Convergence of cAMP and phosphoinositide pathways during rat parotid secretion

Role of NAADP for calcium signaling in the salivary gland

Coordination of intracellular Ca²⁺ signaling in parotid acini is crucial for controlling the secretion of primary saliva. Previous work from our lab has demonstrated acidic-organelle Ca²⁺ release as a participant in agonist-evoked signaling dynamics of the parotid acinar cell. Furthermore, results implicated a potential role for the potent Ca²⁺ releasing second messenger NAADP in these events. The current study interrogated a direct role of NAADP for Ca²⁺ signaling in the parotid salivary gland acinar cell. Use of live-cell Ca²⁺ imaging, patch-clamp methods, and confocal microscopy revealed for the first time NAADP can evoke or enhance Ca²⁺ dynamics in parotid acini. These results were compared with pancreatic acini, a morphologically similar cell type previously shown to display NAADP-dependent Ca²⁺ signals. Findings presented here may be relevant in establishing new therapeutic targets for those suffering from xerostomia produced by hypofunctioning salivary glands.

Enhancement of carbachol-induced amylase secretion in parotid glands from rats with experimental periodontitis

OBJECTIVE

In a previous study we observed that parotid glands from rats with experimental periodontitis showed an increase in basal amylase release as a result of an increase in cAMP accumulation induced by PGE(2) production. The aim of this work was to study whether this change in amylase release influences the secretory effect of carbachol.

DESIGN

Experimental periodontitis was induced through placing a black thread around the cervix of the two lower first molars. Experiments were done 22 days after ligature induced periodontitis. Amylase release was evaluated in vitro and determined using a colorimetric method which uses starch as substrate.

RESULTS

The effect of carbachol was increased in parotid glands from periodontitis rats. The effect of 10(-6)M carbachol was inhibited by 4-DAMP (10(-6)M), U-73122 (5 × 10(-6)M) and trifluoperazine (5 × 10(-6)M) in both groups. No changes were observed in the binding sites and affinity in parotid membranes from rats with experimental periodontitis. The inhibition of the adenylyl cyclase and the cyclooxygenase induced a right shift of the carbachol concentration-response curve in periodontitis group whilst the opposite effect was observed in control group in the presence of db-cAMP and PGE(2).

CONCLUSIONS

Parotid glands from rats with experimental periodontitis release more amylase in response to carbachol suggesting an interaction between Ca(2+) and cAMP in the fusion/exocytosis step of secretory vesicles.

Distribution and axonal projections of neurons coexpressing thyrotropin-releasing hormone and urocortin 3 in the rat brain

Thyrotropin-releasing hormone (TRH) decreases food intake when administered intracerebroventricularly or into the ventromedial hypothalamus. However, it is unknown which population of TRH neurons exerts this anorexigenic function. In the rostral perifornical area, the pattern of TRH-expressing neurons is reminiscent of the distribution of neurons expressing urocortin3 (Ucn3) that also inhibits feeding when injected into the hypothalamic ventromedial nucleus (VMN). Since colocalization of TRH and Ucn3 may help to identify feeding-related TRH neurons, the putative coexpression of the two peptides was examined using fluorescent in situ hybridization combined with immunofluorescence. Almost all (95.5 +/- 0.2%) Ucn3-immunoreactive neurons in the perifornical area expressed pro-TRH mRNA, while 50.2 +/- 1.6% Ucn3 neurons were double-labeled in the bed nucleus of the stria terminalis (BNST). Only a few Ucn3/pro-TRH neurons were found outside these two areas. The distribution of axons containing both Ucn3 and TRH was examined by dual immunofluorescence. Ucn3/TRH fibers heavily innervated the VMN. In addition, high densities of double-labeled axons were observed in the lateral septal nucleus, posterior division of the BNST, medial amygdaloid nucleus, amygdalohippocampal area, and ventral hippocampus, forebrain areas associated with psychological stress and anxiety. We conclude that Ucn3 and TRH are coexpressed in a discrete, continuous population of neurons in the perifornical area and BNST, making Ucn3 a neurochemical marker to define a distinct subset of TRH neurons. The distribution of their axons suggests that Ucn3/TRH neurons may coordinate feeding and behavioral responses to stressful stimuli.

