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

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.

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.

Human urotensin II-induced aorta ring contractions are mediated by protein kinase C, tyrosine kinases and Rho-kinase: inhibition by somatostatin receptor antagonists

Human urotensin II-(1-11) and its N-terminally shortened analogues, human urotensin II-(4-11)-OH and human urotensin II-(4-11)-NH2 are potent vasoconstrictor peptides in isolated rat thoracic aorta. Human urotensin II-induced tonic aorta ring contractions are inhibited by the Ca2+ channel antagonists, verapamil, nitrendipine and diltiazem; D609 (Tricyclodecan-9-yl-xanthogenate, K), selective inhibitor of phosphatidylcholine-specific phospholipase C and partially by phospholipase C inhibitor U-73122 [1-[6-((17ss-3 Methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-25-dione] and a selective inhibitor of phosphatidyl-inositol-specific phospholipase C-ET-18-OCH3 (Edelfosine,1-O-octadecyl-2O-methyl-rac-glycero-3-phosphorylcholine); protein kinase C inhibitors, chelerythrine and NPC-15437 [S-2,6-diamino-N-[[1-(1-oxotridecyl)-2-piperidinyl]methyl]-hexanamide dihydrochloride]; tyrosine kinase inhibitors, genistein and tyrphostin B42 and Rho-kinase inhibitor HA-1077 [1-(5-isoquinolinylsulfonyl)-homopiperazine dihydrochloride]. This indicates that human urotensin II-induced tonic contractions of the rat aorta are mediated by phospholipase C, protein kinase C, tyrosine kinases and Rho-kinase related pathways. In the high K+ medium, human urotensin II induces dose-dependent phasic oscillations of aortic rings. These are inhibited by Ca2+ channel antagonists, the phospholipase C inhibitor, U-73122 and protein kinase C inhibitors, chelerythrine and NPC-15437, indicating that human urotensin II-induced phasic oscillations of the rat aorta are mediated by phospholipase C and protein kinase C-dependent pathways. Given their close structural similarity, several somatostatin analogues, importantly containing DCys5 and DTrp7 and expressing different degrees of somatostatin receptor antagonist activity, were tested for possible inhibitory effects on human urotensin II-induced contractions of the rat aorta rings. Pre-incubation of rat aorta rings in the presence of somatostatin analogues, which are preferentially sst2 specific binders: PRL-2882; PRL-2903 and PRL-2915 at micro-molar concentrations significantly blocked the development of human urotensin II-induced tonic contractions. Somatostatin receptor antagonists dose-dependently inhibited human urotensin II-induced Ca2+ transients in rat thoracic aorta rings. These somatostatin receptor antagonists displayed moderate affinities for recombinant rat and human urotensin II receptor binding sites. The data support the suggestion that urotensin II receptor and somatostatin type 2/5 receptors display similar surface topologies and that analogues of somatostatin could provide useful lead compounds for the development of more potent urotensin II receptor antagonists.

Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder, Platichthys flesus

The physiological factors that govern activity of the caudal neurosecretory system in teleost fish are poorly understood. Immunocytochemical evidence indicates that the neurosecretory Dahlgren cells are innervated by descending monoaminergic fibres. Using intracellular recording techniques in an isolated preparation of the posterior spinal cord of the flounder (Platichthys flesus) we have demonstrated that superfusion of adrenaline or noradrenaline (10(-7) - 10(-3) M) causes hyperpolarization of Dahlgren cells (up to -30 mV). This hyperpolarization is likely to reflect an inhibitory effect of noradrenergic nerves on the neurosecretory system in vivo, reducing the rate of hormone release. Fluctuations in the input resistance and membrane time constant suggest involvement of a multiplicity of cellular mechanisms, including the opening and closing of populations of ion-selective channels. Superfusion with dopamine (10(-7) - 10(-3) M) had no effect. Superfusion with the beta-adrenoreceptor agonist, isoprenaline, caused hyperpolarization but to a markedly lesser extent than the maximum effect of adrenaline or noradrenaline, suggesting that their effects are mediated, only in part, by a beta-adrenoreceptor subtype. Superfusion of the preparation with a membrane permeable, non-hydrolysable cyclic AMP analogue (8-[4-chlorophenylthio]-cAMP) resulted in a slight hyperpolarization which was accompanied by a small, but significant, increase in input resistance. These data are consistent with at least part of the beta-adrenoreceptor mediated effect involving closure of cAMP-sensitive ion channels. Superfusion with the alpha 1-adrenoreceptor agonist, phenylephrine, had no effect on any electrophysiological parameter studied. However, the alpha 2-adrenoreceptor agonist, clonidine, caused hyperpolarization which again failed to reach the maximum level produced by adrenaline or noradrenaline. Together, these data suggest that the adrenergic inhibition of Dahlgren cell activity is mediated by both alpha 2- and beta-adrenoreceptor subtypes.