Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P61278 (somatostatin)
 22,083 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The purpose of this study was to characterize the adenylate cyclase system in a purified population of normal somatotrophs derived from rat pituitary and to determine the responses of this system to GRF, somatostatin, guanine nucleotides, and cations. Additionally, experiments were performed to evaluate the interrelationships among changes in adenylate cyclase activity, cellular cAMP levels, and GH release induced by GRF and somatostatin. The results obtained using homogenates and membrane preparations from somatotrophs included the following. 1) GRF caused guanine nucleotide-dependent concentration-related (Ka, approximately 10(-8) M) stimulation of adenylate cyclase activity. 2) Guanine nucleotides were effective in stimulating cyclase in the absence of GRF; the concentration of guanine nucleotide required for half-maximal stimulation was decreased more than 10-fold in the presence of GRF. 3) Adenylate cyclase activity increased with increasing concentrations of free Mg2+ (0.25-20 mM); activation by GRF and guanine nucleotide resulted in an approximately 7-fold increase in the enzyme's affinity for free Mg2+. 4) Somatostatin, up to 10(-6) M, did not alter basal or GRF-stimulated adenylate cyclase activity. 5) Ca2+ (0.5-11.9 microM) produced concentration-dependent inhibition of basal (up to 28%) and GRF-stimulated (up to 47%) cyclase activities; the inhibitory effect of Ca2+ was accompanied by a decrement (2- to 3-fold) in the apparent affinities of the enzyme for both GRF and guanine nucleotide. In intact somatotrophs, GRF produced concentration-dependent stimulation of GH release (Ka, approximately 6 x 10(-11) M), preceded by a marked elevation of cAMP levels. While somatostatin blocked GRF-induced GH release, the augmented cAMP levels were only slightly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)
 ...
PMID:Growth hormone-releasing factor-sensitive adenylate cyclase system of purified somatotrophs: effects of guanine nucleotides, somatostatin, calcium, and magnesium. 256 26

The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and thyroliberin exerted additive stimulatory effects on prolactin release and synthesis in rat adenoma GH4C1 pituicytes in culture. Both TPA and thyroliberin activated the adenylate cyclase in broken cell membranes. When combined, the secretagogues displayed additive effects. TPA did not alter the time course (time lag) of adenylate cyclase activation by hormones, guanosine 5'-[beta,gamma-imino]triphosphate or forskolin, nor did it affect the enzyme's apparent affinity (basal, 7.2 mM; thyroliberin-enhanced, 2.2 mM) for free Mg2+. The TPA-mediated adenylate cyclase activation was entirely dependent on exogenously added guanosine triphosphate. ED50 (dose yielding half-maximal activation) was 60 microM. Access to free Ca2+ was necessary to express TPA activation of the enzyme, however, the presence of calmodulin was not mandatory. TPA-stimulated adenylate cyclase activity was abolished by the biologically inactive phorbol ester, 4 alpha-phorbol didecanoate, by the protein kinase C inhibitor polymyxin B and by pertussis toxin, while thyroliberin-sensitive adenylate cyclase remained unaffected. Experimental conditions known to translocate protein kinase C to the plasma membrane and without inducing adenylate cyclase desensitization, increased both basal and thyroliberin-stimulated enzyme activities, while absolute TPA-enhanced adenylate cyclase was maintained. Association of extracted GTP-binding inhibitory protein, Gi, from S49 cyc- murine lymphoma cells with GH4C1 cell membranes yielded a reduction of basal and hormone-stimulated adenylate cyclase activities, while net inhibition of the cyclase of somatostatin was dramatically enhanced. However, TPA restored completely basal and hormone-elicited adenylate cyclase activities in the Gi-enriched membranes. Finally, TPA completely abolished the somatostatin-induced inhibition of adenylate cyclase in both hybrid and non-hybrid membranes. These data suggest that, in GH4C1 cells, protein kinase C stimulation by phorbol esters completely inactivates the n alpha i subunit of the inhibitory GTP-binding protein, leaving the n beta subunit functionally intact. It can also be inferred that thyroliberin conveys its main effect on the adenylate cyclase through activation of the stimulatory GTP-binding protein, Gs.
 ...
PMID:Protein kinase C stimulates adenylate cyclase activity in prolactin-secreting rat adenoma (GH4C1) pituicytes by inactivating the inhibitory GTP-binding protein Gi. 256 96

