Vasoactive Peptides, their Receptors and Drug Development

Pharmacological Potential of Endothelin Receptors Agonists and Antagonists

Endothelins are potent predominantly vasoconstricting agents that act as local autocrine and paracrine mediators. Endothelin-1 is the most potent and sustained vasoconstrictor and pressor substance yet identified. Abnormalities of the endothelin system occur in a range of diseases associated with vasoconstriction, vasospasm, and vascular hypertrophy. ET receptor antagonists were until recently regarded as drugs of great promise in patients with congestive heart failure, pulmonary hypertension and others. The aim of this article is a survey of compounds that affect the endothelin receptors and clinical trials with these agents.

Hypotensive effect induced by microinjection of Alamandine, a derivative of angiotensin-(1-7), into caudal ventrolateral medulla of 2K1C hypertensive rats

In the present study we evaluated the cardiovascular effects produced by microinjection of the new component of the renin-angiotensin system, alamandine, into caudal ventrolateral medulla of urethane-anesthetized normotensive and hypertensive 2K1C rats. The participation of different angiotensin receptors in the effects of alamandine was also evaluated. Microinjection of angiotensin-(1-7) was used for comparison. The microinjection of 4, 40 and 140pmol of alamandine or angiotensin-(1-7) into caudal ventrolateral medulla induced similar hypotensive effects in Sham-operated rats. However, contrasting with angiotensin-(1-7), in 2K1C rats the MAP response to the highest dose of alamandine was similar to that observed with saline. The microinjection of A-779, a selective Mas receptor antagonist, blunted the angiotensin-(1-7) effects but did not block the hypotensive effect of alamandine in Sham or in 2K1C rats. However, microinjection of D-Pro⁷-angiotensin-(1-7), a Mas/MrgD receptor antagonist, blocked the hypotensive effect induced by both peptides. Furthermore, microinjection of PD123319, a putative AT2 receptor antagonist blocked the hypotensive effect of alamandine, but not of angiotensin-(1-7), in Sham and 2K1C rats. Microinjection of the AT1 receptor antagonist, losartan, did not alter the hypotensive effect of angiotensin-(1-7) or alamandine in both groups. These results provide new insights about the differential mechanisms participating in the central cardiovascular effects of alamandine and angiotensin-(1-7) in normotensive and 2K1C hypertensive rats.

Endothelin receptor B2 (EDNRB2) is responsible for the tyrosinase-independent recessive white (mo(w) ) and mottled (mo) plumage phenotypes in the chicken

A mutation that confers white plumage with black eyes was identified in the Minohiki breed of Japanese native chicken (Gallus gallus domesticus). The white plumage, with a few partially pigmented feathers, was not associated with the tyrosinase gene, and displayed an autosomal recessive mode of inheritance against the pigmented phenotype. All F1 offspring derived from crosses with mottled chickens (mo/mo), which show characteristic pigmented feathers with white tips, had plumage with a mottled-like pattern. This result indicates that the white plumage mutation is a novel allele at the mo locus; we propose the gene symbol mo(w) for this mutant allele. Furthermore, the F1 hybrid between the mo(w) /mo(w) chicken and the panda (s/s) mutant of Japanese quail (Coturnix japonica), whose causative gene is the endothelin receptor B2 (EDNRB2) gene, showed a mo(w)/mo(w) chicken-like plumage, suggesting the possibility that the mutations in parental species are alleles of the same gene, EDNRB2. Nucleotide sequencing of the entire coding region of EDNRB2 revealed a non-synonymous G1008T substitution, which causes Cys244Phe amino acid substitution in exon 5 (which is part of the extracellular loop between the putative fourth and fifth transmembrane domains of EDNRB2) in the mutant chicken. This Cys244Phe mutation was also present in individuals of four Japanese breeds with white plumage. We also identified a non-synonymous substitution leading to Arg332His substitution that was responsible for the mottled (mo/mo) plumage phenotype. These results suggest that the EDN3 (endothelin 3)-EDNRB2 signaling is essential for normal pigmentation in birds, and that the mutations of EDNRB2 may cause defective binding of the protein with endothelins, which interferes with melanocyte differentiation, proliferation, and migration.

