Cardioprotective effects of bosentan in 5-fluorouracil-induced cardiotoxicity

5-fluorouracil (5-FU) is a widely used chemotherapeutic agent but cardiotoxicity challenges its clinical usefulness. Thus, searching for more cardioprotective drugs is highly required to prevent the accompanied cardiac hazards. Up to date, the different mechanisms involved in 5-FU cardiotoxicity are still unclear and there is no evaluation of bosentan's role in controlling these cardiac complications. This forced us to deeply study and evaluate the possible cardiopreserving properties of bosentan and different mechanisms involved in mediating it. 32 Wistar albino rats were included in our experiment and induction of cardiotoxicity was performed via administration of 5-FU (150 mg/kg) on 5th day of the experiment by intraperitoneal (i.p.) injection with or without co-administration of bosentan (50 mg/kg/day) orally for 7days. Our data revealed that 5-FU could induce cardiotoxicity which was detected as significant increases of troponin I, lactate dehydrogenase (LDH), creatine kinase- MB (CK-MB), endothelin receptors, malondialdehyde (MDA), toll like receptor4 (TLR4), myeloid differentiation primary response 88 (MyD88), nuclear factor kappa B (NFκB), and caspase 3 levels. However, there is marked decrease in endothelial nitric oxide synthase (eNOS), reduced glutathione (GSH) and total antioxidant capacity (TAC). In addition, the histopathological examination showed severe toxic features of cardiac injury. Interestingly, co-administration of bosentan could ameliorate 5-FU-induced cardiotoxicity via improving the detected biochemical and histopathological changes besides modulation of TLR4/MyD88/NFκB signaling pathway, eNOS, and endothelin receptors. Bosentan had a significant cardioprotective effect against 5-FU induced cardiac damage. This effect may be attributed to its ability to inhibit endothelin receptors, stimulates eNOS, anti-oxidant, anti-inflammatory, anti-apoptotic properties with modulation of TLR4/MyD88/NFκB signaling pathway.

Endothelin: 30 Years From Discovery to Therapy

Discovered in 1987 as a potent endothelial cell-derived vasoconstrictor peptide, endothelin-1 (ET-1), the predominant member of the endothelin peptide family, is now recognized as a multifunctional peptide with cytokine-like activity contributing to almost all aspects of physiology and cell function. More than 30 000 scientific articles on endothelin were published over the past 3 decades, leading to the development and subsequent regulatory approval of a new class of therapeutics-the endothelin receptor antagonists (ERAs). This article reviews the history of the discovery of endothelin and its role in genetics, physiology, and disease. Here, we summarize the main clinical trials using ERAs and discuss the role of endothelin in cardiovascular diseases such as arterial hypertension, preecclampsia, coronary atherosclerosis, myocardial infarction in the absence of obstructive coronary artery disease (MINOCA) caused by spontaneous coronary artery dissection (SCAD), Takotsubo syndrome, and heart failure. We also discuss how endothelins contributes to diabetic kidney disease and focal segmental glomerulosclerosis, pulmonary arterial hypertension, as well as cancer, immune disorders, and allograft rejection (which all involve ET_A autoantibodies), and neurological diseases. The application of ERAs, dual endothelin receptor/angiotensin receptor antagonists (DARAs), selective ET_B agonists, novel biologics such as receptor-targeting antibodies, or immunization against ET_A receptors holds the potential to slow the progression or even reverse chronic noncommunicable diseases. Future clinical studies will show whether targeting endothelin receptors can prevent or reduce disability from disease and improve clinical outcome, quality of life, and survival in patients.

Modulation of Calcium Signaling of Angiotensin AT1, Endothelin ETA, and ETB Receptors by Silibinin, Quercetin, Crocin, Diallyl Sulfides, and Ginsenoside Rb1

Angiotensin II and endothelin-1 are potent vasoconstrictive peptides that play a central role in blood pressure regulation. Both peptides exert their pleiotropic effects via binding to their respective G-protein-coupled receptors, i.e., angiotensin AT1 and endothelin type A and type B receptors. In the present study, we have selected six structurally different plant-derived compounds with known cardioprotective properties to evaluate their ability to modulate calcium signaling of the above-mentioned receptors. For this purpose, we used and validated a cellular luminescence-based read-out system in which we measured intracellular calcium signaling in Chinese hamster ovary cells that express the calcium sensitive apo-aequorin protein. Firstly, silibinin, a flavanolignan that occurs in milk thistle (Silybum marianum), was investigated and found to be an antagonist for the human angiotensin AT1 receptor with an affinity constant of about 9 µM, while it had no effect on endothelin type A or type B receptor activation. Quercetin and crocin partially impeded intracellular calcium signaling resulting in a non-receptor-related reduction of the responses recorded for the three investigated G-protein-coupled receptors. Two organosulfur compounds, diallyl disulfide and diallyl trisulfide, as well as the triterpene saponin ginsenoside Rb1 did not affect the activation of the angiotensin AT1 and endothelin type A and type B receptors. In conclusion, we were able, by using a nonradioactive cellular read-out system, to identify a novel pharmacological property of the flavanolignan silibinin.

sGC stimulation totally reverses hypoxia-induced pulmonary vasoconstriction alone and combined with dual endothelin-receptor blockade in a porcine model

AIM

Stimulation of soluble guanylate cyclase (sGC) with BAY 41-8543 was hypothesized to attenuate acute hypoxic pulmonary vasoconstriction alone and combined with dual endothelin (ET)-receptor antagonist tezosentan.

METHODS

Measurements were taken in 18 anaesthetized pigs with a mean ± SEM weight of 31.1 ± 0.4 kg, in normoxia (FiO(2)~0.21) and hypoxia (FiO(2)~0.10) without (control protocol, n = 6), and with right atrial infusion of BAY 41-8543 at 1, 3, 6, 9 and 12 μg min(-1) per kg (protocol 2, n = 6) or tezosentan at 5 mg kg(-1) followed by BAY 41-8543 at 1, 3 and 6 μg min(-1) per kg (protocol 3, n = 6).

RESULTS

Hypoxia (n = 18) increased (P < 0.001) mean pulmonary artery pressure (MPAP) and pulmonary vascular resistance (PVR) by 14.2 ± 0.6 mmHg and 2.8 ± 0.3 WU respectively. During sustained hypoxia without treatment, MPAP and PVR remained stable. BAY 41-8543 (n = 6) dose-dependently decreased (P < 0.001) MPAP and PVR by 15.0 ± 1.2 mmHg and 4.7 ± 0.7 WU respectively. Tezosentan (n = 6) decreased (P < 0.001) MPAP and PVR by 11.8 ± 1.2 mmHg and 2.0 ± 0.2 WU, respectively, whereafter BAY 41-8543 (n = 6) further decreased (P < 0.001) MPAP and PVR by 6.6 ± 0.9 mmHg and 1.9 ± 0.4 WU respectively. Both BAY 41-8543 and tezosentan decreased (P < 0.001) systemic arterial pressure and systemic vascular resistance. Blood-O(2) consumption remained unaltered (P = ns) during all interventions.

CONCLUSION

BAY 41-8543 totally reverses the effects of acute hypoxia-induced pulmonary vasoconstriction, and enhances the attenuating effects of tezosentan, without affecting oxygenation. Thus, sGC stimulation, alone or combined with dual ET-receptor blockade, could offer a means to treat pulmonary hypertension related to hypoxia and potentially other causes.

Normothermic perfusion of ischaemic porcine kidneys: an evaluation of ex vivo function and endothelin receptor antagonism

BACKGROUND

Non-heart-beating donors suffer a period of warm ischaemia (WI) injury, resulting in higher rates of delayed graft function compared with heart-beating donors. Endothelin is believed to play a pathophysiological role in the intense vasospasm associated with WI. This study aimed to investigate renal function using normothermic perfusion and potential therapeutic intervention with endothelin receptor antagonism.

METHODS

Porcine kidneys, subjected to varying periods of in situ WI (<10, 30 and 60 minutes), underwent normothermic machine perfusion with a perfluorodecalin perfusate. Intrarenal vascular resistance (IRR) was measured throughout perfusion, and urine and perfusate biochemical analysis was performed after 1 and 3 hours. Endothelin receptor antagonism was tested by administration of BQ-123 and BQ-788.

RESULTS

Initial IRR during perfusion increased with WI. IRR reduced to similar levels in all WI groups after 90 minutes of perfusion, but subsequently rose after 150 minutes. BQ-123 considerably increased the initial IRR; however, it abolished the rise seen towards the end of perfusion. BQ-788 had little effect. Renal metabolism and function deteriorated with increasing WI. Perfusion had adverse effects on renal function and metabolism. Endothelin receptor antagonism had little effect on renal function.

