Specificity of response of intestinal ion transport systems to a pair of natural peptide hormone analogs: somatostatin and urotensin II

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

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, from fish to human

The cyclic peptide urotensin II (UII) was originally isolated from the urophysis of teleost fish on the basis of its ability to contract intestinal smooth muscle. The UII peptide has subsequently been isolated from frog brain and, later on, the pre-proUII cDNA has been characterized in mammals, including humans. A UII paralog called urotensin II-related peptide (URP) has been identified in the rat brain. The UII and URP genes originate from the same ancestral gene as the somatostatin and cortistatin genes. In the central nervous system (CNS) of tetrapods, UII is expressed primarily in motoneurons of the brainstem and spinal cord. The biological actions of UII and URP are mediated through a G protein-coupled receptor, termed UT, that exhibits high sequence similarity with the somatostatin receptors. The UT gene is widely expressed in the CNS and in peripheral organs. Consistent with the broad distribution of UT, UII and URP exert a large array of behavioral effects and regulate endocrine, cardiovascular, renal, and immune functions.

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.

Urotensin II and renal function in the rat

Urotensin II (UII) is a potent vasoactive hormone in mammals. However, despite its well-known effects on epithelial sodium transport in fish, little is known about its actions on the mammalian kidney. The aim of this study was to determine the effects of UII on renal function in the rat. Using standard clearance methods, the effects of rUII and the rat UII receptor (UT) antagonist, urantide, were studied. UII was measured in plasma and urine by radioimmunoassay. UII and UT were localized in the kidney by immunohistochemistry and mRNA expression quantified. Rat urinary [UII] was 1,650-fold higher than that in plasma. Immunoreactive-UII was localized to the proximal tubules, outer and inner medullary collecting ducts (IMCD); UT receptor was identified in glomerular arterioles, thin ascending limbs, and IMCD. UII and UT mRNA expression was greater in the medulla; expression was higher still in spontaneously hypertensive rats (SHRs) associated with raised plasma (UII). Injection of rUII induced reductions in glomerular filtration rate (GFR), urine flow, and sodium excretion. Urantide infusion resulted in increases in these variables. Endogenous UII appears to contribute to the regulation of GFR and renal sodium and water handling in the rat. While hemodynamic changes predominate, we cannot rule out the possibility of a direct tubular action of UII. Increased expression of UII and UT in the SHR suggests that UII plays a role in the pathophysiology of cardiovascular disease.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

NaCl and fluid secretion by the intestine of the teleostFundulus heteroclitus: involvement of CFTR

Sections of posterior intestine of the euryhaline killifish Fundulus heteroclitus adapted to sea water were stimulated by the calcium ionophore ionomycin (1 μmol l–1) in combination with agents to elevate intracellular cyclic AMP levels, 0.5 mmol l–1 dibutyryl-cyclic AMP (db-cAMP) with 0.1 mmol l–1 3-isobutyl-1-methylxanthine (IBMX). Intestinal bag preparations from recently fed animals (but not from overnight unfed animals) changed from fluid absorption (+18.9±8.30 μl cm–2 h–1 , N=8) in the untreated control period to net fluid secretion after stimulation (–7.43±1.30 μl cm–2 h–1, N=8, P<0.01; means ± s.e.m.), indicative of the capacity of teleost intestine to undergo secretion. Posterior intestinal pieces mounted in vitro in Ussing-style membrane chambers showed net Cl– uptake (+2.245±0.633 μequiv cm–2 h–1, N=7) that turned to net secretion following stimulation by ionomycin + db-cAMP + IBMX (–3.809±1.22 μequiv cm–2 h–1, N=7, P<0.01). Mucosal application of the anion channel blocker 1 mmol l–1 diphenylamine-2-carboxylate (DPC) after ionomycin + db-cAMP + IBMX treatment significantly reduced serosal-to-mucosal unidirectional Cl– flux (P<0.001), net Cl– flux (P<0.05), short-circuit current (Isc, P<0.001) and tissue conductance (Gt, P<0.001), while 0.1 mmol l–1 4,4′-diisothiocyano-2,2′-stilbene-disulphonic acid (DIDS, a blocker of anion exchange) was without effect. Stimulation by db-cAMP + IBMX (no ionomycin) significantly increased unidirectional fluxes, Isc and Gt but did not produce net Cl– secretion. Ionomycin alone produced a transient increase in Isc but had no effect on Gt and caused no significant changes in unidirectional or net Cl– fluxes. Addition of db-cAMP + IBMX after ionomycin treatment produced net secretion of Cl– and large increases in unidirectional fluxes and Gt. Cystic fibrosis transmembrane conductance regulator (CFTR) was immunocytochemically localized with a monoclonal mouse antibody to the carboxy terminus and found to be present in the cytoplasm and basolateral membranes of all enterocytes and in the brush-border membrane of some cells, whereas NKCC immunofluorescence, demonstrating the presence of the Na+/K+/2Cl– cotransporter, was present in the cytoplasm and brush-border membrane. We conclude that the teleost intestine is capable of salt and fluid secretion only if intracellular Ca2+ and cyclic AMP pathways are stimulated together and that this secretion appears to involve activation of CFTR ion channels in the apical membrane of a subpopulation of enterocytes.

