Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “SECRETION”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMed2004

Salmonella type III secretion-associated chaperones confer secretion-pathway specificity.

Type III protein secretion systems (TTSSs) are ancestrally related to the flagellar export system and are essential for the virulence of many bacteria pathogenic for humans, animals and plants. Most proteins destined to travel the TTSS pathway possess at least two domains that specifically target them to the secretion apparatus. One of the domains is located within the amino terminal first approximately 20 amino acids and the second domain, located within the first approximately 140 amino acids, serves as a binding site for specific chaperones. It has been previously proposed that these two secretion signals are capable of operating independently of one another to facilitate secretion into the extracellular environment. We have found that in the absence of their chaperone-binding domains, the Salmonella typhimurium TTSS-secreted proteins SptP and SopE are no longer targeted for secretion through their cognate TTSS and, instead, are secreted through the flagellar export pathway. These results indicate the existence of an 'ancestral' flagellar secretion signal within TTSS-exported proteins that is revealed in the absence of the chaperone-binding domain. Furthermore, we found that secretion into culture supernatants as well as translocation into host cells by the cognate TTSS require both, the amino terminal and chaperone-binding domains. We conclude from these studies that a critical function for the TTSS-associated chaperones is to confer secretion-pathway specificity to their cognate secreted proteins.

Amino Acid Sequence↗
PubMed2004

The crystal structure of filamentous hemagglutinin secretion domain and its implications for the two-partner secretion pathway.

Filamentous hemagglutinin (FHA), the major 230-kDa adhesin of the whooping cough agent Bordetella pertussis, is one of the most efficiently secreted proteins in Gram-negative bacteria. FHA is secreted by means of the two-partner secretion (TPS) pathway. Several important human, animal, and plant pathogens also secrete adhesins and other virulence factors by using this mode of secretion. A TPS system is composed of two separate proteins, with TpsA the secreted protein and TpsB its associated specific outermembrane transporter. All TPS-secreted proteins contain a distinctive N-proximal module essential for secretion, the TPS domain. We report here the 1.7- A structure of a functionally secreted 30-kDa N-terminal fragment of FHA. It reveals that the TPS domain folds into a beta-helix, with three extrahelical motifs, a beta-hairpin, a four-stranded beta-sheet, and an N-terminal capping, mostly formed by the nonconserved regions of the TPS domain. The structure thus explains why the TPS domain is able to initiate folding of the beta-helical motifs that form the central domain of the adhesin, because it is itself a beta-helical scaffold. It also contains less conserved extrahelical regions most likely involved in specific properties, such as the recognition of the outer-membrane transporter. This structure is representative of the TPS domains found so far in >100 secreted proteins from pathogenic bacteria. It also provides a mechanistic insight into how protein folding may be linked to secretion in the TPS pathway.

Adhesins, Bacterial↗
PubMed1976

Stimulus-secretion coupling: role of cyclic AMP, cyclic GMP and calcium in mediating enzyme (kallikrein) secretion in the submandibular gland.