Endogenous signalling system involved in parotid gland adenosine A(1) receptor-amylase release

AIM

In this study, we have determined signalling pathways involved in adenosine A(1) receptor (A(1) receptor)-dependent stimulation of amylase release in rat parotid gland.

METHODS

Amylase release, binding and cyclic adenosine monophosphate (cAMP) assays, inositol phosphates (IPs) production and nitric oxide synthase (NOS) activity in the presence of cyclopentyl-1,3-dipropylxanthine (CPA) alone or in the presence of different inhibitory drugs were performed.

RESULTS

The binding parameters of specific A(1) antagonist [(3)H]-cyclopentyl 1,3-dipropilxanthine ([(3)H]-DPCPX) in parotid gland membranes show a population of high affinity sites with K(d) (nm) 0.53 +/- 0.06 and B(max) (fmol mg(-1) protein) 122.6 +/- 10.2. CPA stimulation of A(1) receptor exerts an increase in amylase release, IPs accumulation, cAMP production and NOS activity. All these A(1) agonist effects were blocked by the A(1) receptor antagonist DPCPX. Inhibitors of phospholipase C (PLC), calcium/calmodulin (CaM), protein kinase C (PKC), and adenylate cyclase, but not NOS, activities attenuated the CPA stimulatory effect on amylase release. The effect of CPA on amylase release significantly correlated with its action either on cAMP or on IPs accumulation.

CONCLUSION

These results suggest that CPA activation of parotid gland A(1) receptor induces a stimulatory effect on amylase release associated with increased production of cAMP and IPs accumulation. The mechanism appears to occur secondarily to stimulation of phosphoinositide turnover via PLC activation. This, in turn, triggers cascade reactions involving CaM and PKC. The CPA stimulation of NOS does not appear to participate in amylase release.

Expression and biological effects of CB1 cannabinoid receptor in rat parotid gland

Experiments were designed to determine whether cannabinoids affect salivary gland function. For this purpose, the effect of anandamide on cAMP accumulation, amylase release and Na+-K+-ATPase activity was studied in rat parotid glands. Anandamide induced a concentration-dependent increase in cAMP and led to amylase release but inhibited Na+-K+-ATPase activity. These effects were blocked by the CB1 cannabinoid receptor antagonist, AM281. The inhibition of adenylyl cyclase activity by SQ 22536 impaired amylase release and Na+-K+-ATPase inhibition. The effect of anandamide on cAMP accumulation significantly correlated with its action either on amylase release or on Na+-K+-ATPase activity. Such correlation strongly supports the view that the effect of anandamide on amylase release and Na+-K+-ATPase activity is the result of cAMP accumulation. The relative potencies of the CB1 cannabinoid receptor antagonist, AM281, to block these three functional responses were similar, supporting the view that anandamide actions in parotid glands were achieved through a single receptor subtype, the CB1. Binding studies using the selective cannabinoid CB1 receptor antagonist, [3H]SR141716A, indicated the presence of the specific binding site. It may be concluded that in parotid glands the endogenous cannabinoid anandamide, bound to the CB1 cannabinoid receptor subtype, induces cAMP accumulation which in turn leads to amylase release and Na+-K+-ATPase inhibition.

Ca2+-dependent protein kinase--a modulation of the plasma membrane Ca2+-ATPase in parotid acinar cells

Cross-talk between cAMP and [Ca(2+)](i) signaling pathways represents a general feature that defines the specificity of stimulus-response coupling in a variety of cell types including parotid acinar cells. We have reported recently that cAMP potentiates Ca(2+) release from intracellular stores, primarily because of a protein kinase A-mediated phosphorylation of type II inositol 1,4,5-trisphosphate receptors (Bruce, J. I. E., Shuttleworth, T. J. S., Giovannucci, D. R., and Yule, D. I. (2002) J. Biol. Chem. 277, 1340-1348). The aim of the present study was to evaluate the functional and molecular mechanism whereby cAMP regulates Ca(2+) clearance pathways in parotid acinar cells. Following an agonist-induced increase in [Ca(2+)](i) the rate of Ca(2+) clearance, after the removal of the stimulus, was potentiated substantially ( approximately 2-fold) by treatment with forskolin. This effect was prevented completely by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) with La(3+). PMCA activity, when isolated pharmacologically, was also potentiated ( approximately 2-fold) by forskolin. Ca(2+) uptake into the endoplasmic reticulum of streptolysin-O-permeabilized cells by sarco/endoplasmic reticulum Ca(2+)-ATPase was largely unaffected by treatment with dibutyryl cAMP. Finally, in situ phosphorylation assays demonstrated that PMCA was phosphorylated by treatment with forskolin but only in the presence of carbamylcholine (carbachol). This effect of forskolin was Ca(2+)-dependent, and protein kinase C-independent, as potentiation of PMCA activity and phosphorylation of PMCA by forskolin also occurred when [Ca(2+)](i) was elevated by the sarco/endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid and was attenuated by pre-incubation with the Ca(2+) chelator, 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The present study demonstrates that elevated cAMP enhances the rate of Ca(2+) clearance because of a complex modulation of PMCA activity that involves a Ca(2+)-dependent step. Tight regulation of both Ca(2+) release and Ca(2+) efflux may represent a general feature of the mechanism whereby cAMP improves the fidelity and specificity of Ca(2+) signaling.

