Regulation of prostacyclin and prostaglandin E(2) receptor mediated responses in adult rat dorsal root ganglion cells, in vitro

1. Primary cultures of adult rat dorsal root ganglia (DRG) were prepared to examine the properties of prostacyclin (IP) receptors and prostaglandin E(2) (EP) receptors in sensory neurones. 2. IP receptor agonists, cicaprost and iloprost, stimulated adenylyl cyclase activity with EC(50) values of 22 and 28 nM, respectively. Prostaglandin E(1) (PGE(1)) and prostaglandin E(2) (PGE(2)) were 7 fold less potent than cicaprost and iloprost, with PGE(2) displaying a lower maximal response. 3. Adenylyl cyclase activation by iloprost, PGE(1) and PGE(2), but not by forskolin, was highly dependent on DRG cell density. Although the potency of iloprost and PGE(2) for stimulating adenylyl cyclase was unchanged, their maximal responses were significantly increased at low cell density. 4. Both IP and EP(2/4) receptors could be down-regulated by agonist pretreatment, however the presence of cyclo-oxygenase (COX) inhibitors did not prevent this apparent down-regulation of IP and EP(2/4) receptors at high DRG cell densities. 5. Stimulation of adenylyl cyclase by the neuropeptide calcitonin gene-related peptide was also decreased at high DRG cell density, whereas the responses to beta-adrenoceptor agonists were increased at high DRG cell density. 6. Addition of nerve growth factor (NGF), or the addition of anti-neurotrophin antibodies during the 5-day culture of DRG cells, had no effect on IP receptor-mediated responses. 7. These results indicate that G(s)-coupled receptors involved in nociception are regulated in a variable manner in adult rat sensory neurones, and that this cell density-dependent regulation may be agonist-independent for IP and EP(2/4) receptors.

Angiotensin II AT1 receptors preserve vasodilator reactivity in skeletal muscle resistance arteries

Resistance arteries (100-150 microm) were isolated from the gracilis muscle of normotensive Sprague-Dawley rats placed on a high-salt (HS) diet (4.0% NaCl) for 3-7 days. Exposure to the HS diet eliminated vascular relaxation in response to hypoxia (PO2 reduction to 35-40 Torr) and iloprost, a stable analog of prostacyclin. Vasodilator responses were restored in arteries isolated from chronically instrumented HS rats receiving a continuous intravenous infusion of either angiotensin II (ANG II; 5-6 ng x kg(-1) x min(-1)) or ANG II plus the AT2 receptor blocker PD-123319 (5 microg x kg(-1) x min(-1)) for 3 days before the isolated vessel studies. In contrast, coinfusion of the AT1 receptor blocker losartan (20 microg x kg(-1) x min(-1)) or coinfusion of both receptor blockers with ANG II eliminated the protective effect of ANG II to restore dilator responses to hypoxia and iloprost. Neither a HS diet nor ANG II infusion affected the dilation of gracilis arteries in response to direct activation of adenylyl cyclase by forskolin, suggesting that the effect of both the HS diet and the ANG II on the vasculature is mediated upstream from second messenger systems. These findings indicate that the protective effect of ANG II to maintain vasodilator reactivity in resistance arteries of rats on a HS diet is mediated via the AT1 receptor subtype.

Permeability of endothelial monolayers to albumin is increased by bradykinin and inhibited by prostaglandins

Using monolayers of bovine aortic endothelial cells (BAEC) in modified Boyden chambers, we examined the role of prostaglandins (PGs) in the bradykinin (BK)-induced increase of albumin permeability. BK induced a concentration-dependent increase of the permeability of BAEC, which reached 49.9 +/- 1% at the concentration of 10(-8) M. Two inhibitors of the prostaglandin G/H synthase, indomethacin (2.88 microM) and ibuprofen (10 microM), potentiated BK-induced permeability 1.8- and 3.9-fold, respectively. Exogenously administered PGE2 and iloprost, a stable analog of prostacyclin, attenuated the effect of BK in a concentration-dependent manner. Butaprost equally reduced the effect of BK, suggesting the participation of the EP2 receptor in this phenomenon. However, the EP4-selective antagonist AH-23848 did not significantly inhibit the protective effect of PGE2. The inhibitory effect of PGE2 was reversed by the adenylate cyclase inhibitor MDL-12330A (10 microM). These results suggest that BK-induced increase of permeability of BAEC monolayer to (125)I-labeled albumin is negatively regulated by PGs. This postulated autocrine activity of PGs may involve an increase in the intracellular level of cAMP.