1. The role of adenosine 3':5'-phosphate (cyclic AMP) and guanosine 3':5'-phosphate (cyclic GMP) as second messengers for the enzyme secretory response evoked by the autonomic neurotransmitters, noradrenaline and acetylcholine, is examined in this in vitro study on the guinea-pig submandibular gland. 2. Noradrenaline increased enzyme (kallikrein) secretion. The initial stimulation of enzyme release appeared to be dose-dependent. The time course of cumulative kallikrein secretion revealed a complex pattern. Isoprenaline and phenylephrine were almost as potent as noradrenaline in releasing kallikrein. Both propranolol and phentolamine were required to fully inhibit the noradrenaline-stimulated enzyme secretion. 3. The cumulative secretion of kallikrein evoked by acetylcholine was dose-dependent. The onset of secretion showed a significantly greater time-lag than that observed with noradrenaline. Atropine effectively blocked the release of kallikrein by acetylcholine. 4. Dibutyryl cyclic AMP stimulated enzyme secretion. Dibutyryl cyclic GMP caused an initial increase which was not maintained. 5. The cyclic nucleotide phosphodiesterase inhibitors, theophylline and papaverine, increased basal kallikrein secretion. The action of the cyclic phosphodiesterase inhibitors on the secretory response to noradrenaline, acetylcholine, dibutyryl cyclic AMP and dibutyryl cyclic GMP was complex. In general, the increase in enzyme release produced by the secretagogues was additively enhanced by both inhibitors. 6. Omission of calcium inhibited both acetylcholine and dibutyryl cyclic GMP stimulated kallikrein release, but to a lesser degree than that of noradrenaline and dibutyryl cyclic AMP. High concentrations of extracellular calcium (10 mM) appeared to enhance the action of acetylcholine. 7. Noradrenaline produced a rise in the intracellular level of cyclic AMP. The increase preceded the stimulated secretion of kallikrein. Of the various adrenergic agonists, noradrenaline and isoprenaline were the most potent, whereas phenylephrine was significantly less effective in raising basal cyclic AMP values. Acetylcholine was without effect, even in the presence of a cyclic phosphodiesterase inhibitor. 8. Acetylcholine and noradrenaline raised intracellular levels of cyclic GMP only when the tissue incubations were performed in the presence of a cyclic phosphodiesterase inhibitor. The increase in cyclic GMP produced by acetylcholine preceded enzyme secretion. 9. Morphological data substantiated the finding that the in vitro release of kallikrein evoked by the secretagogues was associated with the depletion of secretory granules and vacuolations in acinar cells of the gland slices. 10. The molecular mechanisms which control enzyme secretion in the exocrine submandibular gland are discussed. Models are presented for the role of transmitter-specific cyclic nucleotides and calcium in stimulus-secretion coupling.

Acetylcholine↗
PubMed2006

Pseudomonas syringae HrpJ is a type III secreted protein that is required for plant pathogenesis, injection of effectors, and secretion of the HrpZ1 Harpin.

The bacterial plant pathogen Pseudomonas syringae requires a type III protein secretion system (TTSS) to cause disease. The P. syringae TTSS is encoded by the hrp-hrc gene cluster. One of the genes within this cluster, hrpJ, encodes a protein with weak similarity to YopN, a type III secreted protein from the animal pathogenic Yersinia species. Here, we show that HrpJ is secreted in culture and translocated into plant cells by the P. syringae pv. tomato DC3000 TTSS. A DC3000 hrpJ mutant, UNL140, was greatly reduced in its ability to cause disease symptoms and multiply in Arabidopsis thaliana. UNL140 exhibited a reduced ability to elicit a hypersensitive response (HR) in nonhost tobacco plants. UNL140 was unable to elicit an AvrRpt2- or AvrB1-dependent HR in A. thaliana but maintained its ability to secrete AvrB1 in culture via the TTSS. Additionally, UNL140 was defective in its ability to translocate the effectors AvrPto1, HopB1, and AvrPtoB. Type III secretion assays showed that UNL140 secreted HrpA1 and AvrPto1 but was unable to secrete HrpZ1, a protein that is normally secreted in culture in relatively large amounts, into culture supernatants. Taken together, our data indicate that HrpJ is a type III secreted protein that is important for pathogenicity and the translocation of effectors into plant cells. Based on the failure of UNL140 to secrete HrpZ1, HrpJ may play a role in controlling type III secretion, and in its absence, specific accessory proteins, like HrpZ1, may not be extracellularly localized, resulting in disabled translocation of effectors into plant cells.

Arabidopsis↗
PubMed1997

Linker mutagenesis of the Caulobacter crescentus S-layer protein: toward a definition of an N-terminal anchoring region and a C-terminal secretion signal and the potential for heterologous protein secretion.