Gene duplication of endothelin 3 is closely correlated with the hyperpigmentation of the internal organs (Fibromelanosis) in silky chickens

During early development in vertebrates, pluripotent cells are generated from the neural crest and migrate according to their presumptive fate. In birds and mammals, one of the progeny cells, melanoblasts, generally migrate through a dorsolateral route of the trunk region and differentiate to melanocytes. However, Silky is an exceptional chicken in which numerous melanoblasts travel via a ventral pathway and disperse into internal organs. Finally, these ectopic melanocytes induce heavy dermal and visceral melanization known as Fibromelanosis (Fm). To identify the genetic basis of this phenotype, we confirmed the mode of inheritance of Fm as autosomal dominant and then performed linkage analysis with microsatellite markers and sequence-tagged site markers. Using 85 backcross progeny from crossing Black Minorca chickens (BM-C) with F(1) individuals between White Silky (WS) and BM-C Fm was located on 10.2-11.7 Mb of chicken chromosome 20. In addition, we noticed a DNA marker that all Silky chickens and the F(1) individuals showed heterozygous genotyping patterns, suggesting gene duplication in the Fm region. By quantitative real-time PCR assay, Silky line-specific gene duplication was detected as an ~130-kb interval. It contained five genes including endothelin 3 (EDN3), which encoded a potent mitogen for melanoblasts/melanocytes. EDN3 with another three of these duplicated genes in Silky chickens expressed almost twofold of those in BM-C. Present results strongly suggest that the increase of the expression levels resulting from the gene duplication in the Fm region is the trigger of hypermelanization in internal organs of Silky chickens.

Hemifacial microsomia: from gestation to childhood

Hemifacial microsomia (HFM) is a variable, complex malformation involving asymmetric hypoplasia of the face and ear. Little is known about the risk factors for or consequences of HFM. In this study, we describe 3 studies that have been or are currently being conducted to further our understanding of this malformation. The first completed study examined whether HFM risk is related to maternal exposures that may affect blood flow. In that case-control study, interview data from 230 mothers of children in the case group and 678 mothers of children in the control group suggested that maternal use of vasoactive medications in the first trimester, particularly in combination with cigarette smoking, was associated with increased risks of HFM. The second study is currently underway, in which we are evaluating whether HFM risk is related to genetic variation in pathways associated with vasculogenesis and hemostasis, using DNA collected in the first study. The third ongoing study observes children with HFM to identify psychosocial, cognitive, dental, and medical sequelae. When the children from the original case-control study are 6 or 7 years of age, mothers and teachers complete self-administered questionnaires that cover a wide range of psychosocial development domains. Preliminary analyses of 115 case and 314 control children suggest that children with HFM may have worse teacher-reported academic performance and possibly higher levels of internalizing behavior problems than control children. When data on the full study sample are available, further analyses will determine whether the preliminary findings remain and if they vary by HFM phenotype, parenting style, or indicators of social risk.

Endothelin A receptor blockade influences apoptosis and cellular proliferation in the developing rat kidney

Endothelin systems are believed to play important roles in the emergence and maintenance of functions of various organs during perinatal development, including the kidney. The present study was designed to investigate the roles of endothelin systems on physiologic renal growth and development. Newborn rat pups were treated with either Bristol-Myers Squibb-182874 (30 mg/kg/day), a selective endothelin A receptor (ET(A)R) antagonist, or vehicle for 7 days. To identify cellular changes, kidneys were examined for apoptotic cells by terminal deoxynucleotide transferase-mediated nick-end labeling stain and proliferating cell nuclear antigen (PCNA) by immunohistochemical (IHC) stain. To clarify the molecular control of these processes, immunoblots and reverse transcriptase-polymerase chain reaction for Clusterin, Bcl-2, Bcl-X(L), Bax, and p53 were performed. ETAR antagonist treatment resulted in reduced kidney weights, decreased PCNA-positive proliferating cells, and increased apoptotic cells. The protein expressions of renal Bcl-X(L) and Bax in the ETAR antagonist-treated group were significantly decreased, whereas the mRNA expressions of these genes were not changed. There were no significant differences in the expressions of Clusterin, Bcl-2, and p53. In conclusion, inhibition of endogenous endothelins impairs renal growth, in which decreased cellular proliferation, increased apoptosis and decreased expressions of renal Bcl-X(L) and Bax are possibly implicated.

