Simultaneous perfusion of rat isolated superior mesenteric arterial and venous beds: comparison of their vasoconstrictor and vasodilator responses to agonists

Venous endothelial function in cardiovascular disease

The essential role of the endothelium in vascular homeostasis is associated with the release of endothelium-dependent relaxing and contractile factors (EDRF and EDCF, respectively). Different from arteries, where these factors are widely studied, the vasoactive factors derived from the venous endothelium have been given less attention. There is evidence for a role of the nitric oxide (NO), endothelium-dependent hyperpolarization (EDH) mechanism, and cyclooxygenase (COX)-derived metabolites as EDRFs; while the EDCFs need to be better evaluated since no consensus has been reached about their identity in venous vessels. The imbalance between the synthesis, bioavailability, and/or action of EDRFs and/or EDCFs results in a pathological process known as endothelial dysfunction, which leads to reduced vasodilation and/or increased vasoconstriction. In the venous system, endothelial dysfunction is relevant since reduced venodilation may increase venous tone and decrease venous compliance, thus enhancing mean circulatory filling pressure, which maintains or modify cardiac workload contributing to the etiology of cardiovascular diseases. Interestingly, some alterations in venous function appear at the early stages (or even before) the establishment of these diseases. However, if the venous endothelium dysfunction is involved in these alterations is not yet fully understood and requires further studies. In this sense, the present study aims to review the current knowledge on venous endothelial function and dysfunction, and the general state of the venous tone in two important cardiovascular diseases of high incidence and morbimortality worldwide: hypertension and heart failure.

An investigation into the relationship between small intestinal fluid secretion and systemic arterial blood pressure in the anesthetized rat

The effects of changes in the steady level of diastolic blood pressure on fluid flux across the jejunum has been investigated in the anesthetized rat during perfusion with a nutrient-free and Na⁺-free solution. Diastolic blood pressure was manipulated by intravenous infusions, during the jejunal perfusions, of vasodilators (vasoactive intestinal polypeptide, acetyl-β-methylcholine, and phentolamine) and a vasoconstrictor (arginine vasopressin), each of which acts through a different cellular mechanism. The outcome was that fluid flux was related by a parabolic relationship with diastolic blood pressure in which net secretion occurred over the range 40–100 mmHg, whereas net absorption was recorded at diastolic pressures exceeding 100 mmHg and below 40 mmHg. Against a background of normal absorption promoted by perfusion with 145 mmol L⁻¹ Na⁺/5 mmol L⁻¹ glucose solution, reductions in diastolic blood pressure markedly reduced the mean rate of fluid absorption by 58% overall, whereas the rate of glucose absorption remained unchanged. Our results were explained on the basis that vasodilatation led to increased capillary pressure and then to net filtration of fluid from the mesenteric capillary bed. Experiments in which Escherichia coli heat-stable toxin was added to the jejunal perfusate confirmed the absence of a secretory response, which was consistent with the absence of effect of the toxin on diastolic blood pressure.

Regulation of Vascular Tone, Angiogenesis and Cellular Bioenergetics by the 3-Mercaptopyruvate Sulfurtransferase/H2S Pathway: Functional Impairment by Hyperglycemia and Restoration by DL-α-Lipoic Acid

Hydrogen sulfide (H2S), as a reducing agent and an antioxidant molecule, exerts protective effects against hyperglycemic stress in the vascular endothelium. The mitochondrial enzyme 3-mercaptopyruvate sulfurtransferase (3-MST) is an important biological source of H2S. We have recently demonstrated that 3-MST activity is inhibited by oxidative stress in vitro and speculated that this may have an adverse effect on cellular homeostasis. In the current study, given the importance of H2S as a vasorelaxant, angiogenesis stimulator and cellular bioenergetic mediator, we first determined whether the 3-MST/H2S system plays a physiological regulatory role in endothelial cells. Next, we tested whether a dysfunction of this pathway develops during the development of hyperglycemia and μmol/L to diabetes-associated vascular complications. Intraperitoneal (IP) 3-MP (1 mg/kg) raised plasma H2S levels in rats. 3-MP (10 1 mmol/L) promoted angiogenesis in vitro in bEnd3 microvascular endothelial cells and in vivo in a Matrigel assay in mice (0.3-1 mg/kg). In vitro studies with bEnd3 cell homogenates demonstrated that the 3-MP-induced increases in H2S production depended on enzymatic activity, although at higher concentrations (1-3 mmol/L) there was also evidence for an additional nonenzymatic H2S production by 3-MP. In vivo, 3-MP facilitated wound healing in rats, induced the relaxation of dermal microvessels and increased mitochondrial bioenergetic function. In vitro hyperglycemia or in vivo streptozotocin diabetes impaired angiogenesis, attenuated mitochondrial function and delayed wound healing; all of these responses were associated with an impairment of the proangiogenic and bioenergetic effects of 3-MP. The antioxidants DL-α-lipoic acid (LA) in vivo, or dihydrolipoic acid (DHLA) in vitro restored the ability of 3-MP to stimulate angiogenesis, cellular bioenergetics and wound healing in hyperglycemia and diabetes. We conclude that diabetes leads to an impairment of the 3-MST/H2S pathway, and speculate that this may contribute to the pathogenesis of hyperglycemic endothelial cell dysfunction. We also suggest that therapy with H2S donors, or treatment with the combination of 3-MP and lipoic acid may be beneficial in improving angiogenesis and bioenergetics in hyperglycemia.

Agonist-dependent modulation of arterial endothelinA receptor function

BACKGROUND AND PURPOSE Endothelin-1 (ET-1) causes long-lasting vasoconstrictions. These can be prevented by ET(A) receptor antagonists but are only poorly reversed by these drugs. We tested the hypothesis that endothelin ET(A) receptors are susceptible to allosteric modulation by endogenous agonists and exogenous ligands. EXPERIMENTAL APPROACH Rat isolated mesenteric resistance arteries were pretreated with capsaicin and studied in wire myographs, in the presence of L-NAME and indomethacin to concentrate on arterial smooth muscle responses. KEY RESULTS Endothelins caused contractions with equal maximum but differing potency (ET-1 = ET-2 > ET-3). ET-1(1-15) neither mimicked nor antagonized these effects in the absence and presence of ET(16-21). 4(Ala) ET-1 (ET(B) agonist) and BQ788 (ET(B) antagonist) were without effects. BQ123 (peptide ET(A) antagonist) reduced the sensitivity and relaxed the contractile responses to endothelins. Both effects depended on the agonist (pK(B): ET-3 = ET-1 > ET-2; % relaxation: ET-3 = ET-2 > ET-1). Also, with PD156707 (non-peptide ET(A) antagonist) agonist-dependence and a discrepancy between preventive and inhibitory effects were observed. The latter was even more marked with bulky analogues of BQ123 and PD156707. CONCLUSIONS AND IMPLICATIONS These findings indicate allosteric modulation of arterial smooth muscle ET(A) receptor function by endogenous agonists and by exogenous endothelin receptor antagonists. This may have consequences for the diagnosis and pharmacotherapy of diseases involving endothelins.

Vascular mechanisms involved in angiotensin II-induced venoconstriction in hypertensive rats

To investigate the venoconstrictor effect of angiotensin II (Ang II) in spontaneously hypertensive rats (SHR), we used preparations of mesenteric venular beds and the circular muscle of the portal veins. Vessels were tested with Ang II in the presence or absence of losartan, PD 123319, HOE 140, L-NAME, indomethacin, or celecoxib. In the mesenteric venular bed of SHR, the effect of Ang II (0.1 nmol) was nearly abolished by losartan and enhanced by HOE 140, indomethacin, and celecoxib, while PD123319 and L-NAME had no effect. In portal vein preparations, cumulative-concentration response curves (CCRC) to Ang II (0.1-100 nmol/L) exhibited a lower maximal response (E(max)) in SHR compared to Wistar rats. AT(1) receptor expression was similar in the two strains, while AT(2) receptor levels were lower in SHR portal veins when compared to Wistar. In SHR portal veins, losartan shifted the CCRC to Ang II to the right, while indomethacin and HOE 140 increased the E(max) to Ang II. PD 123319, celecoxib, and L-NAME had no effect. Taken together, our results suggest that Ang II-induced venoconstriction in SHR is mediated by activation of AT(1) receptors and this effect may be counterbalanced by kinin B(2) receptor and COX metabolites. Furthermore, our data indicate that there are different cellular and molecular mechanisms involved in the regulation of venous tonus of normotensive and hypertensive rats. These differences probably reflect distinct factors that influence arterial and venous bed in hypertension.

Hydrogen sulfide-induced dual vascular effect involves arachidonic acid cascade in rat mesenteric arterial bed

Hydrogen sulfide (H(2)S), a novel gaseous transmitter, is considered a physiological regulator of vascular homeostasis. Recent evidence suggests H(2)S as an endothelium-hyperpolarizing factor (EDHF) candidate. To address this issue, we evaluated the vascular effect of sodium hydrogen sulfide (NaHS), an H(2)S donor, on the rat mesenteric arterial bed. NaHS concentration-response curve was performed on preconstricted mesenteric arterial bed. To assess the contribution of EDHF, we performed a pharmacologic dissection using indomethacin, N(G)-nitro-l-arginine methyl ester (l-NAME), or apamin and charybdotoxin as cyclooxygenase, nitric-oxide synthase, and calcium-dependent potassium channel inhibitors, respectively. In another set of experiments, we used 4-(4-octadecylphenyl)-4-oxobutenoic acid, baicalein, or proadifen as phospholipase A(2) (PLA(2)), lipoxygenase, and cytochrome P450 inhibitors, respectively. Finally, an immunofluorescence study was performed to support the involvement of PLA(2) in mesenteric artery challenged by NaHS. NaHS promoted a dual vascular effect (i.e., vasoconstriction and vasodilation). l-NAME or baicalein administration affected neither NaHS-mediated vasodilation nor vasoconstriction, whereas apamin and charybdotoxin significantly inhibited NaHS-induced relaxation. Pretreatment with PLA(2) inhibitor abolished both the contracting and the relaxant effect, whereas P450 cytochrome blocker significantly reduced NaHS-mediated relaxation. The immunofluorescence study showed that NaHS caused a migration of cytosolic PLA(2) close to the nucleus, which implicates activation of this enzyme. Our data indicate that H(2)S could activate PLA(2), which in turn releases arachidonic acid leading, initially, to vasoconstriction followed by vasodilation mediated by cytochrome P450-derived metabolites. Because EDHF has been presumed to be a cytochrome P450 derivative of the arachidonic acid, our results suggest that H(2)S acts through EHDF release.

