Urotensin II and renal function in the rat

Urotensin II system in chronic kidney disease

Chronic kidney disease (CKD) is a progressive and long-term condition marked by a gradual decline in kidney function. CKD is prevalent among those with conditions such as diabetes mellitus, hypertension, and glomerulonephritis. Affecting over 10% of the global population, CKD stands as a significant cause of morbidity and mortality. Despite substantial advances in understanding CKD pathophysiology and management, there is still a need to explore novel mechanisms and potential therapeutic targets. Urotensin II (UII), a potent vasoactive peptide, has garnered attention for its possible role in the development and progression of CKD. The UII system consists of endogenous ligands UII and UII-related peptide (URP) and their receptor, UT. URP pathophysiology is understudied, but alterations in tissue expression levels of UII and UT and blood or urinary UII concentrations have been linked to cardiovascular and kidney dysfunctions, including systemic hypertension, chronic heart failure, glomerulonephritis, and diabetes. UII gene polymorphisms are associated with increased risk of diabetes. Pharmacological inhibition or genetic ablation of UT mitigated kidney and cardiovascular disease in rodents, making the UII system a potential target for slowing CKD progression. However, a deeper understanding of the UII system's cellular mechanisms in renal and extrarenal organs is essential for comprehending its role in CKD pathophysiology. This review explores the evolving connections between the UII system and CKD, addressing potential mechanisms, therapeutic implications, controversies, and unexplored concepts.

Urotensin II Receptor Modulation with 1,3,4-Benzotriazepin-2-one Tetrapeptide Mimics

Urotensin II receptor (UT) modulators that differentiate the effects of the endogenous cyclic peptide ligands urotensin II (UII) and urotensin II-related peptide (URP) offer potential for dissecting their respective biological roles in disease etiology. Selective modulators of hUII and URP activities were obtained using 1,3,4-benzotriazepin-2-one mimics of a purported bioactive γ-turn conformation about the Bip-Lys-Tyr tripeptide sequence of urocontrin ([Bip4]URP). Considering an active β-turn conformer about the shared Phe-Trp-Lys-Tyr sequence of UII and URP, 8-substituted 1,3,4-benzotriazepin-2-ones were designed to mimic the Phe-Bip-Lys-Tyr tetrapeptide sequence of urocontrin, synthesized, and examined for biological activity. Subtle 5- and 8-position modifications resulted in biased signaling and selective modulation of hUII- or URP-induced vasoconstriction. For example, p-hydroxyphenethyl analogs 17b-d were strong Gα13 and βarr1 activators devoid of Gαq-mediated signaling. Tertiary amides 15d and 17d negatively modulated hUII-induced vasoconstriction without affecting URP-mediated responses. Benzotriazepinone carboxamides proved to be exceptional tools for elucidating the pharmacological complexity of UT.

Identification and signaling characterization of four urotensin II receptor subtypes in the western clawed frog, Xenopus tropicalis

Urotensin II (UII) is involved, via the UII receptor (UTR), in many physiological and pathological processes, including vasoconstriction, locomotion, osmoregulation, immune response, and metabolic syndrome. In silico studies have revealed the presence of four or five distinct UTR (UTR1-UTR5) gene sequences in nonmammalian vertebrates. However, the functionality of these receptor subtypes and their associations to signaling pathways are unclear. In this study, full-length cDNAs encoding four distinct UTR subtypes (UTR1, UTR3, UTR4, and UTR5) were isolated from the western clawed frog (Xenopus tropicalis). In functional analyses, homologous Xenopus UII stimulation of cells expressing UTR1 or UTR5 induced intracellular calcoum mobilization and phosphorylation of extracellular signal-regulated kinase 1/2. Cells expressing UTR3 or UTR4 did not show this response. Furthermore, UII induced the phosphorylation of cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) through the UII-UTR1/5 system. However, intracellular cAMP accumulation was not observed, suggesting that UII-induced CREB phosphorylation is caused by a signaling pathway different from that involving Gs protein. In contrast, the administration of UII to cells increased the phosphorylation of guanine nucleotide exchange factor-H1 (GEF-H1) and myosin light chain 2 (MLC2) in all UTR subtypes. These results define four distinct UTR functional subtypes and are consistent with the molecular evolution of UTR subtypes in vertebrates. Further understanding of signaling properties associated with UTR subtypes may help in clarifying the functional roles associated with UII-UTR interactions in nonmammalian vertebrates.

Urotensin II in the development and progression of chronic kidney disease following ⅚ nephrectomy in the rat

NEW FINDINGS

What is the central question of this study? Urotensin II is upregulated in patients in the later stages of chronic kidney disease (CKD), particularly in individuals requiring dialysis. Could treatment with a urotensin II receptor antagonist slow progression of renal disease? What is the main finding and its importance? In the rat, expression of urotensin II and its receptor increased, extending into cortical structures as CKD progressed towards end-stage renal failure. Subchronic treatment with a urotensin receptor antagonist slowed but did not prevent progression of CKD. This suggests that urotensin II contributes to the decline in renal function in CKD.

ABSTRACT

Elevated serum and urine urotensin II (UII) concentrations have been reported in patients with end-stage chronic kidney disease (CKD). Similar increases in UII and its receptor, UT, have been reported in animal models of CKD, but only at much earlier stages of renal dysfunction. The aim of this study was to characterize urotensin system expression as renal disease progresses to end-stage failure in a ⅚ subtotal nephrectomy (SNx) rat model. Male Sprague-Dawley rats underwent SNx or sham surgery and were killed at 8 weeks postsurgery [early (E)] or immediately before end-stage renal failure [30 ± 3 weeks postsurgery; late (L)]. Systolic blood pressure, urinary albumin:creatinine ratio and glomerulosclerosis index were all increased in SNx-E rats compared with sham-E by 8 weeks postsurgery. These changes were associated with an increase in renal immunoreactive UII staining but little change in UT expression. As CKD progressed to end-stage disease in the SNx-L group, markers of renal function deteriorated further, in association with a marked increase in immunoreactive UII and UT staining. Subchronic administration of a UT antagonist, SB-611812, at 30 mg kg^-1  day^-1 for 13 weeks, in a separate group of SNx rats resulted in a 2 week delay in the increase in both systolic blood pressure and urinary albumin:creatinine ratio observed in vehicle-treated SNx but did not prevent the progression of renal dysfunction. The urotensin system is upregulated as renal function deteriorates in the rat; UT antagonism can slow but not prevent disease progression, suggesting that UII plays a role in CKD.