Expression of endothelin receptors in frog, chicken, mouse and human pigment cells

Several reports have shown the participation of vasoactive endothelins (ETs) in the regulation of vertebrate pigment cells. In the present study, we identified ET receptors in pigment cells of vertebrate species by RT-PCR assays, and compared the differential expression of the various subtypes in each species by quantitative PCR. RT-PCR was performed with specific primers for ETC, ETA(X) or ETA in Xenopus laevis melanophores, ETA or ETB(2) in chicken melanocytes, ETA or ETB in murine (B-16, S-91 or Melan-A) or human (SK-Mel 23 or SK-Mel 28) melanoma cells, and the products obtained were confirmed by cloning and sequencing. The results showed the presence of ETA(X), but not ETA mRNA, and confirmed the expression of ETC in X. laevis melanophores. ETA and ETB(2) mRNAs were also demonstrated in chicken melanocytes. ETA and ETB receptor were identified in S-91, B16 and Melan-A murine cells. In human melanoma cells, SK-Mel 23 and SK-Mel 28, we confirmed the presence of ETB mRNA, and also found ETA mRNA. The comparison between the two subtypes present in the pigment cell of each species and among species demonstrated that the expression of ETAs in chicken, mouse, and human melanocytes is negligible, as is the expression of ETA(X) in Xenopus melanophores. The relative expression, as determined by quantitative PCR, was as follows: chicken ETB>SK-Mel 23 ETB>S91 ETB>>>Xenopus ETC, suggesting that the endothelin system plays a major role in avian and mammalian pigment cell regulation, as compared to lower vertebrates. The phylogenetic analysis revealed that subtype A receptors were probably the most primitive ET receptors, directly deriving from the ancestral type; all the other receptors, B subtypes and C, originated from diverse derivative molecules.

Vasoactive peptides in cardiovascular (patho)physiology

Numerous vasoactive agents play an important physiological role in regulating vascular tone, reactivity and structure. In pathological conditions, alterations in the regulation of vasoactive peptides result in endothelial dysfunction, vascular remodeling and vascular inflammation, which are important processes underlying vascular damage in cardiovascular disease. Among the many vasoactive agents implicated in vascular (patho)biology, angiotensin II (Ang II), endothelin (ET), serotonin and natriuretic peptides appear to be particularly important because of their many pleiotropic actions and because they have been identified as potential therapeutic targets in cardiovascular disease. Ang II, ET-1, serotonin and natriuretic peptides mediate effects via specific receptors, which belong to the group of G-protein-coupled receptors. ET, serotonin and Ang II are primarily vasoconstrictors with growth-promoting actions, whereas natriuretic peptides, specifically atrial, brain and C-type natriuretic peptides, are vasodilators with natriuretic effects. Inhibition of vasoconstrictor actions with drugs that block peptide receptors, compounds that inhibit enzymes that generate vasoactive peptides or agents that increase levels of natriuretic peptides are potentially valuable therapeutic tools in the management of cardiovascular diseases. This review focuses on ET, natriuretic peptides and serotonin. The properties and distribution of these vasoactive agents and their receptors, mechanisms of action and implications in cardiovascular (patho)physiology will be discussed.

A Novel Pathway Involving Progesterone Receptor, Endothelin-2, and Endothelin Receptor B Controls Ovulation in Mice

The steroid hormone progesterone (P) plays a pivotal role during ovulation. Mice lacking P receptor (Pgr) gene fail to ovulate due to a defect in follicular rupture. The P receptor (PGR)-regulated pathways that modulate ovulation, however, remain poorly understood. To identify these pathways, we performed gene expression profiling using ovaries from mice subjected to gonadotropin-induced superovulation in the presence and in the absence of CDB-2914, a synthetic PGR antagonist. Prominent among the genes that were down-regulated in response to CDB-2914 was endothelin (ET)-2, a potent vasoactive molecule. ET-2 mRNA was transiently induced in mural granulosa cells of the preovulatory follicles immediately preceding ovulation. This induction was absent in the ovaries of PGR null mice, indicating a critical role of this receptor in ET-2 expression. To investigate the functional role of ET-2 during ovulation, we employed selective antagonists of endothelin receptors, ETR-A and ETR-B. Mice treated with an ETR-B antagonist exhibited a dramatic (>85%) decline in the number of released oocytes. Strong expression of ETR-B was observed in the mural and cumulus granulosa cells of the preovulatory follicles as well as in the capillaries lining the inner border of the theca interna. We also identified cGMP-dependent protein kinase II, a previously reported PGR-regulated gene, as a downstream target of ET-2 during ovulation. Collectively, our studies uncovered a unique pathway in which ET-2, produced by PGR in mural granulosa cells, acts in a paracrine or autocrine manner on multiple cell types within the preovulatory follicle to control the final events leading to its rupture.