Captopril reduces the severity of bowel damage in a neonatal rat model of necrotizing enterocolitis

BACKGROUND/PURPOSE

Selective mesenteric ischemia may result from activation of the renin-angiotensin system during periods of shock and is implicated in the pathogenesis of neonatal necrotizing enterocolitis (NEC). We investigated the effectiveness of captopril, an angiotensin-converting enzyme inhibitor, in reducing the severity of bowel damage in a neonatal rat model of NEC.

METHODS

Necrotizing enterocolitis was induced by a combination of gavage feeding of hypertonic formula, hypoxia, and oral lipopolysaccharide (LPS). Rats were randomly divided into 3 groups: group A, control (breast fed; n = 20); group B, NEC (gavage/hypoxia/LPS; n = 31); group C, NEC with captopril 20 mg/kg per dose with the formula for 4 days (gavage/hypoxia/LPS/captopril; n = 35). Pups were killed after 4 days. Incidence of NEC was evaluated microscopically.

RESULTS

Severity of bowel damage was higher in the NEC group compared to controls and was reduced by administration of captopril. Dilatation of the intestinal vasculature was observed in the captopril group. There were no cases of NEC in the controls; the incidence increased to 55% in NEC group and reduced to 29% by captopril.

CONCLUSIONS

In this model of neonatal NEC, captopril supplementation of formula reduces the severity of intestinal damage and the incidence of NEC, presumably by affecting mesenteric blood flow.

Characterization of the non-adrenergic/non-cholinergic response to perivascular nerve stimulation in the double-perfused mesenteric bed of the mouse

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP), a capsaicin-sensitive neuromodulator of splanchnic vascular tone in several animal species, remains poorly investigated in mouse models. We therefore assessed whether endogenous CGRP is a non-adrenergic/non-cholinergic (NANC) neuromodulator in the mesenteric vascular bed of the mouse.

EXPERIMENTAL APPROACH

Arterial and venous changes in perfusion pressure in response to perivascular nerve stimulation (PNS) were monitored in the mouse mesenteric bed under basal conditions or precontracted with KCl (artery) or U46619 (vein) in circuits pretreated with guanethidine, atropine, indomethacin and prazosin. Arterial responses to NANC were also characterized with a CGRP1 antagonist, halphaCGRP8-37. Finally, the PNS-induced release of arterial CGRP was measured by enzyme immunoassay.

KEY RESULTS

HalphaCGRP8-37 enhanced PNS-induced arterial increases in perfusion pressure under basal tone. PNS-induced stimulation of NANC triggered an halphaCGRP8-37 or capsaicin- sensitive reduction in perfusion pressure of the pre-contracted arterial bed only. Chemical removal of the endothelium inhibited PNS- and halphaCGRP- induced reduction in perfusion pressure in the arterial mesenteric bed. Responses to NANC nerves were reduced by guanylate and adenylate cyclase inhibitors (1H-[1,2,4]oxadiazole[4,3-a] quinoxalin-1-one (ODQ)) and [9-(tetrahydro-2-furanyl)-9H-purin-6-amine] (SQ 22,536), respectively. A neuronal NOS inhibitor (7-nitroindazole; 7-NI) also enhanced the response to NANC in vessels from wild-type, eNOS KO but not iNOS KO mice. Finally, PNS enhanced the release of immunoreactive CGRP from the perfused arterial mesenteric bed.

CONCLUSIONS AND IMPLICATIONS

Our study demonstrates a role for CGRP in the NANC-dependent reduction in perfusion pressure of the arterial but not venous mesenteric bed of the mouse.

The Serotonin 5-Hydroxytryptaphan1A Receptor Agonist, (+)8-Hydroxy-2-(di-n-propylamino)-tetralin, Stimulates Sympathetic-Dependent Increases in Venous Tone during Hypovolemic Shock

Adjuvant treatment of hypovolemic shock with vasoconstrictors is controversial due to their propensity to raise arterial resistance and exacerbate ischemia. A more advantageous therapeutic approach would use agents that also promote venoconstriction to augment perfusion pressure through increased venous return. Recent studies indicate that 5-hydroxytryptophan (5-HT)(1A) receptor agonists increase blood pressure by stimulating sympathetic drive when administered after acute hypotensive hemorrhage. Given that venous tone is highly dependent upon sympathetic activation of alpha(2)-adrenergic receptors, we hypothesized that the 5-HT(1A) receptor agonist, (+)8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), would increase venous tone in rats subject to hypovolemic shock through sympathetic activation of alpha(2)-adrenergic receptors. Systemic administration of 8-OH-DPAT produced a sustained rise in blood pressure (+44 +/- 3 mm Hg 35 min after injection, P < 0.01 versus saline) and mean circulatory filling pressure (+4.2 +/- 0.7 mm Hg, P < 0.01 versus saline) in conscious rats subjected to hypovolemic shock. An equipressor infusion of epinephrine failed to influence mean circulatory filling pressure (MCFP). Ganglionic blockade, alpha(1)-, or peripheral alpha(2)-adrenergic receptor blockade prevented the rise in MCFP observed with 8-OH-DPAT, but only alpha(1)-adrenergic receptor blockade diminished the pressor effect of the drug (P < 0.01). 8-OH-DPAT raises blood pressure in rats in hypovolemic shock through both direct vascular activation and sympathetic activation of alpha(1)-adrenergic receptors. The sympathoexcitatory effect of 8-OH-DPAT contributes to elevated venous tone through concurrent activation of both alpha(1)- and alpha(2)-adrenergic receptors. The data suggest that 5-HT(1A) receptor agonists may provide an advantageous alternative to currently therapeutic interventions used to raise perfusion pressure in hypovolemic shock.

Angiotensin II-induced venoconstriction involves both AT1 and AT2 receptors and is counterbalanced by nitric oxide

The venoconstrictor effect of Angiotensin II (Ang II) was investigated in the rat mesenteric venules and portal vein. Mesenteric venules were perfused at a constant rate and reactivity to Ang II (0.1 nmol) was evaluated as changes in the perfusion pressure. Rings of portal vein were mounted in organ baths and curves to Ang II (0.1-100 nmol/L) were generated. In venules, Ang II-contraction (10.6+/-1.1 mmHg) was abolished by losartan (0.9+/-0.3 mmHg*), reduced by PD 123,319 (5.8+/-0.9 mmHg*), increased by L-NAME (16.5+/-1.8 mmHg*) and not altered by indomethacin. In portal veins, curves to Ang II (-logEC50: 8.9+/-0.1 mol/L) were shifted to the right by losartan (-log EC50: 7.5+/-0.1 mol/L*) and by PD 123,319 (-logEC50: 8.0+/-0.1 mol/L*). L-NAME increased the maximal response to Ang II (Emax: 0.91+/-0.1g versus 1.62+/-0.3g*) and indomethacin had no effect. In conclusion, Ang II induces venoconstriction by activating AT1 and AT2 receptors. Data obtained with L-NAME provide evidence that the basal nitric oxide release from the endothelium of the venous system can modulate the Ang II-induced venoconstriction.

Effect of des-aspartate-angiotensin I on the actions of angiotensin II in the isolated renal and mesenteric vasculature of hypertensive and STZ-induced diabetic rats

The present study investigated the action of des-aspartate-angiotensin I (DAA-I) on the pressor action of angiotensin II in the renal and mesenteric vasculature of WKY, SHR and streptozotocin (STZ)-induced diabetic rats. Angiotensin II-induced a dose-dependent pressor response in the renal vasculature. Compared to the WKY, the pressor response was enhanced in the SHR and reduced in the STZ-induced diabetic rat. DAA-I attenuated the angiotensin II pressor action in renal vasculature of WKY and SHR. The attenuation was observed for DAA-I concentration as low as 10(-18) M and was more prominent in SHR. However, the ability of DAA-I to reduce angiotensin II response was lost in the STZ-induced diabetic kidney. Instead, enhancement of angiotensin II pressor response was seen at the lower doses of the octapeptide. The effect of DAA-I was not inhibited by PD123319, an AT2 receptor antagonist, and indomethacin, a cyclo-oxygenase inhibitor in both WKY and SHR, indicating that its action was not mediated by angiotensin AT2 receptor and prostaglandins. The pressor responses to angiotensin II in mesenteric vascular bed were also dose-dependent but smaller in magnitude compared to the renal vasculature. The responses were significantly smaller in SHR but no significant difference was observed between STZ-induced diabetic and WKY rat. Similarly, PD123319 and indomethacin had no effect on the action of DAA-I. The findings reiterate a regulatory role for DAA-I in vascular bed of the kidney and mesentery. By being active at circulating level, DAA-I subserves a physiological role. This function appears to be present in animals with diseased state of hypertension and diabetes. It is likely that DAA-I functions are modified to accommodate the ongoing vascular remodeling.

