Endothelin secretion is regulated by cyclic AMP and phosphatase 2A in endothelial cells

Involvement of the metabolic sensor GPR81 in cardiovascular control

GPR81 is a receptor for the metabolic intermediate lactate with an established role in regulating adipocyte lipolysis. Potentially novel GPR81 agonists were identified that suppressed fasting plasma free fatty acid levels in rodents and in addition improved insulin sensitivity in mouse models of insulin resistance and diabetes. Unexpectedly, the agonists simultaneously induced hypertension in rodents, including wild-type, but not GPR81-deficient mice. Detailed cardiovascular studies in anesthetized dogs showed that the pressor effect was associated with heterogenous effects on vascular resistance among the measured tissues: increasing in the kidney while remaining unchanged in hindlimb and heart. Studies in rats revealed that the pressor effect could be blocked, and the renal resistance effect at least partially blocked, with pharmacological antagonism of endothelin receptors. In situ hybridization localized GPR81 to the microcirculation, notably afferent arterioles of the kidney. In conclusion, these results provide evidence for a potentially novel role of GPR81 agonism in blood pressure control and regulation of renal vascular resistance including modulation of a known vasoeffector mechanism, the endothelin system. In addition, support is provided for the concept of fatty acid lowering as a means of improving insulin sensitivity.

Inhibition of basal and stimulated release of endothelin-1 from guinea pig tracheal epithelial cells in culture by beta 2-adrenoceptor agonists and cyclic AMP enhancers

The effects of cyclic AMP-related compounds and beta adrenoceptor agonists on the basal and lipopolysaccharide (LPS)-stimulated release of endothelin-1 (ET-1) from guinea-pig tracheal epithelial cells (GPTEpCs) in culture were studied. Forskolin (a potent activator of adenylyl cyclase), 8-bromo-cyclic AMP (a cyclic AMP analogue), salbutamol and salmeterol (two beta 2-adrenoceptor agonists), were used to increase cyclic AMP levels. Cultured GPTEpCs released ET-1 continuously over a 24 h incubation period. The values reached 1,938 +/- 122 pg/mg of total cell proteins after 24 h. LPS (10 microg/ml) significantly stimulated the release of ET-1 by 1.6- to 1.8-fold, up to 1,262 +/- 56 pg/mg total cell proteins after an 8 h incubation period. Compound 8-bromo-cyclic AMP (10(-5), 10(-4) and 10(-3) M) reduced the basal release of ET-1 from GPTEpCs by up to 31% (P < 0.01) and the LPS stimulated release by up to 42% (P < 0.05), after an 8 h incubation period. Forskolin (10(-6), 10(-5) and 10(-4) M) also inhibited the basal release of ET-1 by up to 28% (P < 0.05) and LPS-stimulated release of ET-1 by up to 50% (P < 0.05), after an 8 h incubation period. At the concentration of 10(-5) M, forskolin increased cyclic AMP levels in GPTEpCs by 17-fold (P < 0.001) in the medium, 15 min after the beginning of the incubation. Salbutamol (10(-8) to 10(-6) M) had no effect on the basal production and release of ET-1 after 8 h. Conversely, this short acting beta 2-adrenoceptor agonist significantly reduced LPS-mediated increase of ET-1 production by up to 55% (P < 0.05) after an 8 h incubation period. Salmeterol (10(-9) M to 10(-5) M) inhibited basal and LPS-stimulated production and release of ET-1 after an 8 h incubation period (between 44 and 51%, P < 0.01). Both salbutamol and salmeterol (10(-6) M) increase cyclic AMP levels by five- and twofold, respectively (P < 0.05). In summary, these observations indicate that beta 2-adrenoceptor agonists or cyclic AMP enhancers can modulate both basal and more markedly, the enhanced production of ET-1 from LPS-activated guinea pig airway EpCs. In addition, these compounds increase cyclic AMP levels in the cells. It is suggested that there is a correlation between cyclic AMP increase and inhibition of ET-1 release by guinea pig airway EpCs. Since ET-1 production was shown to be elevated in asthmatic subjects and in patients suffering from other inflammatory lung disorders, the inhibition of its production by beta adrenoceptor agonists, such as salbutamol and salmeterol, could be added to their therapeutical benefits.