Vasodilatation, oxygen delivery and oxygen consumption in rat hindlimb during systemic hypoxia: roles of nitric oxide

We have investigated the relationship between O2 delivery (DO2) and O2 consumption (VO2) in hindlimb muscle of anaesthetised rats during progressive systemic hypoxia. Since muscle vasodilatation that occurs during hypoxia is nitric oxide (NO) dependent, we examined the effects of the NO synthase (NOS) inhibitor nitro-L-arginine methyl ester (L-NAME). In control rats (n = 8), femoral vascular conductance (FVC) increased at each level of hypoxia. Hindlimb DO2 decreased with the severity of hypoxia, but muscle VO2 was maintained until the critical DO2 value (DO2,crit) was reached at 0.64 +/- 0.06 ml O2 min-1 kg-1; below this VO2 declined linearly with DO2. This is a novel finding for the rat but is comparable to the biphasic relationship seen in the dog. In another group of rats (n = 6), L-NAME caused hindlimb vasoconstriction and attenuated the hypoxia-evoked increases in FVC DO2 was so low after L-NAME administration that VO2 was dependent on DO2 at all levels of hypoxia. In a further group (n = 8), femoral blood flow and DO2 were restored after L-NAME by infusion of the NO donor sodium nitroprusside (20 g x kg(-1) x min(-1). Thereafter, hypoxia-evoked increases in FVC were fully restored. Nevertheless, DO2,crit was increased relative to control (0.96 +/- 0.07 ml O2 min(-1) x kg(-1), P < 0.01). As NOS inhibition limited the ability of muscle to maintain VO2 during hypoxia, we propose that hypoxia-induced dilatation of terminal arterioles, which improves tissue O2 distribution, is mediated by NO. However, since the hypoxia-evoked increase in FVC was blocked by L-NAME but restored by the NO donor, we propose that the dilatation of proximal arterioles is dependent on tonic levels of NO, rather than mediated by NO.

[Whole blood flow cytometry for detection of activated platelets. I. Platelet identification, blood collection and storage]

In order to assess platelet activation by flow cytometry and activation-dependent monoclonal antibodies, a minute volume(2.5 microliters) of citrated whole blood was mixed for 15 minutes with a cocktail of monoclonal antibodies(MoAb), including PAC1, (a MoAb specific for fibrinogen receptors), and a MoAb against CD62P, (an alpha-granule membrane protein which associates with platelet surface membranes when platelets are activated). After fixation with 1% formaldehyde, the percentage of platelets positive for PAC1 and/or MoAb-CD62P was measured by flow cytometry. Even in resting platelets without stimulation, about 15% of platelets were found to be PAC1-positive and 0.6% of platelets were CD62P-positive. If there was a one-minute delay after needle puncture and before blood collection, the number of PAC1-positive platelets was increased compared to that of blood obtained immediately after puncture. If blood was allowed to stand at room temperature, there was a gradual increase in the number of PAC1-positive platelets, and after 60 minutes, this increase became statistically significant. The addition of Iloprost, (a prostacyclin analogue), immediately after venipuncture did not completely prevent the increase in PAC1-positive platelets after 60 minutes. If the inhibitor was added after first incubating for 60 minutes, however, the percentage of PAC1-positive platelets was reduced to preincubation value. Although flow cytometry is a simple and powerful tool to assess platelet activation, there remain several methodological problems to be resolved before this method may be employed in routine clinical use.

Early protective effects of iloprost after experimental spinal cord ischemia in rabbits

The potential role of Iloprost, a stable analogue of prostocyclin, in treating spinal cord ischemia was investigated in rabbits subjected to aortic occlusion for 15 minutes. Ten adult rabbits weighing 2-2.5 kg received an intravenous infusion of saline (SF) as a control group and 14 rabbits received an intravenous infusion of Iloprost, 25 microg/kg/h. Iloprost infusion was started immediately after clamping of the aorta and continued 60 minutes thereafter. Cortical somatosensorial evoked potentials (CSEP) were recorded during the pre-ischemic period as a baseline and post-ischemic readings were taken at 15, 30 and 60 minutes. There was no statistically significant difference between CSEP of the saline and Iloprost treated groups (p < 0.05). All animals were examined neurologically by using a modification of Tarlov scale and all subjects were then deeply anesthetized and their spinal cords were removed for light and electron microscopic examinations at 24 h after spinal cord ischemia. In order to obtain an accurate comparison of ultrastructural changes between saline treated and Iloprost treated groups, a grading scale was performed. The light microscopic and ultrastructural analysis of the Iloprost treated group revealed that there was moderate protection of the myelin and axons and edema was attenuated. Findings of this study suggest that Iloprost exerts a protective effect on spinal cord ischemia. However, further studies are needed to reveal possible mechanisms of protection provided by Iloprost.

Evidence for platelet alphaIIb beta3 activation despite elevated cytosolic cAMP

Cyclic nucleotides, such as cAMP, are known to inhibit the multistep cascade that results in platelet aggregation. In the present study we provide evidence that it is possible to bypass cAMP inhibitory effect on fibrinogen binding site exposure induced by the thromboxane A2 synthetic analogue U46619, the snake venom toxin convulxin, or by the direct PKC activator OAG, by concomitantly activating a G1-coupled receptor by means of epinephrine or by inducing cytosolic calcium influx by means of ionomycin. In fact, in our study we demonstrate that, in iloprost-treated platelets, the inhibition of both platelet aggregation and fibrinogen binding was overcome by adding epinephrine or ionomycin. To further confirm this, we used the cAMP analogue dibutyryl cAMP and we obtained platelet aggregation in response to U46619, convulxin or OAG plus epinephrine. Moreover, a complete inhibition of platelet aggregation in the presence of high concentrations of cAMP was observed only in the case of U46619, while a small percentage of aggregation persisted when convulxin or OAG were used, due to the small amount of ADP that both convulxin and OAG are able to release. Since PKC inhibition didn't allow platelet aggregation to occur in response to the concomitant activation of U46619 or convulxin, plus epinephrine or ionomycin, we can conclude that cAMP-induced inhibition of aggregation can be counteracted by the simultaneous activation of PKC in the presence of an activated G1-coupled receptor or of an induced calcium influx.