VASOPRESSIN MAY BE USEFUL IN THE TREATMENT OF SYSTEMIC ANAPHYLAXIS IN RABBITS

Recent studies demonstrate that vasopressin is useful when treating hemorrhagic and septic shock. The effect of vasopressin on systemic anaphylaxis has not been investigated except in clinical case reports. Vasopressin increases blood pressure because of vasoconstriction through the V1 receptor. Thus, we evaluated the effect of vasopressin on circulatory depression and bronchoconstriction provoked by systemic anaphylaxis and survival rates in rabbits. In the first set of experiments, 15 nonsensitized rabbits received normal saline (control) and vasopressin at 0.8 or 0.08 U/kg. In the second set, 40 sensitized rabbits received horse serum to induce anaphylaxis, and then received the same drugs as in the first set. In the first set, mean arterial pressure (MAP) in vasopressin groups increased by 18% to 24% compared with the control. Vasopressin at 0.8 U/kg decreased MAP insignificantly before the increases of MAP occurred. In the second set, vasopressin at 0.08 U/kg improved the survival rate. At 45 min after antigen challenge, 69% of the rabbits that received vasopressin at 0.08 U/kg were alive, whereas 29% of the control rabbits and 23% of the rabbits that received vasopressin at 0.8 U/kg were alive. Vasopressin increased MAP by 36% to 109% compared with the control within 5 min, however, at 2 min, vasopressin at 0.8 U/kg had no effect on MAP. Pulmonary dynamics were similar. In conclusion, vasopressin at 0.08 U/kg improved survival rates and severe hypotension provoked by systemic anaphylaxis, suggesting that this agent may be useful in the treatment of systemic anaphylaxis.

m-CPP, a 5-HT2C Receptor Agonist That Modifies the Perfusion Pressure of the Hindquarter Vascular Bed of Anesthetized Rat

In the present work we studied the actions of the intra-arterial administration of meta-chlorophenylpiperazine (m-CPP - a 5-HT(2C) receptor agonist) in the hindquarters of the anesthetized rat. The lowest doses used (0.001, 0.01, 0.1, 0.25 and 0.5 microg/kg) induced vasodilatation whereas the highest doses produced vasoconstriction (1, 6.25, 12.5 and 25 microg/kg). Both vasodilatation and vasoconstriction were inhibited by the 5-HT(1,2 )receptor antagonist methiothepin, whereas the 5-HT(2 )receptor antagonist ritanserin blocked only the vasoconstrictor responses. 1-[4-(1-Adamantanecarboxamido)butyl]-4-(2-methoxyphenyl)piperazine (a 5-HT(1A) receptor antagonist) and ICI 118,551 (a beta(2)-receptor antagonist) failed to modify the vasodilator responses of m-CPP. Both BRL 15572 (a 5-HT(1D) receptor antagonist) and GR 55562 (a 5-HT(1B) receptor antagonist) only partially inhibited this action. Our data reveal that m-CPP induces the 5-HT(1 )and/or non-specific vasodilator effect and 5-HT(2) vasoconstrictor effects in the hindquarter vascular bed of the rat.

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.

Myogenic responses and compliance of mesenteric and splenic vasculature in the rat

In the rat, the spleen is a major site of fluid efflux out of the blood. By contrast, the mesenteric vasculature serves as a blood reservoir. We proposed that the compliance and myogenic responses of these vascular beds would reflect their different functional demands. Mesenteric and splenic arterioles ( approximately 150-200 microm) and venules (<250 microm) from rats anesthetized with pentobarbital sodium were mounted in a pressurized myograph. Mesenteric arterial diameter decreased from 146 +/- 6 to 133 +/- 6 microm on raising intraluminal pressures from 80 to 120 mmHg. This response was enhanced in the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME; 139 +/- 6 to 112 +/- 7 microm). There was no such myogenic response in the splenic arterioles, except in the presence of l-NAME (194 +/- 4 to 164 +/- 4.2 microm). We propose that, whereas mesenteric arterioles exhibit myogenic responses, this is normally masked by NO-mediated dilation in the splenic vessels. The mesenteric venules were highly distensible (active, 184 +/- 15 to 320 +/- 30.9 microm; passive in Ca(2+)-free media, 209 +/- 31 to 344 +/- 27 microm; 4-8 mmHg) compared with the splenic vessels (active, 169 +/- 11 to 184 +/- 16 microm; passive, 187 +/- 12 to 207 +/- 17 microm). We conclude that, in response to an increase in perfusion pressure, mesenteric arterial diameter would decrease to limit the changes in flow and microvascular pressure. In addition, mesenteric venous capacitance would increase. By contrast, splenic arterial diameter would increase, while there would be little change in venous diameter. This would enhance the increase in intrasplenic microvascular pressure and increase fluid extravasation.

Characterization of the NTPDase activities in the mesentery pre- and post-capillary circuits of the guinea pig

NTPDase is one of the principal enzymes involved in the sequential hydrolysis of ATP. In the present study, the presence and functionality of NTPDase in the mesenteric vein and artery were examined. Adenosine triphosphate (ATP) (0.01-1000 pmol) induces a dose-dependent vasodilation in the isolated arterial and venous mesenteric vasculatures of the guinea pig. Adenosine diphosphate (ADP) (0.01-1000 pmol) but not adenosine monophosphate (AMP) (0.01-1000 pmol) induces a similar response in the mesenteric vascular circuit. L-NAME, a nitric oxide synthase inhibitor (200 microM, 30 min), significantly reduces the arterial dilatory effect of ATP and abolishes the responses to ADP and AMP. Complete removal of the endothelium with 3-[(3-cholamidopropyl) dimethylammonio]-1-propansulfonate (CHAPS) (20 mM, 2 x 45 s) abolishes ATP-induced responses. Infusion of ATP in the vascular circuit generated detectable amounts of ADP and AMP, as measured by HPLC. CHAPS treatment significantly reduced the level of ATP and the production of AMP in the arterial mesenteric circuit. In contrast to the arterial mesenteric vasculature, endothelium removal in the venous circuit triggered a marked potentiation of ADP release and, interestingly, a marked reduction in the release of AMP. Moreover, a specific inhibitor of NTP diphosphohydrolase, 1-hydroxynaphthlene-3,6-disulfonic acid BGO 136 (10 mM for 20 min), significatively reduced AMP production in both vascular preparations. These results confirm that the endothelium contributes to the vasoactive properties of ATP, ADP, and AMP. Our data also demonstrated a significant role of endothelium in NTPDase activity on ADP and AMP production prior to exogenous administration of ATP. The activity of this particular enzyme appears to be different from the reaction products viewpoint (i.e., the production of ADP) in the pre- and post-mesenteric circuits, suggesting two different isoforms with different substrate specificities.

The role of angiotensin II in hypertension due to adenosine receptors blockade

The renin-angiotensin system may be involved in hypertension induced by adenosine receptors blockade with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX). Contractions of the mesenteric vasculature to angiotensin II, noradrenaline and potassium chloride were studied in DPSPX-induced hypertension. Male Wistar rats received infusions of saline or DPSPX (90 microg kg(-1) h(-1), i.p.) for 3 or 7 days. Blood pressure was determined by the tail-cuff method. On days 3 or 14, concentration-response curves were obtained on mesenteric arteries and veins. Plasma angiotensin II levels, measured by radioimmunoassay, were higher in DPSPX-hypertensive rats. The maximum contractile effect of angiotensin II was lower in vessels from DPSPX-hypertensive rats while that for noradrenaline was higher. Potassium chloride-induced contractions were larger in veins from DPSPX-hypertensive rats but similar in arteries, when compared with control rats. We conclude that raised angiotensin II levels and altered vascular reactivity are consistent with a renin-angiotensin-mediated hypertension.

Mesenteric vasoconstrictor response to 5-hydroxytryptamine in the in situ blood autoperfused rat mesentery: involvement of 5-HT(2B) and/or 5-HT(2C) receptor activation

Using a number of agonist and antagonist compounds, we attempted to characterize the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT) in the in situ blood perfused rat mesentery. An intra-arterial (i.a.) bolus injection of 5-HT increased mesenteric perfusion pressure in a dose-dependent way but did not change the systemic blood pressure. The selective 5-HT(2) receptor agonists alpha-methyl-5-HT, 1-(3-chlorophenyl)piperazine dihydrochloride (m-CPP) and (+/-)-1-(4-iodo-2, 5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI), caused a local vasoconstrictor effect in the autoperfused vascular mesenteric bed. Intra-arterial injection of 5-carboxamidotryptamine (5-CT) and 1-(m-chlorophenyl)-biguanide (m-CPBG) did not modify the mesenteric perfusion pressure. The vasoconstrictor effect elicited by 5-HT and alpha-methyl-5-HT was significantly decreased by ritanserin and by a selective 5-HT(2B/2C) receptor antagonist, N-3-pyridinyl-3, 5-dihydro-5-methyl-benzo[1,2-b:4,5-b']dipyrrole-1(2H)-carboxamide hydrochloride (SB 206553), but was not modified by prazosin, propranolol, indomethacin or enalapril pretreatment. Our data suggest that the vasoconstrictor serotonergic response induced in the in situ autoperfused rat mesenteric vascular bed is mainly mediated by activation of 5-HT(2B) and/or 5-HT(2C) receptors.

ATP-induced, P2U purinoceptor-mediated constriction of isolated, perfused mesenteric beds of the rat

alpha,beta-Methylene ATP (alpha, beta-mATP), ATP and UTP dose dependently increased the perfusion pressure of rat mesenteric arteries with a potency order of alpha, beta-mATP >> ATP > UTP. In the veins, while alpha, beta-mATP did not affect the pressure, both ATP and UTP equi-potently increased it. The arterial ATP response was attenuated to some degree by suramin (100 microM), but markedly and to a similar extent by pyridoxal-phosphate-6-azophenyl-2',4-disulphonic acid (PPADS 30 microM) and alpha, beta-mATP (100 nmol). The venous response was not affected by PPADS or alpha, beta-mATP, but was slightly attenuated by suramin. Thus, ATP seems to elicit arterial constriction predominantly by stimulating P2X, but venous constriction by stimulating P2U purinoceptors.

Changes in the regulation by endothelium of norepinephrine response in isolated, perfused mesenteric vascular bed of rats at different ages

Age-related changes of norepinephrine response were studied in isolated perfused mesenteric arteries and veins of rats at 4, 32 and 75 weeks of age. Norepinephrine (0.1-100 nmole) significantly and dose-dependently increased perfusion pressure of the arteries, whereas it only slightly changed that of the veins. The arterial response increased, but the venous response decreased with age. Indomethacin at 5 x 10(-6) M did not change the arterial response at any of the ages. The combination of indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) at 5 x 10(-6) M, and endothelium denudation similarly augmented the response in 4-week-old rats, but neither of these affected the responses in 32- and 75-week-old rats. At raised tone by 100 mM potassium chloride, acetylcholine-induced decrease in perfusion pressure and inhibition of the response by L-NAME were attenuated gradually with age. The raised tone in 75-week-old rats was higher than that in either 4- or 32-week-old rats, but the difference disappeared in the presence of L-NAME. These results demonstrate that the arterial portion is predominant in norepinephrine response and the arterial and venous responses change oppositely with age. It is also suggested that neither prostanoids nor endothelial nitric oxide regulate the arterial response in mature animals, while only in 4-week-old rats, nitric oxide may counteractively regulate the response. Additionally, the roles of nitric oxide in the regulation of transient norepinephrine response, in the regulation of the potassium-induced tonic contracture, and in the vasodilation through muscarinic receptor stimulation seem to change somewhat differently with age.