Nitric oxide signalling in salivary glands

Nitric oxide (NO) plays multiple roles in both intracellular and extracellular signalling mechanisms with implications for health and disease. This review focuses on the role of NO signalling in salivary secretion. Attention will be paid primarily to endogenous NO production in acinar cells resulting from specific receptor stimulation and to NO-regulated Ca2+ homeostasis. Due to the fact that NO readily crosses membranes by simple diffusion, endogenous NO may play a physiological role in processes as diverse as modifying the secretory output, controlling blood supply to the gland, modulating transmitter output from nerve endings, participating in the host defence barrier, and affecting growth and differentiation of surrounding tissue. Furthermore, the role of NO in the pathogenesis of human oral diseases will be considered.

Influence of castration on isoprenaline-induced amylase release in parotid gland from male rats

The purpose of this study was to determine the influence of testosterone, the male sex hormone, on beta-adrenergic agonist-induced amylase secretion from rat parotid glands. Isoprenaline (isoproterenol)-induced amylase secretion was measured in vitro from the parotid glands of control and castrated rats with and without testosterone replacement. The isoprenaline-induced amylase release was reduced in parotid glands from castrated rats compared to controls. The reduction of amylase release by isoprenaline in parotid glands of castrated rats, could be reversed by administration of testosterone. Furthermore, beta-adrenergic receptor density and the level of isoprenaline-evoked cAMP in parotid glands from castrated rats was lower compared to intact rats. Using SQ-22536 (an adenylyl cyclase inhibitor), dibutyryl cAMP (a cAMP analogue) and verapamil (a calcium channel blocker), we conclude that the impairment of amylase release from parotid glands after castration was not related to either adenylyl cyclase activity or cAMP accumulation. Amylase release from the parotid glands of castrated rats appears to be mediated by an increase in calcium ion influx.

Phosphorylation of inositol 1,4,5-trisphosphate receptors in parotid acinar cells. A mechanism for the synergistic effects of cAMP on Ca2+ signaling

Acetylcholine-evoked secretion from the parotid gland is substantially potentiated by cAMP-raising agonists. A potential locus for the action of cAMP is the intracellular signaling pathway resulting in elevated cytosolic calcium levels ([Ca(2+)](i)). This hypothesis was tested in mouse parotid acinar cells. Forskolin dramatically potentiated the carbachol-evoked increase in [Ca(2+)](i), converted oscillatory [Ca(2+)](i) changes into a sustained [Ca(2+)](i) increase, and caused subthreshold concentrations of carbachol to increase [Ca(2+)](i) measurably. This potentiation was found to be independent of Ca(2+) entry and inositol 1,4,5-trisphosphate (InsP(3)) production, suggesting that cAMP-mediated effects on Ca(2+) release was the major underlying mechanism. Consistent with this hypothesis, dibutyryl cAMP dramatically potentiated InsP(3)-evoked Ca(2+) release from streptolysin-O-permeabilized cells. Furthermore, type II InsP(3) receptors (InsP(3)R) were shown to be directly phosphorylated by a protein kinase A (PKA)-mediated mechanism after treatment with forskolin. In contrast, no evidence was obtained to support direct PKA-mediated activation of ryanodine receptors (RyRs). However, inhibition of RyRs in intact cells, demonstrated a role for RyRs in propagating Ca(2+) oscillations and amplifying potentiated Ca(2+) release from InsP(3)Rs. These data indicate that potentiation of Ca(2+) release is primarily the result of PKA-mediated phosphorylation of InsP(3)Rs, and may largely explain the synergistic relationship between cAMP-raising agonists and acetylcholine-evoked secretion in the parotid. In addition, this report supports the emerging consensus that phosphorylation at the level of the Ca(2+) release machinery is a broadly important mechanism by which cells can regulate Ca(2+)-mediated processes.