Nitric oxide in the potassium-induced response of the rat middle cerebral artery: a possible permissive role

In the middle cerebral artery (MCA), the presence of nitric oxide (NO) is responsible for maintaining a more dilated state than in its absence during increases in extracellular K(+) and osmolality. The purpose of the present study was to determine whether the involvement of NO was due to (a) a direct effect of the K(+)/osmolality (K(hyper)) on the endothelium or (b) a 'permissive' role of NO. MCAs (approximately 210 microm o.d.) were isolated, cannulated with glass micropipettes, and pressurized to 85 mmHg. When K(+) (KCl) in the extraluminal bath was increased to 21 mM, the diameter increased by 15-20% with the magnitude of dilation diminishing with further increases in K(hyper). The addition of N(G)-nitro-L-arginine methyl ester (L-NAME, 10(-5) mM), an inhibitor of nitric oxide synthase, had no significant effect on dilations at lower K(hyper) concentrations but constricted the arteries relative to the control at 51, 66, and 81 mM K(hyper). In the presence of L-NAME, the addition of an exogenous NO donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-8) M) or an analog of cGMP, 8-bromo-cGMP (6x10(-5) M), tended to restore the response of K(hyper)to near the original response. We conclude that the basal release of NO from the endothelium plays a permissive role in the K(hyper)-induced response.

Significance of endothelial prostacyclin and nitric oxide in peripheral and pulmonary circulation

BACKGROUND

Vasoprotective function of endothelial cells is associated, among others, with biosynthesis and release of nitric oxide (NO), prostacyclin (PGI2), prostaglandin E2 (PGE2), carbon monoxide (CO) and plasminogen activator (t-PA). These endothelial mediators calm down activated platelets and leukocytes, prevent the occurrence of parietal thrombotic events, promote thrombolysis, maintain tissue perfusion and protect vascular wall against acute damage and against chronic remodeling. Endothelial dysfunction in patients suffering from atherosclerosis or diabetes type 2 is associated not only with suppression in release of the above mediators but also with deleterious discharge of prostaglandin endoperoxides (PGH2, PGG2), superoxide anion (O2-, peroxynitrite (ONOO-), and plasminogen activator inhibitor (PAI-1). We looked for mechanisms of protective endothelial function, with a special respect to the differences between peripheral and pulmonary circulation.

METHODS

Cultured endothelial cells of bovine aorta (BAEC) were used to study physiological and pharmacological mechanisms of increasing free cytoplasmic calcium [Ca2+]i. A porphyrinic sensor quantified the release of NO from BAEC. In cultured human umbilical vein endothelial cells (HUVEC) we looked for induction by bradykinin (Bk) of mRNAs for a number of enzymes. In blood perfused rat lungs we studied protective role of NO against injury inferred by lipopolysaccharide on pulmonary microcirculation that was accomplished by thromboxane A2 (TXA2), platelet activating factor (PAF), cysteinyl-leukotrienes (cyst-LTs) and the complement system. In vivo we analyzed the influence of Bk, perindopril and quinapril ('tissue type' angiotensin converting enzyme inhibitors, ACE-Is) on endothelial function in entire circulation of anaesthetized rats using a thrombolytic bioassay and EIA for 6-keto-PGF1 alpha and t-PA antigen.

RESULTS

In BAEC Bk via kinin B2 receptors raised in a concentration-dependent manner (1 pM-10 nM) free cytoplasmic calcium ions [Ca2+]i, that triggered the release of NO from BAEC. Calcium ionophore (A23187, 1-100 nM) as well as receptor agonists such as adenosine diphosphate (ADP, 10 nM-1 microM), adrenaline (Adr, 1-10 microM) or acetylcholine (Ach, 10-100 microM) produced a similar rise in endothelial [Ca2+]i as did Bk at a nanomolar concentration. 'Tissue type' ACE-Is, e.g. quinapril or perindopril acted through accumulation of endogenous Bk. However, the potency of ACE-I to change endothelial function is by several orders of magnitude lower than that for exogenous Bk. In vivo the major difference between thrombolytic actions by quinapril or perindopril on one hand, and by exogenous Bk on the other was longevity of thrombolysis by ACE I and a distinct hypotensive action of exogenous Bk. Still, the long-lasting isolated thrombolytic effect of ACE I was mediated entirely by endogenous BK as evidenced by the preventive action of icatibant, a kinin B2 receptor antagonist. Moreover, in vivo the immediate thrombolysis by ACE-I was mediated by PGI2 rather than by NO or t-PA, as shown by pharmacological analysis, and by direct blood assays of 6-keto-PGF1 alpha and t-PA antigen. Bradykinin as a mediator of pleiotropic endothelial action of several cardiovascular drugs (e.g. ACE-I) may complete its mission not only through B2 receptor and [Ca2+]i--mediated release of PGI2 or NO. Here, we describe a new route of the Bk action. Bk mediated induction of the [Ca2+]i-independent, so called 'inducible', endothelial isoenzymes required for generation of CO, PGI2 and PGE2. After 4 hours of incubation of HUVEC with Bk (10 nM) it induced mRNAs for haemooxygenase 1 (HO-1), cyclooxygenase 2 (COX-2), prostaglandin E synthase (PGE-S) whereas mRNA for nitric oxide synthase 2 (NOS-2) was weakly affected. We proved also that unlike in peripheral circulation, in pulmonary circulation only NO but not PGI2 would play a protective role. In the blood-perfused lung, endotoxaemia liberates lipids, such as TXA2, PAF and cyst-LTs. These toxic lipids along with the activated complement mediate pulmonary damage. Pulmonary endothelial nitric oxide is the only local protector against lung injury evoked by the phagocytised bacterial lipopolysaccharide.