Inhaled nitric oxide combined with prostacyclin and adrenomedullin in acute respiratory failure with pulmonary hypertension in piglets

Our aim was to evaluate if the combined inhalation of both nitric oxide (iNO) and aerosolized prostacyclin or iNO and adrenomedullin (ADM) is more effective in lowering pulmonary arterial pressure (PAP) and improving oxygenation than nitric oxide alone in an animal model with pulmonary hypertension (PH). Moreover, we studied the effect on pulmonary mechanics, surfactant activity, and pulmonary oxidative stress of the different treatments. Twenty-eight piglets with acute lung injury induced by lung lavages with saline were randomized to receive nitric oxide, nitric oxide plus prostacyclin, nitric oxide plus ADM or saline, after. Dynamic compliance, tidal volume, and airway resistance were measured. Lung tissue oxidation was evaluated by measuring total hydroperoxide and advanced oxidation protein products in bronchial aspirate samples. Surface surfactant activity was studied using Capillary Surfactometer. Inhaled nitric oxide combined with prostacyclin or ADM was more effective than nitric oxide alone in lowering PAP and improving oxygenation. Nitric oxide alone or combined increased lung compliance and tidal volume, and decreased airway resistance. No effects on surfactant surface activity and lung tissue oxidation were observed. The treatment with nitric oxide alone or combined with prostacyclin or ADM were effective in decreasing mean PAP and improving oxygenation in a piglet model of PH. However, nitric oxide plus prostacyclin and nitric oxide plus ADM were more effective than nitric oxide alone. The combination of aerosolized prostacyclin and ADM with nitric oxide might have a role in the treatment of infants with PH refractory to nitric oxide alone.

Pilot intervention: aerosolized adrenomedullin reduces pulmonary hypertension

In pulmonary hypertension, systemic infusion of adrenomedullin (ADM), a potent vasodilator peptide, leads to pulmonary vasodilatation. However, systemic blood pressure declines alike. The present study investigated the effect of aerosolized ADM on pulmonary arterial pressure in surfactant-depleted newborn piglets with pulmonary hypertension. Animals randomly received aerosolized ADM (ADM, n = 6), aerosolized ADM combined with intravenous application of NG-nitro-l-arginine methylester to inhibit nitric-oxide (NO) synthases (ADM + l-NAME, n = 5), or aerosolized normal saline solution (control, n = 6). Aerosol therapy was performed in 30-min intervals for 5 h. After a total experimental period of 8 h, mRNA expression of endothelial and inducible NO synthase and endothelin-1 (ET-1) in lung tissue was quantified using TaqMan real-time polymerase chain reaction. Aerosolized ADM reduced mean pulmonary artery pressure (MPAP) compared with control (p < 0.001; at the end of the study, Delta-MPAP -13.5 +/- 1.4 versus -6.2 +/- 2.4 mm Hg). PaO2 significantly increased in the ADM (DeltaPaO2 243.3 mm Hg) and the ADM + l-NAME group (DeltaPaO2 217.4 mm Hg) compared with the control group (DeltaPaO2 82.9 mm Hg; p < 0.001). Aerosolized ADM did not influence mean systemic arterial pressure (baseline 63.2 +/- 2.7 versus end of the study 66.3 +/- 6.5 mm Hg; not significant). NO synthases gene expressions were 20 to 30% lower with ADM compared with control. ET-1 gene expression was significantly reduced (>50%) after ADM aerosol therapy (p < 0.001). Aerosolized adrenomedullin significantly reduced MPAP without lowering the systemic arterial pressure and improved profoundly the arterial oxygen tension. This effect seems to be mediated at least in part by the reduction of ET-1.

Regulation of endothelin-1 production in cultured rat vascular smooth muscle cells

Endothelin-1 (ET-1) is secreted from all rat vascular smooth muscle cells (VSMCs) examined, in addition to endothelial cells (ECs). An average secretion rate of ET-1 from rat VSMCs was determined to be 10% that excreted from ECs. We examined the effects of 22 substances on ET-1 secretion from VSMCs and compared them with those from ECs. Transforming growth factor-beta1 (TGF-beta), acidic and basic fibroblast growth factors, epidermal growth factor, angiotensin II, and adrenaline stimulated ET-1 secretion from VSMCs, whereas forskolin, thrombin, and platelet-derived growth factor-BB reduced it. Only TGF-beta and phorbol ester elicited consistent effects on ET-1 secretion from VSMCs and ECs. Regulation of ET-1 and adrenomedullin secretion from VSMCs was distinctly different. These data suggest that ET- 1 production in VSMCs is regulated by a mechanism separate from that in ECs and from adrenomedullin production in VSMCs. Chromatographic analysis showed immunoreactive ET-1 secreted from VSMCs was mainly composed of big ET- 1, whereas approximately 90% of that from ECs was ET-1. By TGF-beta stimulation of VSMCs, the ratio of big ET-1 to ET-1 was further increased. Because big ET-1 is converted into ET-1 only on the surface of the ECs in the culture system, big ET-1 secreted from the VSMCs may function as a mediator transmitting a signal from VSMCs to ECs in vivo.

