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W Feniuk

Publications and source records attributed to W Feniuk.

At least 37 records · Page 2Linked to original sources
PubMed1997

Somatostatin5 receptor-mediated [35S]guanosine-5'-O-(3-thio)triphosphate binding: agonist potencies and the influence of sodium chloride on intrinsic activity.

We studied the activation of the human somatostatin5 receptor recombinantly expressed in CHO-K1 cells by using some newly available agonists and antagonists. Somatostatin-28 bound to this receptor with a higher affinity than somatostatin-14 and was more potent in increasing [35S]guanosine-5'-O-(3-thio)triphosphate ([35S]GTPgammaS) binding. Somatostatin-14-induced [35S]GTPgammaS binding to membranes from this cell line was decreased in a concentration-related manner by increasing concentrations of GDP and sodium chloride. At 50 mM (low) sodium, agonist EC50 values for stimulating [35S]GTPgammaS binding were lower than those at 150 mM (high) sodium and were closer to their respective affinity estimates (dissociation equilibrium constants) for binding to the receptor in the absence of sodium. Both agonist binding to the high affinity state of the receptor and agonist-induced [35S]GTPgammaS binding were abolished by pertussis toxin pretreatment. The putative somatostatin5 receptor-selective ligand L-362,855, unlike somatostatin-14 and somatostatin-28, showed differential intrinsic activity for stimulation of [35S]GTPgammaS binding, behaving as a partial agonist in high sodium and a full agonist in low sodium. In contrast, BIM-23056 did not behave as an agonist under any conditions studied but was able to antagonize somatostatin-14-induced [35S]GTPgammaS binding. We conclude that measurement of [35S]GTPgammaS binding mediated by somatostatin receptor activation in the presence of different concentrations of sodium chloride provides a useful functional assay for assessing the relative agonist efficacies of novel ligands identified from radioligand binding studies.

Animals↗
PubMed1996

Identification and characterisation of heterogeneous somatostatin binding sites in rat distal colonic mucosa.

We have previously shown that the somatostatin (SRIF) sst2 receptor-selective peptide, BIM-23027, is a potent antisecretory agent in rat isolated distal colonic mucosa (RDCM) and in radioligand binding studies in RDCM membranes, it only maximally inhibited approximately 40% of [125I]-Tyr11-SRIF-14 binding (McKeen ES, Feniuk W, Humphrey PPA (1995) Naunyn-Schmiedeberg's Arch Pharmacol 352:402-411). The aim of this study was to characterise the BIM-23027-sensitive and -insensitive SRIF binding sites in more detail and to compare their properties with those of the recombinant sst2 receptor stably expressed in mouse fibroblast (Ltk-) cells. SRIF-14, SRIF-28, CGP-23996 and D Trp8-SRIF-14 abolished [125I]-Tyr11-SRIF-14 binding (pIC50 values, 8.7-9.7) but the competition curves had Hill slopes which were less than unity. Octreotide and L-362,855 inhibited binding over a wide concentration range (0.1 nM-1 microM) and inhibition of binding was incomplete at the highest concentration studied. BIM-23056 (pIC50 < 6.5) was a weak inhibitor of [125]-Tyr11-SRIF-14 binding. GTP gamma S decreased [125I]-Tyr11-SRIF-14 binding by 40%. Further binding experiments with [125I]-Tyr11-SRIF-14 were carried out in RDCM in the continuous presence of BIM-23027 (1 microM). Under these conditions, seglitide had no effect on [125I]-Tyr11-SRIF-14 binding at concentrations up to 10 microM, whilst SRIF-14 and SRIF-28 abolished specific [125I]-Tyr11-SRIF-14 binding in a manner which was consistent with the ligand binding to two sites. SRIF-14 and SRIF-28 displayed high affinity (pIC50 values of 7.8 and 7.3) for the remaining sites. Octreotide, L-362,855 and BIM-23056 were weak inhibitors of [125I]-Tyr11-SRIF-14 binding (pIC50 < 6.5). [125I]-BIM-23027 labelled a single population of SRIF binding sites in RDCM membranes and mouse fibroblast (Ltk-) cells stably expressing the human recombinant sst2 receptor. There was a significant correlation between the affinity estimates of a range of SRIF analogues at inhibiting [125I]-BIM-23027 binding in RDCM membranes and binding to the recombinant sst2 receptor in Ltk- cells, suggesting that the sites labelled by [125I]-BIM-23027 in RDCM are similar to the sst2 receptor. GTP gamma S (100 microM) decreased [125I]-BIM-23027 binding in RDCM by 60%. The results from these studies demonstrate that [125I]-Tyr11-SRIF-14 labels a heterogeneous population of high affinity SRIF binding sites in RDCM membranes. The majority of these sites are insensitive to GTP gamma S and display negligible affinity for the cyclic hexapeptides, BIM-23027 and seglitide. The remaining high affinity binding sites can be selectively labelled with [125I]-BIM-23027, are sensitive to GTP gamma S and show similar characteristics to the recombinant sst2 receptor which appears to mediate the antisecretory effects of SRIF in the mucosa (McKeen ES, Feniuk W, Humphrey PPA (1995) Naunyn-Schmiedeberg's Arch Pharmacol 352:402-411).