Linker insertion mutagenesis was used to modify the paracrystalline surface layer (S-layer) protein (RsaA) of the gram-negative bacterium Caulobacter crescentus. Eleven unique BamHI linker insertions in the cloned rsaA gene were identified; at the protein level, these linker insertions introduced 4 to 6 amino acids at positions ranging from the extreme N terminus to the extreme C terminus of the 1,026-amino-acid RsaA protein. All linker-peptide insertions in the RsaA N terminus caused the secreted protein to be shed into the growth medium, suggesting that the RsaA N terminus is involved in cell surface anchoring. One linker-peptide insertion in the RsaA C terminus (amino acid 784) had no effect on S-layer biogenesis, while another (amino acid 907) disrupted secretion of the protein, suggesting that RsaA possesses a secretion signal lying C terminal to amino acid 784, near or including amino acid 907. Unlike extreme N- or C-terminal linker-peptide insertions, those more centrally located in the RsaA primary sequence had no apparent effect on S-layer biogenesis. By using a newly introduced linker-encoded restriction site, a 3' fragment of the rsaA gene encoding the last 242 C-terminal amino acids of the S-layer protein was expressed in C. crescentus from heterologous Escherichia coli lacZ transcription and translation initiation information. This C-terminal portion of RsaA was secreted into the growth medium, confirming the presence of a C-terminal secretion signal. The use of the RsaA C terminus for the secretion of heterologous proteins in C. crescentus was explored by fusing 109 amino acids of an envelope glycoprotein from infectious hematopoietic necrosis virus, a pathogen of salmonid fish, to the last 242 amino acids of the RsaA C terminus. The resulting hybrid protein was successfully secreted into the growth medium and accounted for 10% of total protein in a stationary-phase culture. Based on these results and features of the RsaA primary sequence, we propose that the C. crescentus S-layer protein is secreted by a type I secretion system, relying on a stable C-terminal secretion signal in a manner analogous to E. coli alpha-hemolysin, the first example of an S-layer protein secreted by such a pathway.

Bacterial Outer Membrane Proteins↗
PubMed1986

Effects of red dim illumination and surgery on prolactin secretion during the estrous cycle and early pseudopregnancy in the rat: different regulatory mechanisms for prolactin secretion.

The effects of surgery and red light on prolactin (Prl) secretion were investigated in cycling and in pseudopregnant (PSP) rats. Secretion patterns of Prl were determined at hourly intervals from 07.00 to 22.00 h. Different regulatory mechanisms for Prl secretion were hypothesized for three time periods: a nocturnal (07.00-11.00 h), a prediurnal (14.00-17.00 h), and a diurnal (19.00-22.00 h) period. The results demonstrate that red light can affect significantly Prl secretion, in particular nocturnal and prediurnal Prl secretion in estrous, diestrous day 1 and PSP rats. The effect of surgery varied with the time of the day and was dependent upon whether the animals, during the dark period, were maintained under full darkness or constant red dim illumination. In PSP rats the regulation of Prl secretion was different for the three time periods. In PSP day 0 rats there was a prediurnal surge of Prl secretion, comparable in timing and regulation to a prediurnal surge in estrous rats. This prediurnal surge was not evident on the other days of PSP. The regulation of nocturnal as well as diurnal Prl secretion was similar in PSP rats, but differed from cycling rats. The afternoon surge of Prl secretion on proestrus lasted the full afternoon and was basically one surge, distinct from all other surges. In diestrous rats Prl levels were low but showed a circadian variation. In summary, the effects of red light and surgery on Prl secretion varied with the physiological state and with the time of the day, indicating that the regulation of Prl secretion is complex and multimodal in nature.

Animals↗
PubMed2004

Yersinia enterocolitica type III secretion: evidence for the ability to transport proteins that are folded prior to secretion.

BACKGROUND: Pathogenic Yersinia species (Y. enterocolitica, Y. pestis, Y. pseudotuberculosis) share a type three secretion system (TTSS) which allows translocation of effector proteins (called Yops) into host cells. It is believed that proteins are delivered through a hollow needle with an inner diameter of 2-3 nm. Thus transport seems to require substrates which are essentially unfolded. Recent work from different groups suggests that the Yersinia TTSS cannot accommodate substrates which are folded prior to secretion. It was suggested that folding is prevented either by co-translational secretion or by the assistance of specific Yop chaperones (called Sycs). RESULTS: In this study we have fused YopE secretion signals of various length to the mouse dihydrofolate reductase (DHFR) in order to analyse the DHFR folding state prior to secretion. We could demonstrate that secretion-deficient as well as secretion-competent YopE-DHFR fusions complexed to SycE can be efficiently purified from Yersinia cytosol by affinity chromatography using methotrexate-agarose. This implies the folding of the DHFR fusion moiety despite SycE binding and contradicts the previously presented model of folding inhibition by chaperone binding. Secretion-deficient YopE-DHFR fusions caused severe jamming of the TTSS. This observation contradicts the co-translational secretion model. CONCLUSIONS: We present evidence that the Yersinia TTSS is familiar with the processing of transport substrates which are folded prior to secretion. We therefore predict that an unfoldase is involved in type III secretion.