Urotensin II: an inflammatory cytokine

Urotensin II (UII) is a polypeptide molecule with neurohormone-like activity. It has been confirmed that UII is widely distributed in numerous organs of different animal species from fish to mammals, including humans. The UII receptor is orphan G-protein coupled receptor 14, also known as UT. The tissue distribution of UII and UT is highly consistent, and their expression may be regulated by autocrine and paracrine mechanisms. In the body, UII has many physiological and pathophysiological activities, such as vasoconstrictor and vasodilatory actions, cell proliferation, pro-fibrosis, neuroendocrine activity, insulin resistance, and carcinogenic and inflammatory effects, which have been recognized only in recent years. In fact, UII is involved in the process of inflammatory injury and plays a key role in the onset and development of inflammatory diseases. In this paper, we will review the roles UII plays in inflammatory diseases.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

International Union of Basic and Clinical Pharmacology. XCII. Urotensin II, urotensin II-related peptide, and their receptor: from structure to function

Urotensin II (UII) is a cyclic neuropeptide that was first isolated from the urophysis of teleost fish on the basis of its ability to contract the hindgut. Subsequently, UII was characterized in tetrapods including humans. Phylogenetic studies and synteny analysis indicate that UII and its paralogous peptide urotensin II-related peptide (URP) belong to the somatostatin/cortistatin superfamily. In mammals, the UII and URP genes are primarily expressed in cholinergic neurons of the brainstem and spinal cord. UII and URP mRNAs are also present in various organs notably in the cardiovascular, renal, and endocrine systems. UII and URP activate a common G protein-coupled receptor, called UT, that exhibits relatively high sequence identity with somatostatin, opioid, and galanin receptors. The UT gene is widely expressed in the central nervous system (CNS) and in peripheral tissues including the retina, heart, vascular bed, lung, kidney, adrenal medulla, and skeletal muscle. Structure-activity relationship studies and NMR conformational analysis have led to the rational design of a number of peptidic and nonpeptidic UT agonists and antagonists. Consistent with the wide distribution of UT, UII has now been shown to exert a large array of biologic activities, in particular in the CNS, the cardiovascular system, and the kidney. Here, we review the current knowledge concerning the pleiotropic actions of UII and discusses the possible use of antagonists for future therapeutic applications.

Urotensin II upregulates migration and cytokine gene expression in leukocytes of the African clawed frog, Xenopus laevis

Urotensin II (UII) exhibits diverse physiological actions including vasoconstriction, locomotor activity, osmoregulation, and immune response via the UII receptor (UTR) in mammals. However, in amphibians the function of the UII-UTR system remains unknown. In the present study, we investigated the potential immune function of UII using leukocytes isolated from the African clawed frog, Xenopus laevis. Stimulation of male frogs with lipopolysaccharide increased mRNA expression of UII and UTR in leukocytes, suggesting that inflammatory stimuli induce activation of the UII-UTR system. Migration assays showed that both UII and UII-related peptide enhanced migration of leukocytes in a dose-dependent manner, and that UII effect was inhibited by the UTR antagonist urantide. Inhibition of Rho kinase with Y-27632 abolished UII-induced migration, suggesting that it depends on the activation of RhoA/Rho kinase. Treatment of isolated leukocytes with UII increased the expression of several cytokine genes including tumor necrosis factor-α, interleukin-1β, and macrophage migration inhibitory factor, and the effects were abolished by urantide. These results suggest that in amphibian leukocytes the UII-UTR system is involved in the activation of leukocyte migration and cytokine gene expression in response to inflammatory stimuli.

Molecular evolution of GPCRs: Somatostatin/urotensin II receptors

Somatostatin (SS) and urotensin II (UII) are members of two families of structurally related neuropeptides present in all vertebrates. They exert a large array of biological activities that are mediated by two families of G-protein-coupled receptors called SSTR and UTS2R respectively. It is proposed that the two families of peptides as well as those of their receptors probably derive from a single ancestral ligand-receptor pair. This pair had already been duplicated before the emergence of vertebrates to generate one SS peptide with two receptors and one UII peptide with one receptor. Thereafter, each family expanded in the three whole-genome duplications (1R, 2R, and 3R) that occurred during the evolution of vertebrates, whereupon some local duplications and gene losses occurred. Following the 2R event, the vertebrate ancestor is deduced to have possessed three SS (SS1, SS2, and SS5) and six SSTR (SSTR1-6) genes, on the one hand, and four UII (UII, URP, URP1, and URP2) and five UTS2R (UTS2R1-5) genes, on the other hand. In the teleost lineage, all these have been preserved with the exception of SSTR4. Moreover, several additional genes have been gained through the 3R event, such as SS4 and a second copy of the UII, SSTR2, SSTR3, and SSTR5 genes, and through local duplications, such as SS3. In mammals, all the genes of the SSTR family have been preserved, with the exception of SSTR6. In contrast, for the other families, extensive gene losses occurred, as only the SS1, SS2, UII, and URP genes and one UTS2R gene are still present.

RGS2 regulates urotensin II-induced intracellular Ca2+ elevation and contraction in glomerular mesangial cells

Urotensin II (UII), a vasoactive peptide modulates renal hemodynamics. However, the physiological functions of UII in glomerular cells are unclear. In particular, whether UII alters mesangial tone remains largely unknown. The present study investigates the physiological effects of UII on glomerular mesangial cells (GMCs). This study also tested the hypothesis that the regulator of G-protein signaling (RGS) controls UII receptor (UTR) activity in GMCs. RT-PCR, Western immunoblotting, and immunofluorescence revealed UTR expression in cultured murine GMCs. Mouse UII (mUII) stimulated Ca(2+) release from intracellular stores and activated store-operated Ca(2+) entry (SOCE) in the cells. mUII also caused a reduction in planar GMC surface area. mUII-induced [Ca(2+)]i elevation and contraction were attenuated by SB 657510, a UTR antagonist, araguspongin B, an inositol 1,4,5-trisphosphate receptor antagonist, thapsigargin, a sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor, and La(3+), a store-operated Ca(2+) channel blocker, but not nimodipine, an L-type Ca(2+) channel blocker. In situ proximity ligation assay indicated molecular proximity between endogenous RGS2 and UTR in the cells. Treatment of GMCs with mUII elevated plasma membrane expression of RGS2 by ∼2-fold. mUII also increased the interaction between RGS2 and UTR in the cells. siRNA-mediated knockdown of RGS2 in murine GMCs increased mUII-induced [Ca(2+)]i elevation and contraction by ∼35 and 31%, respectively. These findings indicate that mUII-induced SOCE results in murine GMC contraction. These data also suggest that UTR activation stimulates RGS2 recruitment to GMC plasma membrane as a negative feedback mechanism to regulate UTR signaling.

The urotensin system is up-regulated in the pre-hypertensive spontaneously hypertensive rat

Urotensin II (UII) concentrations are raised both in humans with hypertension and in spontaneously hypertensive rats (SHR). Since the urotensin system acts to regulate glomerular filtration in the kidney it may play a greater role in the pre-hypertensive SHR in which renal dysfunction is known to precede the onset of severe hypertension. This study aimed to determine the renal actions and expression of the urotensin system in the young SHR. Intravenous rat UII (6 pmol. min^-1. 100 g body weight^-1) had no significant effect on GFR; however urotensin-related peptide (URP) reduced GFR (P<0.05) in 4-5 week old SHR. Administration of the UT antagonist SB-706375 evoked marked increases in GFR (baseline 0.38 ± 0.07 vs antagonist 0.76 ± 0.05 ml. min^-1. 100 g body weight^-1, P<0.05), urine flow and sodium excretion (baseline 2.5 ± 0.4 vs antagonist 9.1 ± 2.1 µmol. min^-1. 100 g body weight^-1, P<0.05) in the SHR. Normotensive Wistar-Kyoto rats showed little response to UT antagonism. Quantitative RT-PCR showed that neither UII nor UT mRNA expression differed between the kidneys of young SHR and WKY rats; however expression of URP was 4-fold higher in the SHR kidney. Renal transcriptional up-regulation indicates that URP is the major UT ligand in young SHR and WKY rats. Enhanced tonic UT activation may contribute to known renal dysfunction in pre-hypertensive SHR.

