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Culp DJ, Zhang Z, Evans RL. VIP and muscarinic synergistic mucin secretion by salivary mucous cells is mediated by enhanced PKC activity via VIP-induced release of an intracellular Ca 2+ pool. Pflugers Arch 2020; 472:385-403. [PMID: 31932898 DOI: 10.1007/s00424-020-02348-7] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2019] [Revised: 12/09/2019] [Accepted: 01/06/2020] [Indexed: 12/12/2022]
Abstract
Mucin secretion by salivary mucous glands is mediated predominantly by parasympathetic acetylcholine activation of cholinergic muscarinic receptors via increased intracellular free calcium ([Ca2+]i) and activation of conventional protein kinase C isozymes (cPKC). However, the parasympathetic co-neurotransmitter, vasoactive intestinal peptide (VIP), also initiates secretion, but to a lesser extent. In the present study, cross talk between VIP- and muscarinic-induced mucin secretion was investigated using isolated rat sublingual tubuloacini. VIP-induced secretion is mediated by cAMP-activated protein kinase A (PKA), independently of increased [Ca2+]i. Synergistic secretion between VIP and the muscarinic agonist, carbachol, was demonstrated but only with submaximal carbachol. Carbachol has no effect on cAMP ± VIP. Instead, PKA activated by VIP releases Ca2+ from an intracellular pool maintained by the sarco/endoplasmic reticulum Ca2+-ATPase pump. Calcium release was independent of phospholipase C activity. The resultant sustained [Ca2+]i increase is additive to submaximal, but not maximal carbachol-induced [Ca2+]i. Synergistic mucin secretion was mimicked by VIP plus either phorbol 12-myristate 13-acetate or 0.01 μM thapsigargin, and blocked by the PKC inhibitor, Gö6976. VIP-induced Ca2+ release also promoted store-operated Ca2+ entry. Synergism is therefore driven by VIP-mediated [Ca2+]i augmenting cPKC activity to enhance muscarinic mucin secretion. Additional data suggest ryanodine receptors control VIP/PKA-mediated Ca2+ release from a Ca2+ pool also responsive to maximal carbachol. A working model of muscarinic and VIP control of mucous cell exocrine secretion is presented. Results are discussed in relation to synergistic mechanisms in other secretory cells, and the physiological and therapeutic significance of VIP/muscarinic synergism controlling salivary mucous cell exocrine secretion.
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Affiliation(s)
- David J Culp
- Center for Oral Biology, University of Rochester Medical Center, 601 Elmwood Ave, Rochester, NY, 14642, USA. .,Department of Oral Biology, UF College of Dentistry, P.O. Box 100424, Gainesville, FL, 32610-3003, USA.
| | - Z Zhang
- Center for Oral Biology, University of Rochester Medical Center, 601 Elmwood Ave, Rochester, NY, 14642, USA
| | - R L Evans
- Center for Oral Biology, University of Rochester Medical Center, 601 Elmwood Ave, Rochester, NY, 14642, USA.,Unilever Research & Development, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, CH63 3JW, UK
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2
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Abstract
It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.
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Affiliation(s)
- B. J. Baum
- Clinical Investigations and Patient Care Branch, National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland 20892
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3
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Rosignoli F, Pérez Leirós C. Nitric oxide synthase I and VIP-activated signaling are affected in salivary glands of NOD mice. J Neuroimmunol 2002; 130:109-16. [PMID: 12225893 DOI: 10.1016/s0165-5728(02)00223-0] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The autoimmune sialadenitis developed by non-obese diabetic (NOD) mice is considered a suitable model to study the ethiopathogenic mechanisms leading to sicca symptoms in Sjögren's syndrome (SS). Evidence supporting a neural rather than immune origin of the secretory dysfunction has been provided. As both nitric oxide and vasoactive intestinal peptide (VIP) are common messengers to nervous and immune systems mediating secretory and inflammatory responses, we examined nitric oxide synthase (NOS) activity with special focus on VIP-mediated effects in salivary glands of NOD mice. We found a decreased NOS activity and expression in major salivary glands of NOD mice with respect to control mice. In addition, there was a deficient VIP-activated signaling associated with a reduced saliva and amylase secretion in response to VIP. Our results support the hypothesis of an impaired balance of neuroimmune interactions in salivary glands as early events to take place in the progressive loss of secretory function of NOD mice.