Bacterial Outer Membrane Proteins↗
PubMed1982

Evidence of dopamine receptors in human growth hormone (GH)-secreting adenomas with concomitant study of dopamine inhibition of GH secretion in a perifusion system.

Dopamine (DA) is known to influence human GH release both in vivo and in vitro. The direct control of GH secretion at the pituitary level has been observed using human GH-secreting pituitary adenomas in organ culture, but no dose-response relationship between DA and GH inhibition was demonstrated. Such a dose-response relationship was obtained using GH-secreting adenomatous cells in a perifusion column. The IC50 of DA was 5 X 10(-8) M. Binding studies with [3H]apomorphine on human GH-secreting pituitary adenoma membranes demonstrated the presence of a dopaminergic receptor (Kd = 0.73 +/- 0.24 nM; maximum binding, 18.5 +/- 3.6 fmol/mg protein), which presumably mediates the effect of DA on GH release. In comparison to prior studies with PRL-secreting adenomas, several differences were apparent: 1) the maximum inhibition of GH release by DA is lower than that of PRL (50% vs. 80%), 2) the maximum number of binding sites is much lower (5-20 times) in GH-secreting adenomas than in PRL-secreting adenomas, and 3) [3H]domperidone does not show specific binding on GH-secreting adenomas but does on PRL-secreting adenomas. These experimental results indicate that the dopaminergic control of GH secretion is different from that of PRL secretion, as DA is less effective on GH release than on PRL release from human pituitary adenomas.

Adenoma↗
PubMed1996

Regulation of mouse placental lactogen secretion by factors secreted from the pituitary in vitro.

The effect of factors secreted from the pituitary on mouse placental lactogen I (mPL-I) and mPL-II secretion in vitro was examined. Co-culture of mouse placental cells from day 7 of pregnancy with the pituitary cells of the mother significantly stimulated mPL-I secretion but did not regulate mPL-II secretion. The effect on mPL-I secretion was dependent on the number of pituitary cells. The conditioned medium of pituitary cells also significantly stimulated mPL-I secretion but did not regulate mPL-II secretion. The stimulatory effect of mPL-I secretion was dependent on the volume of the conditioned medium. The number of cells containing mPL-I assessed by immunocytochemistry was increased by the co-culture in a cell number-dependent manner. Northern blot analysis for mPL-I indicated that treatment of placental cells with the pituitary-conditioned medium results in an increase of mPL-I gene expression. These findings suggest that factors secreted from the pituitary directly regulate mPL-I secretion, but not mPL-II secretion, before midpregnancy in vivo.

Animals↗
PubMed1990

Fasting decreases apolipoprotein B mRNA editing and the secretion of small molecular weight apoB by rat hepatocytes: evidence that the total amount of apoB secreted is regulated post-transcriptionally.

Two different molecular weight forms of apoB are produced from a common initial transcript via editing of a Gln codon (CAA) to a stop codon (UAA), leading to a truncated translation product (apo BS) that consists of the amino terminal half of the larger form (apoBL). Previous studies have shown that fasting coordinately decreases lipogenesis and the secretion of very low density lipoprotein (VLDL) lipids and apoBS. Secretion of the apoBL is unaffected by fasting. We studied whether editing of apoB RNA is repressed by fasting, thus accounting for the selective decreased secretion of apoBS. Column chromatography of [35S]methionine-labeled lipoproteins secreted by hepatocytes from fed rats showed that essentially all of apoBL is secreted in the VLDL fraction, whereas a significant amount (15%) of apoBS is secreted associated as lipoproteins eluting in the HDL fractions. Fasting decreased the relative amount of apoBS that eluted in the VLDL fractions and increased the amount secreted in the HDL fractions. Consistent with previous results, hepatocytes from fasted rats show a selective twofold decrease in apoBS secretion. Fasting did not affect the relative abundance of apoB RNA, determined by slot blot hybridization assays using two different 32P-labeled cDNA probes coding either for both molecular weight forms or for only the large molecular weight form. However, quantitative of the editing of apoB RNA showed that fasting caused a 60% decrease in the amount of apoB RNA possessing the stop codon. These data show that the editing of apoB RNA is sensitive to metabolic state (i.e., fasting) resulting in a selective decrease in the secretion of apoBS. However, since the total secretion of apoB was decreased by fasting, while apoB mRNA levels remained constant, additional (post-transcriptional) mechanisms play a role in regulating apoB secretion.