Urotensin II exerts antiapoptotic effect on NRK-52E cells through prostacyclin-mediated peroxisome proliferator-activated receptor alpha and Akt activation

Urotensin II (UII) is a cyclic vasoactive peptide which is mainly expressed in kidneys. Although elevated plasma UII levels are associated with renal impairment, the influence of UII on renal injury is unclear. In this study, we monitored the influence of UII on gentamicin-induced apoptosis in rat tubular cells (NRK-52E). We found that UII significantly reduced gentamicin-induced apoptosis and apoptotic signals. Blocking endogenous UII secretion caused cells to be more susceptible to gentamicin. In gentamicin-treated mice, UII also expressed protective effect on renal tubular cells. UII was also found to induce prostacyclin (PGI2) production, which caused peroxisomal proliferator-activated receptor α (PPARα) activation as revealed by both PGI2 synthase siRNA transfection and piroxicam treatment. Blockage of PPARα by siRNA transfection inhibited UII-induced Akt phosphorylation and the antiapoptotic effect of UII. Our results suggest that UII can protect renal tubular cells from gentamicin-induced apoptosis through PGI2-mediated PPARα and Akt activation.

Discovery of new antagonists aimed at discriminating UII and URP-mediated biological activities: insight into UII and URP receptor activation

BACKGROUND AND PURPOSE

Recent evidence suggested that urotensin II (UII) and its paralog peptide UII-related peptide (URP) might exert common but also divergent physiological actions. Unfortunately, none of the existing antagonists were designed to discriminate specific UII- or URP-associated actions, and our understanding, on how these two endogenous peptides can trigger different, but also common responses, is limited.

EXPERIMENTAL APPROACH

Ex vivo rat and monkey aortic ring contraction as well as dissociation kinetics studies using transfected CHO cells expressing the human urotensin (UT) receptors were used in this study.

KEY RESULTS

Ex vivo rat and monkey aortic ring contraction studies revealed the propensity of [Pep(4)]URP to decrease the maximal response of human UII (hUII) without any significant change in potency, whereas no effect was noticeable on the URP-induced vasoconstriction. Dissociation experiments demonstrated the ability of [Pep(4)]URP to increase the dissociation rate of hUII, but not URP. Surprisingly, URP, an equipotent UII paralog, was also able to accelerate the dissociation rate of membrane-bound (125)I-hUII, whereas hUII had no noticeable effect on URP dissociation kinetics. Further experiments suggested that an interaction between the glutamic residue at position 1 of hUII and the UT receptor seems to be critical to induce conformational changes associated with agonistic activation. Finally, we demonstrated that the N-terminal domain of the rat UII isoform was able to act as a specific antagonist of the URP-associated actions.

CONCLUSION

Such compounds, that is [Pep(4)]URP and rUII(1-7), should prove to be useful as new pharmacological tools to decipher the specific role of UII and URP in vitro but also in vivo.

Urotensin II and the kidney

PURPOSE OF REVIEW

Urotensin II (UTS2), the most potent vasoconstrictor identified thus far, is an undecapeptide hormone with a structure that is highly conserved through mammalian phylogeny. In spite of its broad expression across the invertebrate and vertebrate world, the precise role of UTS2 in physiology and disease is still unknown. The first description of human UTS2 and its receptor brought initial promise of a potential therapeutic target for progressive renal disease, with vasoconstrictive and profibrotic actions within an autocrine and paracrine system and local renal generation that was upregulated with renal pathology.

RECENT FINDINGS

However, the last decade has not brought the successful development of new treatments first hoped for, with one small human clinical trial bearing negative results. What has become apparent is that the spectrum of actions of UTS2 is broad and often paradoxical. This ancient hormone has both vasoconstrictor and vasodilatory actions, has both profibrotic and antiapoptotic activity, as well as actions which are highly contextual on the particular vascular bed studied and on the presence or absence of superimposed disease state.

SUMMARY

With current development of newer UTS2 antagonists attempting to more closely replicate the ligand-receptor kinetics of UTS2 and its receptor, the focus on potential clinical applications of UTS2 inhibition has moved away from the kidney to the treatment of chronic lung and cardiovascular diseases.

Pressor and renal regional hemodynamic effects of urotensin II in neonatal pigs

Renal expression of the peptide hormone urotensin II (UII) and its receptor (UTR) are dependent on kidney maturation and anatomical regions. However, renal regional hemodynamic effects of UII in neonates are unclear. Here, we investigated regional hemodynamic responses to acute intrarenal arterial administration of UII in newborn pigs. Western immunoblotting and immunofluorescence confirmed UTR expression and membrane localization in newborn pig renal afferent arterioles and afferent arteriolar smooth muscle cells respectively. Intrarenal arterial bolus injections of human UII (hUII; 1-100  ng/kg) resulted in a dose-dependent decrease in total renal blood flow (RBF) and an increase in mean arterial pressure (MAP) and renal vascular resistance (RVR) in newborn pigs. Moreover, hUII dose dependently reduced cortical blood flow (CBF) but increased medullary blood flow (MBF) in the piglets. hUII-induced MAP elevation and hemodynamic changes were inhibited by urantide, a UTR antagonist, but not losartan, a type 1 angiotensin II receptor antagonist. U-73122, a phospholipase C (PLC) inhibitor, and 2-aminoethoxydiphenyl borate, an inositol 1,4,5 trisphosphate (IP₃) receptor antagonist, attenuated hUII-induced MAP and RVR elevations, RBF and CBF reductions, but not MBF increase. These findings indicate that intrarenal arterial administration of hUII elevates blood pressure and induces region-selective renal hemodynamic changes in newborn pigs. Our data also suggest that the PLC/IP₃ signaling pathway contributes to hUII-induced alterations in MAP, RBF, RVR, and CBF but not MBF in newborn pigs.