L-Glutamate, N-methyl-D-aspartic acid (NMDA), quisqualate, and kainate were found to increase endogenous somatostatin release from primary cultures of rat cortical neurons in a dose-dependent manner. The rank order of potency calculated from the dose-response curves was quisqualate greater than glutamate = NMDA greater than kainate, with EC50 values of 0.4, 20, and 40 microM, respectively. Alanine, glutamine, and glycine did not modify the release of somatostatin. The stimulation of somatostatin release elicited by L-glutamate was Ca2+ dependent, was decreased by Mg2+, and was blocked by DL-amino-5-phosphonovaleric acid (APV) and thienylphencyclidine (TCP), two specific antagonists of NMDA receptors. The NMDA stimulatory effect was strongly inhibited by APV in a competitive manner (IC50 = 50 microM) and by TCP in a noncompetitive manner (IC50 = 90 nM). The release of somatostatin induced by the excitatory amino acid agonists was not blocked by tetrodotoxin (1 microM), a result suggesting that tetrodotoxin-sensitive, sodium-dependent action potentials are not involved in the effect. Somatostatin release in response to NMDA was potentiated by glycine, but the inhibitory strychnine-sensitive glycine receptor did not appear to be involved. Our data suggest that glutamate exerts its stimulatory action on somatostatin release essentially through an NMDA receptor subtype.
 ...
PMID:Actions of excitatory amino acids on somatostatin release from cortical neurons in primary cultures. 257 Jan 26

We have characterized specific receptors for tetradecapeptide somatostatin (SS-14) of rat brain using 125I-labeled Tyr11-SS-14([125I-Tyr11]SS-14) as radioligand. [125I-Tyr11]SS-14 binding was sensitive to time, pH, temperature and ionic strength of the buffer, and was optimal in 50mM HEPES/KOH buffer, pH 7.5, at 25 degrees C for 60 minutes. Scatchard analysis indicated that the rat forebrain membranes had a single binding site with an apparent dissociation constant (Kd) of 0.522 +/- 0.044 nM and maximum binding capacity (Bmax) was 233 +/- 37 fmol/mg protein (mean +/- SEM). Ca2+, Mg2+, Mn2+ and Co2+ had a significant effect on the specific binding, which indicates that these metal ions might affect somatostatin receptor activity in the brain. Among CNS acting drugs, Ca2+ antagonists, antischizophrenic drugs, antidepressants and anticholinergic drugs had relative effects on [125I-Tyr11]SS-14 bindings to rat cerebral cortex membranes.
 ...
PMID:Biochemical and pharmacological characterization of somatostatin receptors in rat brain. 257 1

In the present work, we have characterized by film radioautography the effects of divalent cations and guanine nucleotide on specific receptor for somatostatin (SRIF) using 125I-TyrO-DTrp8-SRIF14 (125I-ToD8-SRIF) as a ligand. The experiments were performed on coronal 20-microM-thick sections cut at the level of the amygdala, thus allowing to study binding sites in several regions enriched in binding sites (frontal cortex, hippocampus CA1 and dentate gyrus, habenula, basolateral nucleus of the amygdala). In a preliminary set of experiments using brain cortical membranes it was found that 3 mM Mg2+ ions doubled the specific binding of 125I-ToD8-SRIF. However, Mg2+ enhanced equally by a factor of 3 affinities of high- and low-affinity binding sites as evidenced by SMS 201.995 displacement curves without modifying the ratio between high and low affinity sites. In radioautographic studies while SRIF14 and SRIF28 elicited monophasic displacement curves, SMS 201.995 displaced 125I-ToD8-SRIF binding in a biphasic manner in all regions tested but the baso-lateral nucleus of the amygdala. Radioautographic distribution of 125I-ToD8-SRIF binding sites was identical whether the sections were incubated with MgCl2 or with MnCl2 and almost undetectable in the absence of ions. In all structures investigated increasing concentrations of GTP totally inhibited 125I-ToD8-SRIF binding with an IC50 value of 3 microM. In conclusion, our results demonstrate that 125I-ToD8-SRIF-binding sites in brain occur on two different affinity states as assessed by a displacement curve using endogenous ligands and SMS 201.995. According to the comparable effects of divalent cations and GTP, the two subtypes of 125I-ToD8-SRIF-binding sites discriminated by SMS 201.995 are likely to correspond to interconvertible forms of the same receptor coupled to a G protein-transducing system.
 ...
PMID:Regional distribution of somatostatin receptor affinity states in rat brain: effects of divalent cations and GTP. 257 46