Animals↗
PubMed1990

[Effect of dopamine and bromocriptine on secretion of growth hormone by pituitary growth hormone secreting tumor in cell culture].

This article reports the effect of dopamine (DA) and its agonist bromocriptine (CB154) on the secretion of growth hormone (GH) by pure GH-secreting pituitary tumor in cell culture as well as a comparison of the effects of these dopaminergic drugs and SMS201-295 (SMS). DA of 10(-8) and 10(-7) mol/L reduced GH secretion to 50.6 and 44.4% of the control, respectively, in 1 out of 6 tumors. CB154 of 10(-7) and 10(-6) mol/L suppressed GH secretion to 59.0 +/- 8.9% of the control in 3 out of 4 tumors. CB154 was at least 10 times less potent than SMS vis a vis GH secretion. CB154 of 10(-6) mol/L inhibited GH secretion to 63.3 +/- 13. 8% (n = 4), but SMS of 10(-7) mol/L induced GH secretion to 45.5 +/- 13.1% (n = 4), the concentration difference between CB154 and SMS was 10 times. CB154 suppressed not only GH secretion, but also GH synthesis in two tumor cell cultures. The major role of SMS in GH secretion was inhibition. The results suggest that DA and CB154 have direct inhibitory effects on GH secretion, at least in some pure pituitary GH secreting tumors. The activities of DA and CB154 are not entirely the same as that of SMS.

Adenoma, Acidophil↗
PubMed1994

A carboxyl-terminal four-amino acid motif is required for secretion of the metalloprotease PrtG through the Erwinia chrysanthemi protease secretion pathway.

PrtG is an extracellular metalloprotease secreted by the Gram-negative bacterium Erwinia chrysanthemi through a signal peptide-independent secretion pathway. Previous studies showed that the PrtG secretion signal is COOH-terminal and located in the last 56 residues of PrtG. We have now performed a deletion and elongation mapping of a short secretion competent COOH-terminal peptide CterG. This approach allowed us to show that: (i) the smaller COOH-terminal sequence containing the information necessary to promote the secretion of a small polypeptide is contained in the last 29 residues of PrtG; (ii) a low but significant level of secretion can be promoted by the last 15 residues of PrtG when fused to the COOH terminus of a non-secreted PrtG derivative; (iii) the extreme COOH-terminal sequence Dxxx, where xs are hydrophobic residues, is a conserved motif in all constructs that are secreted through the E. chrysanthemi transporter. (vi) This motif has to be COOH terminally exposed since addition of even one amino acid impairs the secretion of CterG. The extent of the secretion defect observed with the COOH terminally extended variants correlates with the length of the extension. These results indicate a key role for the COOH-terminal exposition of the last four amino acids in the secretion of PrtG.

Amino Acid Sequence↗
PubMed2004

Cholecystokinin inhibits gastrin secretion independently of paracrine somatostatin secretion in the pig.

BACKGROUND: Cholecystokinin inhibits the secretion of gastrin from antral G cells, an effect that is speculated to be mediated by D cells secreting somatostatin. The aim of the study was to test directly whether cholecystokinin inhibition of antral gastrin secretion is mediated by somatostatin. METHODS: The effects of CCK on gastrin and somatostatin secretion were studied in isolated vascularly perfused preparations of pig antrum before and after immunoneutralization brought about by infusion of large amounts of a high affinity monoclonal antibody against somatostatin. RESULTS: CCK infusion at 10(-9) M and 10(-8) M decreased gastrin output to 70.5% +/- 7.6% (n = 8) and 76.3% +/- 3.6% (n = 7) of basal output, respectively. CCK at 10(-10) M had no effect (n = 6). Somatostatin secretion was dose-dependently increased by CCK infusion and increased to 268 +/- 38.2% (n = 7) of basal secretion during infusion of CCK at 10(-8) M. Immunoneutralization of somatostatin caused a doubling of the basal secretion of gastrin, but did not affect the CCK-induced decrease in gastrin secretion. CONCLUSION: CCK inhibits gastrin secretion independently of paracrine somatostatin secretion.