Urotensin II receptor (UTR) exists in hyaline chondrocytes: a study of peripheral distribution of UTR in the African clawed frog, Xenopus laevis

Urotensin II (UII) and UII-related peptide (URP) exhibit diverse physiological actions including vasoconstriction, locomotor activity, osmoregulation, and immune response through UII receptor (UTR), which is expressed in the central nervous system and peripheral tissues of fish and mammals. In amphibians, only UII has been identified. As the first step toward elucidating the actions of UII and URP in amphibians, we cloned and characterized URP and UTR from the African clawed frog Xenopus laevis. Functional analysis showed that treatment of UII or URP with Chinese hamster ovary cells transfected with the cloned receptor increased the intracellular calcium concentration in a concentration-dependent manner, whereas the administration of the UTR antagonist urantide inhibited UII- or URP-induced Ca(2+) mobilization. An immunohistochemical study showed that UTR was expressed in the splenocytes and leukocytes isolated from peripheral blood, suggesting that UII and URP are involved in the regulation of the immune system. UTR was also localized in the apical membrane of the distal tubule of the kidney and in the transitional epithelial cells of the urinary bladder. This result supports the view that the UII/URP-UTR system plays an important role in osmoregulation of amphibians. Interestingly, immunopositive labeling for UTR was first detected in the chondrocytes of various hyaline cartilages (the lung septa, interphalangeal joint and sternum). The expression of UTR was also observed in the costal cartilage, tracheal cartilages, and xiphoid process of the rat. These novel findings probably suggest that UII and URP mediate the formation of the cartilaginous matrix.

Urotensin-II: More Than a Mediator for Kidney

Human urotensin-II (hU-II) is one of the most potent vasoconstrictors in mammals. Although both hU-II and its receptor, GPR14, are detected in several tissues, kidney is a major source of U-II in humans. Recent studies suggest that U-II may have a possible autocrine/paracrine functions in kidney and may be an important target molecule in studying renal pathophysiology. It has several effects on tubular transport and probably has active role in renal hemodynamics. Although it is an important peptide in renal physiology, certain diseases, such as hypertension and glomerulonephritis, may alter the expression of U-II. As might be expected, oxidative stress, mediators, and inflammation are like a devil's triangle in kidney diseases, mostly they induce each other. Since there is a complex relationship between U-II and oxidative stress, and other mediators, such as transforming growth factor β1 and angiotensin II, U-II is more than a mediator in glomerular diseases. Although it is an ancient peptide, known for 31 years, it looks like that U-II will continue to give new messages as well as raising more questions as research on it increases. In this paper, we mainly discuss the possible role of U-II on renal physiology and its effect on kidney diseases.

Ontogeny of the renal urotensin II system in the rat

Urotensin II (UII), a peptide hormone which influences glomerular filtration rate and urine concentration, and its receptor, UT, are expressed in the adult rat kidney. The ability of the kidney to reabsorb sodium and water starts to develop in utero and matures during early postnatal life in the rat, yet little is known about the ontogeny of the renal UII system. This study mapped renal expression of the urotensin system during the fetal and postnatal periods and determined renal activity of UII in the immature rat. Urotensin II peptide and mRNA were present in Sprague-Dawley (SD) rat metanephroi from the earliest stage examined, embyonic day 19 (E19; rat gestation 22 days); levels increased to peak at 4 weeks of age. In contrast, UT protein and mRNA expression declined rapidly between E19 and birth and remained at a similar level postnatally. Infusion of rat UII [6-60 pmol min(-1) (100 g body weight)(-1)] or rat urotensin-related peptide [6 pmol min(-1) (100 g body weight)(-1)] in anaesthetized 4-week-old SD rats had no influence on measured renal parameters; however, infusion of UT antagonist, SB-706375 (0.01 mg kg(-1) min(-1)), provoked a pronounced diuresis [vehicle 23.5 ± 1.9 versus antagonist 75.3 ± 12.5 μl min(-1) (100 g body weight)(-1); P < 0.001] and natriuresis, accompanied by modest increases in effective renal blood flow and glomerular filtration rate [vehicle 0.4 ± 0.1 versus antagonist 1.1 ± 0.2 ml min(-1) (100 g body weight)(-1); P < 0.0001] and a significant increase in fractional sodium excretion. These results indicate that the endogenous rat UII system may influence renal sodium and water excretion before the onset of full urine concentrating capacity in the SD rat.

Update on the urotensinergic system: new trends in receptor localization, activation, and drug design

The urotensinergic system plays central roles in the physiological regulation of major mammalian organ systems, including the cardiovascular system. As a matter of fact, this system has been linked to numerous pathophysiological states including atherosclerosis, heart failure, hypertension, diabetes as well as psychological, and neurological disorders. The delineation of the (patho)physiological roles of the urotensinergic system has been hampered by the absence of potent and selective antagonists for the urotensin II-receptor (UT). Thus, a more precise definition of the molecular functioning of the urotensinergic system, in normal conditions as well as in a pathological state is still critically needed. The recent discovery of nuclear UT within cardiomyocytes has highlighted the cellular complexity of this system and suggested that UT-associated biological responses are not only initiated at the cell surface but may result from the integration of extracellular and intracellular signaling pathways. Thus, such nuclear-localized receptors, regulating distinct signaling pathways, may represent new therapeutic targets. With the recent observation that urotensin II (UII) and urotensin II-related peptide (URP) exert different biological effects and the postulate that they could also have distinct pathophysiological roles in hypertension, it appears crucial to reassess the recognition process involving UII and URP with UT, and to push forward the development of new analogs of the UT system aimed at discriminating UII- and URP-mediated biological activities. The recent development of such compounds, i.e. urocontrin A and rUII(1-7), is certainly useful to decipher the specific roles of UII and URP in vitro and in vivo. Altogether, these studies, which provide important information regarding the pharmacology of the urotensinergic system and the conformational requirements for binding and activation, will ultimately lead to the development of potent and selective drugs.

Potential Clinical Implications of the Urotensin II Receptor Antagonists

Urotensin II (UII) binds to its receptor, UT, playing an important role in the heart, kidneys, pancreas, adrenal gland, and central nervous system. In the vasculature, it acts as a potent endothelium-independent vasoconstrictor and endothelium-dependent vasodilator. In disease states, however, this constriction–dilation equilibrium is disrupted. There is an upregulation of the UII system in heart disease, metabolic syndrome, and kidney failure. The increase in UII release and UT expression suggest that UII system may be implicated in the pathology and pathogenesis of these diseases by causing an increase in acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) activity leading to smooth muscle cell proliferation and foam cell infiltration, insulin resistance (DMII), as well as inflammation, high blood pressure, and plaque formation. Recently, UT antagonists such as SB-611812, palosuran, and most recently a piperazino-isoindolinone based antagonist have been developed in the hope of better understanding the UII system and treating its associated diseases.