CONCLUSIONS

This study suggests that ex vivo assessment after normothermic perfusion correlates with warm ischaemic damage. However, endothelin receptor antagonism does not ameliorate the rise in IRR or renal function after kidneys are subjected to a warm ischaemic insult.

Role of endothelin in neonatal morphine tolerance

Management of neonatal opioid tolerance and withdrawal symptoms remains a major challenge in neonatal intensive care units. We provide evidence that central endothelin (ET) mechanisms are involved in the development of morphine tolerance in neonatal rats. Pregnant rats were rendered tolerant to morphine and rat pups were delivered by cesarean section. The effect of morphine tolerance on characteristics of ET receptors in neonatal rats was determined. The affinity (Kd) and density (Bmax) of [I]ET-1 binding in the brain was found to be similar in placebo and morphine-tolerant neonatal rats. Morphine and ET-1-induced G-protein stimulation was determined in placebo and morphine-tolerant neonatal rats by [S]GTPgammaS binding assay. Morphine produced significantly lower (P < 0.05) maximal stimulation in morphine-tolerant neonatal rats (33.10%) when compared with placebo-treated neonatal rats (90.90%). Maximal stimulation produced by ET-1 in morphine-tolerant neonatal rats (41.26%) was also significantly lower (P < 0.05) as compared with placebo-treated neonatal rats (92.23%). This is the first report indicating the involvement of ET in neonatal morphine tolerance as evidenced by attenuation of ET-1-induced stimulation of GTP binding in neonatal rats tolerant to morphine.

[The relationship between endothelin receptors and chronic venous insufficiency of lower extremities]

OBJECTIVE

To investigate the effect of endothelin receptors in chronic venous insufficiency (CVI) in lower extremities.

METHODS

Ten cases of varicose veins from CVI patients (as case group) and ten cases of non-varicose veins (as control group) were investigated in this study. The two groups were divided into two groups respectively: endothelium-intact group and de-endothelium groups. The vasoconstriction mediated by endothelin A (ETA) and endothelin B (ETB) receptors was recorded with myography. The distribution of ETA and ETB receptors was detected by immunohistochemistry method.

RESULTS

Endothelin-1 (ET-1) and sarafotoxin 6c (S6c) induced concentration-dependent contraction in the veins. In endothelium-intact veins, the E(max) and pD(2) of contraction curve induced by ET-1 were 132.30% +/- 43.42% and 6.03 +/- 0.35, respectively in control group;and were 19.24% +/- 12.94% and 6.78 +/- 0.46, respectively in case group. The E(max) and pD(2) in case group were much lower than in control group (P < 0.05). The E(max) and pD(2) induced by S6c were 30.10% +/- 12.90% and 6.54 +/- 0.36, respectively in control group, and were 9.61% +/- 1.32% and 6.75 +/- 0.29, respectively in case group; The E(max) in case group was lower than in control group (P < 0.05). In de-endothelium veins, E(max) and pD(2) of S6c were 146.18% +/- 32.33% and 6.50 +/- 0.17 in control group, and 32.93% +/- 3.00% and 6.69 +/- 0.39 in case group; The E(max) in case group was significantly lower than in control group (P < 0.05). ETA receptors was located in endothelium mainly, and ETB receptors in smooth muscle cells mainly. The sites of both ETA and ETB receptors were decreased in case group obviously.

CONCLUSIONS

The contraction mediated by ETA receptor and ETB receptor was decreased with a decrease of ETA receptor and ETB receptor sites in varicose veins of CVI. The contraction insufficiency and down-expression of ETA receptor and ETB receptor are correlated with CVI.

Endogenous nitric oxide reduces the efficacy of the endothelin system to maintain blood pressure during high epidural anaesthesia in conscious dogs

BACKGROUND AND OBJECTIVE

During high epidural anaesthesia, endothelin only contributes minimally to blood pressure stabilization. This phenomenon could result from the inhibitory action of nitric oxide on the endothelin system. To clarify this, we studied the interaction between nitric oxide and endothelin during high epidural anaesthesia in conscious dogs, in comparison to the interaction of nitric oxide and vasopressin.

METHODS

Six animals were used in 45 individual experiments randomly arranged as follows: N-omega-nitro-arginine-methylester 0.3-10 mg kg-1 under physiological conditions or during high epidural anaesthesia (lidocaine 1%) and N-omega-nitro-arginine-methylester (l-NAME) 0.3-10 mg kg-1 after preceding endothelin (Tezosentan(R)) or vasopressin (beta-mercapto-beta,beta-cyclo-penta-methylene-propionyl-O-Me-Tyr-Arg-vasopressin) receptor blockade under physiological conditions or during high epidural anaesthesia. During control experiments normal saline was injected either intravenously (n = 5) or into the epidural space (n = 4).

RESULTS

N-omega-nitro-arginine-methylester increased mean arterial pressure dose-dependently in all groups. However, this effect was substantially reduced in the presence of the endothelin receptor antagonist compared to N-omega-nitro-arginine-methylester alone, both under control conditions (7 +/- 3 vs. 21 +/- 3 mmHg; P < 0.05) and during high epidural anaesthesia (17 +/- 3 vs. 30 +/- 1 mmHg; P < 0.05). Blockade of vasopressin showed no similar relationship with N-omega-nitro-arginine-methylester.

CONCLUSIONS

The diminished increase in mean arterial pressure after injection of N-omega-nitro-arginine-methylester only during endothelin receptor blockade indicates that endogenous nitric oxide inhibits the action of endothelin during high epidural anaesthesia and might thus explain the reduced efficacy of endothelin in maintaining blood pressure during high epidural anaesthesia.

Endothelin receptor blockade does not improve hypoxemia following acute pulmonary thromboembolism

We studied the roles of endothelins in determining ventilation (V̇a) and perfusion (Q̇) mismatch in a porcine model of acute pulmonary thromboembolism (APTE), using a nonspecific endothelin antagonist, tezosentan. Nine anesthetized piglets (∼23 kg) received autologous clots (∼20 g) via a central venous catheter at time = 0 min. The distribution of V̇a and Q̇ at five different time points (−30, −5, 30, 60, 120 min) was mapped by fluorescent microspheres of 10 different colors. Five piglets ( group 1) received tezosentan (courtesy of Actelion) starting at time = 40 min for 2 h, and four piglets ( group 2) received only saline and served as control. Our results showed that, in all of the animals at 30 min following APTE but before tezosentan, the mean V̇a/Q̇ was increased, as was V̇a/Q̇ heterogeneity (log SD V̇a/Q̇), which represented a widening of its main peak. Afterwards, tezosentan attenuated the pulmonary hypertension in group 1 but also produced moderate systemic hypotension. However, it did not improve arterial Po2 or V̇a/Q̇ mismatch. We concluded that endothelin antagonism had minimal impact on gas exchange following APTE and confirmed our earlier observation that the main mechanism for hypoxemia in APTE was due to the mechanical redistribution of pulmonary regional blood flow away from the embolized vessels, resulting in the creation of many divergent low and high V̇a/Q̇ regions.

Endothelin

In humans, the endothelins (ETs) comprise a family of three 21-amino-acid peptides, ET-1, ET-2 and ET-3. ET-1 is synthesised from a biologically inactive precursor, Big ET-1, by an unusual hydrolysis of the Trp21 -Val22 bond by the endothelin converting enzyme (ECE-1). In humans, there are four isoforms (ECE-1a-d) derived from a single gene by the action of alternative promoters. Structurally, they differ only in the amino acid sequence of the extreme N-terminus. A second enzyme, ECE-2, also exists as four isoforms and differs from ECE-1 in requiring an acidic pH for optimal activity. Human chymase can also cleave Big ET-1 to ET-1, which is cleaved, in turn, to the mature peptide as an alternative pathway. ET-1 is the principal isoform in the human cardiovascular system and remains one of the most potent constrictors of human vessels discovered. ET-1 is unusual in being released from a dual secretory pathway. The peptide is continuously released from vascular endothelial cells by the constitutive pathway, producing intense constriction of the underlying smooth muscle and contributing to the maintenance of endogenous vascular tone. ET-1 is also released from endothelial cell-specific storage granules (Weibel-Palade bodies) in response to external stimuli. ETs mediate their action by activating two G protein-coupled receptor sub-types, ETA and ET(B). Two therapeutic strategies have emerged to oppose the actions of ET-1, namely inhibition of the synthetic enzyme by combined ECE/neutral endopeptidase inhibitors such as SLV306, and receptor antagonists such as bosentan. The ET system is up-regulated in atherosclerosis, and ET antagonists may be of benefit in reducing blood pressure in essential hypertension. Bosentan, the first ET antagonist approved for clinical use, represents a significant new therapeutic strategy in the treatment of pulmonary arterial hypertension (PAH).

