Endothelin-1 reduces microvascular fluid permeability through secondary release of prostacyclin in cat Skeletal muscle

Effect of the endothelin receptor antagonist tezosentan on alpha-naphthylthiourea-induced lung injury in rats

Acute lung injury is an inflammatory syndrome that increases the permeability of the blood-gas barrier, resulting in high morbidity and mortality. Despite intensive research, treatment options remain limited. We investigated the protective efficacy of tezosentan, a novel, dual endothelin receptor antagonist, in an experimental model of alpha-naphthylthiourea (ANTU)-induced acute lung injury in rats. ANTU was intraperitoneally (i.p.) injected into rats at a dose of 10 mg/kg. Tezosentan was injected 30 minutes before ANTU was subcutaneously (s.c.) injected at doses of 2, 10, or 30 mg/kg, 60 minutes before ANTU was injected at doses of 2, 10, or 30 mg/kg (i.p.), and 90 minutes before ANTU at a dose of 10 mg/kg (i.p.). Four hours later, the lung weight/body weight (LW/BW) ratio and pleural effusion (PE) were measured. When injected 30 minutes before ANTU at doses of 2, 10, or 30 mg/kg (s.c.), tezosentan had no effect on lung pathology. When injected 60 minutes before ANTU at doses of 2, 10, or 30 mg/kg (i.p.) or 90 minutes before ANTU (10 mg/kg, i.p.), tezosentan significantly decreased the PE/BW ratio and had a prophylactic effect on PE formation at all doses. Therefore, tezosentan may attenuate lung injury. Furthermore, its acute and inhibitory effects on fluid accumulation were more effective in the pleural cavity than in the interstitial compartment in this experimental model.

Endothelin-1 attenuates increases in hydraulic conductivity due to platelet-activating factor via prostacyclin release

We previously showed that endothelin-1 (ET-1) and prostacyclin (PGI(2)) similarly attenuate increases in microvascular permeability induced by platelet-activating factor (PAF). This led us to hypothesize that ET-1 attenuates trans-endothelial fluid flux during PAF through PGI(2) release. We tested this hypothesis in three phases. First, bovine pulmonary artery endothelial cells were exposed to 0.008-8 μM ET-1 and assayed for PGI(2) release. Second, to determine whether increased transmonolayer flux after PAF could be attenuated by ET-1 or PGI(2) and reversed by PGI(2) synthesis inhibition or PGI(2) receptor blockade, we measured endothelial cell transmonolayer flux after cells were exposed to 10 nM PAF plus 10 μM PGI(2) or 80 pM ET-1, with or without 500 μM tranylcypromine (PGI(2) synthase inhibitor) or 20 μM CAY-10441 (PGI(2) receptor blocker). Finally, hydraulic conductivity (L(p)) was measured in rat mesenteric venules in vivo after exposure to 10 nM PAF and 80 pM ET-1 with or without tranylcypromine (100 and 500 μM) or CAY-10441 (2 and 20 μM). We found that in vitro, ET-1 stimulated a dose-dependent increase in PGI(2) production (from 126 to 217 pg/ml, P < 0.01). Compared with PAF alone, PGI(2) plus PAF and ET-1 plus PAF decreased transmonolayer flux similarly by 52 and 46%, respectively (P < 0.01), while tranylcypromine and CAY-10441 reversed these effects by 92 and 47%, respectively (P < 0.05). In vivo, PAF increased L(p) fourfold (P < 0.01) and ET-1 attenuated this effect by 83% (P < 0.01). Tranylcypromine and CAY-10441 reversed the ET-1 attenuation in L(p) during PAF by 55 and 45%, respectively (P < 0.01). We conclude that ET-1 may stimulate endothelial cell PGI(2) release to attenuate the increases in transmonolayer flux and hydraulic conductivity secondary to PAF.

Endothelin 1 and prostacyclin attenuate increases in hydraulic permeability caused by platelet-activating factor in rats

We have previously documented that endothelin 1 (ET-1) and prostacyclin (PGI2) decrease basal state hydraulic permeability (Lp). The aim of this study was to investigate the ability of ET-1 and PGI2 to modulate transendothelial fluid flux during situations in which Lp was artificially elevated with platelet-activating factor (PAF). We hypothesized that ET-1 and PGI2 administration before PAF exposure would prevent the increase in Lp secondary to PAF. In addition, in a potentially more clinically relevant situation, we also hypothesized that ET-1 and PGI2 administration after PAF exposure would attenuate the increase in Lp secondary to PAF. Microvascular Lp was measured in rat mesenteric postcapillary venules. Exposure to 10 nM PAF increased Lp 4-fold (P < 0.001). If the administration of 80 pM ET-1 or 10 microM PGI2 was completed before PAF exposure, no PAF-associated increase in Lp was observed (P < 0.001). The administration of ET-1 or PGI2 after PAF exposure attenuated the peak increase in Lp caused by PAF alone by 55% and 57%, respectively (P < 0.001). We conclude that ET-1 and PGI2 administration before PAF exposure prevents PAF-induced elevations in Lp, and in a more clinically relevant situation, ET-1 and PGI2 administered after PAF exposure attenuate the PAF-induced increase in Lp. Endothelin 1 and PGI2 receptors may provide important therapeutic targets for decreasing the microvascular fluid leak-associated morbidity resulting from shock and sepsis.