Animals↗
PubMed1996

GR127935: a potent and selective 5-HT1D receptor antagonist.

GR127935 is the most potent 5-HT1D receptor antagonist yet described, possessing nanomolar affinity at human 5-HT1D receptors. Sumatriptan-induced contractions of the dog isolated basilar artery and saphenous vein are antagonised by GR127935 in an insurmountable manner indicative of its slow dissociation from the 5-HT1D receptor. 5-HT1D receptor-mediated hypothermia and rotational behaviour in guinea-pigs are antagonised potently, and with long duration, by GR127935, administered by a variety of routes. GR127935 also blocks central 5-HT1D autoreceptors in vitro and in vivo. GR127935 has much lower affinity at other 5-HT, and non-5-HT, receptors. In functional studies, GR127935 fails to affect 5-HT2 receptor-mediated 'wet dog shakes' in guinea-pigs and 5-HT1A receptor-mediated inhibition of 5-HT release in rat dorsal raphé nucleus. The compound has a good safety profile in all species tested. It is concluded that GR127935 is a useful pharmacological tool to characterise 5-HT1D receptor function.

Animals↗
PubMed1996

A quantitative autoradiographical study on the distribution of somatostatin sst2 receptors in the rat central nervous system using [125I]-BIM-23027.

The kinetic properties, steady state binding characteristics and autoradiographic distribution of the somatostatin (SRIF) sst2 receptor-selective ligand, [125I]-BIM-23027, have been investigated in the rat central nervous system. Analysis of kinetic, saturation and competition binding data in rat hippocampal membranes was consistent with [125I]-BIM-23207 binding to a single population of non-interacting binding sites. Competition studies, using different SRIF ligands suggested that [125I]-BIM-23027 was binding to sites similar to that of the recombinant sst2 receptor. The rank order of affinity for displacing specific binding was BIM-23027 = SRIF > L-362855 > > BIM-23056. There was a widespread distribution of [125I]-BIM-23027 binding sites in the rat central nervous system. The highest density of binding was observed in the dentate gyrus, medial habenular, amygdala, claustrum and lateral septum as well as in the piriform, cingulate and parietal cortex. The cervical and lumbar spinal cord also displayed moderate levels of binding localized to the substantia gelatinosa. The cellular localization of [125I]-BIM-23027 binding was found to be associated with dendritic terminal fields. In contrast, the cellular signal for sst2 receptor mRNA was restricted to cell somata. The widespread distribution of [125I]-BIM-23027 binding sites within the brain suggests that receptors similar to the recombinant sst2 receptor may mediate a variety of different physiological effects within the central nervous system.

Animals↗
PubMed1996

Differences in the operational characteristics of the human recombinant somatostatin receptor types, sst1 and sst2, in mouse fibroblast (Ltk-) cells.