SUMMARY

Summing up, in peripheral circulation endogenous Bk is the most efficient activator of protective endothelial function. For instance, thrombolytic action of 'tissue type' ACE-I depends on the Bk-released PGI2. Acting as an agonist of endothelial B2 kinin receptors Bk rises [Ca2+]i with a subsequent activation of constitutive COX 1 and NOS-3. This is followed by an immediate release of PGI2 and NO. Moreover, acting as 'microcytokine' Bk induces mRNAs for HO-1, COX-2 and PGE S, the isoenzymes responsible for a delayed endothelial biosynthesis of CO, PGI2 and PGE2. (ABSTRACT TRUNCATED)

Iloprost inhibits inositol-1,4,5-trisphosphate-mediated calcium mobilization stimulated by angiotensin II in cultured preglomerular vascular smooth muscle cells

In a previous study of cultured preglomerular vascular smooth muscle cells, it was demonstrated that, although the stable prostacyclin analog iloprost alone had no effect on the intracellular calcium concentration ([Ca2+](i)), it did significantly attenuate the increase in [Ca2+](i) stimulated by angiotensin II (AngII). In this study, the mechanisms by which iloprost interacts with calcium signaling pathways stimulated by AngII were examined. [Ca2+](i) was assessed using the calcium-sensitive fluorescent dye fura-2. Initial studies identified two major components of the [Ca2+](i) response to AngII in this homogeneous preparation of vascular smooth muscle cells from renal resistance vessels. Mobilization of internal stores was evident as an immediate TMB-8-sensitive peak increase in [Ca2+](i) (52 +/- 6 to 297 +/- 26 nM, P: < 0.001) in a calcium-free medium. After [Ca2+](i) had returned to baseline levels during continued AngII stimulation, a nifedipine-sensitive entry pathway was revealed by the sustained stimulatory effect of added external calcium, which increased [Ca2+](i) to 112 +/- 13 nM (P: < 0.001). Coadministration of iloprost with AngII attenuated both the immediate peak (154 +/- 14 nM) and sustained plateau (61 +/- 9 nM) phases. Increases in endogenous levels of cAMP induced by the phosphodiesterase inhibitor milrinone mirrored the actions of iloprost, suggesting that the prostacyclin analog exerted its actions via cAMP activation. Blockade of cAMP-dependent protein kinase with KT 5720 reversed the effects of both iloprost and milrinone. When iloprost or milrinone was introduced after the initial mobilization peak had dissipated, the plateau phase of calcium entry was unchanged (92 +/- 9 nM). The concept that iloprost does not directly modulate calcium entry was further supported by data showing that the activation of L-type calcium channels by BAY-K 8644 was unchanged during iloprost treatment. On the basis of the observation that iloprost did not alter thapsigargin stimulation of Ca(2+)-ATPase activity, it is concluded that the actions of cAMP are distinct from increasing calcium uptake into the sarcoplasmic reticulum. This study provides new information on the ability of iloprost to primarily attenuate inositol-1,4,5-triphosphate-mediated calcium mobilization via cAMP, with secondary inhibition of L-type calcium entry channels. These data clarify the mechanism by which prostaglandins buffer AngII constriction in resistance arterioles.

Prostacyclin analogues differentially inhibit growth of distal and proximal human pulmonary artery smooth muscle cells

BACKGROUND

Prostacyclin has proved to be a beneficial treatment for patients with severe pulmonary hypertension. We postulated that the response may reflect, at least in part, inhibition of pulmonary artery smooth muscle cell (PASMC) growth.