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Affiliation(s)
- Florencia Rosignoli
- Departamento de Química Biológica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires-CONICET, Buenos Aires, Argentina
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4
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Dehaye JP, Verhasselt V. Interaction of amiloride with rat parotid muscarinic and alpha-adrenergic receptors. GENERAL PHARMACOLOGY 1995; 26:155-9. [PMID: 7536173 DOI: 10.1016/0306-3623(94)00153-e] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
1. In rat parotid acini, amiloride inhibited the secretion of amylase and the efflux of calcium and rubidium in response to carbamylcholine and to norepinephrine. 2. Amiloride competitively inhibited the binding of [3H]N-methylscopolamine and [3H] is thus a competitive antagonist of muscarinic and norepinephrine alpha-adrenergic receptors. 3. Amiloride did not affect the response to substance P with respect to secretion or ion movements. 4. Thus the Na+/H+ antiporter is not involved in the short-term regulation of amylase secretion and calcium and potassium movements in rat parotid gland function.
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Affiliation(s)
- J P Dehaye
- Department of Biochemistry, Université libre de Bruxelles, Belgium
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5
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Goll R, Poulsen JH, Schmidt P, Schjoldager B, Poulsen SS, Holst JJ. Peptide-evoked release of amylase from isolated acini of the rat parotid gland. REGULATORY PEPTIDES 1994; 51:237-54. [PMID: 7524117 DOI: 10.1016/0167-0115(94)90070-1] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Investigations of the effects of the neuropeptides, substance P (SP), neurokinin A (NKA), neuropeptide K (NPK), gastrin releasing peptide (GRP), calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP), and of acetylcholine on amylase secretion have been carried out on isolated acini of the rat parotid gland. Furthermore, the occurrence and location of the peptides in the gland was studied. Finally, binding of 125I-BH-SP to isolated acini were studied in order to characterize their tachykinin receptor(s) and their binding kinetics. Only SP, NKA, NPK and VIP stimulated amylase release. VIP, however, with a rather low potency (EC50 at 155 nmol/l). Simultaneous stimulation with two compounds elicited additive responses, except for VIP and acetylcholine which elicited an effect significantly above additive response. Only SP, NKA, VIP and CGRP could be identified in extracts of the gland. The immunoreactivity of these peptides could be located to varicose nerve fibers in the gland. Binding of labeled SP to the isolated acini exhibited the characteristics of a genuine agonist/receptor interaction, and the rank order of displacement potencies indicated the presence of NK1-receptors. Thus, the results of the present study support previous suggestions that the tachykinins and VIP are likely to be involved in amylase secretion in the rat parotid gland.
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Affiliation(s)
- R Goll
- Department of Medical Physiology, Panum Institute, University of Copenhagen, Denmark
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6
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Quissell DO, Watson E, Dowd FJ. Signal transduction mechanisms involved in salivary gland regulated exocytosis. CRITICAL REVIEWS IN ORAL BIOLOGY AND MEDICINE : AN OFFICIAL PUBLICATION OF THE AMERICAN ASSOCIATION OF ORAL BIOLOGISTS 1992; 3:83-107. [PMID: 1730072 DOI: 10.1177/10454411920030010701] [Citation(s) in RCA: 49] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- D O Quissell
- Department of Basic Sciences and Oral Research, University of Colorado School of Dentistry, Denver 80262
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7
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Dehaye JP, Turner RJ. Isolation and characterization of rat submandibular intralobular ducts. THE AMERICAN JOURNAL OF PHYSIOLOGY 1991; 261:C490-6. [PMID: 1716052 DOI: 10.1152/ajpcell.1991.261.3.c490] [Citation(s) in RCA: 56] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Intralobular (granular) salivary ducts were purified by isopycnic centrifugation after collagenase/hyaluronidase digestion of the rat submandibular gland. The resulting ductal fraction (density, 1.056 +/- 0.003) was highly enriched in kallikrein (a ductal cell marker) and contained little amylase activity (an acinar cell marker). The resting intracellular calcium level in the ductal preparation was 103 +/- 4 nM. Increased intracellular calcium concentrations (2-3 times resting levels) were observed in response to muscarinic (carbachol) and alpha-adrenergic (epinephrine) agonists, but little response was observed to substance P, suggesting the absence of substance P peptidergic receptors on rat submandibular ducts. Intracellular adenosine 3',5'-cyclic monophosphate levels were increased 35-fold in response to beta-adrenergic stimulation (isoproterenol) and forskolin. The ducts secreted kallikrein in response to epinephrine, carbachol, and isoproterenol but not in response to substance P. Epinephrine was the most potent inducer of kallikrein release with a K0.5 of approximately 3 microM and a maximal secretory rate approximately nine times unstimulated levels. Taken together, these results provide strong evidence for the functional integrity of the ductal preparation. This preparation should prove useful for the further elucidation of the properties of intralobular salivary ducts structures which heretofore have only been studied indirectly.