To investigate whether somatostatin receptors couple to guanine nucleotide inhibitory protein, Ni, on rat pancreatic acinar membranes, the effects of guanine nucleotide analogs or pretreatment of acini with islet activating protein (IAP), pertussis toxin on labeled somatostatin binding were examined. Guanine nucleotides reduced labeled somatostatin binding to acinar membranes up to 80%, with rank order of potency being guanyl-5'-yl imidodiphosphate (Gpp(NH)p) greater than GTP greater than GDP greater than GMP. Scatchard analysis of the labeled somatostatin binding revealed that the decrease in somatostatin binding caused by Gpp(NH)p was due to the decrease in the maximum binding capacity without a significant change in the binding affinity. The inhibitory effect of Gpp(NH)p was partially abolished in the absence of Mg2+ and Na+ also reduced labeled somatostatin binding. Furthermore, inhibitory effects of 100mM Na+ and Gpp(NH)p were additive in reducing labeled somatostatin binding. A half maximal inhibitory concentration of Gpp(NH)p was decreased to 10(-7)M in the presence of 100mM Na+ and 5mM Mg2+ as compared to 10(-6)M in the presence of 5mM Mg2+ alone. Results therefore suggest that Gpp(NH)p requires Mg2+ for Ni activation and Na+ increases sensitivity of Ni to guanine nucleotide analogs. When pancreatic acini were treated for 4 hours with varying concentrations of IAP, which has been shown to uncouple Ni-mediated communication between inhibitory receptors and adenylate cyclase catalytic unit, subsequent labeled somatostatin binding to the acinar membranes was decreased in a dose dependent manner. These results indicate that somatostatin receptors on pancreatic acinar membranes couple to guanine nucleotide inhibitory protein, Ni and thus somatostatin probably functions in the pancreas to regulate intracellular signal transduction via Ni.
 ...
PMID:[Coupling of guanine nucleotide inhibitory protein to somatostatin receptors on rat pancreatic acinar membranes]. 282 26

Preincubation with an alpha 2-adrenergic agonist sensitized subsequent forskolin- and vasoactive intestinal peptide-stimulated cyclic AMP production in HT29 cells, a human colonic adenocarcinoma cell line. Preincubation with somatostatin, another agonist negatively coupled to adenylate cyclase, sensitized forskolin-stimulated cyclic AMP production to a lesser extent. alpha 2-Adrenergic agonist preincubation also resulted in desensitization as indicated by a shift to the right in the dose-response curve of a subsequent challenge by an alpha 2-adrenergic agonist. In an effort to elucidate the mechanism for sensitization, we examined protein kinase C and the Na+/H+ antiporter. Whereas these components had marked effects on forskolin stimulation, there was no effect on sensitization. Changes in the concentration of extra-cellular Ca2+ or Mg2+ had no effect on either forskolin stimulation or sensitization. Pertussis toxin pretreatment caused a time-dependent decrease in sensitization, an attenuation of inhibition of cyclic AMP production, and a decrease in subsequent [32P]ADP-ribosylation by pertussis toxin. The time course for these three events was similar, implicating the inhibitory guanine nucleotide regulatory protein in the mechanism for alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production. In addition, pertussis toxin dramatically decreased forskolin-stimulated cyclic AMP production, although with a different time course. These results suggest that the mechanism of sensitization is via an as yet undefined sequence of biochemical events that includes the inhibitory guanine nucleotide regulatory protein, but does not include inhibition of adenylate cyclase nor activation of the Na+/H+ antiporter.
 ...
PMID:Characterization and possible mechanisms of alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production in HT29 cells. 284 62