1. The human recombinant somatostatin (SRIF) receptors, sst1 and sst2, have been stably expressed in mouse fibroblast (Ltk-) cells. Two stable clones, LSSR 1/20 and LSSR 11/13, expressing sst1 and sst2 receptors, respectively, have been used to characterize these receptor types using radioligand binding assays as well as measurements of changes in extracellular acidification rates using microphysiometry. 2. [125I]-[Tyr11]-SRIF bound to sst1 and sst2 receptors expressed in Ltk- cells with high affinity, Kd values being 1.52 nM, and 0.23 nM respectively. 3. In Ltk- cells expressing sst1 receptors, SRIF, SRIF-28, [D-Trp8]-SRIF and CGP 23996 all displaced [125I]-[Tyr11]-SRIF binding with high potency (IC50 values of 0.43 - 1.27 nM) whilst seglitide, BIM-23027, BIM-23056 and L-362855 were either weak inhibitors of binding or were ineffective. 4. In contrast MK-678 (seglitide) and BIM-23027 were the most potent inhibitors of [125I]-[Tyr11]-SRIF binding in Ltk- cells expressing sst2 receptors with IC50 values of 0.014 and 0.035 nM, respectively. 5. SRIF and a number of SRIF agonists, including seglitide and BIM-23027, caused concentration-dependent increases in extracellular acidification rates in Ltk- cells expressing sst2 receptors but not in Ltk- cells expressing sst1 receptors. The maximum increase in acidification rate produced by SRIF was 11.3 +/- 0.7% above baseline (0.1-0.28 pH unit min-1). The relative potencies of the SRIF agonists examined in causing increases in extracellular acidification rates in Ltk- cells expressing sst2 receptors correlated well with their relative potencies in inhibiting [125I]-[Tyr11] -SRIF binding (r = 0.94). 6. The increase in extracellular acidification produced by SRIF was markedly inhibited by pretreatment of cells with pertussis toxin (100 ng ml-1) indicating the involvement of pertussis toxin-sensitive G proteins. 7. SRIF (1 microM) had no effect on basal cyclic AMP levels in Ltk- cells expressing sst1 or sst2 receptors nor did it inhibit forskolin stimulated increases in cyclic AMP levels in either cell type. 8. The results from the present study describe the operational characteristics of human sst2 receptors expressed in Ltk- cells where receptor activation causes increases in extracellular acidification rates. This receptor is coupled to a pertussis toxin-sensitive G protein. In contrast, activation of sst1 receptors, at a similar transfection density, did not cause increases in extracellular acidification rates.

Amino Acid Sequence↗
PubMed1996

Operational characteristics of somatostatin receptors mediating inhibitory actions on rat locus coeruleus neurones.