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Affiliation(s)
- J P Dehaye
- Department of General and Human Biochemistry, Free University of Brussels, Belgium
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Nilsson C, Fahrenkrug J, Lindvall-Axelsson M, Owman C. Epithelial cells purified from choroid plexus have receptors for vasoactive intestinal polypeptide. Brain Res 1991; 542:241-7. [PMID: 1851455 DOI: 10.1016/0006-8993(91)91573-j] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Using the choroid plexus from pig a method has been developed to purify the epithelial cells from the underlying vascularized connective tissue stroma. An epithelial cell fraction was obtained that showed a purity of at least 95%, as determined by light microscopic analysis. The epithelial cells were investigated for the presence of binding sites for the neurotransmitter peptide, vasoactive intestinal polypeptide (VIP). Suspensions of epithelial cells were found to have high affinity binding sites for 125I-labelled VIP, with maximum binding obtained after 30 min incubation at 20 degrees C with a concentration of 50 micrograms cell protein per sample. Competition experiments with displacement of [125I]VIP binding by increasing concentrations of unlabeled VIP indicated the presence of a single class of binding sites with a Kd of 3 nM and a binding capacity of 970 pmol/g cell protein. Cross-linking of [125I]VIP to epithelial cells with disuccinimido dithiobis (propionate) (DSP), followed by SDS-polyacrylamide gel electrophoresis, demonstrated binding to a single 55 kD protein. The receptor was highly specific for VIP as binding was only inhibited in the presence of high concentrations of the related peptides helodermin, growth hormone-releasing factor, secretin, and peptide histidine isoleucine. This is the first demonstration of VIP-binding to choroid plexus epithelial cells.
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Affiliation(s)
- C Nilsson
- Department of Medical Cell Research, University of Lund, Sweden
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9
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Dehaye JP, Caulier C, Delporte C. Response of rat parotid acini to barium. Arch Oral Biol 1991; 36:335-40. [PMID: 1714713 DOI: 10.1016/0003-9969(91)90002-c] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
The effect of barium on isolated acini was tested. Barium in the 0.1-10 mM concentration range non-competitively inhibited the efflux of 86Rb+ stimulated by carbamylcholine or substance P. This inhibition was independent of the presence of calcium in the extracellular medium. In the same preparation, barium did not affect the efflux of 45Ca2+ but, at a 10 mM concentration, it increased amylase release by 70%. Removal of extracellular calcium decreased basal amylase release and the response to carbamylcholine. Adding back calcium or barium to the incubation medium increased basal and carbamylcholine-stimulated amylase secretion, but calcium was more effective than barium. These results suggest that barium has two opposite effects on calcium-regulated processes in rat parotid gland: (1) it is an inhibitor of calcium-activated potassium channels; (2) it is a partial agonist of calcium-activated amylase secretion.