Two populations of brain somatostatin (SS) receptors, one with high affinity for the somatostatin octapeptide analogue SMS 201-995 (SS1 type) and one poorly sensitive to this analogue (SS2 type) have been characterised in regard to their ionic requirements using two radioligands, the iodinated Tyr3 derivative of the octapeptide SS analog SMS 201-995 and the iodinated [Tyr11]-SS. Specific binding of 125I-[Tyr11]-SS to rat cortex membrane homogenates can be increased by approximately 180% in presence of 5 mM Mg2+. The increase in number of binding sites seen by Mg2+ is not accompanied by a marked increase in affinity for SS but for SMS 201-995: the low affinity binding for SMS 201-995 seen in absence of Mg2+ is replaced in part by higher affinity binding in presence of these ions. SMS 201-995 sensitive SS1 receptor subpopulation measured with 125I-204-090, a specific ligand for SS1 subpopulation, is massively increased in presence of Mg2+. However, SMS 201-995 insensitive SS2 receptor population measured with 125I-[Tyr11]-SS in presence of excess SMS 201-995 is unchanged in presence of Mg2+. The Mg2+-dependency can also be observed with autoradiography for extra cortical, i.e. hippocampal, brain SS receptors. 120 mM Na+ does not affect the total brain SS receptor population, but reduces the specific binding of SS1 receptors and increases that of SS2 receptors. Therefore, the rat brain, in particular the cortex, possesses a SMS 201-995-sensitive, Mg2+-dependent SS receptor subpopulation (SS1) as well as a SMS 201-995-insensitive, Mg2+-independent SS population (SS2).
 ...
PMID:Different ionic requirements for somatostatin receptor subpopulations in the brain. 287 93

Guanine nucleotides and pertussis toxin were used to investigate whether somatostatin receptors interact with the guanine nucleotide inhibitory protein (Ni) on pancreatic acinar membranes in the rat. Guanine nucleotides reduced 125I-[Tyr1]somatostatin binding to acinar membranes up to 80%, with rank order of potency being 5'-guanylyl imidodiphosphate [Gpp(NH)p] greater than GTP greater than GDP greater than GMP. Scatchard analysis revealed that the decrease in somatostatin binding caused by Gpp(NH)p was due to the decrease in the maximum binding capacity without a significant change in the binding affinity. The inhibitory effect of Gpp(NH)p was partially abolished in the absence of Mg2+. When pancreatic acini were treated with 1 microgram/ml pertussis toxin for 4 h, subsequent 125I-[Tyr1]somatostatin binding to acinar membranes was reduced. Gpp(NH)p further decreased somatostatin binding to islet-activating protein (IAP)-treated acinar membranes. Pertussis toxin treatment also abolished the inhibitory effect of somatostatin on vasoactive intestinal peptide-stimulated increase in cellular content of adenosine 3',5'-cyclic monophosphate (cAMP) in the acini. Furthermore, exposure of acini to IAP caused ADP ribosylation of a membrane protein with Mr = 41,000 in parallel to the inhibition of cAMP accumulation in acini. The present results suggest, therefore, that 1) somatostatin probably functions in the pancreas to regulate adenylate cyclase enzyme system via Ni, 2) the extent of modification of Ni is correlated with the ability of somatostatin to inhibit cAMP accumulation in acini, and 3) guanine nucleotides also inhibit somatostatin binding to its receptor.
 ...
PMID:Coupling of guanine nucleotide inhibitory protein to somatostatin receptors on pancreatic acinar membranes. 288 15

The action of excitatory amino acid agonists on endogenous somatostatin release was examined in primary cultures of rat diencephalic neurons. Increasing concentrations of glutamate stimulated somatostatin release in a dose-dependent manner. Since this effect was decreased by Mg2+, all experiments were performed in Mg2+-free media. We found that excitatory amino acid agonists evoked somatostatin release in the following order of potency: quisqualate greater than glutamate = N-methyl-D-aspartate (NMDA) greater than kainate, as calculated from the dose-response curves. The increase in somatostatin release elicited by glutamate or NMDA was selectively antagonized by DL-2-amino-5-phosphonovaleric acid and by thyenyl-phencyclidine, two specific antagonists of NMDA receptors. The NMDA effect was strongly inhibited: in a competitive manner by APV and in a noncompetitive manner by TCP with IC50 of 90 microM and 0.2 microM, respectively. Glutamate-induced somatostatin release was not blocked by tetrodotoxin (1 microM) suggesting that tetrodotoxin-sensitive sodium-dependent action potentials are not involved in this effect. Our data suggest the presence of functionally active excitatory amino acid receptors in somatostatinergic neurons. Glutamate seems to exert its stimulatory action on somatostatin release essentially through NMDA type receptor sites.
 ...
PMID:Glutamate stimulates somatostatin release from diencephalic neurons in primary culture. 290 50


<< Previous 1 2 3 4 Next >>