Urotensin-II receptor antagonism does not improve renal haemodynamics or function in rats with endotoxin-induced acute kidney injury

1. Urotensin-II (U-II) is a vasoactive peptide that influences renal haemodynamics and kidney function. The aim of the present study was to examine the effects of the selective U-II receptor antagonist, urantide, on renal haemodynamics, oxygenation and function in endotoxaemic rats. 2. Endotoxaemia was induced in Sprague-Dawley rats by an intraperitoneal dose of lipopolysaccharide (LPS; Escherichia coli O127:B8, 7.5 mg/kg). At 16 h after endotoxin was given, renal clearance experiments were carried out in thiobutabarbital anaesthetized rats. Group 1, sham-saline; group 2, sham-urantide; group 3 LPS-saline; and group 4, LPS-urantide received isotonic saline or urantide (0.2 mg/kg bolus intravenously, followed by an infusion of 1.2 mg/kg/h throughout) after baseline measurements. Kidney function, renal blood flow (RBF), and cortical and outer medullary perfusion (laser-Doppler flowmetry) and oxygen tension (Clark-type microelectrodes) were analysed during 2 h of drug administration. 3. At baseline, endotoxaemic rats showed approximately 50% reductions in glomerular filtration rate (GFR) and RBF (P < 0.05), a decline in cortical and outer medullary perfusion and pO(2) (P < 0.05), and a significant increase in mean arterial pressure (MAP; P < 0.05) compared with saline-injected controls. In sham animals, urantide in a dose that did not significantly influence MAP or RBF, increased GFR (P < 0.05 time × treatment interaction) and filtration fraction (P < 0.05 treatment effect). However, urantide had no statistically significant effects on any of the investigated variables in endotoxaemic rats. 4. These findings show that U-II, through the UT receptor, does not contribute to abnormalities in renal haemodynamics and function in endotoxaemic rats.

Renal impairment, hypertension and plasma urotensin II

BACKGROUND

Human urotensin II (UII) is a potent mammalian vasoconstrictor thought to be produced and cleared by the kidneys. Conflicting data exist regarding the relationship between UII concentrations, kidney function and blood pressure (BP). We measured the associations between kidney function [including end-stage renal disease (ESRD)] and levels of BP with plasma concentrations of UII.

METHODS

Ninety-one subjects were enrolled. Thirty-one subjects had ESRD (undergoing haemodialysis), 30 subjects had chronic kidney disease (CKD) and 30 control subjects had no kidney disease. Plasma UII concentrations were measured by radioimmunoassay.

RESULTS

Mean plasma UII concentrations were highest in controls, lower in subjects with ESRD and lowest in subjects with non-ESRD CKD (P<0.0001). UII concentrations correlated negatively with serum creatinine (P=0.0012) and CKD stage, and positively with creatinine clearance (P=0.013). In ESRD subjects, plasma UII (P=0.008) increased after dialysis, while SBP (P=0.007), DBP (P=0.009), serum creatinine (P<0.0001) and serum urea nitrogen (P<0.0001) decreased. UII concentrations were lower in patients with a history of hypertension (HTN) (P=0.016). Age, race and gender did not appear to be associated with UII concentration. However, the distribution of African American race and male gender appear to be associated with increasing stages of chronic kidney disease.

CONCLUSIONS

These data suggest a potential vasodilatory role of UII in humans with kidney disease or hypertension. The reduction in UII levels in CKD also suggests either reduced production or greater clearance, or both, of UII.

Expression of urotensin II and its receptor in human liver cirrhosis and fulminant hepatic failure

OBJECTIVES

Urotensin II [U-II] plasma levels are increased in liver cirrhosis [LC] and are discussed as an important mediator of portal hypertension since the U-II antagonist palosuran has beneficial effects on portal hypertension by increasing splanchnic resistance. Nevertheless, no data are available on the intrahepatic expression of U-II and its receptor [UT] in humans.

METHODS

U-II and UT expression were analyzed in the livers of patients with LC, fulminant hepatic failure [FHF], and normal controls [NC] using immunohistochemistry.

RESULTS

Both U-II and UT were expressed in the liver on endothelial cells from arteries, veins, and bile ducts as well as on Kupffer cells. In LC, the total number of U-II-expressing cells was 20% lower compared to NC (P < 0.001), while expression of UT did not differ between LC and NC. In contrast, significant enhanced number of U-II and UT positive cells were found in FHF compared to LC and NC (P < 0.001). U-II and UT expression was also found in portal veins, without differences between LC and NC.

CONCLUSIONS

Our data demonstrate that U-II and UT are not elevated in human cirrhotic livers but are in livers of patients with FHF.

Role of urotensin II in health and disease

Urotensin II (UII) is an 11 amino acid cyclic peptide originally isolated from the goby fish. The amino acid sequence of UII is exceptionally conserved across most vertebrate taxa, sharing structural similarity to somatostatin. UII binds to a class of G protein-coupled receptor known as GPR14 or the urotensin receptor (UT). UII and its receptor, UT, are widely expressed throughout the cardiovascular, pulmonary, central nervous, renal, and metabolic systems. UII is generally agreed to be the most potent endogenous vasoconstrictor discovered to date. Its physiological mechanisms are similar in some ways to other potent mediators, such as endothelin-1. For example, both compounds elicit a strong vascular smooth muscle-dependent vasoconstriction via Ca2+ release. UII also exerts a wide range of actions in other systems, such as proliferation of vascular smooth muscle cells, fibroblasts, and cancer cells. It also 1) enhances foam cell formation, chemotaxis of inflammatory cells, and inotropic and hypertrophic effects on heart muscle; 2) inhibits insulin release, modulates glomerular filtration, and release of catecholamines; and 3) may help regulate food intake and the sleep cycle. Elevated plasma levels of UII and increased levels of UII and UT expression have been demonstrated in numerous diseased conditions, including hypertension, atherosclerosis, heart failure, pulmonary hypertension, diabetes, renal failure, and the metabolic syndrome. Indeed, some of these reports suggest that UII is a marker of disease activity. As such, the UT receptor is emerging as a promising target for therapeutic intervention. Here, a concise review is given on the vast physiologic and pathologic roles of UII.

The renin-angiotensin system, adrenomedullins and urotensin II in the kidney: possible renoprotection via the kidney peptide systems

The incidence of chronic kidney disease, such as diabetic nephropathy, is increasing throughout the world. Many biologically active peptides play important roles in the kidney. The classical example is the renin-angiotensin system (RAS). Angiotensin II plays critical roles in the progression of chronic kidney disease through its vasoconstrictor action, stimulatory action on cell proliferation, and reactive oxygen-generating activity. A renin inhibitor, aliskiren, has recently been shown to be a clinically effective drug to reduce proteinuria in patients with diabetic nephropathy. (Pro)renin receptor, a specific receptor for renin and prorenin, was newly identified as a member of the RAS. When bound to prorenin, (pro)renin receptor activates the angiotensin I-generating activity of prorenin in the absence of cleavage of the prosegment, and directly stimulates the pathway of mitogen-activated protein kinase independently from the RAS. The kidney peptides that antagonize the intrarenal RAS may have renoprotective actions. Adrenomedullins, potent vasodilator peptides, have been shown to have renoprotective actions. On the other hand, urotensin II, a potent vasoconstrictor peptide, may promote the renal dysfunction in chronic kidney disease together with the renal RAS. Thus, in addition to the renin inhibitor and (pro)renin receptor, adrenomedullins and urotensin II may be novel targets to develop therapeutic strategies against chronic kidney disease.