Differences in responses to norepinephrine and adenosine triphosphate in isolated, perfused mesenteric vascular beds between normotensive and spontaneously hypertensive rats

The responses to norepinephrine and adenosine 5' triphosphate (ATP) of isolated, perfused mesenteric vascular beds were compared between spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. Norepinephrine (0.01-100 nmol) dose-dependently increased perfusion pressure in the intact bed and the arteries, but not in the veins. The maximal responses in SHRs were larger than those in WKY rats. ATP (0.1-3,000 nmol) increased perfusion pressure in all preparations. The responses of the intact bed and the veins were larger in SHRs, whereas there was no strain difference in the arteries. Indomethacin (5 x 10(-6) M) enlarged the norepinephrine responses of both strains only in the intact beds and did not affect the ATP responses, except the veins in SHRs, where it was reduced. N(G)-nitro-L-arginine methyl ester (5 x 10(-6) M), in combination with indomethacin, potentiated the responses, except the arterial response to high doses of norepinephrine in SHRs, which was not affected. Endothelium denudation in the arteries produced similar changes to those after the combined treatment. UK14,304-induced and ADPbetaS-induced decreases in perfusion pressure at increased tone were similar between the strains. Thus neither the vasodilation induced by the stimulation of alpha2-adrenoceptors nor of P2y receptors seems to affect the response to norepinephrine or to ATP, respectively. These results demonstrate that the intact mesenteric vascular bed of SHRs shows potentiated responses not only to norepinephrine, but also to ATP, as compared with WKY rats, and that the critical regions for determining the strain differences for norepinephrine are overall arteries, and that for ATP are the vessels downstream from arterioles. In the intact beds, neither regulation by endogenous prostanoids nor that by endothelium-derived relaxing factors (EDRFs) is implicated in the strain difference. However, these two types of regulation differ markedly between different kinds of vessels.

Pharmacology of endothelins in vascular circuits of normal or heterozygous endothelin-A or endothelin-B knockout transgenic mice

Endothelin-1 (ET-1; 0.001-1 nmol) and the ETB receptor agonist IRL-1620 (0.01-1 nmol) induced a dose-dependent vasoconstriction of the arterial and venous mesenteric circuits and of the kidney in normal mice. BQ-123 (10(-7) M) or BQ-788 (10(-7) M) abolished the vasoconstriction induced by ET-1 in the arterial mesenteric and renal vasculatures without affecting that of norepinephrine (NE). In the venous mesenteric vasculature, only BQ-123 reduced the response to ET-1 but not to NE. In other experiments we compared the mesenteric and renal vascular reactivities to ET-1 and IRL-1620 in ETA or ETB heterozygous knockout mice with those of the wild-type strain. We observed a significant reduction in vascular reactivity to ET-1 but not to IRL-1620 in the arterial mesenteric and renal but not the venous mesenteric circuits of ETA knockout mice. In contrast, there was a significant reduction in vascular reactivity to ET-1 and IRL-1620 in the arterial mesenteric and renal circuits of ETB knockout mice. In the venous mesenteric vasculature, only the vasoconstriction induced by IRL-1620 was significantly reduced in the same ETB knockout strain. Our results suggest that, in the mouse, arterial mesenteric and renal vasoconstriction to ET-1 is mediated by both subtypes of ET receptors, whereas venous mesenteric vasoconstriction appears to be mediated uniquely by the ETA receptor subtype. Knockout of only one allele of the ETA or ETB gene appears to be sufficient for reduction of the ET-1 or IRL-1620 vasoconstrictor effects in the mesenteric and renal vascular beds of the mouse.

Elevated plasma endothelin-1 level in streptozotocin-induced diabetic rats and responsiveness of the mesenteric arterial bed to endothelin-1

1. Both the plasma endothelin-1 (ET-1) levels and the plasma glucose levels were markedly elevated in streptozotocin (STZ)-induced diabetic rats. 2. The maximum contractile response of the mesenteric arterial bed to ET-1 was significantly reduced, and the vasodilatation induced by the ET(B)-receptor agonist IRL-1620 in the mesenteric arterial bed was significantly reduced in STZ-induced diabetic rats. 3. ET-1 (10(-8) M) caused a transient vasodilatation followed by a marked vasoconstriction in methoxamine-preconstricted mesenteric arterial beds. The ET-1-induced vasodilatation was significantly larger in beds from diabetic rats than in those from age-matched controls. By contrast, the ET-1-induced vasoconstriction was significantly smaller in STZ-induced diabetic rats than in the controls. 4. Both removal of the endothelium with Triton X-100 and preincubation with BQ-788 (10(-6) M) (ET(B)-receptor antagonist) abolished the ET-1-induced vasodilatation. Preincubation with BQ-485 (10(-6) M) or BQ-123 (3 x 10(-6)) (ET(A)-receptor antagonist) significantly augmented the ET-1-induced vasodilatation in control mesenteric arterial beds, but not that in beds from diabetic rats. 5. These results demonstrate that marked increases not only in plasma glucose, but also in plasma ET-1 occur in STZ-induced diabetic rats. We suggest that the decreased contractile response and the increased vasodilator response of the mesenteric arterial bed to ET-1 may both be due to desensitization of ET(A) receptors, though ET(B) receptors may also be desensitized. This desensitization may result from the elevation of the plasma ET-1 levels seen in STZ-induced diabetic rats.

Pharmacology of kinins in the arterial and venous mesenteric bed of normal and B2 knockout transgenic mice

We have tested the vasoactive effects of kinins in addition to various other endothelium-dependent or independent agonists in the arterial and venous perfused mesenteric circuits of the mouse. Bradykinin (0.1 pmol-100 nmol), but not des-Arg9-bradykinin (10 nmol) induced a dose-dependent vasodilation of the precontracted arterial and venous mesenteric vasculature of the mouse. Furthermore, acetylcholine (2.5 nmol) also induced a marked arterial vasodilation but was without effect on the venous side. Other endothelium-dependent vasodilators, such as platelet-activating factor (PAF) (1 nmol), tachykinin NK1 selective agonist ([Sar9,Met(O2)(l1) ]substance P) (0.5 nmol) and adenosine diphosphate (5 nmol), were without effect on either side of the mesenteric bed of the mouse. The bradykinin B2 receptor selective antagonist (HOE 140) abolished the arterial and venous vasodilation induced by bradykinin without affecting that of acetylcholine or sodium nitroprusside. In addition, the bradykinin B1 receptor antagonist des-Arg9-[Leu8]bradykinin was without effect on the responses induced by bradykinin. A nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) markedly reduced, whereas removal of the endothelium with 3-[3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) abolished dilatation to bradykinin and acetylcholine (arterial side only) without affecting that induced by sodium nitroprusside in the mouse arterial and venous mesenteric circuits. In the same two circuits of transgenic B2 knockout mice, the vasodilatory responses to bradykinin were absent, whereas the arterial circuit still responded to acetylcholine by a L-NAME-sensitive vasodilation. Our results suggest the exclusive contribution of B2 receptors located on the endothelium in the vasodilatory effects of bradykinin in the arterial and venous mesenteric circuits of the mouse.

Roles of NO-synthase and cyclooxygenase in sex- and pregnancy-dependent arterial and venous pressures in the rat

The roles of NO synthase (NOS) and cyclooxygenase on vascular pressures were studied as a function of sex and pregnancy. After anesthesia, mean arterial pressure (MAP) and mean circulatory filling pressure were lower in pregnant rats compared with male and virgin rats, but N(G)-nitro-L-arginine methyl ester (L-NAME; 30 mg/kg) induced similar increases in MAP. Pithing abolished these pressure differences, suggesting a diminished autonomic reflex in pregnancy, and led in pregnant rats to a lower arterial and venous NO modulation. In separately perfused mesenteries, the lower responses to KCI observed in venous beds of female compared with male rats do not involve any dysfunction of NOS activity in the mesenteries isolated from virgin and pregnant rats. The cyclooxygenase pathway is implicated in the KCl-induced responses of vessels taken from male rats and of venous mesentery from pregnant rats. But prostanoids do not share in the acetylcholine (ACh)-induced relaxations in the arterial and venous K+-contracted mesenteric vasculatures isolated from any of the groups of rats.

Effects of endothelin-1 on arterial and venous resistances in anaesthetized rats

The effects of endothelin-1 and vehicle (0.9% NaCl) on mean arterial pressure, heart rate, mean circulatory filling pressure, systemic arterial resistance, cardiac output and venous resistance were studied in four groups of pentobarbitone-anaesthetized rats, either in presence or absence of phentolamine. I.v. bolus injections of endothelin-1 at 0.5, 1 and 2 nmol/kg dose dependently increased mean arterial pressure (22, 34 and 40 mmHg), arterial resistance (33, 93 and 122% over baseline), venous resistance (40, 117 and 143% over baseline) and mean circulatory filling pressure (1.0, 1.7 and 1.8 mmHg), but decreased heart rate (-16, -21 and -17 beats/min) and cardiac output (-6, -28 and -35% below baseline). The vehicle did not significantly alter any of these variables. During the continuous infusion of phentolamine (300 microg/kg per min), endothelin-1 caused similar increases in arterial resistance, venous resistance and mean circulatory filling pressure, similar reduction in cardiac output but significantly greater pressor and bradycardic responses, suggesting that the arterial and venous constrictor effects of endothelin-1 are not due to sympathetic activation and the stimulation of alpha-adrenoceptors. The results show that endothelin-1 raised mean arterial pressure via the increment in systemic arterial resistance, since cardiac output was markedly reduced. This decrease in cardiac output was mediated by increases in arterial as well as venous resistances. The vasoconstrictor and venoconstrictor effects of endothelin-1 were independent of sympathetic tone.