The effect of endothelin-1 on alveolar fluid clearance and pulmonary edema formation in the rat

BACKGROUND

Endothelin-1 (ET-1) is thought to play a pivotal role in pulmonary edema formation. The underlying mechanisms remain uncertain but may include alterations in capillary pressure and vascular permeability. There are no studies investigating whether ET-1 also affects alveolar fluid clearance which is the primary mechanism for the resolution of pulmonary edema. Therefore, we performed this study to clarify effects of ET-1 on alveolar reabsorption and fluid balance in the rat lung.

METHODS

Alveolar fluid clearance was measured in fluid instilled rat lungs using a 5% albumin solution with or without ET-1 (10(-7) M) and/or amiloride (100 microM). Net alveolar fluid balance, time course of edema formation, pulmonary capillary pressure, and alveolar permeability to albumin were measured in the isolated, ventilated, constant pressure perfused rat lung with or without ET-1 (0.8 nM) added to the perfusate.

RESULTS

In the fluid-instilled lung, ET-1 reduced alveolar fluid clearance by about 65%, an effect that was related to a decrease in amiloride-sensitive transepithelial Na(+) transport (P < 0.001). The ET-1-induced inhibition was completely prevented by the endothelin B receptor antagonist BQ788 (P = 0.006), whereas the endothelin A receptor antagonist BQ123 had no effect (P = 0.663). In the isolated, ventilated, perfused rat lung ET-1 caused a net accumulation of alveolar fluid by about 20% (P = 0.011 vs control), whereas lungs of control rats cleared about 20% of the instilled fluid. ET-1 increased pulmonary capillary pressure (+9.4 cm H(2)O), decreased perfusate flow (-81%), accelerated lung weight gain and reduced lung survival time (P < 0.001). Permeability to albumin was not significantly affected by ET-1 (P = 0.24).

CONCLUSION

ET-1 inhibits alveolar fluid clearance of anesthetized rats by inhibition of amiloride-sensitive epithelial Na(+) channels. The inhibitory effect of ET-1 results from activation of the endothelin B receptor. These findings suggest a mechanism by which ET-1, in addition to increasing capillary pressure, contributes to pulmonary edema formation.

Endothelin-1 reduces mesenteric microvascular hydraulic permeability via cyclic AMP and protein kinase A signal transduction

We have previously shown that endothelin-1 (ET-1) decreases microvascular hydraulic permeability. In this study, we tested the hypothesis that ET-1 exerts its permeability-decreasing effect through cAMP, cGMP, and protein kinase A (PKA) by determining the effect of ET-1 on venular fluid leak during inhibition of cAMP synthesis, inhibition of cGMP degredation, and inhibition of PKA. Rat mesenteric venules were cannulated to measure hydraulic permeability, L(p) (units x 10(-7)cm/(s cmH(2)O)). L(p) was measured during continuous perfusion of 80 pM ET-1 and either (1) an inhibitor of cAMP synthesis (10 microM 2',5'ddA), (2) an inhibitor of cGMP degradation (100 microM Zaprinast), or (3) an inhibitor of PKA (10 microM H-89). Inhibition of cAMP synthesis blocked the permeability decreasing effects of ET-1. The peak L(p) of the cAMP inhibitor alone and with ET-1 was 4.11+/-0.53 and 3.86+/-0.19, respectively (p=0.36, n=6). Inhibition of cGMP degradation did not block the permeability decreasing effects of ET-1. The peak L(p) during inhibition of cGMP degradation alone and with ET-1 was 2.26+/-0.15 and 1.44+/-0.09, respectively (p<0.001, n=6). Inhibition of PKA activation blocked the permeability decreasing effects of ET-1. The peak L(p) of the PKA inhibitor alone and with ET-1 was 2.70+/-0.15 and 2.59+/-0.15, respectively (p=0.38, n=6). The data support the notion that the signal transduction mechanism of ET-1 with regard to decreasing microvascular fluid leak involves cAMP production and PKA activation, but not cGMP degradation. Further understanding of intracellular mechanisms that control microvascular fluid leak could lead to the development of a pharmacologic therapy to control third space fluid loss in severely injured or septic patients.

