitric oxide synthase (iNOS) stimulator, lipopolysaccharide (LPS), on noradrenaline (NA) responses and on NOS activity and its expression in intact mesenteric resistance arteries (MRAs) from Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats. 2. In MRAs from WKY, LPS (10 microg ml(-1); 1-5 h) reduced the vasoconstrictor responses to NA (0.1 - 30 microM) in the presence, but not in the absence of L-arginine (L-Arg, 10 microM). However, in SHR arteries, LPS induced an incubation-time dependent reduction of NA responses in the absence, as well as the presence, of L-Arg. The LPS inhibitory effect was reduced by the non-specific NOS inhibitor L-N(G)-nitroarginine methyl ester (L-NAME, 100 microM) and the selective iNOS inhibitor, aminoguanidine (100 microM). 3. L-NAME alone similarly shifted the concentration-response curve to NA leftward in arteries from both strains, while aminoguanidine had no effect. L-Arg shifted the curve to NA rightward only in SHR MRAs. 4. Basal activity of both iNOS and constitutive NOS (conversion of [(3)H]-L-Arg to [(3)H]-L-citrulline) was similar in arteries from both strains. After 5 h incubation with LPS, only iNOS activity in arteries from SHR was increased. 5. Basal iNOS protein expression was undetectable; basal endothelial (eNOS) protein expression was similar in arteries from both strains, while neuronal (nNOS) was greater in arteries from SHR. LPS induced iNOS protein expression, that was higher in arteries from SHR than in those from WKY. 6. These results indicate that NO production, via iNOS induction, is greater than in those from MRAs from SHR to WKY.

Induction of nitric oxide synthase and dual effects of nitric oxide and cyclooxygenase products in regulation of arterial contraction in human septic shock

BACKGROUND

The role of endogenous nitric oxide (NO) and cyclooxygenase metabolites was investigated in contractile responses of small omental arteries from patients with hyperdynamic septic shock.

METHODS AND RESULTS

Expression of inducible NO synthase (immunostaining) and a high but variable level of NO production (NO spin trapping) was detected in arteries from patients with septic shock. In these vessels, ex vivo contractile responses to the thromboxane A2 analogue U46619 and to low concentrations of norepinephrine (NE) (up to 10 micromol/L) were not significantly different from controls. However, higher concentrations of NE caused pronounced fading of contraction in septic but not in nonseptic arteries. Exposure to either the NO synthase inhibitor NG-nitro-L-arginine methyl ester or the cyclooxygenase inhibitor indomethacin had no effect in control vessels. However, both inhibitors increased the response to the contractile effects of the 2 agonists only in patients with septic shock. In contrast to NG-nitro-L-arginine methyl ester, which decreased the threshold concentration of the fading effect of NE, indomethacin abolished this effect in arteries from septic patients.

CONCLUSIONS

These results provide direct evidence for the induction of NO synthase in small arteries from patients with septic shock. They suggest that in these arteries, increased production of NO, in conjunction with vasodilatory cyclooxygenase metabolites, contributes to counteract hyperreactivity to agonists and decreases the cyclooxygenase product-mediated pronounced fading of contraction caused by a high concentration of NE.

The inducible nitric oxide synthase in vascular and cardiac tissue

Expression of the inducible form of nitric oxide synthase (iNOS) has been reported in a variety of cardiovascular diseases. The resulting high output nitric oxide (NO) formation, besides the level of iNOS expression, depends also on the expression of the metabolic pathways providing the enzyme with substrate and cofactor. NO may trigger short and long term effects which are either beneficial or deleterious, depending on the molecular targets with which it interacts. These interactions are governed by local factors (like the redox state). In the cardiovascular system, the major targets involve not only guanylyl cyclase, but also other haem proteins, protein thiols, iron-non-haem complexes, and superoxide anion (forming peroxynitrite). The latter has several intracellular targets and may be cytotoxic, despite the existence of endogenous defence mechanisms. These interactions may either trigger NO effects or represent releasable NO stores, able to buffer NO and prolong its effects in blood vessels and in the heart. Besides selectively inhibiting iNOS, a number of other therapeutic strategies are conceivable to alleviate deleterious effects of excessive NO formation, including peroxynitrite (ONOO-) scavenging and inhibition of metabolic pathways triggered by ONOO-. When available, these approaches might have the advantage to preserve beneficial effects of iNOS induction. Counteracting vascular hyper-responsiveness to endogenous vasoconstrictor agonists in septic shock, or inducing cardiac protection against ischaemia-reperfusion injury are examples of such beneficial effects of iNOS induction.