Expression and localization of Rab3D in rat parotid gland

Rab3 proteins (isoforms A, B, C and D) are low molecular weight GTP-binding proteins proposed to be involved in regulated exocytosis. In the present study, Rab3 protein expression and localization was examined in rat parotid gland by reverse transcription (rt) PCR, Western blotting and immunocytochemistry. An approximately 200 bp PCR product was obtained from parotid RNA by rtPCR and this fragment was cloned and sequenced. Nucleotide and deduced amino acid sequences obtained from five clones were identical to rab3D. Membrane and cytosolic fractions prepared from parotid acini were immunoblotted with antisera specific for each of the four Rab3 isoforms. A 28 kDa protein was detected with Rab3D-specific antisera in both fractions with staining being more intense in the membrane fraction. No other Rab3 isoforms were detected by immunoblotting, a result consistent with those obtained by rtPCR. Rab3D was enriched in zymogen granule membranes and Triton X-114 extraction revealed that this isoform is predominantly lipid-modified in parotid. Localization of Rab3D was done on frozen sections of parotid gland by immunofluorescence microscopy. Staining was observed primarily in the acinar cells and was adjacent to the acinar lumen. Incubation of dispersed acini with isoproterenol and substance P stimulated amylase secretion 4- and 2-fold above basal, respectively. Isoproterenol, but not substance P, induced redistribution of Rab3D from the cytosol to the membrane fraction in dispersed parotid acini. Consistent with these findings, isoproterenol injections into fasted rats also resulted in increased membrane-associated Rab3D in the parotid acini. These results indicate that Rab3D is: (1) the major Rab3 isoform expressed in rat parotid gland; (2) localized to zymogen granule membranes; and (3) involved with regulated enzyme secretion in acinar cells.

Rat serum induces a differentiated phenotype in a rat parotid acinar cell line

To establish a continuous cell line, freshly prepared rat parotid acinar cells were stably transfected with a plasmid vector containing the SV40 large T antigen. The acinar origin of these cells was confirmed by Western blotting, enzyme analysis, and morphological analysis. Transformed cells grown in 10% rat serum showed a modest reduction in cell number after 7 days and a concentration- and time-dependent increase in amylase levels approximately 16 times greater than those observed in fetal bovine serum-treated cells. Ultrastructural analysis revealed that cells grown in rat serum harbored protein-filled secretory granules localized adjacent to the endoplasmic reticulum, and punctate amylase-specific immunofluorescence distributed throughout the cytoplasm was consistent with the presence of amylase in secretory organelles. Clonal cells express tissue-specific proline-rich proteins and the four protein kinase C isozymes present in primary culture. Carbachol and isoproterenol stimulated [3H]protein secretion and isoproterenol enhanced amylase secretion from cells grown in rat serum. Moreover, norepinephrine, carbachol, and substance P produced a time- and concentration-dependent rise in cytoplasmic Ca2+. This continuous cell line of parotid acinar cells, which after treatment with rat serum retains the basic structural and functional properties of primary culture cells, will be utilized as a model system for studying long-term biological processes that regulate parotid cell function.

Cyclic AMP induces ryanodine-sensitive Ca2+ release from microsomal vesicles of rat parotid acinar cells

The effect of cAMP on a ryanodine-sensitive Ca2+ release from microsomal vesicles of rat parotid acinar cells was studied. After a steady state of ATP-dependent 45Ca2+ uptake into the vesicles, cAMP was added to the medium with thapsigargin (TG) to block a reuptake of 45Ca2+. The addition of cAMP (1.0 mM) with TG released about 10% of the 45Ca2+ that had been taken up. The cAMP-induced 45Ca2+ release was strongly inhibited by pretreatment of the vesicles with 500 microMM ryanodine. Preincubation with cAMP (1 mM) abolished ryanodine (10 microM)-induced 45Ca2+ release. The presence of a specific inhibitor of cAMP-dependent protein kinase (PKA) H-89 (10 microMM) inhibited the cAMP-induced 45Ca2+ release. These results indicate that in rat parotid acinar cells, cAMP can activate a ryanodine-sensitive Ca2+ release mechanism in the endoplasmic reticulum and that this activation is via a PKA-dependent process.