METHODS AND RESULTS

Human PASMCs were derived from distal (<1-mm external diameter, n=8) and proximal (>8-mm external diameter, n=12) pulmonary arteries obtained at transplant surgery and pneumonectomy. The effects of the stable prostacyclin analogues on [methyl-(3)H]thymidine incorporation and cell proliferation were investigated by using immunohistochemically characterized cells. Distal cells proliferated faster than did proximal PASMCs and displayed a distinct sensitivity to cicaprost and iloprost. Both analogues inhibited thymidine uptake over 24 hours (20% to 60%, P<0.001; n=8) and abolished stimulation of DNA synthesis by platelet-derived growth factor-BB (10 ng/mL) in distal but not proximal cells. The inhibitory effect of cicaprost was mimicked by isoproterenol (10(-5) mol/L), forskolin (10(-5) mol/L), and dibutyryl cAMP (5x10(-4) mol/L) and was potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (5x10(-5) mol/L). Cicaprost (10(-10) to 10(-6) mol/L) inhibited the proliferation of PASMCs, which had been stimulated with either platelet-derived growth factor-BB or serum, and increased cAMP production. These effects were potentiated by 3-isobutyl-1-methylxanthine and attenuated by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (10(-5) to 10(-4) mol/L).

CONCLUSIONS

++Cicaprost and iloprost inhibit DNA synthesis and proliferation to a greater extent in distal compared with proximal human PASMCs, acting at least in part via a cAMP-dependent mechanism. The results are consistent with the hypothesis that prostacyclin analogues inhibit vascular remodeling in pulmonary hypertension and demonstrate heterogeneity among human PASMCs.

Bradykinin causes endothelium-independent hyperpolarisation and neuromodulation by prostanoid synthesis in hamster mesenteric artery

The mechanism of bradykinin-induced hyperpolarisation and purinergic neuromodulation was examined in the hamster superior mesenteric artery using intracellular microelectrode techniques. Bradykinin induced a concentration-dependent hyperpolarisation both in endothelium-intact and -denuded preparations. Indomethacin blocked this hyperpolarisation. Prostacyclin and iloprost also hyperpolarised the membrane of mesenteric artery, while prostaglandin E(2) did not evoke any membrane hyperpolarisation. The bradykinin-, prostacyclin- and iloprost-induced hyperpolarisation were inhibited by glibenclamide. Bradykinin also inhibited the amplitude of the purinergic excitatory junction potentials (e.j.p.s), both in endothelium-intact and -denuded preparations. Indomethacin blocked this inhibitory effect. Prostaglandin E(2) inhibited the e.j. p. in a concentration-dependent manner. Focally applied ATP-induced depolarisation was not modified by bradykinin or prostaglandin E(2.) These findings suggest that bradykinin via prostanoids production pre-synaptically, inhibit the amplitude of purinergic e.j.p., resulting inhibitory purinergic neuromodulation. In addition, bradykinin-released prostanoids elicits membrane hyperpolarisation of smooth muscle cells through opening of K(ATP) channels.

Agonist-induced long-term desensitization of the human prostacyclin receptor

Phosphorylation of the human prostacyclin (PGI(2)) receptor (hIP-R) by diacylglycerol-regulated protein kinase C (PKC) has been reported to be responsible for its rapid desensitization in HEK293 cells. In this study we demonstrate, that human fibroblasts reveal a much slower hIP-R desensitization kinetics, which was neither affected by stimulation nor inhibition of PKC by either phorbol 12-myristate-13-acetate or GF-109203X suggesting a different cellular mechanism. Although agonist-promoted sequestration of a C-terminally green fluorescent protein-tagged hIP-R was demonstrated, it did not account for the long-term desensitization. Concanavalin A did not abolish, but accelerated receptor desensitization kinetics. Resensitization of hIP-R involved receptor recycling and/or de novo synthesis of receptor protein, depending on the duration of prior desensitization. This is the first study investigating the mechanisms of hIP-R desensitization in intact human cells naturally expressing hIP-R. Our data suggest, that a hitherto unknown mechanism of hIP-R long-term desensitization, which is independent of receptor phosphorylation by conventional and novel type PKC isoforms or endocytosis, is a key event in regulating the cellular responsiveness to PGI(2).

Internalization and sequestration of the human prostacyclin receptor

Prostacyclin (PGI(2)), the major product of cyclooxygenase in macrovascular endothelium, mediates its biological effects through its cell surface G protein-coupled receptor, the IP. PKC-mediated phosphorylation of human (h) IP is a critical determinant of agonist-induced desensitization (Smyth, E. M., Hong Li, W., and FitzGerald, G. A. (1998) J. Biol. Chem. 273, 23258-23266). The regulatory events that follow desensitization are unclear. We have examined agonist-induced sequestration of hIP. Human IP, tagged at the N terminus with hemagglutinin (HA) and fused at the C terminus to the green fluorescent protein (GFP), was coupled to increased cAMP (EC(50) = 0.39 +/- 0.09 nm) and inositol phosphate (EC(50) = 86. 6 +/- 18.3 nm) generation when overexpressed in HEK 293 cells. Iloprost-induced sequestration of HAhIP-GFP, followed in real time by confocal microscopy, was partially colocalized to clathrin-coated vesicles. Iloprost induced a time- and concentration-dependent loss of cell surface HA, indicating receptor internalization, which was prevented by inhibitors of clathrin-mediated trafficking and partially reduced by cotransfection of cells with a dynamin dominant negative mutant. Sequestration (EC(50) = 27.6 +/- 5.7 nm) was evident at those concentrations of iloprost that induce PKC-dependent desensitization. Neither the PKC inhibitor GF109203X nor mutation of Ser-328, the site for PKC phosphorylation, altered receptor sequestration indicating that, unlike desensitization, internalization is PKC-independent. Deletion of the C terminus prevented iloprost-induced internalization, demonstrating the critical nature of this region for sequestration. Internalization was unaltered by cotransfection of cells with G protein-coupled receptor kinases (GRK)-2, -3, -5, -6, arrestin-2, or an arrestin-2 dominant negative mutant, indicating that GRKs and arrestins do not play a role in hIP trafficking. The hIP is sequestered in response to agonist activation via a PKC-independent pathway that is distinct from desensitization. Trafficking is dependent on determinants located in the C terminus, is GRK/arrestin-independent, and proceeds in part via a dynamin-dependent clathrin-coated vesicular endocytotic pathway although other dynamin-independent pathways may also be involved.