Urotensin II induces transactivation of the epidermal growth factor receptor via transient oxidation of SHP-2 in the rat renal tubular cell line NRK-52E

Urotensin-II (UII) is a potent vasoactive peptide that has been implicated in cardiac fibrosis and renal diseases. However, the role played by UII in renal tissues is largely unknown. In this study, we investigated the effects of human UII (hUII) on rat renal proximal tubular cells of the NRK-52E line and the role of Src homology 2-containing phosphotyrosine phosphatase (SHP-2) in the hUII-induced transactivation of the epidermal growth factor receptor (EGFR). Exposure to hUII at low concentrations significantly induced proliferation in NRK-52E cells; this effect was inhibited by treatment with an ERK1/2 inhibitor (PD98059). UII treatment increased the phosphorylation of EGFR and induced the generation of reactive oxygen species (ROS). Treatment of the ROS scavenger N-acetyl-cysteine (NAC) inhibited EGFR transactivation and ERK phosphorylation induced by hUII. SHP-2 was found to interact with EGFR and be transiently oxidized following the hUII treatment. In SHP-2 knockdown cells, UII-induced phosphorylation of EGFR was less influenced by NAC, and significantly suppressed by heparin binding (HB)-EGF neutralizing antibody. Our data suggest that the ROS-mediated oxidation of SHP-2 is essential for the hUII-induced mitogenic pathway in NRK-52E cells.

Increased expression of urotensin II-related peptide and its receptor in kidney with hypertension or renal failure

Urotensin II-related peptide (URP) is a novel vasoactive peptide that shares urotensin II receptor (UT) with urotensin II. In order to clarify possible changes of URP expression in hypertension and chronic renal failure (CRF), the expressions of URP and UT were studied by quantitative RT-PCR and immunohistochemistry in kidneys obtained from spontaneous hypertensive rats (SHR), Wistar-Kyoto rats (WKY), and WKY with CRF due to 5/6 nephrectomy. Expression levels of URP mRNA and UT mRNA were significantly higher in the kidneys obtained from SHR compared with age-matched WKY (at 5-16 and 16 weeks old, respectively). A dissection study of the kidney into three portions (inner medulla, outer medulla and cortex) showed that the expression levels of URP mRNA and UT mRNA were highest in the inner medulla and the outer medulla, respectively, in both SHR and WKY. The expression levels of URP and UT mRNAs were greatly elevated in the remnant kidneys of CRF rats at day 56 after nephrectomy, compared with sham-operated rats (about 6.5- and 11.9-fold, respectively). Immunohistochemistry showed that URP immunostaining was found mainly in the renal tubules, vascular smooth muscle cells and vascular endothelial cells. UT immunoreactivity was localized in the renal tubules and vascular endothelial cells. These findings suggest that the expressions of URP and UT mRNAs in the kidney are enhanced in hypertension and CRF, and that URP and its receptor have important pathophysiological roles in these diseases.

Characterization of the insulinostatic effect of urotensin II: a study in the perfused rat pancreas

We have investigated the effect of urotensin II (UII) on insulin secretion at normal and high glucose concentrations as well as induced by secretagogues acting on the B cell via different mechanisms. The study was performed in the perfused rat pancreas. UII, at 1 nM, blocked the insulin response to an increase in perfusate glucose concentration from 5.5 to 9 mM while failed to significantly modify insulin secretion at higher glucose levels (from 9 to 13 mM). The insulinotropic effect of this glucose challenge was reduced by 10 nM UII. UII, at 1 nM, inhibited tolbutamide-induced insulin secretion, whereas, it did not affect KCl-induced insulin release. UII inhibited exendin-4-induced insulin secretion, an effect not observed in pertussis toxin-treated rats.

CONCLUSION

1) B cells are less sensitive to UII at a high glucose level than at a low glucose. 2) The inhibitory effect of UII on both glucose and tolbutamide-induced insulin release, suggests the implication of ATP-dependent K(+) channels. The insulinostatic effect of UII was not observed during KCl stimulation, a condition in which these channels are overridden. 3) The insulinostatic effect of UII can also be mediated by its inhibitory action on the adenylate cyclase/cAMP system via a pertussis toxin-sensitive G(i) protein.

Diabetes-induced upregulation of urotensin II and its receptor plays an important role in TGF-beta1-mediated renal fibrosis and dysfunction

Urotensin II (UII) was identified as the ligand for a novel G protein-coupled receptor, GPR14. UII was found not only to have a potent vasoconstrictive action but also to have profibrotic effects in the heart. The present study was to define whether UII and GPR14 also play important roles in diabetes-induced renal fibrosis and dysfunction. Diabetic rats were induced using streptozotocin, and the rat proximal tubular epithelial cells (NRK-52E) were used for the in vitro mechanism study. Results showed that expression of UII and GPR14 was significantly upregulated at both mRNA and protein levels in the diabetic kidneys compared with controls. The upregulated expressions of UII and GPR14 in the kidney were accompanied by significant increases in the renal profibrotic factor transforming growth factor (TGF)-beta1 expression, the renal extracellular matrix (fibronectin and collagen IV) accumulation, and the renal dysfunction (increases in urinal N-acetyl-beta-d-glucosaminidase content, 24-h urinary retinol-binding protein excretion rate, and decrease in creatinine clearance rate). Exposure of NRK-52E cells to 10(-8) mol/l UII for 48 h caused a significant increase of TGF-beta1, but not ANG II, production that was GPR14- and calcium-dependent, since GPR14 small-interfering RNA and calcium channel blocker nimodipine or calcium chelator EDTA all could abolish the induction of TGF- beta1 by UII. Furthermore, exposure of NRK-52E cells to TGF-beta1 or ANG II also increased UII and GPR14 mRNA expressions. These results suggested that diabetes-induced upregulation of UII and GPR14, most likely through autocrine and/or paracrine mechanisms, plays an important role in TGF-beta1-mediated renal fibrosis and dysfunction.

Enhanced renal sensitivity of the spontaneously hypertensive rat to urotensin II

Urotensin II (UII) has been implicated widely in cardiovascular disease. The mechanism(s) through which it contributes to elevated blood pressure is unknown, but its emerging role as a regulator of mammalian renal function suggests that the kidney might be involved. The aim of this study was to determine the effect of UII on renal function in the spontaneously hypertensive rat (SHR). UII infusion (6 pmol.min(-1).100 g body wt(-1)) in anesthetized SHR and control Wistar-Kyoto (WKY) rats produced marked reductions in glomerular filtration rate (DeltaGFR WKY, n=7, -0.3+/-0.1 vs. SHR, n=7, -0.6+/-0.1 ml.min(-1).100 g body wt(-1), P=0.03), urine flow, and sodium excretion rates, which were greater in SHR by comparison with WKY rats. WKY rats also showed an increase in fractional excretion of sodium (DeltaFE(Na); +0.6+/-0.1%, P=0.02) in contrast to SHR in which no such change was observed (DeltaFE(Na) -0.6+/-0.2%). Blockade of the UII receptor (UT), and thus endogenous UII activity, with urantide evoked an increase in GFR which was greater in SHR (+0.3+/-0.1) compared with WKY rats (+0.1+/-0.1 ml.min(-1).100 g body wt(-1), P=0.04) and was accompanied by a diuresis and natriuresis. UII and UT mRNA expression were greater in the renal medulla than the cortex of both strains; however, expression levels were up to threefold higher in SHR tissue. SHR are more sensitive than WKY to UII, which acts primarily to lower GFR thus favoring salt retention in this model of hypertension.