Animals↗
PubMed2004

Tacrolimus impairment of insulin secretion in isolated rat islets occurs at multiple distal sites in stimulus-secretion coupling.

Tacrolimus causes posttransplant diabetes mellitus, although the pathogenetic mechanisms remain controversial. We studied the mechanism of tacrolimus-induced impairment of insulin secretion using isolated rat pancreatic islets. Tacrolimus caused reductions in DNA and insulin contents per islet during 7-d culture. Tacrolimus time-dependently suppressed glucose-stimulated insulin secretion, and at a therapeutic concentration of 0.01 micromol/liter, it suppressed glucose-stimulated insulin secretion to 32 +/- 5% of the control value after 7-d incubation. Tacrolimus did not change islet glucose utilization and oxidation, ATP production, insulin mRNA expression, or the capacity for high glucose to increase intracellular Ca(2+), but altered the rapid frequency oscillations of Ca(2+) concentration. Tacrolimus suppressed insulin secretion stimulated by mitochondrial fuel (combination of l-leucine and l-glutamine, and alpha-ketoisocaproate) and glibenclamide, but not by l-arginine. Tacrolimus suppressed insulin secretion induced by carbachol and by a protein kinase C agonist in the presence or absence of extracellular Ca(2+). Under stringent Ca(2+)-free conditions, tacrolimus did not affect mastoparan-induced insulin secretion, but suppressed its glucose augmentation. Our results suggest that tacrolimus impairs glucose-stimulated insulin secretion downstream of the rise in intracellular Ca(2+) at insulin exocytosis, and that protein kinase C-mediated (Ca(2+)-dependent and independent) and Ca(2+)-independent GTP signaling pathways may be involved. However, tacrolimus-induced impaired insulin secretion was reversed 3 d after removal of the drug. Our study demonstrated that tacrolimus impairs insulin secretion at multiple steps in stimulus-secretion coupling.

Adenosine Triphosphate↗
PubMed1988

GTP-dependent inhibition of insulin secretion by epinephrine in permeabilized RINm5F cells. Lack of correlation between insulin secretion and cyclic AMP levels.

The mechanism by which alpha 2-adrenergic agonists inhibit exocytosis was investigated in electrically permeabilized insulin secreting RINm5F cells. In this preparation alpha 2-adrenoceptors remain coupled to adenylate cyclase, since basal- and forskolin-stimulated cyclic AMP production was lowered by epinephrine and clonidine by 30-50%. Cyclic AMP levels did not correlate with the rate of insulin secretion. Thus, at low Ca2+, forskolin enhanced cyclic AMP levels 5-fold without eliciting secretion, and Ca2+-stimulated secretion was associated with decreased cyclic AMP accumulation. Epinephrine (plus propranolol) inhibited Ca2+-induced insulin secretion in a GTP-dependent manner. The maximal inhibition (43%) occurred at 500 microM GTP. Clonidine also inhibited Ca2+-stimulated secretion. Replacement of GTP by GDP or by the nonhydrolyzable GTP analog guanosine 5'-(3-O-thio)triphosphate as well as treatment of the cells with pertussis toxin prior to permeabilization abolished epinephrine inhibition of insulin secretion. Pertussis toxin did not affect Ca2+-stimulated secretion. Insulin release stimulated by 1,2-didecanoyl glycerol was also lowered by epinephrine suggesting an effect distal to the activation of protein kinase C (Ca2+/phospholipid-dependent enzyme). These results taken together with the ability of epinephrine to inhibit ionomycin-induced insulin secretion in intact cells suggest that alpha 2-adrenergic inhibition is distal to the generation of second messengers. A model is proposed for alpha 2-adrenoceptor coupling to two effector systems, namely the adenylate cyclase and the exocytotic site in insulin-secreting cells.

Adenylate Cyclase Toxin↗
PubMed1983

Synthesis and secretion of apolipoproteins by pig intestinal mucosa in organ culture. Lack of inhibition of apolipoprotein A-I secretion by colchicine.