1. In order to characterize somatostatin (SRIF) receptor inhibiting spontaneous firing of rat locus coeruleus neurones, and their transduction mechanism(S), extracellular recordings were obtained from a pontine slice preparation of rat brain containing the locus coeruleus (LC). LC neurones were identified by electrophysiological and pharmacological properties; spontaneous firing (characteristically 0.5-5 Hz) was reversibly and concentration-dependently inhibited by exogenously applied noradrenaline. 2. Spontaneous firing of LC neurones was reversibly and concentration-dependently inhibited by SRIF and the N-terminally extended form, somatostatin-28 (SRIF-28), with EC50 values of 15.1 and 19.4 nM, respectively. The synthetic SRIF analogues (octreotide, MK-678, BIM-23027 and L-362,855) also caused concentration-dependent inhibition of LC neurone firing with a rank order of agonist potencies compatible with actions at a receptor resembling the recombinant sst2 receptor. The putative sst3 selective agonist, BIM-23056, was without agonist or antagonist effect. 3. Addition of 100 nM desipramine significantly increased the efficacy of exogenously applied noradrenaline (EC50 values, 2.96 and 0.13 microM, absence and presence of desipramine, respectively) but did not significantly affect SRIF-induced inhibition (EC50 values, 15.6 and 8.0 nM, respectively). Furthermore, application of phenoxybenzamine (3 microM) abolished responses to NA, but did not affect responses to SRIF (EC50 = 14.1 nM). 4. Application of the cyclic AMP analogue, 8-bromoadenosine-cyclic monophosphate (8-Br-cyclic AMP; 500 microM), significantly increased the spontaneous firing rate of all neurones tested (223 +/- 24% over basal rate). Concentration-effect curves for SRIF constructed in the absence and presence of 8-Br-cyclic AMP had similar threshold concentrations, maxima and EC50 values. 5. Incubation of pontine slices in a modified artificial CSF containing 500 ng ml-1 pertussis toxin (PTX) for 18 h prior to extracellular recording affected neither the spontaneous firing of LC neurones, nor the inhibitory responses to muscimol (EC50 2.2 and 1.2 microM, absence and presence of PTX). However, inhibitory responses to SRIF were markedly attenuated. 6. We conclude that the inhibitory actions of SRIF on spontaneous firing of LC neurones are mediated directly by activation of somatodendritic SRIF receptors, and not indirectly by release of noradrenaline. The SRIF receptors involved appear to couple via a pertussis toxin sensitive G-protein, and elicit their response by a mechanism apparently independent of inhibition of cyclic AMP formation. The agonist profile of several selective and novel SRIF analogues suggests the identity of this receptor to be similar to the recombinant sst2 receptor.

Adrenergic Uptake Inhibitors↗
PubMed1996

Potent antagonism by BIM-23056 at the human recombinant somatostatin sst5 receptor.

We have investigated the effects of somatostatin (SRIF) and the linear octapeptide BIM-23056 on changes in intracellular calcium ion concentration ([Ca2+]i) and on the formation of inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) in CHO-K1 cells transfected with the human recombinant SRIF sst5 receptor. SRIF elicited concentration-dependent increases in [Ca2+]i, with a pEC50 of 7.02 +/- 0.06, while BIM-23056 (1 x 10(-7) M) behaved not as an agonist but as a potent, surmountable antagonist of these increases in [Ca2+]i. The SRIF concentration-effect curve for increases in [Ca2+]i was shifted rightward producing an estimated pKB for the antagonist of 8.0. BIM-23056 (1 x 10(-7) M) also significantly attenuated Ins(1,4,5)P3 increases due to SRIF, but had no effect on either basal or uridine 5'-triphosphate (UTP) (1 x 10(-4) M) stimulated increases in the levels of [Ca2+]i or Ins(1,4,5)P3.

Animals↗
PubMed1996

Somatostatin sst2 receptor-mediated inhibition of parietal cell function in rat isolated gastric mucosa.

1. The aim of this study was to determine the location and functional characteristics of the somatostatin (SRIF) receptor type(s) which mediate inhibition of acid secretion in rat isolated gastric mucosa. 2. Gastrin (1 nM-1 microM), dimaprit (10 microM-300 microM) and isobutyl methylxanthine (IBMX, 1 microM-100 microM) all caused concentration-dependent increases in acid output. Responses to gastrin were almost completely inhibited by ranitidine (10 microM) at a concentration which abolished the secretory response to dimaprit. In contrast, responses to IBMX were not changed by ranitidine suggesting that IBMX acts directly on the parietal cell and not indirectly by releasing histamine from enterochromaffin-like (ECL) cells. 3. SRIF-14 (1 nM-1 microM) had no effect on basal acid output, but inhibited acid output produced by gastrin, dimaprit and IBMX in a concentration-dependent manner with respective EC50 values of 46, 54 and 167 nM. The peptidase inhibitors, amastatin (10 microM) and phosphoramidon (1 microM), had no effect on SRIF-induced inhibition of dimaprit stimulated gastric acid secretion. 4. The inhibitory effect of a range of SRIF analogues on gastrin-, dimaprit- and IBMX-induced acid secretion was also studied. Irrespective of the secretagogue used to increase acid output, the rank order of potencies was similar (BIM-23027 = seglitide = octreotide > SRIF-14 = SRIF-28 > L-362,855). The linear peptide BIM-23056 was devoid of agonist or antagonist activity in concentrations up to 1 microM. 5. The sst2 receptor selective peptides, BIM-23027, seglitide and octreotide were the most potent inhibitors of gastrin-, dimaprit- and IBMX-induced acid secretion suggesting that SRIF receptors resembling the recombinant sst2 receptors are involved. Furthermore, since dimaprit and IBMX stimulate gastric acid secretion independently of histamine release, sst2 receptor-mediated inhibition must occur at the level of the parietal cell itself.