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Affiliation(s)
- J P Dehaye
- Department of Biochemistry, Faculty of Medicine, Free University of Brussels, Belgium
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10
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Månsson B, Nilsson BO, Ekström J. Effects of repeated infusions of substance P and vasoactive intestinal peptide on the weights of salivary glands subjected to atrophying influences in rats. Br J Pharmacol 1990; 101:853-8. [PMID: 1707705 PMCID: PMC1917844 DOI: 10.1111/j.1476-5381.1990.tb14170.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
1. The long-term influence of substance P (SP) and vasoactive intestinal peptide (VIP) on rat salivary gland weight was investigated after parasympathetic denervation or on feeding soft food. 2. The parotid gland lost about one-third of its weight within 4-5 days following parasympathetic post-ganglionic denervation or change in dietary regimen, from pellets to liquid diet, thought to reduce nerve reflex activity. 3. Daily i.v. infusions with SP or VIP diminished or largely prevented the fall in parotid gland weight, whereas infusions with pentagastrin, bethanechol and saline had no effect. The infusions were preceded by administration of alpha- and beta-adrenoceptor antagonists; these antagonists were also given to the control animals. 4. The effect of SP and VIP on the parotid gland weight appeared to be related to cell size rather than to cell number, as judged by measurements of RNA and DNA. 5. Observations on the two other major salivary glands underlined the fact that different gland types in the same animal behave differently. Parasympathetic preganglionic denervation (decentralization) lowered the weights of the sublingual and submandibular glands, whereas liquid diet only reduced the weight of the sublingual gland. SP and VIP did not affect the weights of the submandibular glands, but VIP prevented the slight fall in sublingual gland weight induced by liquid diet. 6. The present results suggest a trophic role in rats for SP and VIP on parotid glands and for VIP on sublingual glands. Such an influence may be exerted naturally as a result of their release from nerves containing these peptides around acini.
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Affiliation(s)
- B Månsson
- Department of Physiology and Biophysics, Lund University, Sweden
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11
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Spearman TN, Butcher FR. Cellular Regulation of Amylase Secretion by the Parotid Gland. Compr Physiol 1989. [DOI: 10.1002/cphy.cp060304] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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12
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Abstract
Vasoactive intestinal peptide (VIP) is a putative neurotransmitter found in the salivary glands of many species, including the rat parotid gland. Parasympathetic denervation has been reported to deplete VIP in the rat parotid gland and to lead to supersensitivity to this peptide in vivo. We have compared the effects of VIP on acini isolated from parasympathetically denervated and unoperated parotid glands to examine possible supersensitivity to the peptide in vitro. VIP normally produced responses similar to those obtained with a low concentration of the beta adrenergic agonist isoproterenol (ISO), but strikingly different from the effects obtained with the muscarinic agonist carbachol (CARB). In parotid membrane preparations, VIP stimulated adenylate cyclase activity. Dissociated acini treated with VIP showed increases in cAMP accumulation and amylase release which were potentiated by forskolin and also by inhibition of phosphodiesterase. After parasympathetic denervation, maximal effects of VIP on adenylate cyclase, cAMP accumulation and amylase release in intact cells were increased two- to five-fold over contralateral control (or unoperated) parotid responses. The increase in adenylate cyclase-mediated responses after denervation was specific to VIP; there was no increased response nor increased sensitivity of any of these responses to ISO. Specific [125I]VIP binding to parotid acini increased two-fold per gland and three-fold per mg of protein after denervation; this probably explains the observed increases in the response to VIP.
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Affiliation(s)
- M K McMillian
- Department of Neurology, Tufts University School of Medicine, Boston, MA 02111
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13
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Dehaye JP. Calcium-activated potassium channels from rat parotid acinar cells. Pflugers Arch 1989; 414 Suppl 1:S158-9. [PMID: 2780242 DOI: 10.1007/bf00582281] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Affiliation(s)
- J P Dehaye
- Laboratoire de Chimie Biologique et de la Nutrition, Faculté de Médecine de l'Université Libre de Bruxelles, Belgium
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14
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Calvo JR, Guerrero JM, Goberna R. Interaction of thymic peptide thymosin alpha 1 with VIP receptors in rat intestinal epithelial cells: comparison with PHI and secretin. GENERAL PHARMACOLOGY 1989; 20:503-5. [PMID: 2546855 DOI: 10.1016/0306-3623(89)90203-6] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
1. Thymic peptide thymosin alpha 1 (10(-10)-10(-7) M) is shown to inhibit the specific binding of 125I-labelled VIP. 2. Thymosin alpha 1 was 1500 times less potent than VIP at inhibiting 125I-labelled VIP binding. 3. Thymosin alpha 1 (10(-10)-10(-7) M) was weak in stimulating cyclic AMP production. 4. Interaction of thymosin alpha 1 with VIP receptors is compared with PHI and secretin. 5. The order of affinity of different peptides is VIP greater than PHI greater than secretin greater than thymosin alpha 1.