Role of iNOS in the vasodilator responses induced by L-arginine in the middle cerebral artery from normotensive and hypertensive rats

1. The substrate of nitric oxide synthase (NOS), L-arginine (L-Arg, 0.01 microM - 1 mM), induced endothelium-independent relaxations in segments of middle cerebral arteries (MCAs) from normotensive Wistar-Kyoto (WKY) and hypertensive rats (SHR) precontracted with prostaglandin F2alpha (PGF2alpha). These relaxations were higher in SHR than WKY arteries. 2. L-N(G)-nitroarginine methyl ester (L-NAME) and 2-amine-5,6-dihydro-6-methyl-4H-1,3-tiazine (AMT), unspecific and inducible NOS (iNOS) inhibitors, respectively, reduced those relaxations, specially in SHR. 3. Four- and seven-hours incubation with dexamethasone reduced the relaxations in MCAs from WKY and SHR, respectively. 4. Polymyxin B and calphostin C, protein kinase C (PKC) inhibitors, reduced the L-Arg-induced relaxation. 5. Lipopolysaccharide (LPS, 7 h incubation) unaltered and inhibited these relaxations in WKY and SHR segments, respectively. LPS antagonized the effect polymyxin B in WKY and potentiated L-Arg-induced relaxations in SHR in the presence of polymyxin B. 6. The contraction induced by PGF2alpha was greater in SHR than WKY arteries. This contraction was potentiated by dexamethasone and polymyxin B although the effect of polymyxin B was higher in SHR segments. LPS reduced that contraction and antagonized dexamethasone- and polymyxin B-induced potentiation, these effects being greater in arteries from SHR. 7. These results suggest that in MCAs: (1) the induction of iNOS participates in the L-Arg relaxation and modulates the contraction to PGF2alpha; (2) that induction is partially mediated by a PKC-dependent mechanism; and (3) the involvement of iNOS in such responses is greater in the hypertensive strain.

Endotoxin impairs agonist-induced calcium mobilization in bovine aortic myocytes by a nitric oxide-independent mechanism

We hypothesized that endotoxin (LPS) would impair vasoconstrictor-agonist-induced calcium (Ca2+) mobilization by a nitric oxide (NO)-dependent mechanism. We incubated bovine aortic myocytes (passages 16 to 23) for 22 to 24 hours with 0 to 1.0 mg/ml Escherichia coli lipopolysaccharide (LPS). Medium (Dulbecco's modified Eagle's medium (DMEM) + 10% fetal bovine serum (FBS)) was assayed for nitrite (chemiluminescence), and myocytes were loaded with fura-2 acetoxymethyl ester (fura-2AM), after which we assessed basal and thrombin (10 U/ml)-induced peak Ca2+ mobilization by microspectrofluorimetry. LPS (0.01 to 1.0 mg/ml) led to dose-dependent nitrite accumulation, which was blocked by coincubation with N(omega)-nitro-L-arginine methyl ester (L-NAME, 1 mmol/L). LPS also impaired Ca2+ responses in a dose-dependent manner (from -13% at 0.1 mg/ml to -47% at 1.0 mg/ml, n = 8 to 43/dose). However, coincubation with L-NAME did not ameliorate the Ca2+ mobilization defect (peak Ca2+ increments: control = 419 +/-30 nmol/L, vs LPS (1 mg/ml) = 206+/-18 nmol/L (mean+/-SE), n = 15; p < 0.001; control/L-NAME: 417+/-31 nmol/L vs LPS/L-NAME: 212+/-19 nmol/L; n = 17 p < 0.001), despite inhibition of associated nitrite accumulation in the medium (control vs LPS: p < 0.001; control/L-NAME vs LPS/L-NAME: p > 0.05; LPS vs LPS/L-NAME: p < 0.001). Supplemental L-arginine augmented LPS-induced nitrite generation without affecting Ca2+ mobilization. Indomethacin failed to prevent the LPS-induced decrement in thrombin response, but did inhibit LPS-induced myocyte nitrite production, suggesting "crosstalk" between the NO-synthase and cyclo-oxygenase (COX) systems. These experiments suggest that LPS-induced vascular contractile impairment is at least partly mediated by an NO-independent impairment of agonist-induced myocyte Ca2+ mobilization. This further suggests that any important contribution of NO synthesis to LPS-induced contractile dysfunction must depend on impairment of the Ca2+ sensitivity of the contractile apparatus (i.e., pharmacomechanical coupling).