Cyclic AMP potentiates substance P-induced amylase secretion by augmenting the effect of calcium in the rat parotid acinar cells

Potentiation of amylase secretion by the combination of isoproterenol and substance P was examined in perfused rat parotid acinar cells. Combined additions of substance P and isoproterenol evoked biphasic changes in amylase secretion, an initial large peak and the following sustained plateau: the magnitudes of the both responses were higher than the sum of the responses induced by each agonist alone. Isoproterenol also increased the maximum response and the apparent affinity (EC50) for substance P to evoke the initial peak response; the EC50 values were about 20 and 0.8 nM, respectively, in the absence and the presence of isoproterenol. On the other hand, 1 nM substance P was sufficient for evoking the maximum potentiation of the sustained plateau response. Substance P did not change the EC50 for isoproterenol. The effect of isoproterenol was mimicked with dibutyryl cyclic AMP and agonists that increase parotid cyclic AMP. Omission of Ca2+ or addition of 5 mM nickel chloride almost completely abolished the potentiation of the sustained plateau, but little decreased that of the initial peak. Depletion of Ca2+ in InsP3-sensitive intracellular stores with thapsigargin, on the other hand, decreased the initial peak response, but not the sustained plateau, to substance P. The potentiation was also observed between isoproterenol and Ca2+ ionophores. Switching to the solutions containing higher concentrations of Ca2+ during the continuous stimulation with isoproterenol or IBMX evoked a large, but transient, response of amylase secretion. Time course of changes in amylase secretion induced by isoproterenol and substance P in combination was very similar to that of substance P, but not of isoproterenol. Isoproterenol did not enhance the effect of substance P on [Ca2+]i. These results show that the potentiation is mainly, if not totally, caused by cyclic AMP-induced enhancement of the potency and the efficacy in the pathway regulated by Ca2+.

Ryanodine receptor expression is associated with intracellular Ca2+ release in rat parotid acinar cells

The ryanodine receptor mediates intracellular Ca2+ mobilization in muscle and nerve, but its physiological role in nonexcitable cells is less well defined. Like adenosine 3',5'-cyclic monophosphate and inositol 1,4,5-trisphosphate, cyclic ADP-ribose (0.3-5 microM) and ADP (1-25 microM) produced a concentration-dependent rise in cytosolic Ca2+ in permeabilized rat parotid acinar cells. Adenosine and AMP were less effective. Ryanodine markedly depressed the Ca2+-mobilizing action of the adenine nucleotides and forskolin in permeabilized cells and was likewise effective in depressing the action of forskolin in intact cells. Cyclic ADP-ribose-evoked Ca2+ release was enhanced by calmodulin and depressed by W-7, a calmodulin inhibitor. A fluorescently labeled ligand, 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3,4-diaza-s-indac ene-3-propionic acid-glycyl ryanodine, was synthesized to detect the expression and distribution of ryanodine receptors. In addition, ryanodine receptor expression was detected in rat parotid cells with a sequence highly homologous to a rat skeletal muscle type 1 and a novel brain type 1 ryanodine receptor. These findings demonstrate the presence of a ryanodine-sensitive intracellular Ca2+ store in rat parotid cells that shares many of the characteristics of stores in muscle and nerve and may mediate Ca2+-induced Ca2+ release or a modified form of this process.