1-Methyl-3-isobutylxanthine↗
PubMed1995

Somatostatin receptors mediating inhibition of basal and stimulated electrogenic ion transport in rat isolated distal colonic mucosa.

The aim of this study was to examine the potencies of several recently identified selective somatostatin (SRIF)-receptor ligands as inhibitors of electrogenic ion transport in the rat distal colonic mucosa with the view to identifying the SRIF receptor type involved. Under basal conditions, cumulative administration of SRIF and SRIF28 decreased short circuit current (SCC), a measure of electrogenic ion transport, with EC50 values of 4 nM and 9 nM respectively. The peptidase inhibitors, phosphoramidon (1 microM) and amastatin (10 microM), has no effect on the potencies of either SRIF or SRIF28. The inhibitory action of SRIF on basal SCC was suppressed by piretanide and diphenylamine-2-carboxylate, compatible with the assumption that the Na+K+2Cl- co-transporter and Cl- channels, respectively, may be involved in this antisecretory action of SRIF. Tetrodotoxin (1 microM) had no effect on the antisecretory action of SRIF, suggesting that the process was not neuronally mediated. All of the SRIF analogues examined, with the exception of BIM-23056, maximally inhibited basal SCC to a similar extent as SRIF. Seglitide and octreotide were both more potent antisecretory agents than SRIF (respective EC50 values, 0.4 nM and 1.5 nM) suggesting that this effect was mediated by a receptor belonging to the SRIF1 receptor group. The most distinguishing feature of the rank order of agonist potencies was the high potency of the selective sst2 receptor ligand, BIM-23027 (EC50 value 0.32 nM), the weaker potency exhibited by the selective sst5 receptor ligand, L-362855 (EC50 value 21 nM), and the lack of agonist activity displayed by the selective sst3 receptor ligand, BIM-23056 (EC50 value > 1000 nM). This profile is comparable with that observed in binding studies on the recombinant sst2 receptor. Forskolin-stimulated secretion was suppressed by SRIF analogues with the rank order of agonist potencies BIM-23027 > SRIF > L-362855 >> BIM-23056 which resembled that exhibited under basal conditions. However, the absolute potencies of these agonists were lower (respective EC50 values 2 nM, 14 nM< 38 nM and > 1000 nM) whilst the magnitude of inhibition was about three fold greater. BIM-23027 and SRIF (both 30 nM) also inhibited carbachol-stimulated increases in basal SCC by 60-70%, while a similar concentration of L-362855 inhibited these responses by 11%. BIM-23056 (1 microM) had no effect on carbachol-simulated secretion. Radioligand binding studies on rat colonic mucosal membranes using [125I]-Tyr11-SRIF suggested heterogeneity of SRIF binding sites. Thus, SRIF and SRIF28 competed for binding (IC50 values, 0.32 and 0.63 nM, respectively) with Hill slopes less than unity; while seglitide and BIM-23027 both maximally displaced only 30-40% of specific binding with apparent high affinity (respective pIC50 values, 10.1 nM and 10.0). In conclusion, SRIF decreases basal as well as both cAMP and Ca(2+)-dependent Cl- secretion in rat colonic mucosa. The rank order of agonist potencies suggests that receptors resembling the recombinant sst2 receptor mediate inhibition of basal and forskolin-stimulated secretion. Radioligand binding studies suggest that BIM-23027 interacts with a sub-population of [125I]Tyr11-SRIF binding sites in rat colonic mucosal membranes which probably corresponds to the receptors mediating the antisecretory effects described here.