Differential effects of endotoxaemia on pressor and vasoconstrictor actions of angiotensin II and arginine vasopressin in conscious rats

1. Regional haemodynamic responses to arginine vasopressin (AVP; 0.5, 1.0, 5.0 pmol i.v.) and angiotensin II (AII; 5.0, 10.0, 50.0 pmol i.v.) were measured in conscious Long Evans rats at various times (0, 2, 6 and 24 h) during infusion of lipopolysaccharide (LPS, 150 microg kg(-1) h(-1), i.v., n=9) or saline (n=9). Additional experiments were performed in vasopressin-deficient (Brattleboro) rats infused with LPS (n=7) or saline (n=8) to determine whether or not, in the absence of circulating vasopressin, responses to the exogenous peptides differed from those in Long Evans rats. 2. In the Long Evans rats, during the 24 h infusion of LPS, there was a changing haemodynamic profile with renal vasodilatation from 2 h onwards, additional mesenteric vasodilatation at 6 h, and a modest hypotension (reduction in mean arterial blood pressure (MAP) from 103+/-1 to 98+/-2 mmHg) associated with renal and hindquarters vasodilatation at 24 h. 3. In the Brattleboro rats, the changes in regional haemodynamics during LPS infusion were more profound than in the Long Evans rats. At 2 h and 6 h, there was a marked fall in MAP (from 103+/-3 mmHg; to 65+/-3 mmHg at 2 h, and to 82+/-4 mmHg at 6 h) associated with vasodilatation in all three vascular beds. After 24 h infusion of LPS, the hypotension was less although still significant (from 103+/-3 mmHg; to 93+/-4 mmHg, a change of 10+/-4 mmHg), and there was renal and hindquarters vasodilatation, but mesenteric vasoconstriction. 4. During infusion of LPS, at each time point studied, and in both strains of rat, pressor responses to AII and AVP were reduced, but the changes were less marked at 6 h than at 2 h or 24 h. The reduced pressor responses were not accompanied by generalized reductions in the regional vasoconstrictor responses. Thus, in the Long Evans rats, the renal vasoconstrictor responses to both peptides were enhanced (at 6 h and 24 h for AVP; at all times for AII), whereas the mesenteric vasoconstrictor response to AVP was unchanged at 2 h, enhanced at 6 h and reduced at 24 h. The mesenteric vasoconstrictor response to AII was reduced at 2 h, normal at 6 h and reduced at 24 h. The small hindquarters vasoconstrictor responses to both peptides were reduced at 2 h and 6 h, but normal at 24 h. 5. In the Brattleboro rats, the renal vasoconstrictor responses to both peptides were reduced at 2 h and enhanced at 6 h and 24 h, whereas the mesenteric vasoconstrictor response to AVP was normal at 2 h and 6 h, and reduced at 24 h. The response to AII was reduced at 2 h, normal at 6 h and reduced again at 24 h. There were no reproducible hindquarters vasoconstrictions to AVP in the Brattleboro rats. The small hindquarters vasoconstrictor responses to AII were unchanged at 2 h and enhanced at 6 h and 24 h. 6. In isolated perfused mesenteric vascular beds, removed after 24 h of LPS infusion in vivo, there was an increase in the potency of AVP in both strains (Long Evans, ED50 saline: 56.9+/-15.0 pmol, ED50 LPS: 20.4+/-4.8 pmol, Brattleboro, ED50 saline: 38.6+/-4.2, ED50 LPS: 19.6+/-2.9 pmol), but no change in the responses to AII. 7. These findings indicate that a reduced pressor response to a vasoconstrictor challenge during LPS infusion is not necessarily associated with a reduced regional vasoconstriction. The data obtained in the Brattleboro rats indicate a potentially important role for vasopressin in maintaining haemodynamic status during LPS infusion in Long Evans rats. However, it is unlikely that the responses to exogenous AVP (or AII) are influenced by changes in the background level of endogenous vasopressin, since the patterns of change were similar in Long Evans and Brattleboro rats. 8. The results obtained in isolated perfused mesenteric vascular beds differed from those in vivo, possibly due to the conditions pertaining with in vitro perfusion.