Endothelin-1 overexpression leads to further water accumulation and brain edema after middle cerebral artery occlusion via aquaporin 4 expression in astrocytic end-feet

Stroke patients have increased levels of endothelin-1 (ET-1), a strong vasoconstrictor, in their plasma or cerebrospinal fluid. Previously, we showed high level of ET-1 mRNA expression in astrocytes after hypoxia/ischemia. It is unclear whether the contribution of ET-1 induction in astrocytes is protective or destructive in cerebral ischemia. Here, we generated a transgenic mouse model that overexpress ET-1 in astrocytes (GET-1) using the glial fibrillary acidic protein promoter to examine the role of astrocytic ET-1 in ischemic stroke by challenging these mice with transient middle cerebral artery occlusion (MCAO). Under normal condition, GET-1 mice showed no abnormality in brain morphology, cerebrovasculature, absolute cerebral blood flow, blood-brain barrier (BBB) integrity, and mean arterial blood pressure. Yet, GET-1 mice subjected to transient MCAO showed more severe neurologic deficits and increased infarct, which were partially normalized by administration of ABT-627 (ET(A) antagonist) 5 mins after MCAO. In addition, GET-1 brains exhibited more Evans blue extravasation and showed decreased endothelial occludin expression after MCAO, correlating with higher brain water content and increased cerebral edema. Aquaporin 4 expression was also more pronounced in astrocytic end-feet on blood vessels in GET-1 ipsilateral brains. Our current data suggest that astrocytic ET-1 has deleterious effects on water homeostasis, cerebral edema and BBB integrity, which contribute to more severe ischemic brain injury.

Endothelin-1 decreases postcapillary fluid efflux via prostacyclin release

BACKGROUND

Endothelin-1 (ET-1) decreases water efflux across the endothelial barrier (Lp). ET-1 may exert this permeability-decreasing effect by stimulating prostacyclin (PGI2) release. The purposes of this study were to (1) examine the effect of PGI2 on Lp, (2) measure Lp after inhibition of PGI(2) synthesis, and (3) determine the effect of ET-1 on Lp during inhibition of PGI2 production.

METHODS

After microscopic cannulation of mesenteric venules, Lp was measured during PGI2 infusion (0.1 micromol/L, 1 micromol/L, and 10 micromol/L; n = 6 in each group). Lp was also measured after 100 micromol/L of the PGI2 synthase inhibitor, tranylcypromine (TCPN) (n = 6). Finally, the influence of ET-1 on Lp during PGI2 synthase inhibition was assessed (n = 6).

RESULTS

Compared to baseline Lp of 1.05 +/- 0.06, PGI2 decreased Lp at 1 micromol/L (Lp = 0.63 +/- 0.03, P < .003) and 10 micromol/L (Lp = 0.52 +/- 0.04, P < .0001). TCPN increased Lp compared to baseline (P < .0001). Compared to ET-1 alone, venules perfused with TCPN + ET-1 increased Lp (P < .005). Units for Lp ) are 10(-7) cm x sec(-1) x cmH2O(-1).

CONCLUSIONS

We found that (1) PGI2 decreases Lp, (2) inhibition of PGI2 synthesis increases Lp, and (3) permeability-decreasing effects of ET-1 can be blocked by inhibiting PGI2 synthesis. These data suggest that constitutive production of PGI2 modulates basal microvessel permeability and that ET-1 may exert its permeability-decreasing effect via the stimulation of PGI2 release.

Endothelin-1 decreases microvessel permeability after endothelial activation

BACKGROUND

Endothelin-1 (ET-1) is a potent vasoconstrictor that is released during shock and sepsis. We hypothesized that ET-1 plays a role in the modulation of the elevated microvascular permeability state of the activated endothelium.

METHODS

Hydraulic permeability (Lp) was measured using the modified Landis micro-occlusion technique. The effect of different ET-1 doses on Lp was determined by obtaining paired measures of Lp at baseline and after the vessels were perfused with ET-1 at doses of 2.0 pg/mL (n = 6), 20 pg/mL (n = 6), 200 pg/mL (n = 6), or 2,000 pg/mL (n = 6). To evaluate the effects of ET-1 in the activated endothelium, additional vessels were perfused with either 10 micromol/L adenosine triphosphate (ATP) (n = 6) or 1 nmol/L bradykinin (n = 6). The vessels were then perfused with 200 pg/mL ET-1 followed by the final L determination.