Animals↗
PubMed1995

Effects of the 5-HT1D receptor antagonist GR127935 on extracellular levels of 5-HT in the guinea-pig frontal cortex as measured by microdialysis.

The involvement of 5-HT1D receptors in the regulation of 5-HT release in the guinea-pig brain was examined using the novel 5-HT1D receptor blocking drug GR127935. Levels of 5-HT were measured in frontal cortex of anaesthetized guinea-pigs using microdialysis. The infusion of GR127935 (100 nM) through the dialysis probe into frontal cortex caused a significant increase (61 +/- 8%) in cortical extracellular levels of 5-HT. The increase was transient (approximately 40 min) even in the continuous presence of GR127935. The transient increase was abolished by tetrodotoxin (1 microM). The 5-HT1 receptor agonist GR46611 (10 mg/kg s.c.) caused a significant and sustained (> 100 min) reduction in extracellular levels of 5-HT (65 +/- 5%). This response was abolished in animals pre-treated with GR127935, 0.05 mg/kg i.p. Paradoxically, systemic administration of higher doses of GR127935 (0.1-1 mg/kg i.p.) in naive anaesthetized guinea-pigs caused significant and sustained (> 120 min) decreases (> 65%) in cortical levels of 5-HT. The increase in extracellular 5-HT seen following infusion of GR127935 into frontal cortex may be due to GR127935 blocking 5-HT terminal autoreceptors causing a subsequent increase in the outflow of 5-HT from pre-synaptic terminals. This conclusion is supported by the ability of GR127935 to block the decrease in 5-HT induced by the 5-HT1 receptor agonist GR46611.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Classification and nomenclature of somatostatin receptors.

There is considerable controversy about the classification and nomenclature of somatostatin receptors. To date, five distinct receptor genes have been cloned and named chronologically according to their respective publication dates, but two were unfortunately given the same appellation (SSTR4). Consensually, a nomenclature for the recombinant receptors has been agreed according to IUPHAR guidelines (sst1, sst2, sst3, sst4, and sst5). However, a more informative classification is to be preferred for the future, employing all classification criteria in an integrated scheme. It is already apparent that the five recombinant receptors fall into two classes or groups, on the basis of not only structure but also pharmacological characteristics. One class (already referred to by some as SRIF1) appears to comprise sst2, sst3 and sst5 receptor subtypes. The other class (SRIF2) appears to comprise the other two recombinant receptor subtypes (sst1 and sst4). This promising approach is discussed but it is acknowledged that much more data from endogenous receptors in whole tissues are needed before further recommendations on somatostatin receptor nomenclature can be made.

Amino Acid Sequence↗
PubMed1995

Somatostatin-induced contraction of human isolated saphenous vein involves sst2 receptor-mediated activation of L-type calcium channels.

A range of somatostatin (SRIF) analogues have been used to characterize the SRIF receptor-mediating contraction of the human saphenous vein. SRIF produced concentration-dependent contractions with an EC50 value of approximately 20 nM. The peptidase inhibitors phosphoramidon and amastatin did not alter the potency of SRIF. The sst2 receptor-selective peptide BIM-23027 was approximately three times more potent than SRIF in contracting the vein, whereas the sst5 receptor-selective peptide L-362855 was approximately 50 times weaker. The sst3 receptor-selective peptide BIM-23056 did not contract the saphenous vein. Contractions to SRIF were not antagonised by the putative SRIF receptor blocker cyclo(7-aminoheptanoyl-Phe-D Trp-Lys-Thr[Bzl]) (CPP), phentolamine, or indomethacin. Decreasing the external calcium concentration reduced the maximum contraction to SRIF in a concentration-dependent manner without altering the EC50 value. Nifedipine and verapamil also markedly reduced the SRIF-induced contraction. SRIF and several SRIF analogues caused contraction of the human saphenous vein by what appeared to be a direct effect on the smooth muscle. Their relative potencies suggest that their effects were mediated by a somatostatin receptor that is like the recombinant sst2 receptor. The receptor transduction mechanism appears to involve activation of L-type calcium channels and entry of extracellular calcium.