Plasma concentrations of arginine vasotocin and urotensin II are reduced following transfer of the euryhaline flounder (Platichthys flesus) from seawater to fresh water

Plasma concentrations and stored levels of the neuroendocrine peptides arginine vasotocin (AVT) and urotensin II (UII) were measured in the euryhaline flounder (Platichthys flesus) following the acute hypo-osmotic challenge of direct seawater (SW) to fresh water (FW) transfer. Hormone measures, plasma osmolality, and ion concentrations and tissue water content were determined 1, 4, 8, 24, 72, and 144 h after transfer. Plasma AVT concentration fell initially following FW transfer but then returned toward pretransfer levels by day 6. Plasma UII concentration decreased while urophysial UII content was increased following hypo-osmotic challenge relative to SW time-matched controls, suggesting down regulation of the UII system during the initial stages after FW transfer. These changes in neuroendocrine activity were associated with a significant fall in plasma osmolality and major plasma ions. Positive correlations were observed between plasma AVT and osmolality and Cl- and Mg2+ concentrations, suggesting functional association of these plasma parameters with AVT action and/or control of AVT secretion. The initial response to hypotonic challenge involves reduced plasma AVT and UII levels consistent with the proposed role for these hormones, supporting flounder osmoregulation in hypertonic media.

Angiotensin II binding sites in the rat pancreas and their modulation after sodium loading and depletion

Specific 125I angiotensin II binding sites were identified in the rat pancreas by radioreceptor assay, autoradiography and immunohistochemistry. Scatchard analysis of the binding in normal rats yielded a Kd of 0.51 +/- 0.23 nM with a Bmax of 15 +/- 3.5 fmol/mg protein (means +/- SD, n = 6). Changed plasma sodium concentration resulted in modifications in the binding affinity and capacity. Sodium loading depressed both Kd (0.36 +/- 0.1 nM) and Bmax (6.4 +/- 0.1 fmol/mg protein), while sodium depletion elevated both Kd (2.03 +/- 0.3 nM) and Bmax (45 +/- 3.5 fmol/mg protein) (means +/- SD, n = 6). Autoradiography using 125I Ang II and immunohistochemistry of the binding sites saturated with unlabeled Ang II and incubated with Ab-Ile5 Ang II, revealed localization of the binding sites on the islet cell membranes and in the exocrine pancreas.

Recognition by goby intestine of a somatostatin analog, SMS 201-995

The caudal neurosecretory peptide urotensin II and its partial structural analog the hypothalamic and gastroenteropancreatic peptide somatostatin can be distinguished by the goby posterior intestine; urotensin II stimulates Na+ and Cl- absorption whereas somatostatin is without effect. Sandostatin (Sandoz compound SMS 201-995) is a partial structural analog of somatostatin, possessing the active core common to both urotensin II and somatostatin but lacking the C- and N-terminal tails which apparently confer recognition in species specificity. SMS 201-995 mimicked the effects of urotensin II, namely, increased net Cl- absorption and decreased radiochloride backflux consistent with the observed increase in transepithelial resistance and the cellular depolarization typical of transport stimulation in this tissue. Speculation on the role of the tails in species-specific recognition is supported by the nature of amino acid residue substitutions in urotensin II, somatostatin, and SMS 201-995 and suggests the existence of multiple receptor types for this group of peptides.