Pig duodeno-jejunal mucosa was maintained in organ culture for up to 24 h in Eagle's minimum essential medium containing 10% foal serum. Viability was controlled by determination of alkaline phosphatase and sucrase activity in the tissue. [14C]Leucine incorporation into proteins decreased 3-fold between 2 and 24 h. Newly synthesized secreted proteins were analyzed by SDS-polyacrylamide gel electrophoresis of the whole culture medium. Apolipoprotein A-I specifically measured by immunoelectrophoresis represented 10-20% of newly secreted proteins. Only 10% of apolipoprotein A-I secreted was recovered with the lipoprotein fraction (d less than 1.21). Recombination of the medium with porcine lipoproteins or DMPC vesicles prior to ultracentrifugation allowed, respectively, the recovery of 40 and 80% of apolipoprotein A-I secreted. The lipoprotein fractions also contained some apolipoproteins B and C and, after DMPC recombination, an apolipoprotein of Mr 45 000, most likely apolipoprotein A-IV, representing about 3.5% of newly secreted proteins. The d greater than 1.21 fractions all contained a high Mr protein, identified as IgA, and an unidentified protein of Mr approximately 45 000. The addition of colchicine (125 microM) to the culture medium did not significantly modify either tissue enzyme activities or [14C]leucine incorporation. It reduced total secretion by about 40% between 2 and 8 h of incubation, without interfering with apolipoprotein A-I secretion, which then represented up to 35% of secretion products. This raises the question of the mode of secretion of apolipoprotein A-I, which may be related to the high proportion of its which is secreted free.

Animals↗
PubMed1983

Separation of rabbit ileum mucus secretion from electrolyte and water secretion by cholera enterotoxin, verapamil and A23187.

Net water, Na+, Cl- and HCO3- fluxes were measured in in vivo rabbit ileal loops, while mucus secretion was assessed by measuring the glycoprotein or total sialic acid secreted into the lumen, or by measuring the luminal fluid viscosity. Inoculating loops with cholera enterotoxin (CT) produced a sustained secretion of electrolytes and water, but a more transient secretion of mucus. A dose of verapamil was found which, when included in the luminal fluid, inhibited or delayed the CT-induced mucus secretion while not affecting the ongoing electrolyte and water secretion. Exposure of the ileal mucosa to the ionophore, A23187, in the presence of 2mM Ca++ resulted in a brief secretion of mucus, with no change in basal water absorption. Verapamil inhibited this A23187-induced mucus secretion. The ionophore was not effective in the absence of luminal Ca++. Thus rabbit ileum mucus secretion can be separated from electrolyte and water secretion by agents that affect Ca++ movement.

Animals↗
PubMed1996

Retinoic acid receptor transcripts and effects of retinol and retinoic acid on glucagon secretion from rat islets and glucagon-secreting cell lines.

Using intact rat islets, hamster In-R1-G9 cells, and mouse alphaTC-1 clone 9 transgenic tumoral glucagon-secreting cells, we determined the effects of retinol (ROH) and retinoic acid (RA) on glucagon secretion. Since vitamin A effects may be mediated through nuclear RA receptors (RARs) and cytoplasmic ROH- and RA-binding proteins (CRBP and CRABP), cells were also assayed for RARs, CRBP, and CRABP mRNA by Northern blot analyses. Islets and cells were cultured in 2.8 mmol/L glucose and vitamin A-deficient (A-def) medium or in different concentrations of ROH and RA. Using intact islets, RA 10 and 100 nmol/L inhibited glucagon secretion to approximately 60% of control levels. Using In-R1-G9 cells, ROH 0.175 to 5.0 micromol/L inhibited glucagon secretion to 60% to 83% of control levels, and RA 100 and 1,000 nmol/L inhibited glucagon secretion from 72% to 43% of control levels, respectively. Using alphaTC-1 cells, ROH 1.75 micromol/L inhibited glucagon secretion to 80% of control levels, and RA 1 to 100 nmol/L inhibited secretion from 83% to 68% of control levels. Inhibition of secretion was dose-dependent. RARalpha RNA transcripts were detected in alpha TC-1 and In-R1-G9 total RNA extracts; RAR gamma transcripts were detected in alphaTC-1 cells. We conclude the following: (1) ROH and RA inhibit glucagon secretion in cultured rat islets and glucagon-secreting cell lines, and in cell lines the effect of RA is dose-dependent; (2) on a molar basis, RA is on the order of 10- to 100-fold more potent than ROH, a finding consistent with RA being the active metabolite of ROH at the alpha-cell level; and (3) this inhibition may be mediated through classic pathways of retinoid action involving nuclear RARs and gene expression of specific proteins.

Animals↗