Aged↗
PubMed1995

Further evidence from functional studies for somatostatin receptor heterogeneity in guinea-pig isolated ileum, vas deferens and right atrium.

1. Somatostatin (SRIF) causes a concentration-dependent inhibition of neurotransmission in guinea-pig ileum and vas deferens as well as negative inotropy in guinea-pig isolated right atrium. The SRIF receptors mediating these effects have now been further characterized by use of the peptides BIM-23027, BIM-23056 and L-362855, reported as selective for the recombinant SRIF receptor types, sst2, sst3 and sst5, respectively. 2. BIM-23027 was a highly potent agonist at causing an inhibition of neurotransmission in the guinea-pig ileum (EC50 value 1.9 nM), being about 3 times more potent than SRIF (EC50 value 6.8 nM). In contrast, in both guinea-pig vas deferens and right atrial preparations, BIM-23027 was a relatively weak agonist being at least 30-100 times weaker than SRIF. In guinea-pig atria, BIM-23027 (3 microM) antagonized the negative inotropic action of SRIF28 (apparent pKB = 5.9 +/- 0.1) but had no effect on the negative inotropic action of cyclohexyladenosine. 3. The inhibitory effect of BIM-23027 in the guinea-pig ileum was readily desensitized. Prior exposure to BIM-23027 (0.3 microM) markedly attenuated the inhibitory effect of SRIF but had no effect on the inhibitory action of clonidine suggesting that BIM-23027 and SRIF act via a common receptor mechanism. 4. L-362855 caused a concentration-dependent inhibition of neurotransmission in both the guinea-pig ileum and vas deferens as well as causing negative inotropy in the guinea-pig atrium but was at least 30-100 times weaker than SRIF. In guinea-pig isolated atria, L-362855 (3 microM) did not antagonize the negative inotropic action of SRIF28. 5. BIM-23056 in concentrations up to 1 microM was inactive as an agonist in guinea-pig isolated ileum, vas deferens and atrium and did not antagonize the inhibitory actions of SRIF in any of these preparations.6. The results from this study support our previous contention that the sst2 receptor type mediates inhibition of neurotransmission by SRIF in the guinea-pig ileum. The SRIF receptor type mediating inhibition of neurotransmission in the guinea-pig vas deferens appears different, but similar, to that mediating negative inotropy in the atrium. However the characteristics of these latter receptors appear different from that of the recombinant sst2, sst3 and sst5 receptors for SRIF described for rat and man.

Animals↗
PubMed1994

Somatostatin-induced inhibition of neurotransmission in the mouse isolated vas deferens is resistant to pertussis toxin.

The potential effects of pertussis toxin pretreatment on the inhibitory effect of somatostatin (SRIF) and the selective SRIF receptor agonist, seglitide, were studied in mouse vas deferens and these were compared with its effect on the negative chronotropic action of carbachol in mouse atria. Somatostatin and seglitide caused a concentration-dependent inhibition of neurogenically mediated contractile responses in the vas deferens (EC50 values of 15 and 0.6 nM respectively). There was no difference in their potencies in preparations removed from pertussis toxin pretreated mice. In contrast, the negative chronotropic action of carbachol in mouse atria was abolished by pretreatment with pertussis toxin. We conclude that, in contrast to muscarinic receptor activation in mouse atria, the inhibitory effect of somatostatin in the vas deferens is not mediated by a pertussis toxin sensitive G-protein. The high potency of seglitide suggests that the SRIF receptor involved is of the SRIF1 type.

Amino Acid Sequence↗
PubMed1994

Mediation by SRIF1 receptors of the contractile action of somatostatin in rat isolated distal colon; studies using some novel SRIF analogues.