Urotensin II: a new pharmacologic target in the treatment of cardiovascular disease

Urotensin II was first identified over 30 years ago as a potent vasoconstrictor, and the identification of its receptor in the heart, lungs, blood vessels, and brain have made it a potential target for human pharmacotherapy. Current research would suggest that urotensin II plays a major role in the pathophysiology of various cardiovascular disease entities. This article discusses the biologic effects of urotensin under normal and pathophysiologic conditions, and reviews the research experiences with synthetic urotensin blockers in the treatment of various cardiovascular illnesses.

Urotensin II: lessons from comparative studies for general endocrinology

The importance of combining studies across vertebrates to provide insights into the functionality of hormone systems is considered, using recent advances in Urotensin II (UII) biology to illustrate this. The impact of genome analyses on understanding ligand and UII receptor (UT) structures is reviewed, noting their high conservation from fish to mammals. The early linkage of UII with fish osmoregulatory physiology drove our investigation of possible renal actions of UII in mammals. The kidney is a potential major source of UII in mammals and endogenous peptide appears to have tonal influence over renal excretion of water and electrolytes. Blockade of UII actions by administration of UT receptor antagonist, urantide, in anaesthetised rats, indicates that endogenous UII lowers renal filtration rates and excretion of water and ions. These effects are considered in relation to apparent association of UII with a number of human cardiovascular and renal disorders. Following up the sequencing of UT in mammals here we contrast the first fish UT sequences with those in other species. It is now evident that UT expression in fish osmoregulatory tissues, such as the gill and kidney, exhibits considerable plasticity in response to physiological challenge, providing an important component of the adaptive organismal responses. A number of areas of UII research, which will continue to benefit from moving questions between appropriate vertebrate groups, have been highlighted. These comparative approaches will yield improved understanding and further novel actions of this intriguing endocrine and paracrine system, so highly conserved across the vertebrate series.

Increased gene expression of urotensin II-related peptide in the hearts of rats with congestive heart failure

Urotensin II-related peptide (URP) is a novel endogenous ligand for urotensin II receptor (UT-R). To investigate the pathophysiological role of URP in heart failure, we examined URP, UII and UT-R expression in hearts and kidneys of rats with congestive heart failure due to coronary ligation by quantitative RT-PCR and immunocytochemistry. Significantly increased expression levels of URP mRNA were found in the atrium, the right ventricle and the infarcted part of left ventricle of heart failure rats, when compared with sham-operated rats (about 2.2-fold, 2.7-fold and 3.9-fold, respectively). Expression levels of UII mRNA in the heart were about 10% of URP mRNA, and were slightly increased only in the infarcted part of left ventricle of heart failure rats, when compared with sham-operated rats. The expression levels of UT-R mRNA were increased in the atrium of heart failure rats. There was no significant change of URP, UII and UT-R mRNA expression levels in the kidney between heart failure and sham-operated rats. The myocardium was diffusely immunostained with URP in both rats. The blood vessels in the heart were positively immunostained with URP in heart failure rats, but not in sham-operated rats, whereas they were positively immunostained with UT-R in both rats. These findings suggest that the expression of URP, UII and UT-R is enhanced in failing heart, and the UII/URP/UT-R system has important pathophysiological roles in the progression of heart failure.

Hemodynamic-independent anti-natriuretic effect of urotensin II in spontaneously hypertensive rats

The present study aims to test the hypothesis that U-II might have a direct anti-natriuretic action in spontaneously hypertensive rats (SHR). Bolus U-II injection (15 nmol kg(-1)) caused a transient decrease in glomerular filtration rate (GFR), urine flow rate (UV), urinary sodium (UNaV) and potassium excretion (U(K)V) that corresponded with a committed decrease in mean arterial pressure (MAP) and renal blood flow (RBF) during the first 30 min. Continuous U-II infusion (0.2 nmol kg(-1)h(-1)) following a bolus U-II injection (0.3 nmol kg(-1)) caused an anti-natriuretic effect without any significant change in MAP, RBF, GFR, UV and UKV during the entire 1.5-h perfusion period in SHR. The levels of aldosterone and angiotensin II were not altered in the plasma and kidney, while plasma antidiuretic hormone decreased in response to U-II injection (15 nmol kg(-1)). Protein levels of U-II receptors (UT) were significantly increased in the kidney of 17-week-old SHR when compared with the age-matched WKY rats, while mRNA transcripts of both U-II and UT were increased in the kidney, left ventricle and thoracic aorta. In conclusion, U-II exerts a hemodynamic-independent anti-natriuretic action in adult SHR. The anti-natriuretic action of U-II in SHR is probably associated with an increased expression of the U-II-UT system in the kidney, suggesting a potential renal role of U-II in the pathogenesis of hypertension.

Increased circulating urotensin II in cirrhosis: potential implications in liver disease

Urotensin II (UII) is a potent vasoactive mediator which, through interaction with a specific G-protein coupled receptor, can result in either a vasoconstrictive or vasodilatory response. In addition to its effect upon vascular tone, UII possess mitogenic and fibrogenic potential. The influence of UII on vascular tone is to some degree both species-specific and disease-specific. Increased circulating UII levels have been documented in subjects with liver cirrhosis although the significance of this finding with regards to the development of chronic liver disease and portal hypertension has yet to be fully elucidated. In this review we focus on the potential relevance of UII as a vasoactive mediator in the chronic liver disease population and postulate as to the site of overproduction of UII.

Hemodynamic effects of urotensin II and its specific receptor antagonist palosuran in cirrhotic rats

In cirrhosis, splanchnic vasodilation contributes to portal hypertension, subsequent renal sodium retention, and formation of ascites. Urotensin II(U-II) is a constrictor of large conductive vessels. Conversely, it relaxes mesenteric vessels, decreases glomerular filtration, and increases renal sodium retention. In patients with cirrhosis, U-II plasma levels are increased. Thus, we investigated hemodynamic and renal effects of U-II and its receptor antagonist, palosuran, in cirrhotic bile duct-ligated rats (BDL). In BDL and sham-operated rats, we studied acute effects of U-II (3 nmol/kg; intravenously) and palosuran (10 mg/kg; intravenously) and effects of oral administration of palosuran (30 mg/kg/day; 3 days) on hemodynamics and renal function. We localized U-II and U-II-receptor (UTR) in livers and portal veins by immunostaining. We determined U-II-plasma levels by enzyme-linked immunosorbent assay (ELISA), and mesenteric nitrite/nitrate-levels by Griess-reaction. RhoA/Rho-kinase and endothelial nitric oxide synthase (eNOS) pathways were determined by western blot analysis and reverse transcription polymerase chain reaction (RT-PCR) in mesenteric arteries. U-II plasma levels, as well as U-II and UTR-receptor expression in livers and portal veins of cirrhotic rats were significantly increased. U-II administration further augmented the increased portal pressure (PP) and decreased mean arterial pressure (MAP), whereas palosuran decreased PP without affecting MAP. The decrease in PP was associated with an increase in splanchnic vascular resistance. In mesenteric vessels, palosuran treatment up-regulated expression of RhoA and Rho-kinase, increased Rho-kinase-activity, and diminished nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP) signaling. Moreover, palosuran increased renal blood flow, sodium, and water excretion in BDL rats.