RESULTS

ET-1 significantly decreased Lp at doses of 20 pg/mL (p = 0.03), 200 pg/mL (p = 0.03), and 2,000 pg/mL (p = 0.01). Endothelial activation with ATP and bradykinin increased Lp to 4.21 +/- 0.39 (p < 0.0001) and 2.72 +/- 0.24 (p = 0.001), respectively. ET-1 significantly decreased the Lp to 1.99 +/- 0.48 after activation with ATP (p = 0.004). ET-1 also decreased the Lp to 1.10 +/- 0.19 after activation with bradykinin (p = 0.001). Units for Lp are x10(-7) cm x s(-1) x cm H2O(-1).

CONCLUSION

In this model, ET-1 attenuated the increase in microvascular permeability that can be seen in inflamed vessels. In addition to its vasopressor function, ET-1 may be of benefit in pathophysiologic states by decreasing third-space fluid loss. This receptor-mediated function of ET-1 may be amenable to pharmacologic manipulation.

Vasopeptidase inhibition with omapatrilat increases fluid and protein microvascular permeability in cat skeletal muscle

BACKGROUND

Vasopeptidase inhibition is a new antihypertensive approach combining inhibition of angiotensin-converting enzyme (ACE) and neutral endopeptidase (NEP), but severe oedema, mainly angio-oedema, has been reported. As ACE and NEP catalyse degradation of the permeability-increasing peptide bradykinin, and NEP also catalyses degradation of permeability-increasing peptides such as atrial natriuretic peptide, substance P, endothelin-1 and angiotensin II, vasopeptidase inhibition may increase microvascular permeability.

OBJECTIVE

To analyse the effects of vasopeptidase inhibition on permeability.

DESIGN

The study was performed on the autoperfused cat calf skeletal muscle, evaluating the effects on fluid and protein permeability of a clinically relevant dose of the vasopeptidase inhibitor, omapatrilat. The effects were compared with those of the vehicle, of selective ACE and NEP inhibition, and of omapatrilat during bradykinin receptor blockade.

METHODS

Effects on fluid permeability were determined with a capillary filtration coefficient (CFC) technique, and effects on protein permeability were assessed from changes in the osmotic reflection coefficient for albumin.

RESULTS

After 1.5 h of intravenous infusion of omapatrilat (0.35 mg/kg per hour), mean arterial pressure was reduced from 114 mmHg to 86 mmHg (P < 0.01) and skeletal muscle vascular resistance was reduced from 14.5 peripheral resistance units (PRU) to 11.5 PRU (P < 0.05). CFC was increased by 22% (P < 0.01) and the reflection coefficient was decreased by 17% (P < 0.01). Infusion of vehicle had no effects. Inhibition of NEP increased permeability without affecting blood pressure, whereas ACE inhibition decreased blood pressure without affecting permeability. The increase in permeability associated with omapatrilat was reduced by bradykinin blockade.

CONCLUSIONS

A clinically relevant antihypertensive dose of omapatrilat reduces vascular resistance and increases fluid and protein permeability, the permeability effect more by inhibition of NEP than by inhibition of ACE, by a mechanism involving bradykinin.

Endogenous nitric oxide reduces microvascular permeability and tissue oedema during exercise in cat skeletal muscle

Based on a proposed increase in the release of the vasodilators nitric oxide (NO) and prostacyclin during exercise, and the fact that these substances have vascular permeability-reducing properties, this study was designed to evaluate (1) possible effects of exercise on hydraulic permeability, (2) whether permeability and muscle swelling are reduced by an increased release of NO and prostacyclin during exercise and (3) whether NO and prostacyclin are involved in exercise hyperaemia. The study was performed on an autoperfused cat calf muscle preparation with ligated lymph vessels, and exercise was induced by somatomotor nerve stimulation. Change in microvascular hydraulic permeability was estimated by a capillary filtration coefficient (CFC) technique. We found that the marked muscle volume increase after the start of the exercise gradually decreased, reaching an isovolumetric state within 25 min where CFC had decreased by about 25% (p < 0.05). CFC recovered completely after exercise was stopped. The decrease in CFC was abolished during blockade of endogenous NO by the NO synthase inhibitor L-NAME, but was preserved during blockade of endogenous prostacyclin by tranylcypromine. The muscle volume increase during exercise was about 60% greater with L-NAME than during vehicle or tranylcypromine (p < 0.01). Neither L-NAME nor tranylcypromine had any effect on exercise hyperaemia. We conclude that microvascular hydraulic permeability is reduced during exercise, that this effect reduces exercise-induced muscle swelling, and that the effects are mediated via release of NO. NO and prostacyclin are not involved in exercise hyperaemia.