1. The motor effects of somatostatin-14 (SRIF), and several SRIF peptide analogues were investigated on the rat isolated distal colon. The objective of these studies was to characterize the receptor mediating the contractile action of SRIF by comparing the relative agonist potencies of a range of SRIF analogues. 2. SRIF (1 nM-1 microM) produced concentration-dependent contractions with an EC50 value of approximately 10 nM. Contractile responses induced by SRIF were insensitive to atropine (1 microM) or naloxone (1 microM) but abolished by tetrodotoxin (1 microM). Somatostatin-28 (SRIF28), also induced concentration-dependent contractions and was equipotent with SRIF. Phosphoramidon (1 microM) and amastatin (10 microM) did not increase the potency of either SRIF or SRIF28. 3. The SRIF peptide analogues, octreotide, SRIF25, seglitide, angiopeptin and CGP23996 (1 nM-1 microM) produced contractile responses in the rat distal colon, each having similar potency and maximal activity relative to SRIF. The SSTR2 receptor-selective hexapeptide, BIM23027 (0.1 nM-1 microM), and the SRIF stereoisomer, D-Trp8-SRIF (0.1 nM-1 microM), were the most potent agonists examined being approximately 12 and 7 times more potent than SRIF, respectively. In contrast, the SSTR5 receptor-selective analogue, L362,855, was approximately 120 times weaker than SRIF, whilst the SSTR3 receptor-selective analogue, BIM23056, was inactive at concentrations up to 3 microM. 4. The putative SRIF receptor antagonist, (cyclo(7-aminoheptanoyl Phe-D-Trp-Lys-Thr[Bzl]))(CPP) (1 microM), had no agonist activity and had no effect on contractions induced by SRIF. 5. The contractile actions of BIM23027 and seglitide were subject to pronounced desensitization. Desensitization of preparations by BIM23027 (0.3 JIM) abolished the contractile action of SRIF andSRIF28 but had no effect on contractions produced by acetylcholine (0.1 nM-I1M), suggesting thatBIM23027, SRIF and SRIF28 act via a common receptor mechanism.6. In conclusion, the rat isolated distal colon contracts in response to SRIF and a number of SRIF analogues. Seglitide and octreotide exhibited similar potency and maximal activity relative to SRIF,suggesting that in the rat colon the receptor mediating contraction belongs to the SRIF,-receptor group,of which the recombinant SSTR2, SSTR3 and SSTR5 receptors appear to be subtypes. The high potency of BIM23027, the weak agonist activity of L362,855 and the lack of activity exhibited by BIM23056suggests that the SRIF receptor mediating contraction in the rat distal colon is similar to there combinant SSTR2 receptor.

Amino Acid Sequence↗
PubMed1993

Serotonin activates Cl- channels in the apical membrane of rat choroid plexus epithelial cells.

The effects of serotonin on ion channel activity in epithelial cells from rat choroid plexus were examined. Serotonin (50 nM, 500 nM and 1 microM) stimulated channel activity in cell-attached patches. The current-voltage (I-V) relationship for the serotonin-activated channel gave a conductance of 26.6 +/- 1.5 pS and the current reversed at an applied electrode potential (-Vp) = 15.3 +/- 3.3 mV with a KCl-rich electrode solution (n = 8). Similar I-V relationships were obtained using electrode solutions in which K+ was replaced by other cations (Na+ and N-methyl-D-glucamine), suggesting that the serotonin-activated channels are selective to Cl-. The effect of 1 microM serotonin on channel activity was inhibited by ritanserin (30 and 100 nM) which has a high affinity for serotonin 5-HT1C receptors and 5-HT2 receptors. Spiperone (30 nM), which binds weakly to 5-HT1C receptors but has a high affinity for 5-HT2 receptors, did not inhibit the actions of serotonin. These data suggest that serotonin increases Cl- channel activity by acting on the 5-HT1C receptors on the epithelium.

Animals↗