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Affiliation(s)
- J R Calvo
- Departamento de Bioquimica Médica y Biología Molecular, Facultad de Medicina, Sevilla, Spain
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15
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Sundler F, Ekblad E, Grunditz T, Håkanson R, Uddman R. Vasoactive intestinal peptide in the peripheral nervous system. Ann N Y Acad Sci 1988; 527:143-67. [PMID: 3291690 DOI: 10.1111/j.1749-6632.1988.tb26979.x] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Affiliation(s)
- F Sundler
- Department of Pharmacology, University of Lund, Sweden
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16
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Biancani P, Beinfeld MC, Coy DH, Hillemeier C, Walsh JH, Behar J. Dysfunction of the gastrointestinal tract. Vasoactive intestinal peptide in peristalsis and sphincter function. Ann N Y Acad Sci 1988; 527:546-67. [PMID: 2898914 DOI: 10.1111/j.1749-6632.1988.tb27008.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Affiliation(s)
- P Biancani
- Division of Biological and Medical Sciences, Rhode Island Hospital, Brown University, Providence 02902
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17
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Dehaye JP, Winand J, Christophe J. Scorpion venom inhibits carbamylcholine-induced 86rubidium efflux from rat parotid acini. FEBS Lett 1987; 219:451-4. [PMID: 3609303 DOI: 10.1016/0014-5793(87)80270-3] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The muscarinic agonist carbamylcholine stimulated 5-fold 86Rb efflux from preloaded rat parotid acini. Apamin was without effect on this carbamylcholine-induced 86Rb efflux. By contrast, the venom from Leiurus quinquestriatus (a scorpion from Israel) inhibited non-competitively this efflux while being without effect on the carbamylcholine-stimulated 45Ca efflux and amylase release. This heat-resistant inhibitory effect of the venom was destroyed by boiling in the presence of dithiothreitol. These results suggest that the venom from L. quinquestriatus contains a toxin capable to block apamin-insensitive calcium-activated potassium channels in rat parotid acini.
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van Bogaert P, Soukias Y, Dehaye JP, Lambert M, Poloczek P, Winand J, Mayer R, Christophe J. Adenylate cyclase stimulation by VIP in rat and human parotid membranes. REGULATORY PEPTIDES 1987; 17:339-48. [PMID: 3602475 DOI: 10.1016/0167-0115(87)90057-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Adenylate cyclase activity was stimulated by vasoactive intestinal peptide (VIP) in rat parotid membranes, in the presence of 100 microM guanosine triphosphate (GTP). The threshold concentration of VIP was 300 nM and the activity doubled at the maximal VIP concentration tested (30 microM). The relative potency of peptides of the VIP family was: VIP greater than peptide histidine isoleucinamide (PHI) greater than secretin. The beta-adrenergic agent isoproterenol was a more efficient activator of rat parotid adenylate cyclase and its stimulatory effect, like that of VIP, depended on the presence of GTP. The effects of VIP and isoproterenol were both potentiated by 10 microM forskolin. By comparison with rat parotid preparations, membranes from a human parotid gland responded similarly to the VIP family of peptides (VIP greater than PHI greater than secretin). In both rat and human parotid membranes, two proteins (Mr 44 kDa and 53 kDa) of the alpha-subunit of Ns (the guanyl nucleotide-binding stimulatory protein) were labelled by ADP-ribosylation, in the presence of cholera toxin. Taken together, these results indicate that VIP receptors, when coupled to Ns, were able to activate the adenylate cyclase system in rat and human parotid membranes.
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Abstract
It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.
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