Chemically programmed antibodies: endothelin receptor targeting CovX-Bodies

Aryl sulfonamide-based endothelin antagonists were synthesized and covalently linked to the reactive lysine of the m38C2 antibody to create a series of CovX-Bodies. These chemically programmed antibodies behaved as potent endothelin receptor antagonists in vitro and had antitumor efficacy in a prostate cancer xenograft model which, on a molar basis, far exceeded the activity of the parent small molecule.

Current treatment options in children with pulmonary arterial hypertension and experiences with oral bosentan

Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by vasoconstriction and progressive remodelling of the pulmonary arterial wall leading to right ventricular failure and death. Idiopathic PAH (IPAH) and PAH associated with congenital heart defects account for the majority of paediatric patients with PAH. During the last few decades, several pharmacological approaches have been introduced, including calcium channel-blockers (CCBs), prostacyclin analogues, endothelin receptor antagonists and, most recently, phosphodiesterase inhibitors. This paper reviews the treatment options available to children with a special focus on the initial experience with bosentan. Although CCBs have been shown to increase survival in IPAH, the beneficial effect appears to be limited to a small number of patients, defined as 'responders' to the vasoreactivity testing. With the availability of prostacyclin (intravenous epoprostenol) and then prostacyclin analogues, the treatment options have increased markedly and particularly in patients who have not responded to conventional therapy. Although epoprostenol has been shown to be efficacious in PAH, the drug is not ideal owing to serious complications arising from the invasive mode of application, particularly in children. Phosphodiesterase-5 inhibitors have also shown beneficial effects. Targeting the endothelin (ET) system with the oral, dual ET(A)/ET(B) receptor antagonist, bosentan has been demonstrated to improve the cardiopulmonary haemodynamics, exercise capacity, quality-of-life and survival in adult patients with PAH. Specific ET(A) antagonists may also present the same beneficial profile. Recent experience with bosentan in paediatric patients with PAH indicates that the results obtained in adult patients may be extrapolated to children, thus offering a safe and effective therapy that is easy to administer.

[Endothelin-1, angiotensin II and cancer]

Endothelin-1 (ET-1) and angiotensin II (AngII), two potent vasoactive peptides involved in the regulation of cardiovascular homeostasis, also induce mitogenic and pro-angiogenic responses in vitro and in vivo. Both peptides are produced by cleavage of inactive precursors by metalloproteases (endothelin-converting enzyme and angiotensin-converting enzyme, respectively) and activate two subtypes of membrane receptors (ETA-R and ETB-R for ET-1, AT1R and AT2R for AngII) that all belong to the superfamily of G-protein coupled receptors. There is increasing evidence that ETA-R, ETB-R and AT1R are expressed in a variety of cancer cells and tissues, and may play a role on tumor growth, angiogenesis and invasion in vivo. This review summarizes the similarities and differences between the ET-1 and AngII systems with regard to their reported effects on various aspects of cancer. In addition to being expressed on vascular endothelium, ET-1 and AngII receptors participate in tumor angiogenesis through the production of the angiogenic factor VEGF. Furthermore, recent clinical studies indicate that a selective ETA-R antagonist has beneficial effects in prostate cancer, suggesting that a similar approach using ETB-R and AT1R blockers might be envisioned. Experimental data presented here suggest that a combined therapy targeting both ET-1 and AngII systems may prove valuable for future treatments of highly angiogenic tumors.

Aldosterone breakthrough during ras blockade: A role for endothelins and their antagonists?

Activation of the renin-angiotensin system (RAS), with ensuing aldosterone excess, detrimentally affects outcome in patients with hypertension and heart failure (HF). RAS blockade with angiotensin (Ang) 1-converting enzyme inhibitors (ACEIs) or Ang II type 1 receptor blockers (ARBs) is beneficial in such conditions. However, aldosterone secretion can persist despite these treatments. Hence, mechanisms besides Ang II acquire the role of aldosterone secretagogue. The RALES and EPHESUS studies have shown that this aldosterone "escape" or "breakthrough" is an important factor, because it is a determinant of outcome in HF patients. Endothelin (ET)-1, which stimulates aldosterone secretion via both A (ETA) and B (ETB) receptor subtypes, and which is increased in HF, is a candidate for the "aldosterone breakthrough." Moreover, the novel ET peptide ET-1(1-31) is involved in adrenocortical growth. Therefore, findings suggesting a role for the ET-1 system as an aldosterone secretagogue, along with the potential usefulness of endothelin antagonists for the prevention of "aldosterone breakthrough," are discussed.

The role of endothelin-1 in hyperoxia-induced lung injury in mice

Background

As prolonged hyperoxia induces extensive lung tissue damage, we set out to investigate the involvement of endothelin-1 (ET-1) receptors in these adverse changes.

Methods

Experiments were performed on four groups of mice: control animals kept in room air and a group of mice exposed to hyperoxia for 60 h were not subjected to ET-1 receptor blockade, whereas the dual ETA/ETB-receptor blocker tezosantan (TEZ) was administered via an intraperitoneal pump (10 mg/kg/day for 6 days) to other groups of normal and hyperoxic mice. The respiratory system impedance (Zrs) was measured by means of forced oscillations in the anesthetized, paralyzed and mechanically ventilated mice before and after the iv injection of ET-1 (2 μg). Changes in the airway resistance (Raw) and in the tissue damping (G) and elastance (H) of a constant-phase tissue compartment were identified from Zrs by model fitting.

Results

The plasma ET-1 level increased in the mice exposed to hyperoxia (3.3 ± 1.6 pg/ml) relative to those exposed to room air (1.6 ± 0.3 pg/ml, p < 0.05). TEZ administration prevented the hyperoxia-induced increases in G (13.1 ± 1.7 vs. 9.6 ± 0.3 cmH₂O/l, p < 0.05) and H (59 ± 9 vs. 41 ± 5 cmH₂O/l, p < 0.05) and inhibited the lung responses to ET-1. Hyperoxia decreased the reactivity of the airways to ET-1, whereas it elevated the reactivity of the tissues.

Conclusion

These findings substantiate the involvement of the ET-1 receptors in the physiopathogenesis of hyperoxia-induced lung damage. Dual ET-1 receptor antagonism may well be of value in the prevention of hyperoxia-induced parenchymal damage.

Effect of endothelin-1 on lipolysis in rat adipocytes

OBJECTIVE

To explore the role of endothelin-1 (ET-1) on lipid metabolism, we examined the effect of ET-1 on lipolysis in rat adipocytes.

RESEARCH METHODS AND PROCEDURE

Adipocytes isolated from male Sprague-Dawley rats, weighing 400 to 450 grams, were incubated in Krebs-Ringer buffer with or without 10(-7) M ET-1 for various times or with various concentrations of ET-1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET(A)R and ET(B)R blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET-1-stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET-1 treatment. To further explore the underlying mechanism of ET-1 action, we examined the involvement of the extracellular signal-regulated kinase (ERK)-mediated pathways.

RESULTS

Our results showed that ET-1 caused lipolysis in rat adipocytes in a time- and dose-dependent manner. BQ610, a selective ET(A)R blocker, blocked this effect. The adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine, had no effect on ET-1-stimulated lipolysis. ET-1 did not induce an increase in intracellular cAMP levels. In addition, ET-1-induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET-1 and insulin suppressed ET-1-stimulated lipolysis.

DISCUSSION

These findings show that ET-1 stimulates lipolysis in rat adipocytes through the ET(A)R and activation of the ERK pathway. The underlying mechanism is cAMP-independent. However, this non-conventional lipolytic effect of ET-1 is inhibited by the anti-lipolytic effect of insulin.

Endothelin action on pituitary lactotrophs: one receptor, many GTP-binding proteins

The endothelins are a family of hormones that have a biphasic action on pituitary lactotrophs. The initial effect is stimulatory, followed later by inhibition that persists long after the agonist has been removed. Recent research has uncovered several G protein pathways that mediate these effects.