Comparison of the nitric oxide and cyclo-oxygenase pathway in mesenteric resistance vessels of normotensive and spontaneously hypertensive rats

1. The double perfused mesentery was used to compare arterial and venous KCl- and acetylcholine (ACh)-induced responses in tissues taken from normotensive (WKY) and spontaneously hypertensive rats (SHR) in the presence or absence of inhibitors of nitric oxide (NO) synthase (NG-nitro-L-arginine (L-NOARG) and NG-nitro-L-arginine methyl ester (L-NAME)) and cyclo-oxygenase (indomethacin, mefenamic acid). 2. KCl (20 to 120 mM K+) caused concentration-dependent increases in arterial and venous perfusion pressures. The maximal arterial effects were significantly higher in the SHRs than in the WKY, with no differences in the venous pressor responses. 3. L-NAME and L-NOARG (100 microM) had no effect on the basal perfusion pressures in tissues from either WKY or SHRs, and mefenamic acid only induced a significant reduction of the basal perfusion pressures in the venous mesenteric vessels isolated from WKY. 4. L-NAME and L-NOARG (100 microM) potentiated the pressor responses to KCl to the same extent in the venous and arterial beds derived from WKY and SHR, while indomethacin and mefenamic acid (5 microM) only significantly decreased these responses in WKY. 5. Acetylcholine (ACh)-induced relaxations (1 nM to 10 microM) were significantly higher in arterial beds of WKY than in SHR, without differences in the venous relaxant responses. 6. L-NAME (100 microM) inhibited ACh-induced relaxations in arterial and venous beds from both groups of rats. Mefenamic acid was without effect on ACh-induced relaxations in either the arterial or the venous beds from WKY and SHR. 7. In conclusion, the liberation of NO in the perfused mesenteric vasculatures requires an active tone and no dysfunction of NO synthase activity is functionally apparent in the mesenteries isolated from SHRs. The cyclo-oxygenase pathway is only implicated in the KCl-induced responses of tissues derived from WKY, but not in the vasodilatations induced by ACh in either the arterial or the venous vasculatures from WKY and SHR.

Neurotensin induces the release of prostacyclin from human umbilical vein endothelial cells in vitro and increases plasma prostacyclin levels in the rat

Human umbilical vein endothelial cells express high affinity neurotensin receptors which are coupled to phosphoinositide turnover and 45Ca2+ efflux (Schaeffer et al., 1995. J. Biol. Chem. 270, 3409-3413). In order to assess the physiological significance of neurotensin receptor activation in endothelial cells, we have compared the in vitro effect of neurotensin on prostacyclin release and cytosolic free calcium increase ([Ca2+]i) as determined by fura-2 fluorescence experiments to the in vivo effect of neurotensin on blood pressure and haematocrit. Neurotensin increased [Ca2+]i levels at low concentrations (EC50 = 4.2 +/- 0.2 nM, n = 3). At similar concentrations, neurotensin was also able to induce prostacyclin release from human umbilical vein endothelial cells (EC50 = 14 +/- 1 nM, n = 3) as determined by a 6-keto-prostaglandin F1 alpha enzyme immunoassay. The neurotensin (100 nM)-induced [Ca2+]i increase and prostacyclin release were inhibited by the specific non-peptide neurotensin receptor antagonist SR 48692 at similar concentrations (IC50 = 41 +/- 16 nM and 86 +/- 17 nM, respectively, n = 3), confirming that these responses were mediated by high affinity neurotensin receptors. Intravenous injection of neurotensin (1-4 nmol/kg i.v.) in the rat resulted in a drop of blood pressure and increased haematocrit, and nearly doubled the plasma levels of 6-keto-prostaglandin F1 alpha, the stable metabolite of prostacyclin. Whereas indomethacin (10 mg/kg i.v.) pretreatment significantly reduced the effect of neurotensin on blood pressure, it did not alter its effect on haematocrit. These results suggest that prostacyclin release plays a role in the hypotensive effects of neurotensin, but is not involved in its effects on haematocrit.

Role of ATP and related purine compounds on urethral relaxation in male rabbits

BACKGROUND

Non-adrenergic, non-cholinergic (NANC) relaxation evoked by electrical field stimulation (EFS) has been observed in the urethra, with nitric oxide (NO) considered the agent most probably mediating this effect. However, Burnstock's purinergic hypothesis suggests that ATP and related purine compounds are neurotransmitters in NANC relaxation, although the physiological and pharmacological effects of ATP and related purine compounds in the urethra have been little studied.

METHODS

The effects of ATP and related purine compounds, NG-nitro-L-arginine (NOARG; an inhibitor of nitric oxide synthesis from L-arginine), calcitonin gene-related peptide (CGRP), substance P and vasoactive intestinal polypeptide (VIP) on relaxation and smooth muscle tension induced by electrical field stimulation (EFS) were studied in isolated male rabbit circular urethral smooth muscle (functional study). In addition, the outflow of ATP elicited by EFS was measured using the luciferase technique (superfusion study). All experiments were performed in the presence of guanethidine (3 x 10(-3) mol/L) and atropine (10(-6) mol/L).

RESULTS

In preparations contracted with U46619, a prostaglandin peroxidase inhibitor, ATP had almost no effect on EFS-induced relaxation; however, suramin, a non-selective P2Y-purinoceptor antagonist, and NOARG each markedly attenuated this relaxation in a concentration-dependent manner. ATP and related purine compounds (adenosine, AMP and ADP) each reduced U46619-induced tonic contraction in a concentration-dependent manner. The potencies of the relaxant effects of ATP and these purine compounds were almost the same. In preparations contracted with U46619, CGRP and substance P had no effect on tonic contraction, but VIP reduced tonic contraction in a concentration-dependent manner. In the superfusion study, the outflow of ATP into the superfusate was markedly increased by EFS. When NOARG or prazosin was added to the superfusate, the increase in outflow of ATP was unchanged, but when suramin was added to the superfusate, no increase in outflow of ATP was observed.

CONCLUSIONS

These findings suggest that P2Y-purinoceptors exist in the male rabbit urethra, and that ATP and related purine compounds may play a role in non-adrenergic, non-cholinergic neurotransmission. Consequently, the pathways mediating urethral relaxation by ATP, NO and VIP may be different.

Fluoroaluminate induces rapid release of endothelin-1 in the isolated perfused arterial and venous vessels of the rat mesentery

1. Endothelin-1 (ET-1) production from endothelial cells is generally believed to be a process that happens over the course of hours. 2. When fluoroaluminate (AIF-4) was infused in the isolated perfused arterial and venous vessels of the rat mesentery there was an increase in perfusion pressure on both sides. 3. Treatment of mesentery with the endothelin receptor antagonists FR 139317 (ETA receptor selective) or PD 145065 (ETA-ETB receptor nonselective) caused inhibition on both the arterial and venous sides, suggesting that response is mediated predominantly by endothelin-1 through ETA receptors. 4. Endothelial denudation attenuated changes in perfusion pressure of mesenteric circulation generated by fluoroaluminate, but not those caused by exogenously added PGF2 alpha. 5. Our data demonstrate that there is an immediate release of endothelin-1 following fluoroaluminate infusion which could be partially mediated by activation of phospholipase C.

Age-related differences and roles of endothelial nitric oxide and prostanoids in angiotensin II responses of isolated, perfused mesenteric arteries and veins of rats

We examined whether or not cyclo-oxygenase products of arachidonic acid and endothelium-derived relaxing factor (nitric oxide, NO) regulate the vascular response to angiotensin II differently with aging or development. For this purpose angiotensin II responses of isolated, perfused rat mesenteric vascular beds were compared between rats aged 4 weeks and 32 weeks. Angiotensin II increased perfusion pressure in arteries and veins of both rats aged 4 weeks and 32 weeks. In the arteries of rats aged 32 weeks the increase was slight, and less than that in rats aged 4 weeks. In contrast, the veins showed similar increases in perfusion pressure in rats aged 4 weeks and 32 weeks. Indomethacin, an inhibitor of cyclo-oxygenase, at 5 x 10(-6) M depressed the increase in perfusion pressure only in the arteries of rats aged 32 weeks. NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, applied at 5 x 10(-6) M in the presence of indomethacin enlarged the perfusion pressure increase in the arteries of both rats aged 4 weeks and 32 weeks, while it failed to modify that in the veins. After removal of the endothelium from the blood vessels, the perfusion pressure responses in arteries were increased in both rats aged 4 weeks and 32 weeks, whereas those in veins were not affected. Regardless of the endothelium being intact or removed, the increase in arterial perfusion pressure of rats aged 32 weeks all but disappeared with 5 x 10(-6) M furegrelate, an inhibitor of thromboxane A2 synthase, and with a combined application of furegrelate and 10(-6) M SQ29,548, a blocker of thromboxane A2/prostaglandin H2 receptors. These results indicate the following: in rat mesenteric vascular beds the angiotensin II response in the arteries appears to diminish with aging or development, whereas that in the veins does not change. The NO released from the endothelium regulates the arterial response but vasodilating prostanoids have no role in the response. Moreover, in the arteries of rats aged 32 weeks, vasoconstricting prostanoids, such as prostaglandin H2 and thromboxane A2, seem to play a role in angiotensin II-induced vasoconstriction. With aging or development, and depending on the type of blood vessel, NO and prostanoids appear to modify the angiotensin II response differently.

Thromboxane A2 analogue contracts predominantly the hepatic veins in isolated canine liver

Thromboxane A2 (TxA2) is a potent vasoconstrictor and has been implicated as a mediator of liver diseases such as ischemic-reperfusion injury. We determined the effects of TxA2 and the well-known hepatic venoconstrictor histamine, on the vascular resistance distribution and liver weight in isolated canine livers perfused with blood via the portal vein. The stable TxA2 (STA2; 20 micrograms, n = 5) and histamine (5 micrograms, n = 6) similarly increased the hepatic total vascular resistance, 2.5- and 2.4-fold, respectively. The increase in the hepatic venous resistance was significantly greater than that of the portal resistance (threefold vs. 1.9-fold for STA2; threefold vs. 1.8-fold for histamine). Predominant hepatic venoconstriction induced by both agents was confirmed in livers perfused in a reverse direction from the hepatic vein to the portal vein, as shown by marked precapillary vasoconstriction. STA2 transiently increased liver weight loss (-3.6 g/100 g liver weight), followed by a gradual weight gain (9.0 g/100 g). Histamine caused a progressive weight gain (9.1 g/100 g). In conclusion, similar to histamine, TxA2 constricts predominantly the hepatic vein in isolated canine livers.