CONCLUSION

In BDL rats, U-II is a mediator of splanchnic vasodilation, portal hypertension and renal sodium retention. The U-II-receptor antagonist palosuran might represent a new therapeutic option in liver cirrhosis with portal hypertension.

Fish caudal neurosecretory system: a model for the study of neuroendocrine secretion

The caudal neurosecretory system (CNSS) is unique to fish and has suggested homeostatic roles in osmoregulation and reproduction. Magnocellular neuroendocrine Dahlgren cells, located in the terminal segments of the spinal cord, project to a neurohaemal organ, the urophysis, from which neuropeptides are released. In the euryhaline flounder Platichthys flesus Dahlgren cells synthesise at least four peptides, including urotensins I and II and CRF. These peptides are differentially expressed with co-localisation of up to three in a single cell. Dahlgren cells display a range of electrical firing patterns, including characteristic bursting activity, which is dependent on L-type Ca(2+) and Ca-activated K(+)channels. Activity is modulated by a range of extrinsic and intrinsic neuromodulators. This includes autoregulation by the secreted peptides themselves, leading to enhanced bursting. Electrophysiological and mRNA expression studies have examined changes in response to altered physiological demands. Bursting activity is more robust and more Dahlgren cells are recruited in seawater compared to freshwater adapted fish and this is mirrored by a reduction in mRNA expression for L-type Ca(2+) and Ca-activated K(+) channels. Acute seawater/freshwater transfer experiments support a role for UII in adaptation to hyperosmotic conditions. Responses to stress suggest a shared role for CRF and UI, released from the CNSS. We hypothesise that the Dahlgren cell population is reprogrammed, both in anticipation of and in response to changed physiological demands, and this is seen as changes in gene expression profile and electrical activity. The CNSS shows striking parallels with the hypothalamic-neurohypophysial system, providing a highly accessible system for studies of neuroendocrine mechanisms. Furthermore, the presence of homologues of urotensins throughout the vertebrates has sparked new interest in these peptides and their functional evolution.

Urotensin-II induces ear flushing in rats

BACKGROUND AND PURPOSE

While investigating the effects of systemic urotensin II (U-II), a potent vasoactive peptide acting at the UT receptor, we observed ear pinna flushing after systemic administration to conscious rats. In the present study, U-II-induced ear flushing was quantified in terms of ear pinna temperature change and potential mechanisms were explored.

EXPERIMENTAL APPROACH

U-II-induced ear flushing was quantified by measuring lateral ear pinna temperature changes and compared to that of calcitonin gene-related peptide (CGRP), a known cutaneous vasodilator. Further, the effects of a variety of pharmacological agents on U-II-induced ear flushing were explored.

KEY RESULTS

Subcutaneous injection of U-II (9 microg kg(-1))produced localized ear pinna flushing with an onset of approximately 15 min, a duration of approximately 30 min and a maximal temperature change of 9 degrees C. In contrast, CGRP caused cutaneous flushing within multiple cutaneous beds including the ear pinna with a shorter onset and greater duration than U-II. A potent UT receptor antagonist, urantide, blocked U-II-induced ear flushing but did not affect CGRP-induced ear flushing. Pretreatment with indomethacin or L-Nomega-nitroarginine methylester (L-NAME) abolished U-II-induced ear flushing. Mecamylamine or propranolol did not affect this response to U-II. Direct intracerebroventricular injection studies suggested that the ear flushing response to U-II was not mediated directly by the CNS.

CONCLUSION AND IMPLICATIONS

Our results suggest that U-II-induced ear flushing and temperature increase is mediated by peripheral activation of the UT receptor and involves prostaglandin- and nitric oxide-mediated vasodilation of small capillary beds in the rat ear pinna.

Renal and vascular actions of urotensin II

The peptide hormone urotensin II (UII) has been highly conserved through the vertebrates from fish to humans. As it was shown to be the endogenous ligand for the mammalian orphan G-protein-coupled receptor GPR14, now renamed the UT receptor, interest in UII physiology has grown. Initial observations of a potent vasoconstrictor effect have been tempered with the subsequent revelation of an endothelium-dependent vasodilator action. These complex and contrasting vascular actions are both species- and vascular bed-specific. UII also plays a role in body fluid regulation in lower vertebrates, and it now appears that this extends to mammals. The kidney is a major source of both circulating and urinary UII. UII is found in both the proximal tubules and collecting ducts; the UT receptor is localized primarily to the renal medulla, with greatest expression in the inner medullary collecting ducts. Infusion in rats produced conflicting results: exogenous UII has been shown to increase glomerular filtration rate (GFR) and excretion of water and sodium, but also to reduce the same variables. Inhibition of UT receptor activity with the antagonist urantide resulted in an increase in GFR, diuresis, and natriuresis, suggesting that endogenous UII exerts a tonic influence on basal renal function. UII may also play a role in renal disease, being elevated in the circulation or urine of patients with renal failure and in experimental models of cardiovascular disease such as the spontaneously hypertensive rat. It remains to be established whether these changes represent an underlying primary cause or a compensatory response.

Molecular characterization and expression of urotensin II and its receptor in the flounder (Platichthys flesus): a hormone system supporting body fluid homeostasis in euryhaline fish

Urotensin II (UII) is a potent vasoconstrictor in mammals, but the source of circulating UII remains unclear. Investigations of the caudal neurosecretory system (CNSS), considered the major source of UII in fish, alongside target tissue expression of UII receptor (UT), can provide valuable insights into this highly conserved regulatory system. We report UII gene characterization, expression of the first fish UT, and responses to salinity challenge in flounder. The 12-aa UII peptide shares 73% sequence identity with pig and human UII. Flounder UT receptor shares 56.7% identity with rat. Although the CNSS is the major site of UII expression, RT-PCR revealed expression of UII and UT in all tissues tested. Around 30-40% of large CNSS Dahlgren cells expressed UII, alone or in combination with urotensin I and/or corticotrophin releasing hormone. Immunolocalization of UT in osmoregulatory tissues (gill, kidney) was associated with vascular elements. There were no consistent differences in CNSS UII expression or plasma UII between seawater (SW)- and freshwater (FW)-adapted fish, although gill and kidney UT expression was lower in FW animals. After acute transfer from SW to FW, plasma UII and kidney and gill UT expression were reduced, whereas UT expression in kidney was increased after reverse transfer. UII appears to be more important to combat dehydration and salt-loading in SW than the hemodilution faced in FW. Potentially, altered target tissue sensitivity through changes in UT expression, is an important physiological controlling mechanism, not only relevant for migratory fish but also likely conserved in mammals.