The role of protein kinase C in carbachol-induced and of cAMP-dependent protein kinase in isoproterenol-induced secretion in primary cultured guinea pig parotid acinar cells

Stimulation of secretion by muscarinic agonists in guinea pig parotid or pancreatic acini is accompanied by a translocation of protein kinase C (PKC) from the cytosol to the particulate fraction [Machado-De Domenech and Söling (1987) Biochem. J. 242, 749-754] and by a PKC-mediated phosphorylation of the ribosomal protein S6 [Padel and Söling (1985) Eur. J. Biochem. 151, 1-10]. In order to decide whether PKC is directly involved in the secretory process, the effect of down regulation of PKC by phorbol 12-myristate 13-acetate (PMA) was studied in primary cultured guinea pig parotid acinar cells. These cells secrete in response to carbachol and isoproterenol. Only the carbachol response is associated with an increase in cytosolic calcium. Carbachol plus isoproterenol lead to an over-additive stimulation of secretion, an effect which depends completely on the presence of external calcium. Down regulation of PKC by about 90% did not significantly affect carbachol-induced exocytosis, whereas isoproterenol-stimulated secretion was almost doubled. The secretory response to permeable cAMP analogues was also enhanced in PKC-down-regulated acini, indicating a post-receptor effect. The increased response to isoproterenol was also observed in the absence of external calcium. The isoproterenol effect was significantly inhibited by the relatively specific cAMP-dependent protein kinase inhibitor H-89, which had only a minor effect on carbachol-induced exocytosis. Although down regulation of total PKC by up to 90% did not significantly affect the secretory response to carbachol, RO 31-8220, a relatively specific inhibitor of PKC, abolished carbachol-induced secretion in normal as well as in PMA-down-regulated cells. This indicates that a PKC isoform resistant to down regulation by PMA is involved in carbachol- but not in cAMP-mediated secretion.

Effect of adenosine and adenosine agonists on amylase release from rat pancreatic lobules

The effects of adenosine on the amylase secretion from rat pancreatic lobules have been studied. Adenosine induces a dose-dependent stimulation on amylase release, which is maximal at a concentration of 10(-4) M. It has been observed a clear inhibition of this secretory action when atropine was added whereas no amylase release was seen in isolated acini after adenosine. The effect of adenosine is completely blocked by the adenosine receptors antagonist theophylline (10(-4) M), but not by dipyridamole (10(-3) M), a drug that inhibits the transport of adenosine into the cell. The increase of amylase secretion induced by adenosine is inhibited by either the relatively selective A1 receptor antagonist PD116,948 (10(-6) M) and by the A2 receptor antagonist PD115,199 (10(-6) M). Significant increases of amylase release are observed after the relatively selective A1 receptor agonist R-PIA (10(-5) M) and after the relatively selective A2 receptor agonist NECA (10(-4) M). Finally, the effect of R-PIA is not modified by coincubation with PD115,199 and the effect of NECA is not affected by coincubation with PD116,948. These results suggest that the action of adenosine is mediated through the release of acetylcholine and probably by the simultaneous occupation of both A1 and A2 adenosine receptors, whereas the intracellular action of adenosine could be discarded.

Convergent and parallel activation of low-conductance potassium channels by calcium and cAMP-dependent protein kinase

K+ channels, which have been linked to regulation of electrogenic solute transport as well as Ca2+ influx, represent a locus in hepatocytes for the concerted actions of hormones that employ Ca2+ and cAMP as intracellular messengers. Despite considerable study, the single-channel basis for synergistic effects of Ca2+ and cAMP on hepatocellular K+ conductance is not well understood. To address this question, patch-clamp recording techniques were applied to a model liver cell line, HTC hepatoma cells. Increasing the cytosolic Ca2+ concentration ([Ca2+]i) in HTC cells, either by activation of purinergic receptors with ATP or by inhibition of intracellular Ca2+ sequestration with thapsigargin, activated low-conductance (9-pS) K+ channels. Studies with excised membrane patches suggested that these channels were directly activated by Ca2+. Exposure of HTC cells to a permeant cAMP analog, 8-(4-chlorophenylthio)-cAMP, also activated 9-pS K+ channels but did not change [Ca2+]i. In excised membrane patches, cAMP-dependent protein kinase (the downstream effector of cAMP) activated K+ channels with conductance and selectivity identical to those of channels activated by Ca2+. In addition, cAMP-dependent protein kinase activated a distinct K+ channel type (5 pS). These data represent the differential regulation of low-conductance K+ channels by signaling pathways mediated by Ca2+ and cAMP. Moreover, since low-conductance Ca(2+)-activated K+ channels have been identified in a variety of cell types, these findings suggest that differential regulation of K+ channels by hormones with distinct signaling pathways may provide a mechanism for hormonal control of solute transport and Ca(2+)-dependent cellular functions in the liver as well as other nonexcitable tissues.