Regulation of goby intestinal ion absorption by the calcium messenger system

The role of the calcium messenger system in the regulation of ion absorption across the teleost intestine was studied using pharmacological intervention. Radiochloride transport was independent of external Ca2+ over the range 10 microM to 2.5 mM. Treatment with the Ca2+ ionophore A23187 (to hyperpolarization of the apical membrane potential of intestinal epithelial cells. The Ca2+-calmodulin antagonists trifluoperazine (TFP) and calmidazolium (R24571) produced opposite effects, i.e., stimulation of Cl- absorption and cellular depolarization. Treatment with TFP or R24571 will block or override the inhibitory action of A23187. These data suggest a regulatory role for Ca2+ in the control of intestinal NaCl absorption and mediation via calmodulin.

Urotensin II lowers cytoplasmic free calcium concentration in goby enterocytes: measurements using quin2

A technique is described for the measurement of cytoplasmic free calcium concentration ([Ca2+]c) in a suspension of goby intestinal epithelial cells using the fluorescent probe quin2. For 18 cell suspensions, [Ca2+]c was determined to be 142 +/- 14 nM. The caudal neurosecretory peptide urotensin II at 10(-7) to 10(-6) M concentration reduced [Ca2+]c by 20-50%. Together with previous findings linking the calcium-calmodulin system with the stimulation of Na+ and Cl- absorption in this tissue, it appears that alterations in cytoplasmic Ca2+ may mediate urotensin II-induced stimulation of ion transport.

In vitro effects of somatostatin and urotensin II on prolactin and growth hormone secretion in tilapia, Oreochromis mossambicus

The control of release of two recently characterized forms of prolactin (PRL) of molecular mass 24 and 20 kDa was investigated. The rostral pars distalis of male tilapia was incubated singly in a hypotonic modified Krebs-Ringer bicarbonate medium in order to stimulate PRL release; for comparison, the proximal pars distalis containing growth hormone (GH) cells was incubated in isotonic medium with or without 1 microgram/ml cortisol to stimulate GH release. The release of both PRLs and GH into the medium was measured by sodium dodecyl sulfate (SDS)--polyacrylamide gel electrophoresis followed by densitometry. Both somatostatin and synthetic (Gillichthys) urotensin II, a partial somatostatin homolog and analog from the teleost caudal neurosecretory system, significantly inhibited the release of both PRLs. Somatostatin significantly inhibited GH release, but urotensin II had no significant effect.

Both somatostatin and the caudal neuropeptide, urotensin II, stimulate lipid mobilization from coho salmon liver incubated in vitro

The direct effects of somatostatin-14 (SRIF; synthetic ovine) and the fish caudal neuropeptide, urotensin II (UII; synthetic Gillichthys), on fatty acid (FA) release and on lipolytic enzyme (triacylglycerol lipase) activity were determined on coho salmon liver slices incubated in vitro. FA release was continuously measured by pH-stat titration. Additionally, gas chromatographic analysis of the incubation medium was performed to determine the type and relative composition of medium fatty acid constituents. SRIF and UII both stimulated FA release in a dose-dependent manner; the two peptides appeared to stimulate FA release in an equimolar manner. Maximal response was obtained at 1 X 10(-5) M; ED50 was approximately 2 X 10(-7) M. SRIF-stimulated FA release did not result in differential secretion of any particular FA type. Tissue triacylglycerol lipase activity was significantly enhanced by addition of UII or SRIF (2 X 10(-6) M). Dibutyryl cAMP and IBMX both stimulated FA release and lipase activity; dbcAMP stimulated FA release in dose-dependent manner. These results indicate that SRIF and UII directly enhance lipid mobilization from salmon liver slices and suggest that SRIF- and UII-stimulated lipid mobilization from salmon liver slices is mediated through cAMP.