Mechanisms underlying the vascular activity of beta-amyloid protein fragment (beta A(4)25-35) at the level of skin microvasculature

Deposition of beta-amyloid protein (beta A4) in extracellular senile plaques is a pathologic hallmark of Alzheimer's disease (AD). The neurotoxic effect of beta A4 has been ascribed to a discrete 11-amino acid internal sequence (beta A(4)25-35). Substance P (SP) has been found to be depleted in the brain of AD patients while its presence was found to protect against the neurodegenerative effect of beta A(4)25-35. Our previous studies, in vivo, in aged rats showed that beta A(4)25-35 exhibits a potent vasoconstrictor (VC) effect in rat skin microvasculature and can prevent SP but not calcitonin gene-related peptide (CGRP) from inducing a vasodilator (VD) response. It was postulated that beta A(4)25-35 might be interacting with SP at the level of the second messenger system via the phosphoinositide pathway. Using a blister model of inflammation in the rat hind footpad, we examined the ability of beta A(4)25-35 to modulate the vascular activity of bradykinin (BK) and serotonin (5-HT) which also activate the phosphoinositide pathway. In addition, the role of nitric oxide (NO), endothelin (ET, an endothelium-derived constrictor factor) and protein kinase C (PKC) in the vascular effects of beta A(4)25-35 were examined using the NO synthase inhibitor, NG-nitro-L-arginine (L-NOARG), the ET-receptor antagonist, BQ-123, and the PKC inhibitor, bisindolylmaleimide (BIM) respectively. Changes in microvascular blood flow were monitored using laser Doppler flowmetry and the area within the response curve measured. The results showed that beta A(4)25-35 (10 microM) induced a VC effect and inhibited the subsequent VD response to BK (10 microM) and 5-HT (1 microM) in a similar fashion to its effect on SP (1 microM). In the presence of L-NOARG (100 microM), the VD effect of SP was reduced and further attenuated after perfusion of beta A(4)25-35. Superfusion of the blister base with BQ-123 (10 microM) or BIM (1 microM) prior to and during perfusion with beta A(4)25-35 abolished its VC effect and allowed SP to induce a normal VD response in both young and old rats. Based on these results, we suggest that the vascular activity of the active fragment, beta A(4)25-35, is mediated by ET via activation of PKC. This study provides new findings which may help to elucidate the signal transduction mechanisms involved in the vascular activity of beta A(4)25-35. The relevance of these mechanisms to those underlying the pathological effects of beta A4 and their significance in AD remains to be determined.

Altered reactivity of the inferior vena cava to noradrenaline and acetylcholine following the blockade of EDRF-biosynthesis with NG-nitro-L-arginine methyl ester

1. Endothelium-derived relaxing factor (EDRF) has been shown to influence arterial tone, but controversial results have been obtained studying veins. The present study was performed to determine the importance of EDRF for the inferior vena cava in the rabbit and whether blockade of the synthesis of EDRF with NG-nitro-L-arginine methyl ester may influence the reactivity of the inferior vena cava to noradrenaline. 2. The inferior vena cava was excised in six New Zealand white rabbits and 12 rings were prepared for organ bath studies. Concentration-response curves were constructed for acetylcholine (10(-9)-10(-4) mol/L) and noradrenaline (10(-9)-10(-4) mol/L) before and following the administration of NG-nitro-L-arginine methyl ester. 3. All rings showed concentration-dependent relaxation to acetylcholine (mean maximum: 57 +/- 9%) following precontraction with noradrenaline (EC50:10(-6) mol/L). Following NG-nitro-L-arginine methyl ester, this dilation was significantly reduced to a mean maximum relaxation of 13 +/- 6% (P < 0.01). 4. Contraction of the inferior vena cava to increasing doses of noradrenaline reached a maximum of 5.8 +/- 2.8 g before NG-nitro-L-arginine methyl ester (basal tension 1.0 +/- 0.5 g). NG-nitro-L-arginine methyl ester did not affect basal tension, but the constrictor response to noradrenaline was enhanced significantly to a maximum of 9.1 +/- 3.8 g (P < 0.01). 5. Although it cannot be ascertained definitively from the present results, it is suggested that EDRF is mediating vasodilation of the inferior vena cava and that this vasoactive agent may also contributes significantly to the modulation of the reactivity to catecholamines.

Endothelial dysfunction in diabetes mellitus

In the present study, we examined the pattern of Evan's blue (EB) extravasation over time and we verified the effect of two inhibitors of aldose reductase (sorbinil and ARI 509) as well as aminoguanidine, which modulate nitric oxide (NO) production, on streptozotocin-induced capillary extravasation abnormalities in the upper bronchi, heart, kidney, duodenum, pancreas, skeletal muscle and skin. Albumin extravasation was measured using the EB technique (20 mg/kg). On the third day, a transient decrease in EB leakage was observed in the lung (-49%), heart (-29%) and skeletal muscle (-64%). These early changes in EB were transient, and values returned to normal there after. Later on, EB extravasation was significantly enhanced in the skin (+358, +680, +580, +525 and +365, respectively, at 2, 4, 5, 6 and 7 weeks of diabetes), the duodenum (+101, +160, +92, +124 and +76%), the upper bronchus (+70, +113, +70, +41 and +25%) and the pancreas (+43, +102, +46, +15 and +78%). In the kidney, the increase of EB extravasation was significant at 2 weeks (26%), and from 5 to 7 weeks (+12, +22, +36%). The chronic treatment of diabetic rats with aminoguanidine normalized capillary permeability in most tissues, suggesting that NO is involved in the development of endothelium dysfunction in this streptozotocin-induced diabetic model. Treatment with aldose reductase inhibitors selectively normalized EB extravasation in the kidney.

Distribution of endothelin-1-receptor subtypes in rat portal vein

Endothelin-1 (ET-1), a potent vasoactive peptide, was first isolated from cultured porcine endothelial cells. Subsequent studies revealed the existence of two additional related peptides, ET-2 and ET-3, and at least two distinct ET-receptor subtypes, ETA (selective for ET-1) and ETB (nonselective for ET isopeptides). These isopeptides and receptors are widely distributed in many tissues and are involved in numerous biological responses. The aim of this study was to identify the eventual distribution of the two distinct endothelin-receptor subtypes in isolated endothelium-denuded rat portal vein rings (PVRs) and strips (PVSs). BQ-123 (0.6, 1, and 6 microM) and PD-145065 (0.06, 0.1, 0.6, and 6 microM) were used to differentiate the subtypes because they are selective antagonists for ETA and nonselective antagonists for ETA-ETB receptors, respectively. To characterize the ET receptors further, sarafotoxin-S6c (a selective ETB-receptor agonist) and IRL-1038 (a selective ETB-receptor antagonist) were used. In PVRs, cumulative additions of ET-1 (0.1-100 nM) caused graded and slow contractions and potentiated spontaneous rhythmic contractions. The EC50 values and maximal response to 100 nM of ET-1 were 2.72 nM and 0.75 g, respectively (n = 7). PVSs showed ET-1 EC50 values very similar to those of PVRs, but Emax values to 100 nM of ET-1 were significantly lower (Emax = 0.33 g; n = 7). Moreover, ET-1 clearly increased the amplitude and frequency of spontaneous contractions in both types of specimens, although these were greater in the PVSs. Thirty-minute incubation with the selective ETA-receptor antagonist BQ-123 blunted ET-1-induced effects in PVS specimens but only weakly antagonized ET-1-induced contractions in PVRs. In contrast, the nonselective ETA-ETB-receptor antagonist PD-145065 significantly shifted the ET-1 concentration-response curve to the right in PVRs and partially inhibited ET-1 effects in PVSs. Moreover, sarafotoxin-S6c (0.1-100 nM) contracted PVRs and PVSs in a similar manner to ET-1; its effects were antagonized by IRL-1038 only at the PVR level. The differences observed in PVR and PVS specimens in response to agonists and antagonists of ET confirmed the great heterogeneity of endothelin-sarafotoxin receptors. In our experimental models, functionally ETB-like (or non-ETA) receptors seem mostly to mediate vasoconstriction.

Effects of the ETA/ETB receptor antagonist, bosentan on endothelin-1-induced myocardial ischaemia and oedema in the rat

1. The purpose of this study were to assess the role of ETB receptors in mediating endothelin-1 (ET-1)-induced myocardial ischaemia and oedema in rats and to study the inhibitory action of the novel nonpeptide ETA/ETB receptor antagonist, bosentan on these actions of ET-1. 2. Intravenous bolus injection of ET-1 (1 nmol kg-1) into anaesthetized rats produced marked ST segment elevation of the electrocardiogram without causing arrhythmias. ST segment elevation developed within 30-50 s and persisted for at least 30 min following injection of the peptide. 3. Pretreatment of the animals with bosentan (10 mg kg-1, i.v.) inhibited on average by 96% the ST segment elevation elicited by ET-1 (1 nmol kg-1) compared to the 82% inhibition observed with the ETA receptor-selective antagonist, FR 139317 (2.5 mg kg-1, i.v.). 4. Bolus injection of ET-1 (1 nmol kg-1, i.v.) to conscious chronically catheterized rats evoked a transient depressor response followed by a prolonged pressor effect. Corresponding to changes in blood pressure, a transient tachycardia and a sustained bradycardia were observed. ET-1 (1 nmol kg-1) enhanced albumin extravasation by 119 and 93% in the left ventricle and right atrium, respectively, as measured by the local extravascular accumulation of Evans blue dye. 5. Pretreatment of the animals with bosentan (10 mg kg-1) inhibited by 71 and 90% the depressor and pressor actions of ET-1 (1 nmol kg-1) and the accompanying tachycardia and bradycardia, respectively. FR 139317 (2.5 mg kg-1) attenuated the pressor response to ET-1 and accompanying bradycardia by 75%, without affecting the depressor action and accompanying tachycardia. ET-1-induced albumin extravasation was completely inhibited by bosentan (10 mg kg-1) both in the left ventricle and right atrium, compared to the 86% inhibition observed with FR 139317 (2.5 mg kg-1).6. Like ET-1, the ETB receptor-selective agonist, IRL 1620 (0.3 and 1 nmol kg-1, i.v.) also produced dose-dependent ST segment elevation in anaesthetized rats and enhanced albumin extravasation (up to141% of control) in the left ventricle and right atrium, respectively, in conscious rats. These effects ofIRL 1620 were completely prevented by bosentan (10 mg kg-1).7. These results indicate that ETB receptors, albeit to a lesser extent than ETA receptors, are also involved in mediating ET-1-induced myocardial ischaemia and oedema in the rat, and suggest the therapeutic potential for bosentan in the treatment of ischaemic myocardial diseases.