Short-term exposure of renal proximal tubules to gentamicin increases long-term multidrug resistance protein 2 (Abcc2) transport function and reduces nephrotoxicant sensitivity

We previously showed that the function of renal multidrug resistance protein (Mrp) 2 (Abcc2) is reduced by endothelin (ET)-1 signaling through an ET(B) receptor, nitric-oxide synthase (NOS), cGMP, and protein kinase C and that this pathway was activated by several nephrotoxicants (Masereeuw et al., 2000; Terlouw et al., 2001; Notenboom et al., 2002, 2004). Here, we determined the long-term effects on Mrp2-mediated transport (luminal fluorescein methotrexate accumulation) of short-term (30 min) exposure to ET-1 and the aminoglycoside antibiotic, gentamicin. Our data show that over the 3 h following exposure, proximal tubules recovered fully from the initial decrease in Mrp2-mediated transport and that transport activity was not changed 9 h later. However, 24 h after exposure, luminal accumulation of an Mrp2 substrate had increased by 50%. Increased transport at 24 h was accompanied by an increased transporter protein content of the luminal plasma membrane as measured by immunostaining. Blocking ET-1 signaling at the ET(B) receptor or downstream at NOS or guanylyl cyclase abolished both stimulation of transport and increased transporter expression. Thus, regardless of whether signaling was initiated by a short exposure to ET-1 or to a nephrotoxicant, the time course of Mrp2 response to ET(B) signaling was the same and was multiphasic. Finally, when tubules were exposed to gentamicin for 30 min and removed to gentamicin-free medium for 24 h, they were less sensitive to acute gentamicin toxicity than paired controls not initially exposed to the drug. Thus, short-term exposure to ET-1 or gentamicin resulted in long-term protection against a second insult.

A facile synthesis of new 2-carboxamido-3-carboxythiophene and 4,5,6,7-tetrahydro-2-carboxamido-3-carboxythieno[2,3-c]pyridine derivatives as potential endothelin receptors ligands

Endothelins (ETs) are the most ubiquitous, highly potent and unusually long-lasting peptidic constrictors of human vessels known. Elevated levels of the plasma concentration of ETs were observed in several diseases such as hypertension, acute myocardial infarction, congestive heart failure, renal failure, pulmonary hypertension, and atherosclerosis. ETs exert their activities via specific seven-transmembrane, G protein-coupled receptors. To date two receptor subtypes, endothelin A (ET(A)) and endothelin B (ET(B)), have been identified and cloned. A literature survey revealed that a number of compounds that bind ET receptors with affinity and selectivity are known, nevertheless these compounds belong only to few chemical classes. The aim of this work is the identification of an "hit compound" with novel chemical structure endowed with reasonable ET affinity and selectivity. Accordingly, new variously substituted 2-carboxamido-3-carboxythiophene derivatives (29-52) were synthesized. These compounds were tested for their ability to inhibit ETs binding in radioligand binding assay using CHO cells stably expressing human ET(A) and ET(B) receptors.

Multiple-dose pharmacokinetics, pharmacodynamics and tolerability of the oral ET(A) endothelin-receptor antagonist SPP301 in man

BACKGROUND

SPP301 is a competitive antagonist of ET-1 with a high selectivity for the ET(A) receptor. The multiple-dose pharmacokinetics, pharmacodynamics, safety and tolerability of SPP301 were investigated in healthy male subjects.

METHODS

In an ascending-dose, double-blind, placebo-controlled trial doses of 5, 20, 40 and 60 mg SPP301 were given orally on Day 1 and, after a wash-out period of 48 hours, once daily for seven days. At regular intervals, blood pressure and pulse rate, plasma levels of ET-1, and of SPP301 and its hydroxymethyl metabolite as well as urinary excretion of the parent drug and its metabolite were determined.

RESULTS

SPP301 was generally well-tolerated. With the higher doses asymptomatic and transient increases in liver transaminases were observed. No other clinically relevant effects of SPP301 on hematology, biochemistry or urinalysis parameters were observed. Vital signs, ECG parameters and physical examinations showed no time or treatment effect. The pharmacokinetics of SPP301 and its hydroxymethyl metabolite (Ro 68-5925) were linear up to 40 mg SPP301. Steady state was reached after 3-4 days. The apparent terminal half-life of SPP301 and the metabolite was in the range of 7-10 hours after single and repeated doses. At the 60 mg dose level plasma concentrations of SPP301 decreased from the first to the last day of oral treatment. The average decreases in Cmax and AUC were 33% and 37%, respectively. Cmax and AUC of the metabolite amounted to about 4-6% of the values for SPP301 under single and repeated-dose conditions. Urinary excretion of SPP301 and of its metabolite were below 0.1% and 5%, respectively.

CONCLUSION

SPP301 is quite well-tolerated, pharmacokinetics are linear and time-independent up to 40 mg multiple doses. Steady state is reached after 3-4 days. Urinary excretion of the unchanged drug plays a minor role in the elimination process of SPP301. ET-1 plasma concentrations increased about 1.5-fold at 20 mg and all further doses.

Physiological involvement of placental endothelin-1 and prostaglandin F2alpha in uteroplacental circulatory disturbance in pregnant rats exposed to heat stress

Several studies suggest that heat stress affects placental functions including uteroplacental circulation, subsequently leading to pregnancy failure and birth weight reduction. To clarify the involvement of endothelin and placental prostaglandin (PG) systems in the uteroplacental circulation during heat stress, we examined the effects of i.v. administration of the endothelin receptor antagonist bosentan and the cyclooxygenase inhibitor indomethacin on uteroplacental blood flow and on placental PGE2 and PGF2alpha levels and their 13,14-dyhydro-15-keto-metabolites (PGEM and PGFM, respectively) in heat-exposed or non-heat-exposed pregnant rats. The administration of bosentan or indomethacin did not change uteroplacental blood flow in non-heat-exposed pregnant rats. In contrast, heat reduced uteroplacental blood flow in pregnant rats, but the reduction was reversed by the administration of bosentan or indomethacin before heat exposure. Heat did not change placental PGE2 or PGEM levels, but in pregnant rats it increased placental PGF2alpha and PGFM levels, which were reversed by bosentan or indomethacin. Our results suggest that the activation of placental endothelin receptor and PGF2alpha systems are involved in the uteroplacental circulatory disturbances produced by heat. PGF2alpha systems activated by heat may be involved in the vasoconstricting effects of endothelin-A and -B receptors during heat exposure.

Aging decreases vasoconstrictor responses of coronary resistance arterioles through endothelium-dependent mechanisms

OBJECTIVE

Aging decreases coronary blood flow and maximal reserve capacity. Impaired blood flow capacity may be related to an increased vasoconstrictor capacity of coronary resistance vessels. This study tested the hypothesis that aging increases the vasoconstrictor responsiveness of coronary arterioles isolated from myocardium of young (4 months) and old (24 months) Fischer 344 rats.

METHODS

Isolated coronary arterioles were cannulated and pressurized (60 cm H2O) via hydrostatic pressure reservoirs.

RESULTS

Contrary to our hypothesis, aging decreased responsiveness of coronary arterioles to endothelin (ET, 1 x 10(-11)-3 x 10(-8) M), potassium chloride (KCl, 10-100 mM), and pressure-induced myogenic responses (0-140 cm H2O). Removal of the endothelium from coronary arterioles increased vasoconstriction to all agonists; however, age-related KCl vasoconstrictor response differences remained, suggesting that K+ channel activity and/or the relative contribution of specific K+ channels to maintenance of vascular smooth muscle membrane potential may change with age. Removal of the endothelium, in addition to increasing responsiveness, eliminated aging-induced differences in ET- and pressure-induced vasoconstriction. L-NAME (10(-5)) incubation resulted in a greater enhancement of spontaneous tone in arterioles from old rats compared to those of young rats. ETB (BQ-788, 3 x 10(-8)) receptor blockade eliminated the age-associated differences.

CONCLUSION

Collectively, these data suggest an age-associated increase in endothelial modulation of coronary resistance vessel constriction. This enhanced endothelial attenuation of coronary arteriolar constriction appears to result from increased basal release of nitric oxide. These alterations of coronary vascular reactivity may contribute to age-induced redistribution of coronary blood flow and diminished cardiac function.

