Beta-adrenergic stimulation and cAMP mobilize Ca2+ from an IP3-insensitive pool in rat submandibular granular ducts

Structural and functional analysis of salivary intercalated duct cells reveals a secretory phenotype

Currently, all salivary ducts (intercalated, striated and collecting) are assumed to function broadly in a similar manner, reclaiming ions that were secreted by the secretory acinar cells while preserving fluid volume and delivering saliva to the oral cavity. Nevertheless, there has been minimal investigation into the structural and functional differences between distinct types of salivary duct cells. Therefore, in this study, the expression profile of proteins involved in stimulus-secretion coupling, as well as the function of the intercalated duct (ID) and striated duct cells, was examined. Particular focus was placed on defining differences between distinct duct cell populations. To accomplish this, immunohistochemistry and in situ hybridization were utilized to examine the localization and expression of proteins involved in reabsorption and secretion of ions and fluid. Further, in vivo calcium imaging was employed to investigate cellular function. Based on the protein expression profile and functional data, marked differences between the IDs and striated ducts were observed. Specifically, the ID cells express proteins native to the secretory acinar cells while lacking proteins specifically expressed in the striated ducts. Further, the ID and striated duct cells display different calcium signalling characteristics, with the IDs responding to a neural stimulus in a manner similar to the acinar cells. Overall, our data suggest that the IDs have a distinct role in the secretory process, separate from the reabsorptive striated ducts. Instead, based on our evidence, the IDs express proteins found in secretory cells, generate calcium signals in a manner similar to acinar cells, and, therefore, are likely secretory cells. KEY POINTS: Current studies examining salivary intercalated duct cells are limited, with minimal documentation of the ion transport machinery and the overall role of the cells in fluid generation. Salivary intercalated duct cells are presumed to function in the same manner as other duct cells, reclaiming ions, maintaining fluid volume and delivering the final saliva to the oral cavity. Here we systematically examine the structure and function of the salivary intercalated duct cells using immunohistochemistry, in situ hybridization and by monitoring in vivo Ca2+ dynamics. Structural data revealed that the intercalated duct cells lack proteins vital for reabsorption and express proteins necessary for secretion. Ca2+ dynamics in the intercalated duct cells were consistent with those observed in secretory cells and resulted from GPCR-mediated IP3 production.

Co-localization of endogenous Arf6 and its activator EFA6D in the granular convoluted tubule cells of mouse submandibular glands under normal conditions and when stimulated by isoproterenol, noradrenaline and carbachol

OBJECTIVE

This study proposed to investigate the localization at light and electron microscopic levels of Arf6 and its activator EFA6D in the mouse submandibular gland (SMG) under normal conditions and when stimulated by adrenergic or cholinergic agonists.

MATERIALS AND METHODS

SMGs of male adult mice were utilized for immunoblotting and immuno-light and -electron microscopic analyses. Isoproterenol and noradrenalin were used as adrenergics, while carbachol was used for the cholinergic stimulant. SMGs were examined at 15, 30, 60 and 120min after intraperitoneal injection of these agents.

RESULTS

Immunoreactivities for both Arf6 and its activator EFA6D were similarly intense in the basolateral domain of GCTs, but no significant immunoreactivities were seen in the apical domain of GCT cells or any domain of acinar cells under normal conditions. In immuno-electron microscopy, the immunoreactive materials were mainly deposited on the basolateral plasma membranes and subjacent cytoplasm. Shortly after injection of isoproterenol and noradrenaline, but not carbachol, the immunoreactivities for both molecules were additionally seen on the apical plasmalemma of most, if not all, GCT cells, but not acinar cells.

CONCLUSION

The present findings suggest that the direct involvement of Arf6/EFA6D in regulatory exocytosis at the apical plasma membrane of acinar and GCT cells is apparently to be smaller, if present, than that of endocytosis at the basolateral membranes of GCT cells under normal conditions. This also suggests that the two molecules function additionally at the apical membrane of GCT cells for modulation of saliva secretion under β-adrenoceptor stimulation.

Immunohistochemical localization of cannabinoid receptor 1 (CB1) in the submandibular gland of mice under normal conditions and when stimulated by isoproterenol or carbachol

OBJECTIVE

We wished to investigate the subcellular localization of CB1, a receptor for the endocannabinoids in mouse submandibular glands (SMGs) under normal conditions and when stimulated by adrenergic or cholinergic agonists.

MATERIALS AND METHODS

SMGs of both male and female adult mice were utilized for immunoblotting and immuno-light and -electron microscopic analyses. Isoproterenol and carbachol were used as adrenergic and cholinergic stimulants, respectively. SMGs were examined at 15, 30, 60 and 120min after intraperitoneal injection of these agents.

RESULTS

Selective localization of intense immunoreactivity for CB1 in the granular convoluted ductal cells was confirmed by immunoblotting and the antigen absorption test. In SMGs of control male mice, CB1-immunoreactivity was evident on the basolateral plasma membranes, including the basal infoldings, but was absent on the apical membranes in the ductal cells. Localization and intensity of CB1-immunoreactivity were essentially the same in SMGs of female mice. The immunoreactivity was transiently localized in the apical plasmalemma of some acinar and granular ductal cells of male SMGs shortly after stimulation by isoproterenol, but not by carbachol.