Endothelin-A-receptor antagonist LU 302146 inhibits electrostimulation-induced bladder contractions in vivo

OBJECTIVES

Endothelin (ET) is a strong constrictor of smooth muscle structures. The relevance of Endothelin-A receptors in the bladder was demonstrated in several in vitro studies. The aim of this functional study was to evaluate the acute effect of the selective ET-A-antagonist LU 302146 (LU) on neurostimulation-induced bladder contractions in vivo.

METHODS

Eight male mini pigs were anesthesized. The bladder was exposed and a double lumen catheter was inserted to perform intravesical pressure (pves) measurements. Laminectomy was performed for sacral anterior root stimulation (SARS) of S2. Four animals received the selective ET-A-antagonist LU, three atropine and one animal was treated with vehicle. Pves was recorded before and after drug administration as well as before and during neurostimulation. At the end of each LU trial, a supplementary application of 4 mg atropine was administered followed by a final SARS.

RESULTS

In all experiments reproducible pves values were elicited during electrostimulation before administration of the test substance. The selective ET-A-antagonist reduced stimulation-induced bladder contraction by a mean of 57%. Additional administration of atropine inhibited the detrusor contraction almost completely during SARS. The vehicle had no effect on bladder contraction.

CONCLUSIONS

In the presented animal model, ET-1 inhibition with the selective ET receptor-A-antagonist LU 302146 decreases stimulation-induced bladder contraction in vivo. The results suggest that the selective ET-A antagonist LU acts on the atropine-resistant component of efferent detrusor activation since additional administration of atropine almost completely abolish detrusor contraction. This observation in addition to the involvement of ET-1 in bladder smooth muscle proliferation, raises the possibility that ET-receptor antagonists might be beneficial in patients with neurogenic bladder dysfunction or in patients with functional or anatomical BOO.

Effect of endothelins on the cardiovascular system

Endothelins (ETs) exert a persistent constrictor effect on the vessels via an increase in intracellular Ca2+ concentration due to the activation of Na+/H+ and Na+/Ca2+ exchangers of the vascular smooth muscle fibres. They also produce a transient dilator effect via the activation of endothelial nitric oxide synthase mediated by protein kinase B/Akt. ETA and ETB2 receptors are involved in vasoconstriction, whereas transient vasodilatation depends on the activation of ETB1 receptors. Depending on animal species and experimental conditions, ETs can also play a role in cardiac muscle contraction and induce either an increase or a decrease in contractility. It is likely that only ETA, and not ETB, receptors are involved in the ET-induced increase in myocardial contractility. As in the case of vasoconstriction, this inotropic effect depends on an increase in intracellular Ca2+ concentration induced by Na+/H+ and Na+/Ca2+ exchangers. Activation of the Na+/H+ exchanger is stimulated by protein kinase C, which is activated by diacylglycerol released in response to ET activity. It has also been proposed that the positive inotropic effect can occur without the contribution of the Na+/Ca2+ exchanger, if the cell alkalinisation produced by the Na/H exchanger improves myofibrillar Ca2+ sensitivity. A reduction in contractility has been attributed to the involvement of the Gi protein/protein kinase G pathway or to the activation of protein kinase C without an increase in intracellular Ca2+ concentration or in myofibrillar Ca2+ sensitivity. The chronic effect of ETs on the myocardium results in hypertrophy and prevention of apoptosis, two processes that are together responsible for the contradictory effect of ETs in heart failure.

Volume adaptation in chronic liver disease: on the static and dynamic location of water, salt, protein and red cells in cirrhosis

Adequate size and distribution of the circulating medium are important for cardiovascular function, tissue oxygenation and fluid homoeostasis. Patients with cirrhosis have an abnormal distribution of increased blood volume, increased total vascular compliance and increased arterial compliance. The pattern and temporal relations of plasma and blood volume expansion are important for pathophysiological, diagnostic and therapeutic purposes and depend highly on the type of load (water, saline, oncotic material, red blood cells). In some aspects patients with cirrhosis respond differently from healthy subjects. Thus the reaction during volume expansion may be somewhat blunted, and in advanced cirrhosis, the non-central parts of the circulation in particular, including the splanchnic blood volume, are expanded by a volume load. The use of vasoactive drugs has provided important information on the changes in haemodynamic and homoeostatic mechanisms in patients with cirrhosis. Infusion of oncotic material (preferably albumin) may prevent circulatory dysfunction during certain types of stress. Volume expansion in advanced cirrhosis is qualitatively and quantitively different from that of healthy subjects and those with early cirrhosis. Timely volume handling is essential, but difficult as it is a balance between hypovolaemia and excess extravascular volume loading with further circulatory dysfunction in these patients with a hyperdynamic, but hyporeactive circulation.