Novel anticancer targets and drug discovery in post genomic age

Cancer is a serious disease with a complex pathogenesis, which threats human life greatly. Currently, great efforts have been put to the identification of novel anticancer targets and the discovery of anticancer drugs following the progress of chemogenomics, which will be reviewed briefly in this article. Furthermore, during the past 5 years, the global effort of sequencing human genome has provided us with an enormous number of potential targets associated with cancer therapy. As a result, the New Drug Discovery (NDD) is undergoing a transition "from gene to drug". Accordingly, the targets for anticancer drugs studies now are focused on some biological macromolecular targets associated with cancer and several interactive mechanisms involved in the growth and metastasis of cancer cells as well as tumor angiogenesis, such as Matrix Metalloproteinases (MMPs), Aminopeptidase N (APN), Tyrosine Kinase (TK), Farnesyltransferase (FTase) and cell Signal Transduction Pathway and so forth. Among these targets the MMP-2, -9 and APN are the most extensively studied enzymes in our laboratory. The peptidomimetics Matrix Metalloproteinase Inhibitors (MMPIs) and APN inhibitors (APNIs) with the molecular scaffold of pyrrolidine, 3-amino-2-hydroxy-4-phenyl butyric acid (AHPA) and glutamylide, which have been designed and synthesized in our laboratory, will be described in the review, among which the pyrrolidine scaffold is patented with the IC(50) ranging from 1 nM to 300 nM against MMP-2, and MMP-9.

Bosentan inhibits transient receptor potential channel expression in pulmonary vascular myocytes

Bosentan, a dual endothelin receptor blocker, has been used clinically to treat idiopathic pulmonary arterial hypertension (IPAH). However, the mechanism of its antiproliferative effect on pulmonary artery smooth muscle cells (PASMCs) remains unclear. A rise in cytoplasmic Ca2+ stimulates PASMC proliferation and the canonical transient receptor potential (TRPC) channels are an important pathway for Ca2+ entry during PASMC proliferation. Bosentan (20-50 microM) significantly inhibited endothelin-1- or platelet-derived growth factor (PDGF)-mediated PASMC growth and [3H]thymidine uptake. In PASMCs, endothelin-1 (1 microM) and PDGF (10 ng/ml) both upregulated protein expression of TRPC6, whereas bosentan markedly downregulated TRPC6 protein levels. Furthermore, TRPC6 expression in PASMCs from patients with IPAH was greater than in normal PASMCs, and the antiproliferative effect of bosentan was significantly enhanced in IPAH-PASMCs in comparison with normal PASMCs. These observations demonstrate that the antiproliferative effect of bosentan on PASMCs involves the downregulation of TRPC6 channels via a mechanism possibly independent of endothelin receptor blockade. The greater effect of bosentan on IPAH-PASMCs than on normal PASMCs suggests that increased TRPC6 expression and function may be involved in the overgrowth of PASMCs in patients with IPAH.

[Endothelin system and inhalation of iloprost in pulmonary hypertension]

The haemodynamics and vascular proliferation (remodelling) in pulmonary hypertension are unfavourably affected by a malfunction of the endothelial cells of the pulmonary vessels. A characteristic result is reduced formation of endothelial dilators and increased formation of vasoconstrictors. The favourable effects of a blockade of the endothelin (ET) system with receptor antagonists suggest that the increased formation and limited clearance of ET-1 reaches pathogenetic significance. The therapeutic effects of inhaled iloprost on right cardiac haemodynamics and clinical course, could in addition to the direct vasodilatory effect, also be considered to have a favourable influence on the ET system as a pathophysiologically effective mediator system.

Arteries and veins desensitize differently to endothelin

Hypertension is accompanied by increased arterial endothelin-1 (ET-1) and decreased arterial contraction to ET-1. By contrast, veins remain responsive to ET-1 in hypertension. Isometric contraction was used to test the hypothesis that veins do not desensitize to ET-1 to the extent of arteries, possibly because of the presence of functional ETA and ETB receptors on veins and only functional ETA receptors on arteries. Contraction to ET-1 after exposure to ET-1 (100 nmol/L) was abolished in aortae, while in veins 36.3 +/- 0.2% of maximal contraction to ET-1 remained. Aortae were unresponsive to the ETA receptor agonist ET-1(1-31) (100 nmol/L) after ET-1 exposure, while 21.9 +/- 0.6% of maximum venous contraction to ET-1 (1-31) remained. In a similar manner, the venous ETB receptor did not lose responsiveness to the ETB receptor agonist sarafotoxin 6c (S6c, 100 nmol/L); aortae did not contract to S6c. In ET-1-desensitized veins, the ETB receptor antagonist BQ-788 (100 nmol/L) decreased maximum contraction to ET-1, but did not alter potency (-log EC50 control = 8.14 +/- 0.01 mol/L; BQ-788 = 8.13 +/- 0.04 mol/L). The ETA receptor antagonist atrasentan (100 nmol/L) blocked remaining venous contraction to ET-1 (control = 8.05 +/- 0.05 mol/L; atrasentan = unmeasurable). Maintained responsiveness to ET-1 in veins occurs primarily via the ETA receptor, while in arteries the ETA receptor is responsible for desensitization to ET-1.

Alteration in endothelin receptor sub-type responsiveness and in the endothelin-TXA(2) mimetic U46619 interaction, in type-2 hypertensive diabetic Zucker rats

BACKGROUND

Type-2 diabetes is characterized by endotheliopathy, which increases target organ damage and mortality. There is excessive endothelin-1 and TXA(2) production, and abnormal vascular reactivity to endothelin-1, manifested as a paradoxical hypotensive action in Zucker diabetic, but not lean rats. We examined the hypothesis that there is an alteration in the ET-A/ET-B receptor subtype sensitivity, and/or the interaction or cross-talk between ET-1 and TXA(2) in type-2 diabetes, using Zucker diabetic rats and their lean littermates.

MATERIALS AND METHODS

Hemodynamic studies were performed in lean and Zucker fatty diabetic rats of both sexes. Laser doppler flowmetry was used to measure renal cortical (RCF) and medullary blood flow (MBF) responses. Dose response curves for mean arterial blood pressure (MAP), MBF and RCF in response to ET-1, U46619, acetylcholine, and L-NAME (25mg/kg) were constructed after pre-treatment of the rats with either BQ610 1mg/kg i.v. or BQ788 0.5mg/kg i.v. The effects of BQ610 and BQ788 on whole blood impedance aggregation were also assessed.

RESULTS

BQ788, but not BQ610 abolished both the paradoxical hypotensive action of ET-1 in Zucker diabetic rats (n=7 each, P<0.001 ANOVA) as well as the dose-dependent rise in MBF (P<0.001 ANOVA). BQ788, but not BQ610 abolished the difference in response to ET-1 between lean and diabetic Zucker rats. U46619 caused a hypotensive action in male Zucker rats which was abolished by L-NAME 25mg/kg or indomethacin 10mg/kg i.v. The U46619 interaction with BQ788 on both MAP and MBF was significantly (P<0.03 ANOVA) different between lean and diabetic Zucker rats. BQ788, but not BQ610 attenuated both the MAP and MBF responses to acetylcholine or L-NAME P<0.02 ANOVA). However, BQ610 dose-dependently attenuated the slope of platelet aggregation in both lean and Zucker diabetic rats (P<0.02 ANOVA).

CONCLUSION

ET-B receptor antagonism abolished the abnormal vascular reactivity and MBF responses to ET-1, and also normalized the vasoactive responses to the level seen in healthy lean Zucker rats. ET-1 receptor blockade influences the responses to TXA(2) receptor activation. In the systemic and renal circulation, this interaction appears to be mostly ET-B receptor mediated, whilst in platelets, ET-A receptor role may be predominant. The interaction or cross-talk between ET-1 and TXA(2) is altered in the type-2 diabetic state. Collectively, these pathophysiological changes may contribute to the vicious circle of diabetic endotheliopathy.

Contribution of endothelin and its receptors to the regulation of vascular tone during exercise is different in the systemic, coronary and pulmonary circulation

OBJECTIVES

Exercise-induced vasodilation is thought to be mediated through various vasodilator substances, but blunting the influence of vasoconstrictors such as ET may also play a role. However, the role of ET and its receptors in the regulation of systemic, pulmonary and coronary vascular resistance is incompletely understood. The aim of this study was to identify the contribution of ET-1 through the ET(A) and ET(B) receptors to the regulation of tone in the systemic, coronary and pulmonary beds at rest and during exercise.

METHODS

Ten chronically instrumented swine were studied while running on a treadmill before and after ET(A) blockade (EMD122946) or ET(AB) blockade (tezosentan).

RESULTS

At rest, EMD122946 resulted in vasodilation in the coronary and systemic circulation, evidenced by a decrease in coronary and systemic vascular resistance and an increase in coronary and mixed venous O(2)-saturation. These effects waned during exercise. The effect of tezosentan on the systemic vasculature was similar to that of EMD122946, whereas it was smaller in the coronary circulation. EMD122946 had no effect on the pulmonary vasculature, whereas tezosentan decreased pulmonary resistance but only during exercise.