Characterization of receptors for kinins and neurokinins in the arterial and venous mesenteric vasculatures of the guinea-pig

1. In the present work, we have studied the microvascular reactivity of the arterial and venous mesenteric beds of the guinea-pig to bradykinin, neurokinins and other agents. 2. The vasoactive properties of three selective agonists for neurokinin receptors, namely [Sar9, Met (O2)11]SP (NK1), [beta-Ala8]NKA(4-10) (NK2) and [MePhe7]NKB (NK3), were evaluated on precontracted arterial and venous mesenteric vasculatures of the guinea-pig. The NK1-selective agonist, [Sar9,Met(O2)11]SP (1 to 1000 pmol), induced an endothelium-dependent and N omega-nitro-L-arginine methyl ester (L-NAME)-sensitive relaxation of the arterial vasculature precontracted with methoxamine, whereas the NK2 and NK3-selective agonists were virtually inactive at high doses (1000 pmol). 3. The three selective neurokinin receptor agonists were inactive in the non-precontracted arterial and venous mesenteric vasculatures as well as in the precontracted venous mesenteric vasculature. 4. Bradykinin (0.1 to 100 pmol) induced a marked dose- and endothelium-dependent vasodilatation of the precontracted arterial and venous vasculatures. ED50 values were 5.5 pmol on the arterial side and 1.9 pmol on the venous side. In contrast, desArg9-bradykinin was inactive at doses up to 1000 pmol. Furthermore, on the arterial and venous sides, a higher dose of bradykinin (1000 pmol), induced a biphasic effect, a transient constriction followed by a marked and sustained vasodilatation. The vasodilator effects of bradykinin were abolished by Hoe 140 (0.1 microM) and CHAPS, markedly reduced by L-NAME and were unaffected by [Leu8]desArg9-bradykinin (0.1 microM) on both sides of the mesenteric vasculature. Hoe 140 also abolished the arterial vasoconstrictions induced by high doses of bradykinin. 5. Noradrenaline, angiotensin II and endothelin-1 produced contractions on both sides of the mesenteric circulation, while acetylcholine (arterial side) and sodium nitroprusside (arterial and venous sides) caused vasodilatation.6. Our study supports the view that NK1 receptors responsible for vasodilatation are present solely in the endothelium of the arterial mesenteric vasculature of the guinea-pig. On the other hand, bradykinin(0.1 to 100 pmol) exerts predominantly vasodilator effects on both sides of the mesenteric vasculature via selective activation of B2 receptors located on the endothelium. The same receptor type located on the smooth muscle appears to be responsible for the arterial and venous constriction with high doses of bradykinin.

Rat adrenomedullin induces a selective arterial vasodilation via CGRP1 receptors in the double-perfused mesenteric bed of the rat

The present work was undertaken to study the effect of rat adrenomedullin (rADM (1-50) and its C-terminal fragment (11-50)) in the endothelium-intact arterial and venous vasculatures of the rat perfused mesenteric bed. rADM (1-50) and the fragment rADM (11-50)(1-1000 pmol) induced a dose-dependent and endothelium-independent vasodilation on the arterial mesenteric vasculature. However, both peptides were inactive on the venous side of this vascular bed. The CGRP1 receptor antagonist, hCGRP8-37 (1 microM), markedly reduced the vasodilation caused by rADM (1-50) in the arterial mesenteric vasculature. Thus, our results show that rADM (1-50) in the arterial mesenteric vasculature. Thus, our results show that rADM (1-50) and its C-terminal fragment rADM(11-50) share properties similar to those of hCGRP. The blocking effect of hCGRP8-37 supports a role for CGRP1 receptor activation by adrenomedullin in this vascular preparation.

Localization of atrial natriuretic factor receptors in the mesenteric arterial bed. Comparison with angiotensin II and endothelin receptors

Although receptors for atrial natriuretic factor (ANF) and angiotensin II (Ang II) have been reported in rat mesenteric arteries, both peptides induce weak biological responses. Endothelin-1 (ET-1) evokes a potent vasoconstriction in the mesenteric artery. To identify the tissue localization of ANF, Ang II, and ET-1 receptors, radioligand binding experiments with 125I-ANF, 125I-[Sar1,Ile8]Ang II, and 125I-ET-1 were performed in defatted mesenteric arteries and in the surrounding adipose tissue. 125I-ANF binding assays in adipose tissue showed a single class of high-affinity binding sites (Bmax, 420 +/- 16 fmol/mg protein; Kd, 343 +/- 16 pmol/L). In vascular membranes, most 125I-ANF binding was nonspecific. The majority of receptors present in adipose tissue recognized ANF, C-type natriuretic peptide (CNP), and des-[Gln18,Ser19,Gly20,Leu21,Gly22]ANF-(4- 23) (C-ANF) with close affinities, with C-ANF competing for > 98% of the binding sites. In adipocytes, ANF and CNP stimulated cGMP generation. cGMP production by mesenteric arteries was stimulated by sodium nitroprusside but not by ANF or CNP. Autoradiographic localization of 125I-ANF and 125I-ET-1 showed that in the case of ANF, most specific binding occurred in adipocytes, whereas for ET-1, specific binding was present in both adipose tissue and mesenteric arteries. Cross-linking of 125I-ANF followed by SDS-PAGE revealed two receptor species of 130 and 70 kD in adipose membranes and none in vascular tissue. Both were completely displaced by ANF, CNP, and C-ANF. 125I-[Sar1,Ile8]Ang II binding assays in adipose tissue exhibited a single class of binding sites (Bmax, 211 +/- 4 fmol/mg protein; Kd, 520 +/- 10 pmol/L.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute pro-inflammatory actions of endothelin-1 in the guinea-pig lung: involvement of ETA and ETB receptors

1. Although recent observations suggest that endothelin-1 (ET-1) may play a role in the pathogenesis of asthma, to date little is known about the effects of ET-1 on parameters other than bronchoconstriction. The objectives of the present experiments were to study whether intravenously administered ET-1 could exert pro-inflammatory actions in the guinea-pig lung and to assess the involvement of endothelin ETA and ETB receptors in these events by using the ETA receptor-selective antagonist, FR 139317, the novel ETA/ETB receptor antagonist, bosentan and the ETB receptor-selective agonist, IRL 1620. 2. Bolus i.v. injection of ET-1 (0.1-1 nmol kg-1) to anaesthetized guinea-pigs evoked dose-dependent increases in mean arterial blood pressure which lasted for 6-12 min. This was accompanied by a dose-dependent haemoconcentration (8-15% plasma volume losses) and increases (up to 546%) in albumin extravasation in the trachea, upper and lower bronchi, but not in the pulmonary parenchyma. Qualitatively similar changes were observed following i.v. injection of the ETB receptor agonist, IRL 1620 (0.3 and 1 nmol kg-1), although IRL 1620 appeared to be about 3 times less potent than ET-1. The ETA receptor-selective antagonist, FR 139317 (2.5 mg kg-1) inhibited the ET-1 (1 nmol kg-1)-induced pressor response, haemoconcentration and albumin extravasation by 75, 77 and 60-70%, respectively, whereas it did not attenuate IRL 1620 (1 nmol kg-1)-induced changes. The ETA/ETB receptor antagonist, bosentan (10 mg kg-1) almost completely inhibited the pressor, haemoconcentration and permeability effects of both ET-1 and IRL 1620. 3. ET-1, but not IRL 1620 (0.1-1 nmol kg-1), produced a dose-dependent neutropenia with relative lymphocytosis and monocytosis, but did not induce influx of neutrophil granulocytes into pulmonary tissues or the bronchoalveolar space. ET-1 (1 nmol kg-1)-induced neutropenia was prevented by pretreatment of the animals with FR 139317 (2.5 mg kg-1), bosentan (10 mg kg-1) or adrenaline (90 nmol kg-1), indicating that ET-1 caused intravascular sequestration of neutrophil granulocytes. 4. ET-1 or IRL 1620 (10(-10)-10(-6) M) alone did not activate alveolar macrophages in vitro, whereas at a concentration of 10(-8) M, ET-1, but not IRL 1620, markedly potentiated superoxide production in response to f-Met-Leu-Phe (10(-9)-10(-7) M) and platelet-activating factor (PAF, 10(-9)-10(-7) M), but not to phorbol 12-myristate 13-acetate (10(-9) M). ET-1 did not affect f-Met-Leu-Phe- or PAF-induced increases in intracellular free calcium concentration. This potentiating effect of ET-1 was abolished by FR 139317(1.5 X 10-7 M).5. We conclude that, in addition to evoking airway contractions, ET-1 exerts pro-inflammatory actions via activation of the ETA and to a lesser extent the ETB receptors, and therefore, might contribute to the airway inflammation present in asthma. These findings also suggest the therapeutic potential of ETA/ETB receptor and perhaps ETA receptor-selective antagonists in this disease.

Rapid degradation of endothelin-1 by an enzyme released by the rat isolated perfused mesentery

1. In vivo the effects of endothelin-1 (ET-1) are limited by its rapid removal from the circulation and possibly by its metabolism by enzymes such as neutral endopeptidase 24.11, deamidase or carboxypeptidase A. Here, using as a model the isolated perfused mesenteric arterial bed of the rat, we have examined the involvements of these enzymatic activities in the vascular responses to ET-1. 2. Samples of Krebs buffer which had been recirculated through the mesenteric arterial bed for 30 min rapidly destroyed the activity of ET-1 as assessed either by bioassay on rings of rat thoracic aorta or by high performance liquid chromatography (h.p.l.c.). For instance, after 15 min incubation with the recirculated-Krebs solution (recirc-K) the contraction induced by 3 x 10(-9) M ET-1 was reduced by more than 90%. Contractions induced by sarafotoxin 6b (3 x 10(-9) M) were similarly suppressed by preincubation with recirc-K whereas those to Arg-vasopressin (3 x 10(-9) M) were unaffected. 3. The degradation of ET-1 by recirc-K was prevented by 1,10-phenanthroline (10(-3) M), abolished by heating the recirc-K solution to 90 degrees C for 15 min, and reduced by EGTA (5 x 10(-3) M) or ET-1(16-21) (10(-5) M). For instance, in the presence of ET-1(16-21) (n = 6) the contraction induced by ET-1 was reduced by only 40% after 15 min incubation with recirc-K buffer. Leupeptin (3 x 10-4 M), dichloroisocoumarin(5 x 10-5 M), phenylmethyl-sulphonyl fluoride (10-3 M), a combination of bacitracin (300 mg ml-1),bestatin (10-5 M), captopril (10-5 M), phosphoramidon (10-4 M) and thiorphan (10-4 M) or Polypep (aproprietary protein digest) did not inhibit the degradation of ET-1 by recirc-K.4. In experiments examining directly the vascular responses of the isolated perfused mesentery of the rat, the addition of cumulative concentrations of ET-1 to the recirculating Krebs solution caused small concentration-dependent increases in perfusion pressure. The inclusion of ET-1(16-2l), ET-1(17-21), or ET-1(18-21) (10-5M) greatly potentiated these responses, but not those to Arg-vasopressin or methoxamine.The effects of 1,10-phenanthroline or EGTA could not be examined in this system because these agents both depressed non-specifically the vasoconstrictor responses of the mesenteric vascular bed.5. Thus, the rat mesentery releases an enzyme that very rapidly destroys ET-1 or the very closely related peptide, sarafotoxin 6b but not Arg-vasopressin. This enzyme is most probably a metallopeptidase because of its sensitivity to inhibition by 1,10-phenanthroline or EGTA. It is particularly interesting that a simple vascular bed such as the mesentery produces such a powerful endothelin metabolising enzyme. It is tempting, therefore, to speculate that the endothelin degrading enzyme active at neutral pH that- we have found is important in the metabolism of ET-1 throughout the vasculature.