Inhibition of protein phosphatase 1 decreases PTH secretion from isolated dispersed parathyroid cells

To investigate the regulation of parathyroid hormone secretion by phosphatases we examined the effect of okadaic acid, a selective inhibitor of protein phosphatases (PP)-1 and -2A, on isolated, dispersed parathyroid cells. Okadaic acid inhibited secretion from intact bovine, intact human and streptolysin-O permeabilized bovine cells. Approximately 10(-6) M okadaic acid resulted in a 50% decrease in parathyroid hormone (PTH) secretion from both intact and permeabilized cells, consistent with PP-1 being the target of inhibition. Upon subcellular fractionation, PP-1 overlapped but was not identical to either PTH, a marker of the secretory granule, or Na+/K+-ATPase, a plasma membrane marker. In summary, PP-1 activity is involved in Ca2+-dependent but not basal PTH secretion.

Regulation of endothelin-1 in human non-pigmented ciliary epithelial cells by tumor necrosis factor-alpha

Endothelins (ET) are potent vasoactive peptides present in many ocular structures and are formed from precursor Big endothelins (Big ET-1) by the action of an endothelin-converting enzyme (ECE). ET-1 is thought to decrease intraocular pressure by contracting the ciliary muscle thus enhancing the outflow of aqueous humor through the Canal of Schlemm and trabecular meshwork. However, the mechanisms involved in the regulation of endothelin-1 (ET-1) synthesis and release in ocular tissues have not been fully characterized. In this study we examined the effect of tumor necrosis factor-alpha(TNF-alpha; 10 nm), a proinflammatory cytokine, on the cellular mechanisms leading to ET-1 synthesis and release in SV-40 transformed human ciliary non-pigmented epithelial cells (HNPE). ET-1 and Big endothelin-1 (Big ET-1) immunoreactivity was time-dependently increased following TNF-alphatreatment. Phorbol esters (PMA), activators of PKC, also raised the immunoreactive levels of ET-1 and Big ET-1 while, staurosporine, a PKC inhibitor (20 nm), decreased ET-1 levels in TNF-alpha-stimulated cells. Pre-treatment with phosphoramidon (1 micron) an ECE-inhibitor, followed by TNF-alpha stimulation, decreased ir-ET-1 levels. Cycloheximide (9 micron), a protein synthesis inhibitor, decreased TNF-alpha-stimulated levels for ir-ET-1 and ir-Big ET-1, suggesting that TNF-alpha may be directly regulating ET-1 expression at the ET-1 gene. Our data indicates that TNF-alpha regulates ET-1 levels in HNPE cells possibly by activating PKC either to stimulate protein synthesis and/or to enhance ET-1 secretion. These results suggest that ET-1 released from the ciliary body may play an important role in aqueous humor dynamics following cytokine activation.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

Purinergic agonists stimulate the secretion of endothelin-1 in rat thyroid FRTL-5 cells

The aim of the present study was to investigate the mechanisms regulating endothelin-1 (ET-1) secretion in rat thyroid FRTL-5 cells. ET-1 was found to be secreted after stimulation with adenosine and ATP. The release of ET-1 was sensitive to pertussis toxin, indicating a role of G-proteins in the stimulus-secretion coupling. The stimulation evoked by ATP or adenosine was inhibited by the P1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and in the presence of adenosine deaminase the adenosine- and ATP-mediated ET-1 secretion was abolished. These evidences suggest a role of a P1-adenosine receptor in the secretion of ET-1. Increasing cyclic AMP with forskolin decreased the adenosine-mediated secretion. In addition, the intracellular calcium chelator BAPTA or inhibition of calcium entry with Ni2+ prevented the response. Protein kinase C (PKC) is also partly involved in ET-1 secretion in FRTL-5 cells. Activation of PKC with the phorbol ester phorbol 12-myristate 13-acetate (PMA) stimulated the secretion of ET-1 in a time- and dose-dependent manner. Furthermore, downregulation of PKC decreased the secretion of ET-1 stimulated by adenosine. In conclusion, ET-1 secretion in FRTL-5 cells is stimulated via a pertussis toxin-sensitive P1-receptor pathway which is modulated by several signal transduction mechanisms including cAMP, Ca2+, and PKC.