CONCLUSIONS

ET exerts a constrictor influence on the coronary and systemic circulation through the ET(A)-receptor, which decreases during exercise thereby contributing to metabolic vasodilation. ET exerts a tonic vasodilator influence on coronary resistance vessels through the ET(B)-receptor. Finally, ET exerts an ET(B)-mediated constrictor influence in the pulmonary vasculature during exercise.

Actions of the anti-oestrogen agent clomiphene on outward K+ currents in rat ventricular myocytes

1. The effects of clomiphene (CLM) on cardiac outward K+ current components from rat isolated ventricular myocytes were investigated using the whole-cell patch-clamp technique. Clomiphene (10 micromol/L) significantly inhibited both peak (Ipeak) and end-pulse (Ilate) outward currents (elicited by a 500 msec voltage step from -40 to +50 mV in the presence of K+-containing intracellular and extracellular solutions) by approximately 37% (n = 6; P < 0.01) and 49% (n = 6; P < 0.01), respectively. In contrast, CLM had no effect on outward currents when K+-free solutions were used. 2. A double-pulse protocol and Boltzmann fitting were used to separate individual K+ current components on the basis of their voltage-dependent inactivation properties. At potentials positive to -80 mV, two inactivating transient outward components (Ito) and (IKx) and a non-inactivating steady state component (Iss) could be distinguished. 3. Clomiphene inhibited both Ito and Iss. The maximal block of Ito and Iss induced by CLM (100 micromol/L) was approximately 61% (n = 5) and 43% (n = 5) with IC50 values of 1.54 +/- 0.39 and 2.2 +/- 0.4 micromol/L, respectively. In contrast, the peak magnitude of IKx was unaltered by CLM, although its time-course of inactivation was accelerated. 4. Further experiments whereby myocytes were superfused with the vasoactive peptide endothelin (ET)-1 (20 nmol/L) revealed that CLM (10 micro mol/L) completely abolished the ET-1-sensitive component of Iss. 5. Our findings demonstrate, for the first time, the effects of CLM on distinct cardiac K+ current components and show that CLM modulates the voltage-gated K+ current components Ito and IKx and inhibits the steady state outward current Iss in rat ventricular myocytes.

Vasoactive peptides, their receptors and drug development

Vasoactive peptides with vasoconstrictor properties play an important role in many physiological and pathological conditions. The peptides act via specific receptors, most of them belonging to the group of seven transmembrane G-protein coupled receptors. These receptors have become important targets for drugs developed to inhibit vasoconstrictor actions. Alternatively, compounds which inhibit enzymes generating vasoactive peptides have also been demonstrated to represent valuable therapeutic tools. This review will first describe the properties and distribution of two very potent vasoconstrictors, angiotensin II and endothelin. It will further focus on their receptors and on new drugs, which act as antagonists for these receptors. In addition, the properties of indirectly acting drugs like angiotensin-converting enzyme inhibitors and--in analogy--endothelin-converting enzyme inhibitors will be presented.

Lysosomal traffic of liganded endothelin B receptor

The endothelin B receptor (ETB) is an endothelial cell receptor found in caveolae. Studies with GFP-tagged ETB have suggested that the protein is constitutively endocytosed and targeted to lysosomes where it is rapidly degraded. We report that iodinated endothelin-1 ligand (ET-1) is taken up by cells transfected with ETB and remains undegraded for at least 17 h. Analysis of the intracellular traffic of endocytosed ET-1 on isotonic Ficoll gradients shows that it is rapidly internalised to lysosomes by a chloroquine sensitive and cholesterol dependent pathway. Low-temperature nonreducing SDS gels show that the ET-1 initially binds to full-length GFP-tagged ETB, which is rapidly clipped at the amino-terminus and is then stable for at least 6 h. Analysis of GFP tagged ETB on reducing SDS gels shows that it is proteolytically cleaved with a half time of approximately 3 h. However, nonreducing gels show that the receptor is virtually intact, suffering only a similar cleavage to the liganded receptor. We conclude that the ETB receptor shows remarkable stability in lysosomes, held together by disulfide bonds, and maintaining ligand binding for long periods of time.

ETA endothelin receptors are involved in the ouabain-induced haemodynamic effects in the periaqueductal gray area of rats

The aim of the present study was to asses the effects on blood pressure and vascular resistances elicited by microinjections of ouabain (OUA) within the periaqueductal gray area (PAG). We also tested whether peripheral vascular responses caused by exogenous intra-PAG ouabain involve activation of the PAG-endothelin system. In normotensive Sprague-Dawley rats the basal mean arterial blood pressure (MABP) was 114 +/- 3 mmHg. This was significantly increased by OUA (3 micro g, 122 +/- 2 mmHg, p < 0.05; and 6 micro g, 139 +/- 3 mmHg, p < 0.01) microinjected into the PAG area. Increases in MABP were associated with increases in total peripheral resistances (TPR), organ vascular resistances, and with reduced blood flow of almost all the organs tested: kidneys, skeletal muscle, skin, stomach, spleen, testes and intestine. Cardiac output did not change. Changes in the above vascular parameters induced by OUA were significantly (p < 0.01) reduced by intra-PAG microinjections of FR139317 (a selective ETA receptor antagonist, 5 nmol), SB209670 (a non-selective ETA/ETB receptor antagonist, 3 nmol), but not by BQ 788 (a selective ETB receptor antagonist, 5 nmol). In conclusion, OUA into the PAG area of normotensive rats caused significant changes in peripheral vascular parameters that are reduced by ETA receptor antagonists. These results indicate that PAG-ET-1 system via an action on ETA receptors is involved in the OUA effects.

Dexamethasone regulates endothelin-1 and endothelin receptors in human non-pigmented ciliary epithelial (HNPE) cells

Endothelin-1 (ET-1) lowers intraocular pressure (IOP) in animal models by regulating aqueous humour dynamics through both inflow and outflow mechanisms. Moreover, ET's concentration is elevated in glaucoma patients and in animal models of glaucoma. Glucocorticoid therapy often can lead to increase IOP in susceptible individuals including patients with primary open angle glaucoma (POAG). In this study, we examined the effects of dexamethasone (Dex), a frequently used anti-inflammatory glucocorticoid, on the synthesis and release of endothelin-1 and on the expression of endothelin receptors in human non-pigmented ciliary epithelial (HNPE) cells, an established source for ET-1 in the anterior chamber. As measured by ET-1 immunoreactivity, ET-1 was concentration-dependently increased following 24hr Dex treatment, with a maximum concentration (100 nM) causing a threefold increase of ET-1 release. Western blot analysis of HNPE cells showed the expression of endothelin receptor A (ET(A)) and endothelin receptor B (ET(B)) with approximate molecular weights of 40 kDa. Dex treatment decreased ET(A) receptor expression at all Dex doses, but up-regulated ET(B) receptors with 10nM Dex having the greatest effect. Quantitative PCR demonstrated that Dex also increased the mRNA of pre-pro-ET-1 (ppET-1) and ET(B) but decreased the mRNA of ET(A). RU486, a glucocorticoid receptor antagonist, was able to block Dex's actions on ET release and ET(B) receptor expression, but did not block its action on ET(A) receptor expression. Endothelin receptors were minimally expressed in HNPE cells as determined in binding experiments (B(max): ET(A) 17, ET(B) 25 fmolmg(-1) membrane protein). However Dex treatment stimulated a dramatic increase in ET(B) receptor density while decreasing ET(A) receptors (B(max): ET(A) 11, ET(B) 116 fmolmg(-1) membrane protein). The regulation of endothelin and its receptors could be a novel mechanism associated with glucocorticoid's effects on intraocular pressure. The increase in ET-1 and disproportionate regulation in ET receptor expression by Dex could promote dysregulation in ET's mechanism on both inflow and outflow, thus affecting aqueous humour dynamics in the anterior chamber of the eye.

Role of phosphoinositide 3-kinase in the nonselective cation channel activation by endothelin-1/endothelinB receptor

We recently demonstrated that endothelin-1 (ET-1) activates two types of Ca(2+)-permeable nonselective cation channel (designated NSCC-1 and NSCC-2) in Chinese hamster ovarian cells expressing endothelin(B) receptor (CHO-ET(B)R). These channels can be discriminated using the Ca(2+) channel blockers, LOE 908 and SK&F 96365. LOE 908 is a blocker of NSCC-1 and NSCC-2, whereas SK&F 96365 is a blocker of NSCC-2. In this study, we investigated the possible role of phosphoinositide 3-kinase (PI3K) in the ET-1-induced activation of NSCCs in CHO-ET(B)R using wortmannin and LY-294002, inhibitors of PI3K. ET-1-induced Ca(2+) influx was partially inhibited in CHO-ET(B)R pretreated with wortmannin or LY-294002. In contrast, addition of wortmannin or LY-294002 after stimulation with ET-1 did not suppress Ca(2+) influx. The Ca(2+) channels activated by ET-1 in wortmannin- or LY-294002-treated CHO-ET(B)R were sensitive to LOE 908 and resistant to SK&F 96365. In conclusion, NSCC-2 is stimulated by ET-1 via PI3K-dependent cascade, whereas NSCC-1 is stimulated independently of the PI3K pathway. Moreover, PI3K seems to be required for the initiation of the Ca(2+) entry through NSCC-2 but not for its maintenance.