The role of phosphotyrosine signaling pathway in a parotid gland proliferation and function

Tyrosine phosphorylation and the intracellular signaling processes associated with it have been the focus of intense study due to its importance in the regulation of biological processes as diverse as cell proliferation and cell differentiation. While much of what we now understand has been derived from the study of cell lines and tumor cells, the salivary glands provide a model to examine the effects of tyrosine kinases and tyrosine phosphatases in a normal differentiated tissue. This review will focus, therefore, on the role tyrosine kinases and phosphatases play in inducing the transition from stasis to active proliferation and their potential role in mediating secretory function of the salivary glands.

Translocation of the alpha-isozyme of protein kinase C during stimulation of rat parotid acinar cells by phorbol ester and carbachol

Protein kinase C activity was detected in the cytosolic fraction of quiescent parotid acinar cells; the particulate fraction contained a much smaller proportion of the enzyme. Protein kinase C activity was increased in the membrane fraction and decreased in the cytosol after exposure of intact cells to phorbol 12-myristate 13-acetate (PMA) or the muscarinic-receptor agonist carbachol. The effect of PMA was potentiated by a subthreshold concentration of ionomycin. Immunoblot analysis with anti-protein kinase C antibodies revealed that the protein kinase C-alpha isoform is expressed in rat parotid cells. Other Ca(2+)-dependent isoforms were not detected. Further, agonist stimulation caused the redistribution of protein kinase C-alpha from cytosol to a membrane fraction. Agonists may promote parotid acinar cell activity, including amylase secretion, by increasing the affinity of protein kinase C-alpha for the membrane fraction, presumably via a rise in Ca2+ and diacylglycerol derived from polyphosphoinositide hydrolysis.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Secretion of alpha-amylase in human parotid gland epithelial cell culture

The secretions of the salivary gland system are essential for the maintenance of oral health. The nature of cell-specific secretions of the various glands and their regulation is not completely understood. The objective of this study was to establish epithelial cell cultures from the human parotid gland that exhibit the tissue-specific function of alpha-amylase secretion. A specimen of normal human parotid gland was obtained at surgery and used to obtain primary cultures by the explant/outgrowth procedure. The cultures were maintained in keratinocyte basal medium, supplemented with insulin (5 micrograms/ml), EGF (10 ng/ml), hydrocortisone (0.5 micrograms/ml), bovine pituitary extract (25 micrograms/ml), and antibiotics. The cultures were passaged using 0.125% trypsin to dissociate the cells. Phase contrast and ultrastructural observations showed that the cells were polygonal and exhibited desmosomes. Their cytoplasm contained tonofilament bundles and abundant rough endoplasmic reticulum and Golgi complexes. Immunofluorescence studies showed that all cells were positive for cytokeratins. Immunoblot analysis revealed keratins with molecular weights of 58, 56, 52, 50, 48, 46, and 40 KD, which are characteristic of secretory epithelia. The cells have been passaged 35 times so far, undergoing a cumulative 120-140 population doublings. The serially passaged epithelial cell cultures produced and secreted alpha-amylase, a major component of parotid gland acinar cell secretion. The beta-adrenergic agonist, isoproterenol (ISP), stimulated alpha-amylase secretion, which was accompanied by increased intracellular concentrations of cAMP. ISP-induced stimulation of amylase and cAMP was blocked by the beta-adrenergic antagonist, propranolol. Further, dibutyryl cAMP also enhanced the secretion of amylase. Thus we have established a long-term epithelial cell culture model of human parotid gland epithelial cells that exhibits differentiated function and retains the intact beta-adrenergic receptor system.

Effect of adenosine and adenosine analogues on cyclic AMP accumulation in cultured mesangial cells and isolated glomeruli of the rat