Autoradiographic localization of NK1 and NK3 tachykinin receptors in rat kidney

The distribution of neurokinin receptors in rat kidney, renal artery, renal vein, and proximal ureter was evaluated by autoradiography after in vitro labeling of NK1 sites with [125I]Bolton-Hunter substance P (BHSP) or NK3 sites with [125I][MePhe7]neurokinin B ([MePhe7]NKB). Film autoradiography using [125I][MePhe7]NKB revealed specific binding sites associated with the renal vein and its large branches, the renal pelvis, the inner strip of outer renal medulla, and the proximal ureter. High-resolution autoradiograms demonstrated that these sites were localized to the smooth muscle layer in the veins, pelvis, and ureter. Neither the renal arterial system nor the renal cortex contained specific [125I][MePhe7]NKB binding sites although a high level of nonspecific binding was associated with the renal artery. Specific binding of [125I]BHSP was associated with the renal artery and renal pelvis but not the renal veins. Arterial NK1 receptors appeared to be localized to the adventitia. The results indicate that at least two types of tachykinin receptor are present in the rat kidney. The distinct localization observed for most of the NK1 and NK3 receptors suggests that they have different functions.

Characterization of receptors mediating vascular responses to endothelin-1 in the conscious rat

1. The present study has determined the receptors mediating the vascular responses (pressor and depressor actions and vascular permeability effect) to endothelin-1 (ET-1) in the conscious rat by using the novel non-peptide ETA/ETB receptor antagonist, bosentan (Ro 47-0203, 4-tert-butyl-N-[6-(2 hydroxyethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidine- 4-yl]benzene-sulphonamide), the ETA receptor-selective antagonist, FR 139317 and the ETB receptor-selective peptide agonist, IRL 1620. 2. Bolus injection of ET-1 (1 nmol kg-1, i.v.) resulted in a prolonged pressor effect (maximum increase in mean arterial blood pressure (MABP) was 47 +/- 3 mmHg, n = 6) preceded by a transient depressor response (maximum decrease in MABP was 17 +/- 1 mmHg). Both these responses were inhibited by bosentan (1-20 mg kg-1, i.v. bolus) in a dose-dependent manner. The maximum inhibition of ET-induced depressor and pressor responses did not exceed 53 and 87%, respectively. FR 139317 (2.5 mg kg-1, i.v.) attenuated the pressor response to ET-1 by 75% without affecting the depressor response. Furthermore, FR 139317, but not bosentan, prolonged the depressor action of ET-1. Corresponding to changes in blood pressure, a small transient tachycardia (delta heart rate 15 +/- 5 beats min-1) followed by a sustained bradycardia (delta heart rate -48 +/- 10 beats min-1, n = 6) was observed following injection of 1 nmol kg-1 ET-1. FR 139317 and bosentan (10 mg kg-1) inhibited ET-1-induced bradycardia by 79% and 71%, respectively.ET-l-induced tachycardia was significantly attenuated by bosentan,but not FR 139317.3. The ETB receptor agonist, IRL 1620 (0.1-2 micro molkg-1, i.v.) produced biphasic dose-dependent changes in MABP with an initial transient fall followed by a prolonged pressor action. The maximum decrease and increase in MABP were 11 +/- 2 and 19 +/- 3 mmHg, respectively (n = 5). These changes in MABP were accompanied by a transient tachycardia (Delta heart rate 9+/- 3 beats min-1) and prolonged bradycardia (Delta heart rate -17+/-11 beats min-1), respectively. Pretreatment of the animals with FR139317 (2.5 mg kg-1, i.v.) did not affect IRL 1620 (1 nmol kg-1)-induced changes in MABP and heart rate, whereas both the depressor and pressor actions of IRL 1620 and the accompanying tachycardia and bradycardia were almost completely inhibited by bosentan (10mgkg-1).4. ET-1 (1 nmol kg-1) enhanced albumin extravasation in the upper and lower bronchi, spleen, kidney,stomach and duodenum (up to 246%) as measured by the extravasation of Evans blue dye. FR 139317(2.5mgkg-1) completely inhibited ET-l-induced protein extravasation in the stomach and duodenum,whereas 40-75% inhibition was observed in the other vascular beds studied. The permeability effect of ET-l was almost completely inhibited by bosentan (10mgkg-1) in all vascular beds studied.5. IRL 1620 (0.4 or 1 nmol kg-1, i.v.) enhanced albumin extravasation (up to 219%) in the upper and lower bronchi, spleen and kidney in a dose-dependent manner. Unlike ET-1, IRL 1620 failed to increase albumin extravasation in the stomach and duodenum.6. The present study demonstrates in the conscious rat that ETA and ETB receptors are responsible for mediating the majority of the pressor response to ET-l and suggest that a small component of the ET-l pressor response might be mediated via a non-ETA, non-ETB receptor, whereas ETB and perhaps a non-ETA, non-ETB receptor may mediate the depressor action of ET-1. Furthermore, the ET-1 induced albumin extravasation is mediated solely via ETA receptors in the stomach and duodenum, whereas both ETA and ETB receptors are involved in the permeability effect of ET-l in the bronchial, splenic and renal vascular beds.

Role of R-type calcium channels in the response of the perfused arterial and venous mesenteric vasculature of the rat to platelet-activating factor

1. The vasoactive properties of platelet-activating factor (PAF) were studied in the arterial and venous vasculature of the rat double-perfused mesenteric bed. Although PAF (0.01-0.3 pmol) induced a dose-dependent vasodilatation of the arterial mesenteric vasculature, it triggered only vasoconstrictions on the venous side, with an intact endothelium as bradykinin induced a significant venodilatation. 2. NG-nitro-L-arginine methyl ester (L-NAME, 100 microM), a nitric oxide synthase inhibitor, markedly reduced the vasodilatation induced by PAF in the arterial mesenteric vasculature and potentiated the contractile responses of the venous side to the same agent. 3. The PAF antagonist, WEB-2170, markedly reduced the response to PAF on both sides of the mesenteric vasculature. However, the IC50 of WEB-2170 against PAF was reached at a much higher concentration (1 x 10(-8) M) on the arterial side than on the venous side (5.3 x 10(-11) M). Furthermore, a second antagonist of PAF receptors, SRI-63441, although being less potent on the venous vasculature than WEB-2170, was equipotent in antagonizing the venoconstriction and the arterial dilatation induced by PAF (IC50 of SRI-63441, arterial side: 2.9 x 10(-9) M; venous side: 3.1 x 10(-9) M). 4. The dual L- and R-calcium channel blocker, isradipine (PN 200-110), but not the L-type calcium channel blocker, nifedipine, markedly reduced the PAF-induced vasoactive properties on both sides of the mesenteric vasculature. 5. Our results illustrate the differential vasoactive properties of PAF in the mesenteric vasculature of the rat. These vasoactive responses occur following activation of specific receptors for PAF or,alternatively, through activation of R-type calcium channels.

Effect of PAF on rat lung vascular permeability: role of platelets and polymorphonuclear leucocytes

1. The objectives of the present experiments were to assess the contribution of polymorphonuclear leucocytes (PMNLs), platelets and their products such as thromboxane A2 (TxA2), histamine and 5-hydroxytryptamine to platelet activating factor (PAF)-mediated protein extravasation in rat lungs. 2. Intravenous injection of PAF (1.0 and 5.0 micrograms kg-1) increased dose-dependently (up to 7.5 fold) the vascular permeability of the trachea, upper and lower bronchi to Evans blue dye (EB), a marker of albumin extravasation. The permeability of the pulmonary parenchyma was not affected significantly by PAF. 3. Thrombocytopenia induced by administration of the IgG fraction of goat anti-rat platelet serum (APS; 15 mg 100 g-1, i.p., 16-18 h) reduced by 55, 58 and 40% the effects of the lower dose of PAF (1.0 microgram kg-1) and by 31, 23 and 15% the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi respectively to albumin. 4. PMNL depletion induced by administration of rabbit anti-rat polymorphonuclear serum (ANS; 2 mg kg-1, i.v., 24 h) did not reduce significantly the effects of the lower dose of PAF (1.0 microgram kg-1) on the airways, however the effects of the higher dose of PAF (5.0 micrograms kg-1) on the permeability of the trachea, upper and lower bronchi to albumin were reduced by 43, 25 and 23% respectively. 5. The injection of both the anti-platelet and the anti-PMNL sera reduced by 61, 62 and 96% the effects of the lower dose of PAF (1.0 microg kg-1) and by 44, 39 and 47% the effects of the higher dose of PAF (5.0 microg kg-1) on the permeability of the trachea, upper and lower bronchi respectively.6. The combined injection of the TxA2-mimetic (U-44069; 5.0 microg kg-1) and PAF (1.0 and 5.0 microg kg-1)in thrombocytopenic rats overcame the vascular permeability decrease induced by APS treatment.7. Pretreatment of the animals with a combination of antagonists to histamine (mepyramine;3.0 mg kg-1) and 5-hydroxytryptamine (methysergide; 2.5 mg kg-1) did not cause a significant inhibition of the effect of PAF (1.0 and 5.0 microg kg-1) on EB extravasation in the airways.8. These data show that the effect of intravenous PAF on rat vascular permeability is partly modulated by polymorphonuclear leucocyte and platelet activation. Our results suggest that following its release,TxA2 could increase postcapillary hydrostatic pressure by inducing a venoconstriction and potentiate the extravasation elicited by PAF. These results do not suggest a major role for histamine and/or 5-hydroxytryptamine on PAF-induced albumin extravasation.