Hepatic O2 exchange and liver energy metabolism in hyperdynamic porcine endotoxemia: effects of iloprost

OBJECTIVE

To compare the effects of a 12 h continuous infusion of iloprost, a stable prostacyclin analogue, on hepatic blood flow (Qliv), O2 exchange, and energy metabolism during a 24 h hyperdynamic, porcine endotoxemia with volume resuscitation alone.

DESIGN

Prospective, randomized, experimental study with repeated measures.

SETTING

Investigational animal laboratory.

SUBJECTS

Twenty-eight domestic pigs: 16 animals during endotoxemia with volume resuscitation alone (ETX), 12 with endotoxemia, volume resuscitation, and treatment with iloprost (ILO).

INTERVENTIONS

Endotoxemia was initiated by continuous infusion of E. coli lipopolysaccharide. Animals were resuscitated with hetastarch, aimed at maintaining a MAP of > 60 mmHg. After 12 h of endotoxemia, iloprost was administered for 12 h in the treatment group, titrated to avoid pharmacologically induced hypotension (MAP < 60 mmHg).

MEASUREMENTS AND RESULTS

Iloprost significantly increased Qliv, with no effect on hepatic O2 delivery. Mean capillary hemoglobin O2 saturation (HbScO2) on the liver surface, as well as HbScO2 frequency distributions--a measure of microcirculatory O2 availability--remained unchanged. Treatment with iloprost, however, significantly attenuated the endotoxin-induced derangements of cellular energy metabolism as reflected by the diminished progressive decrease in hepatic lactate uptake rate and a blunted increase in hepatic venous lactate/pyruvate ratios. While endotoxin significantly increased endogenous glucose production (EGP) rate, iloprost restored EGP to normal at the end of the experiment.

CONCLUSIONS

Thus, in a clinically relevant model of human sepsis, iloprost did not produce potential adverse effects but rather ameliorated hepatic metabolic disturbances and, thereby, hepatic energy balance.

beta-adrenergic agonists exert their "anti-inflammatory" effects in monocytic cells through the IkappaB/NF-kappaB pathway

In addition to their well-studied bronchodilatory and cardiotonic effects, beta-adrenergic agonists carry anti-inflammatory properties by inhibiting cytokine production by human mononuclear cells. In a model of human promonocytic THP-1 cells stimulated with lipopolysaccharide (LPS), we showed that beta-agonists inhibited tumor necrosis factor-alpha and interleukin-8 production predominantly via the beta(2)-adrenergic receptor through the generation of cAMP and activation of protein kinase A. This effect was reproduced by other cAMP-elevating agents such as prostaglandins and cAMP analogs. Activation and nuclear translocation of the transcription factor nuclear factor-kappaB induced by LPS were inhibited with treatment with beta-agonists, an effect that was prominent at late time points (>1 h). Although the initial IkappaB-alpha degradation induced by LPS was minimally affected by beta-agonists, the latter induced a marked rebound of the cytosolic IkappaB-alpha levels at later time points (>1 h), accompanied by an increased IkappaB-alpha cytoplasmic half-life. This potentially accounts for the observed nuclear factor-kappaB sequestration in the cytoplasmic compartment. We postulate that the anti-inflammatory effects of beta-agonists reside in their capacity to increase cytoplasmic concentrations of IkappaB-alpha, possibly by decreasing its degradation.

The management of idiopathic thrombotic microangiopathy. Changing trends

Thrombotic microangiopathy, including the two related syndromes thrombotic thrombocytopenic purpura and hemolytic-uremic syndrome, is a rare and severe multisystem disorder, due to widespread deposition of intravascular microthrombi consisting mainly of platelets, with subsequent consumption thrombocytopenia, microangiopathic hemolytic anemia, renal abnormalities, and neurologic disturbances. The epidemic, verotoxin-induced hemolytic-uremic syndrome, typically associated with prodromal diarrhea, mainly affects young children in small outbreaks. By contrast, idiopathic thrombotic microangiopathy generally affects adults in a sporadic form; it has a more devastating course and a less favourable outcome. Over 90% of the reported cases in the adult, when untreated, have progressed to death within three months of diagnosis. Since the introduction of plasma exchange, a dramatic change in the prognosis of the disease has taken place, although the mortality rate still remains considerable. Indeed, improved survival is the most striking feature of adult thrombotic microangiopathy compared to some decades ago. In the present article we will focus on the evolving concepts able to exert a considerable impact in the management of the adult idiopathic form of thrombotic microangiopathy.