1. Changes in intracellular levels of adenosine 3':5'-cyclic monophosphate (cyclic AMP) were studied in rat isolated glomeruli and cultured glomerular mesangial cells exposed to adenosine and to the preferential A1 receptor agonist N6-R-1-methyl-2-phenylethyl adenosine (R-PIA), or the potent A2 adenosine receptor agonist 5-(N-ethylcarboxamide)adenosine (NECA). 2. Whereas NECA and adenosine triggered a dose-dependent increase in cyclic AMP values with EC50 values of approximately 10(-6) M and 3 x 10(-5) M respectively, R-PIA lowered cyclic AMP levels at concentrations of 10(-6) M or less and increased them at higher concentrations. 3. The time-course of the increase induced by 10(-6) M NECA was slower than that induced by 10(-4) M adenosine. Adenosine produced a maximal stimulation within the first minute, whereas the effect of NECA in both glomeruli and mesangial cells was noticeable only from the second minute of incubation. 4. The effects of the agonists R-PIA and NECA on the cyclic AMP system were blocked respectively by the A1 adenosine receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthihe (DPCPX) at 10(-6) M and the A2 antagonist N-(2-dimethylaminoethyl)-N-methyl-4-(2, 3, 6, 7-tetrahydro-2,b-dioxo-1, 3-dipropyl-1H-purin-8-yl) benzene sulphonamide (PD115,199) at 10(-6) M. Theophylline, a known antagonist of adenosine receptors, inhibited the action of adenosine on cyclic AMP in mesangial cells. Dipyridamole, an inhibitor of the uptake of adenosine by the cells, enhanced the response to adenosine.5. These results suggest the existence of Al and A2 adenosine receptors with opposite actions on intracellular levels of cyclic AMP in both glomeruli and mesangial cells. Adenosine seems to increase cyclic AMP through the activation of a surface adenosine receptor with pharmacological properties distinct from those exhibited by the A2 adenosine receptor.

Interaction between the calcium and cyclic AMP messenger systems in perifused rat parotid acinar cells

Potentiation of amylase secretion induced by a combination of isoproterenol and carbamylcholine was examined in perifused rat parotid acinar cells. The time course of changes in the augmented amylase secretion induced by isoproterenol plus carbamylcholine was similar to that induced by carbamylcholine alone, but not to that caused by isoproterenol. Concentration-response analysis showed that isoproterenol increased the apparent affinity for carbamylcholine to stimulate amylase secretion with the maximum effect attained by isoproterenol plus carbamylcholine being higher than that attained by isoproterenol or carbamylcholine. 8-Bromo cyclic AMP, forskolin and 3-isobutyl-1-methylxanthine mimicked the effect of isoproterenol. Calcium ionophores (A23187 and ionomycin), but not phorbol 12,13-dibutyrate, mimicked the effect of carbamylcholine. Chelation of intracellular free calcium with 1,2-bis-[2-aminophenoxyl]-ethane-N,N,N',N'-tetraacetic acid, but not that of extracellular calcium with [ethylenebis(oxyethylenenitrile)]-tetraacetic acid (EGTA), abolished the potentiation. Calmodulin antagonists inhibited amylase secretion induced by isoproterenol plus carbamylcholine or carbamylcholine alone, but not that induced by isoproterenol alone. These results suggest that the potentiation is mainly, if not completely, caused by a coordinated interaction between the cyclic AMP system and the Ca2+ system at a step distal to second messenger generation, probably via a cyclic AMP-induced increase in the sensitivity of the Ca2+ response element to calcium.

Microplate reader-based kinetic determination of alpha-amylase activity: application to quantitation of secretion from rat parotid acini

A coupled enzyme assay for measuring alpha-amylase activity was adapted for analysis with a microplate reader. Activity was quantified by monitoring the cleavage of p-nitrophenol from a chemically defined substrate at 405 nm. Features of this assay method include: low sample volume (10 microliters); economical use of reagent (200 microliters); increased precision due to kinetic nature of assay; linearity with amylase content to 2600 U/liter; capability of processing up to 96 samples within 10 min; facilitated data analysis using readily available software. The ability to rapidly measure amylase content of a large number of samples permitted sophisticated analysis of alpha-amylase secretion patterns from dispersed rat parotid acinar cells. Examination of the dose-dependent increase in the rate of amylase secretion stimulated by carbachol revealed a biphasic response at higher concentrations, where a rapid and transient increase in amylase release was consistently observed during the first 10 min. This initial phase of amylase release was additive with the slower, sustained secretion which was stimulated by carbachol in a dose-dependent manner (Kd = 2.5 microM). Such a biphasic release pattern was not seen with other potent secretagogues (isoproterenol, dibutyryl cAMP) that are not thought to act via a Ca-dependent mechanism. These results suggest that parotid secretion patterns should be studied at a number of time points, which is feasible using the method reported herein.