Effect of endothelin-1 (1-31) on the renal resistance vessels

Human chymase produces not only angiotensin II but also endothelin(ET)-1(1-31). We previously reported that ET-1(1-31) had several biological activities in vascular smooth muscle cells. In this study, we investigated the vasoconstrictor effect of ET-1(1-31) on the renal resistance vessels using in vitro microperfused rabbit afferent and efferent arterioles. ET-1(1-31) decreased the lumen diameter of the afferent and efferent arterioles dose-dependently. ET-1(1-31)-induced afferent arteriolar vasoconstriction was not affected by phosphoramidon, an ET converting enzyme inhibitor. ET-1(1-31)-induced renal arteriolar vasoconstriction was inhibited by BQ123, an ETA receptor inhibitor, but not by BQ788, an ETB receptor inhibitor. These results suggest that ET-1(1-31)-induced renal arteriolar vasoconstriction may be mediated by ETA-like receptors.

Role of K+ATP channels, endothelin A receptors, and effect of angiotensin II on blood flow in oral tissues

K+(ATP) channels are involved in CGRP-mediated vasodilation and in the vasoconstriction induced by endothelin or angiotensin II. In this study, we examined the effects of a K+(ATP) channel antagonist and an ET(A) receptor antagonist on resting blood flow in the pulp and gingiva, and observed their role in the vasodilation induced by tooth stimulation. We also investigated whether receptors for angiotensin II exist in the pulp and gingiva. Blood flow was measured with laser-Doppler flowmetry. Under control conditions, the K+(ATP) channel antagonist and angiotensin II caused a significant drop in blood flow in both target tissues. Blocking of ET(A) receptor did not change basal blood flow. The vasodilation observed after tooth stimulation remained unchanged following blockade of K+(ATP) channels and ET(A) receptors. Analysis of the data shows that open K+(ATP) channels exist during resting conditions in the pulp and gingiva, but that CGRP seems to induce vasodilation mainly via mechanisms other than K+(ATP) channels. ET(A) and AT(1) receptors are found in the pulp and gingiva, but ET(A) receptors are not involved in modulation of a basal vascular tone in these tissues or in the vasodilation observed after tooth stimulation.

Xenobiotic export pumps, endothelin signaling, and tubular nephrotoxicants--a case of molecular hijacking

This article is a review on recent studies in intact renal proximal tubules that link tubular nephrotoxicants with endothelin (ET) regulation of xenobiotic export pump function. The data show that transport on p-glycoprotein and Mrp2 decreases rapidly when ET signals through an ET(B) receptor, NO synthase (NOS), and protein kinase C (PKC). Surprisingly, nephrotoxicants, such as radiocontrast agents, aminoglycoside antibiotics, and heavy metal salts, "hijack" this signaling pathway, causing ET release from the tubules, hormone binding to its receptor, activation of NOS and PKC, and reduced xenobiotic transport. These findings suggest a new common mechanism by which nephrotoxicants may act to disrupt renal tubular function.

Mechanism of endothelin-1-induced cytosolic Ca(2+) mobility in cultured H9c2 myocardiac ventricular cells

The effect of endothelin-1 (ET-1) on the intracellular free Ca(2+) ([Ca(2+)](i)) mobility in cultured H9c2 myocardiac ventricular cells was studied after loading with fura-2-AM. In Ca(2+)-containing buffer, ET-1 induced [Ca(2+)](i) rise from 10(-7) to 10(-9) M. ET-1 induced [Ca(2+)](i), which was composed of a first small peak and a secondary persistent plateau. In Ca(2+)-free buffer, pretreatment with 10(-7) M ET-1 inhibited the thapsigargin and carbonylcyanide m-chlorophenylhydrazone (CCCP)-induced [Ca(2+)](i) increase. Meanwhile, pretreatment with thapsigargin and CCCP also inhibited ET-1-induced [Ca(2+)](i) rise. In Ca(2+)-containing buffer, the ET(A) receptor antagonist (BQ123) completely abolished the secondary rising peak and plateau. Conversely, the ET(B) receptor antagonist (BQ788) completely inhibited the first small peak and secondary peak plateau. Nifedipine and La(3+) also abolished the 10(-7) M ET-1-induced [Ca(2+)](i) in the first rising peak. The internal Ca(2+) release induced by ET-1 was inhibited by U73122 (phospholipase C inhibitor), propranolol (phospholipase D inhibitor) and aristolochic acid (phospholipase A2 inhibitor). After incubation of 10(-7) M ET-1 in Ca(2+)-free buffer, the addition of 5 mM CaCl(2) increased Ca(2+) influx, implying that release of Ca(2+) from internal stores further induces capacitative Ca(2+) entry. Taken together, these results suggest that both ET(A) and ET(B) receptors are involved in ET-1-induced [Ca(2+)](i) rise in H9c2 myocardiac ventricular cells. Whereas ET(B) receptor seems to mediate the initial Ca(2+) influx via L-type Ca(2+) channel, ET(A) receptor appears to be involved in the subsequent Ca(2+) release from endoplasmic reticulum and mitochondria Ca(2+) stores.

Myocardial effects of ETB receptor stimulation

INTRODUCTION

Endothelin-1 (ET-1) is an endogenous peptide whose effects are mediated by two distinct types of receptors, ETA and ETB. Whereas the ETA receptors promote vasoconstriction and mitogenesis and increase inotropism, ETB receptors have vasodilatory and anti-mitogenic properties mediated by nitric oxide release. In this study we investigated the myocardial effects of selective ETB receptor stimulation.

METHODS

The study was performed on right papillary muscles (n = 30) from New Zealand white rabbits (Krebs-Ringer; 1.8 mM CaCl2; 35 degrees C). The effects of selective ETB receptor activation by Sarafotoxin S6c (0.2 microM; n = 6) and of non-selective ETA and ETB receptor activation by ET-1 (1 nM; n = 9) were studied. The effects of ET-1 were also evaluated in the presence of a selective ETA receptor antagonist, BQ-123 (0.1 microM; n = 9) and of a selective ETB receptor antagonist BQ-788 (0.1 microM; n = 6). Only significant results (mean +/- SE; p < 0.05) are given, expressed as % baseline.

RESULTS

Sarafotoxin S6c reduced active tension (AT) by 8.1 +/- 5.5% and peak rate of tension development (dT/dtmax) by 8.6 +/- 5.6%. Alone, ET-1 increased AT by 64.2 +/- 18.2% and dT/dtmax by 58.6 +/- 20.2%. These effects of ET-1 were exacerbated in the presence of BQ-788 (AT increased by 82.6 +/- 17.5% and dT/dtmax by 121.3 +/- 26.6%) and inverted in the presence of BQ-123 (AT decreased by 12.8 +/- 2.7% and dT/dtmax by 16.1 +/- 3.0%).

CONCLUSIONS

This study demonstrates, for the first time, that selective stimulation of ETB receptors has a negative inotropic effect. Detailed characterization of the effects of stimulation of each type of ET-1 receptor is of particular relevance as selective and non-selective inhibitors of these receptors are currently being tested for treatment of heart failure.

Endothelin-2 is a macrophage chemoattractant: implications for macrophage distribution in tumors

Endothelins (ET-1, ET-2 and ET-3) are 21-amino acid vasoactive peptides that bind to G-protein-linked transmembrane receptors, ET-RA and ET-RB. As well as modulating vasoconstriction, endothelins regulate growth in several cell types and may also affect differentiation, inflammation and angiogenesis. Both macrophages and endothelins are found in areas of hypoxia in solid tumors and ET-2 expression may be modulated by hypoxia in some tumors. As the peptide structure of mature endothelins is similar to that of CXC chemokines, we asked if endothelins contribute to control of macrophage distribution in tumors. We found that ET-2 is a chemoattractant for macrophages and THP-1 monocytic cells, but not for freshly isolated monocytes. The chemotactic response to ET-2 shows a typical bell-shaped response curve. Experiments with endothelin receptor antagonists showed that migration to ET-2 is mediated via the ET-RB receptor. Moreover, monocytes do not express ET-RB. Chemotaxis towards ET-2 is via the MAPK pathway: p44 and p42 are phosphorylated when THP-1 cells are stimulated with ET-2, and the MAPKK inhibitor PD98059 stops chemotaxis. As with 'classical' chemokines, migration toET-2 is also inhibited by hypoxia and by pertussis toxin. As well as its chemotactic properties, ET-2 leads to activation of macrophages. In human breast tumors that express ET-2, endothelins and ET-RB expressing macrophages often co-localized. While shorter than 'classical' chemokines, ET-2 shares a similar peptide sequence with chemokines and may signal via a similar receptor and MAPK-mediated pathway. Furthermore, ET-2 expression by tumors may modulate the behavior of macrophages such that activated cells accumulate in areas of hypoxia.