Effects of the stable prostacyclin analogue iloprost on the plasma disappearance rate of indocyanine green in human septic shock

OBJECTIVES

To evaluate the effect of the stable prostacyclin analogue iloprost on the plasma disappearance rate of indocyanine green (PDR) in patients with septic shock.

DESIGN AND SETTING

A prospective clinical study in a university hospital intensive care unit.

PATIENTS AND INTERVENTIONS

20 patients in septic shock. Patients received iloprost infusion (1 ng/kg per minute) for 24 h.

MEASUREMENTS AND RESULTS

PDR was determined by a femoral arterial fiberoptic catheter before, 1, 6, and 24 h after start and 1 h after end of iloprost infusion. PDR increased significantly 24 h after start of iloprost infusion (baseline: 13.9 +/- 1.7% vs. 18.6 +/- 2.2%/min) and decreased 1 h after end of infusion (13.7 +/- 1.7%/min; p < 0.002). There was no change in pHi, cardiac index, mean arterial pressure, heart rate, central venous pressure, or intrathoracic blood volume index.

CONCLUSION

Administration of the stable prostacyclin analogue iloprost significantly increases PDR, indicating improvement in liver function.

Permanent life support by kidneys perfused with a vitrifiable (7.5 molar) cryoprotectant solution

BACKGROUND

Vitrification (glass formation) is a potential method for indefinite term organ preservation that eliminates all of the conventional problems of freezing and thawing. A 7.5 M mixture of cryoprotectants known as VS4 is sufficiently concentrated, in combination with applied pressure, to preclude ice formation entirely during cooling to below the glass transition temperature (about -125 degrees C), at which point vitrification takes place, arresting further changes over time.

METHODS

Rabbit kidneys were perfused with VS4 according to three different protocols. The kidneys were evaluated using an autograft model with immediate contralateral nephrectomy.

RESULTS

All three methods permitted long-term survival, but the best results were obtained when the highest concentrations were perfused at about -3 degrees C. Using the latter protocol, the survival rate was 10/10, serum creatinine returned to a normal baseline after transient elevation, other clinical chemistry results normalized, and no histological damage was apparent 3 weeks after autografting.

CONCLUSIONS

The results described provide the strongest evidence to date that it may be possible to bank kidneys for unlimited periods in the absence of ice for later transplantation.

Prostaglandin I(2), a possible thermo-sensory mediator in Paramecium

Thermo-sensory mechanisms are little understood. The protozoan, Paramecium multimicronucleatum, is sensitive and responsive to a cooling stimulus. We found that inhibitors of prostaglandin (PG) biosynthesis reduced the response to the cooling stimulus. Inversely, the response suppressed by the inhibitors was recovered by application of stable PGI(2) analogs, including iloprost. Paramecium cells showed binding activity specific for [(3)H]iloprost. Moreover, an arachidonic acid metabolite, possibly PGI(2), was rapidly increased in response to the cooling stimulus, suggesting that prostaglandin biosynthesis plays a crucial role in the cooling-sensory transduction. Paramecium may be a useful model for the investigation of the molecular basis of thermo-sensory mechanisms.

Long-term treatment of primary pulmonary hypertension with aerosolized iloprost, a prostacyclin analogue

BACKGROUND

Continuous intravenous infusion of epoprostenol (prostacyclin) is an effective treatment for primary pulmonary hypertension. This approach requires the insertion of a permanent central venous catheter, with the associated risk of serious complications. Recently, aerosolized iloprost, a stable prostacyclin analogue, has been introduced as an alternative therapy for severe pulmonary hypertension.

METHODS

We evaluated the effects of aerosolized iloprost on exercise capacity and hemodynamic variables over a one-year period in patients with primary pulmonary hypertension.

RESULTS

Twenty-four patients with primary pulmonary hypertension received aerosolized iloprost at a daily dose of 100 or 150 microg for at least one year. The mean (+/-SD) distance covered in the six-minute walk test increased from 278+/-96 m at base line to 363+/-135 m after 12 months (P<0.001). During the same period, the mean pulmonary arterial pressure before the inhalation of iloprost declined from 59+/-10 mm Hg to 52+/-15 mm Hg (P=0.006), cardiac output increased from 3.8+/-1.4 liters per minute to 4.4+/-1.3 liters per minute (P=0.02), and pulmonary vascular resistance declined from 1205+/-467 dyn x sec x cm(-5) to 925+/-469 dyn x sec x cm(-5) (P<0.001). The treatment was generally well tolerated, except for mild coughing, minor headache, and jaw pain in some patients.

CONCLUSIONS

Long-term treatment with aerosolized iloprost is safe and has sustained effects on exercise capacity and pulmonary hemodynamics in patients with primary pulmonary hypertension.