Endothelin B receptors located on the endothelium provide cardiovascular protection in the hamster

Endothelins (ETs) act through two receptors, namely ET(A) and ET(B). In the cardiovascular system, the activation of both receptors leads to vasoconstriction. However, ET(B) receptors also mediate endothelium-dependent vasodilatation and clearance of plasma ET-1. With regard to these latter properties, we wanted to assess the contribution of ET(B) receptors and the effects of selective and mixed ET receptor blockade on vascular tone in control Syrian Golden hamsters and in Bio 14.6 cardiomyopathic hamsters after bolus injection of ET-1 and IRL-1620, a selective ET(B) agonist. In 12-week-old anaesthetized control hamsters, ET-1 (0.5 nmol/kg) induced a sustained pressor response which was only partly reduced by the selective ET(A) receptor antagonist BQ-123, suggesting a contribution of ET(B) receptor activation to the vasoconstrictive effects of ET-1. This was confirmed by injection of the selective ET(B) receptor agonist IRL-1620 (1 nmol/kg). However, the pressor response to this agonist was always preceded by a transient vasodilatation, indicating activation of endothelium-located ET(B) receptors. When the selective ET(B) receptor antagonist BQ-788 was administered, the hypotensive phase following IRL-1620 injection was abolished. Interestingly, BQ-788 or a mixture of BQ-788 and BQ-123 significantly potentiated the pressor responses to ET-1. In 12-week-old Bio 14.6 cardiomyopathic hamsters, ET-1 and IRL-1620 induced haemodynamic responses similar to those observed in control hamsters, although the IRL-1620-induced pressor increase was lower. No difference in cardiac prepro ET-1 mRNA expression was observed between the two strains of hamsters. In conclusion, we suggest that endothelium-located ET(B) receptors are involved in the physiological antagonism of ET-dependent protracted pressor effects, and thus may play a protective role in both normal hamsters and those with cardiomyopathy.

L-nitro-arginine inhibits increase in endothelin binding sites induced by ischemia and reperfusion

We have demonstrated that ischemia and reperfusion promoted augmented contractile response to endothelin-1 (ET) in coronary arteries in the presence of polymorphonuclear leukocytes (PMN). It has been also reported that ischemia and reperfusion increase ET binding sites in cardiac membrane in isolated rat heart perfused by blood cell-free system. To determine the role of PMN and L-arginine to nitric oxide (NO) pathway in these phenomena, isolated perfused rabbit hearts were subjected to 30 min of global ischemia followed by 30 min of reflow in the absence or presence of PMN and 10(-5)M of L-nitro-arginine (LNA). PMN was prepared with Percoll density gradients from peritoneal exudate elicited by glycogen. PMN activated with 10(-6)M of phorbol myristate acetate or their supernatant were infused into the coronary perfusion circuit after 5 min of reflow. LNA was added to perfusate also after reflow. The effect of superoxide dismutase (SOD: 50 IU/ml) was also determined. After the end of protocols, membrane fraction was isolated from the hearts for (125)I-ET-1 binding assay. ET-1 binding (Bmax) showed a significant increase by ischemia and reperfusion (P<0.01 vs control). That was markedly augmented with addition of activated PMN or their supernatant (both P<0.01), but abolished either by LNA or SOD (P<0.01 and P<0.05, respectively). These results indicate that increase in ET-receptor by ischemia and reperfusion is mediated by free radicals generated via L-arginine to NO pathway.

The response of the lamb ductus arteriosus to endothelin: developmental changes and influence of light

Endothelin-1 (ET-1) is a putative messenger of oxygen in the ductus arteriosus. Since the ability of the vessel to contract to oxygen increases with gestation, we wished to ascertain whether ET-1 action is also developmentally regulated. A corollary objective was to assess whether any gestational variation in the ET-1 contraction is due to a change in the ET(A)-mediated action or to a shift in the balance between opposing, contractile (ET(A) - mediated) and relaxant (ET(B)-mediated), actions. Experiments were performed with isolated ductal strips from preterm (0.7 gestation) and near-term fetal lambs. ET-1 contracted the ductus dose-dependently (10(-10)-10(-7) M) at both ages; however, the peak contraction was about double in magnitude at term. Regardless of age, ET-1 contraction was greater with preparations kept in the dark compared to those exposed to light. This effect of light was not seen after removing the endothelium or when treating the intact tissue with the ET(B) antagonist BQ788 (1 microM). In the dark, however, BQ788 did not modify significantly the ET-1 response at either age. We conclude that ET-1 becomes a stronger ductus constrictor with fetal age, conceivably by acting on ET(A) receptors. Hence, the concept of ET-1 mediating the oxygen contraction is further validated. Peculiarly, the ET-1 contraction is curtailed by light through a hitherto undefined ET(B) receptor-linked process.

Cyclooxygenase inhibitors attenuate endothelin ET(B) receptor-mediated contraction in human temporal artery

It is well documented that endothelin ET(B) receptor-mediated contraction develops in artery segments incubated in culture and that the reaction is augmented by proinflammatory cytokines, but little is known of the mechanisms involved. Segments of human temporal artery were incubated in organ culture for 2 days in the absence or presence of interleukin-1 beta (IL-1 beta), with or without nonsteroidal anti-inflammatory drugs, glucocorticoids or a nitric oxide synthase inhibitor. Thereafter, contractions were induced by the selective endothelin ET(B) receptor agonist, sarafotoxin S6c. Acetylsalicylic acid, indomethacin, nimesulide and rofecoxib were all effective in eliminating the increase in endothelin ET(B) receptor-mediated contraction induced by interleukin-1 beta, but only indomethacin and rofecoxib significantly reduced the spontaneous development of this reaction in cultured arteries. Dexamethasone and methylprednisolone augmented the reaction, and the nitric oxide synthase inhibitor had no effect. The results clearly indicate a role for cyclooxygenase, most likely cyclooxygenase-2, in endothelin ET(B) receptor-mediated contraction in this preparation.

Localization of immunoreactive endothelin and characterization of its receptors in aortic cusps

BACKGROUND AND AIM OF THE STUDY

Aortic valve cusp tissue has been shown to have contractile properties in response to a range of common vasoactive agents. Of these, endothelin (ET) is both the most potent and efficacious.

METHODS

In an attempt to define the mechanism of action and localization of ET, the response of porcine aortic valve cusps to ET and the selective ET(B) receptor agonist sarafotoxin 6c (S6c) was examined, in the presence and absence of ET(A) and ET(B) receptor antagonists. An attempt was made, using immunocytochemical techniques, to localize ET in cusp tissue.

RESULTS

Addition of 90 mM KCl produced a mean contractile response of 1.02+/-0.09 mN (n = 27). ET (10- to 10(-7)M) produced a concentration-dependent contraction of aortic valve cusps, with a maximum response of 116.7+/-12.7% (n = 6) of that obtained with 90 mM KCl. In a similar manner, 10(-5)M of the selective ET(A) receptor antagonist BQ123 (n = 4) and 10(-5)M of the selective ET(B) receptor antagonist BQ788 (n = 4) each partially inhibited the effect of ET. The ET(B)-selective agonist S6c (10(-9) to 10(-7)M) also induced a concentration-dependent contraction of valve cusps (n = 4), with a maximum response of 99.1+/-11.1%. This response was completely inhibited by 10(-5)M BQ788 (n = 4). Immunoreactive ET was localized to the endothelial cells that lined both the ventricular and aortic side of the cusps.

CONCLUSION

These results show that aortic valve cusps contract to ET via an action at both ET(A) and ET(B) receptors. The presence of immunoreactive ET in the endothelial cells of the cusps also suggests that it might play a role in valve function. Further studies are required to elucidate the role of these receptors in the physiology and pathophysiology of the aortic valve.