Activated neutrophils impair gastric cytoprotection role of neutrophil elastase

Neutrophil elastase decreases production of PGI2 by cultured endothelial cells. Thus, neutrophil elastase may play an important role in gastric mucosal injury by decreasing the tissue level of PGI2, an important gastric cytoprotective substance. We examined whether activated neutrophils inhibit gastric PGI2 production in rats subjected to water-immersion restraint stress. Gastric 6-keto-PGF1alpha levels were determined by enzyme immunoassay. Gastric mucosal blood flow was determined by laser-Doppler flowmeter. Gastric microvascular permeability was determined by Evans blue leakage. Gastric levels of 6-keto-PGF1alpha were transiently increased 0.5 hr after the stress, followed by a decrease to below baseline at 6 hr, when mucosal blood flow fell to 60% of baseline. Gastric levels of 6-keto-PGF1alpha were significantly higher in animals with nitrogen mustard-induced leukocytopenia than in controls 1 and 6 hr after the stress. In leukocytopenic animals, levels 6 hr after stress were not lower than those preceding stress. Leukocytopenia markedly limited both the decrease in mucosal blood flow and the increase in gastric microvascular permeability. The level of gastric mucosal injury observed 6 hr after the stress was markedly attenuated by leukocytopenia. Pretreatment with neutrophil elastase inhibitors (ONO-5046 and Eglin C) or an anti-P-selectin monoclonal antibody produced effects similar to leukocytopenia. Neutrophil elastase is involved in the stress-induced gastric mucosal injury by decreasing gastric production of PGI2. Thus, pharmacologic inhibition of neutrophil elastase should help to prevent stress-induced gastric mucosal injury.

Inhibitory effect of iloprost on the contractility of lower uterine segment myometrium from rhesus monkeys in normal-term and androstenedione-induced preterm labor

OBJECTIVE

Iloprost, a combined EP(1) stimulatory and IP inhibitory receptor agonist, was tested in vitro on myometrium from the lower uterine segment of pregnant rhesus monkeys to compare its effects in spontaneous labor and in labor induced by the administration of androstenedione to the mother.

METHODS

Pregnant rhesus monkeys carrying fetuses of known gestational age were instrumented under halothane general anesthesia with femoral artery and vein catheters and uterine electromyogram leads. Experimental animals were infused with androstenedione from 139 days' gestation. Control animals were infused with intralipid vehicle from 139 days' gestation. Lower uterine segment myometrium was removed from control animals either before labor began (n = 6) or in spontaneous labor (n = 4) and from animals undergoing premature labor induced by androstenedione (n = 4). Myometrial contractility in response to iloprost was evaluated using a superfusion system in vitro.

RESULTS

Iloprost was inhibitory on myometrium obtained from the lower uterine segment from androstenedione-treated animals as well as vehicle-infused animals in spontaneous term labor. In contrast, iloprost had no effect on myometrial strips from control animals not in labor.

CONCLUSION

These findings indicate up-regulation of IP receptors which inhibit myometrial contractility and/or down-regulation of EP(1) receptors which stimulate myometrial contractility in the lower uterine segment during labor. A relative increase in inhibitory responses in the lower uterine segment during labor may enable this region to dilate to allow passage of the fetus.

Iloprost protects the spinal cord during aortic cross-clamping in a canine model

BACKGROUND

Surgical procedures on the thoracoabdominal part of the aorta make the spinal cord vulnerable to ischemia. Paraplegia is the most severe complication following thoracoabdominal operations. In this study, iloprost was used as an agent to decrease the severity of ischemia and reperfusion injury to the spinal cord during aortic occlusion and declamping.

METHODS

Twelve adult mongrel dogs weighing 17+/-2 kg were used in this study. The animals were randomly assigned to either group I, which received saline solution (6 dogs), or group II, which received prostacyclin. Group I was referred to as the control group and group II as the iloprost group. After baseline measurements were completed, the aorta was cross-clamped for sixty minutes distal to the left subclavian artery. No pharmacologic agents were used to control blood pressure in group I. Proximal and distal mean arterial pressures (DMAP) were monitored continuously. DMAP were considered as diastolic pressure in preocclusion and reperfusion periods. Iloprost administration was started at a rate of 5 ng/kg/minute five minutes before the aortic occlusion. This dosage was increased to 25 ng/kg/minute during aortic occlusion.

RESULTS

Mean proximal arterial pressure was 147+/-12 mmHg in the control group and 116+/-13 mmHg in the iloprost group at occlusion (p<0.01). Mean distal arterial pressure was 19+/-7 in the control group and 37+/-5 in the iloprost group during clamping (p<0.05). Functional outcome was evaluated according to Tarlov scores 24 hours after the study. Although none of the animals recovered completely from the control group, 4 animals from the iloprost group recovered (p<0.05). Following the neurologic assessment, animals were sacrificed and specimens were taken for the electron microscopic study. Electron microscopic changes documented that severe mitochondrial damage and vacuolisation occurred in the control group. However these changes were more subtle in the iloprost group.

CONCLUSIONS

As a result of this study we concluded that iloprost infused before and during clamping of the thoracic aorta mitigates the spinal cord injury due to ischemia and reperfusion following unclamping.