CONCLUSION

The present finding suggests that CB1 functions primarily in the basolateral membranes of the granular convoluted ductal cells of SMGs under normal conditions, and that the CB1 can function additionally in the apical membrane of acinar and granular ductal cells for modulation of the saliva secretory condition via adrenoceptors.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Analysis of calcium concentration fluctuations in hepatopancreatic R cells of Marsupenaeus japonicus during the molting cycle

In this study we examined the fluctuations of the intracellular calcium concentration in isolated hepatopancreatic R cells during the four molting stages of the prawn Marsupenaeus japonicus. In addition, we used the Fura-2-AM fluorescence technique to investigate the release of calcium from mitochondria and ATP-sensitive calcium stores (endoplasmic reticulum (ER), Golgi, and nucleus) into cytoplasm during the molting cycle. Results demonstrate that both the cytosolic free calcium concentration and the total cell calcium (free, bound to calcium-binding proteins, and stored in amorphous form) in the R cells strictly depend upon the molting cycle. Interestingly, the total cell calcium was higher (approximately 10 mmol l(-1)) in postmolt than in premolt (approximately 1 mmol l(-1)) and intermolt (approximately 0.3 mmol l(-1)). The calcium released from mitochondria was higher during premolt than during postmolt and intermolt, but the amount of calcium released from ATP-sensitive calcium stores was similar during all four stages. All together, our results suggest that the mitochondria-ATP-sensitive calcium stores system does not play a key role in calcium storage during the molting cycle but that it is involved in transcellular calcium flux. We hypothesize that lysosome or membrane-clad concretion vacuoles could represent the main site of calcium storage in hepatopancreatic R cells.

Comparison of agonist-induced Ca2+ responses in rat submandibular acini and ducts

Changes in intracellular Ca(2+) concentration ([Ca(2+)]i) induced by agonists were simultaneously monitored in rat submandibular acini and ducts using a Ca(2+) imaging system. Substance P (SP) elicited marked increases in [Ca(2+)]i in acini but not in ducts. Carbachol (CCh) increased [Ca(2+)]i in both acini and ducts, but the maximal level was higher in acini than in ducts. In contrast, epinephrine (Epi) also induced an increase in [Ca(2+)]i in acini and ducts, but to a greater extent in ducts than in acini. Isoproterenol (ISO) caused a small but significant increase in [Ca(2+)]i in ducts but not acini. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis using total RNA extracted from highly purified acinar and ductal cells showed that substance P receptor mRNA was present in acini at higher levels than in ducts. In contrast, alpha(1a)-adrenoceptor mRNA was more strongly expressed in ducts than in acini. The muscarinic receptors (M(3) and M(5)) and beta-adrenoceptors (beta(1) and beta(2)) were expressed at equivalent levels in both cell types. These results confirm that acini and ducts exhibit significant differences in agonist-induced Ca(2+) responses. Furthermore, substance P- and epinephrine-induced Ca(2+) responses were consistent with receptor mRNA expression in acini and ducts, but carbachol- and isoproterenol-induced [Ca(2+)]i increases were not.

Compound exocytosis of secretory granules containing salivary chromogranin A in granular duct cells in rat submandibular gland: the last study in collaboration with the late Professor Noboru Yanaihara at Yanaihara Institute

Chromogranin A (CgA) is a member of a family of chromogranins, which are co-stored and co-released with adrenaline and noradrenalin (NAd) in the adrenal medulla in response to stimulation of the splanchnic nerve. Double immunohistochemical staining is carried out by use of antibodies against CgA and 1,4,5-trisphosphate receptor type 2 (IP3R2) on the same sections prepared from the isolated and perfused submandibulllar gland of rat. In the control sections prepared from resting state, an intense IP3R2 immunoreactivity (IR) appeared preferentially at the apical pole of subpopulation of the granulated duct cells, in which CgA-like IR distributed throughout the cytoplasm. Electron-micrograph showed that the granular cells in the resting state stored numerous membrane-bound granules in the apical cytoplasm. Stimulation with 1 microM NAd caused rapid immediate increase in secretory responses. Sections prepared from the gland at the peak of secretory responses exhibited that, in the granular duct cells, the apically converge IP3R2 IR became diffuse and indistinguishable, and that the apical half of the cells was occupied, indicating that mobilization of Ca2+ from the IP3-sensitive pool may preferentially be involved in the secretory responses to alpha-adrenergic agonist.

Microarray analysis and description of SMR1 gene in rat penis in a post-radical prostatectomy model of erectile dysfunction

PURPOSE

We focused on the post-radical prostatectomy model to advance the understanding of neurogenic erectile dysfunction. We attempted to identify previously undescribed molecular changes via gene discovery methods using GeneChip (Affymetrix, Santa Clara, California) microarray technology.

MATERIALS AND METHODS

Five male adult 120-day-old Sprague-Dawley rats underwent bilateral cavernous nerve neurectomy. Five age matched controls were prepared simultaneously. The penises were harvested on postoperative day 2 and snap frozen in liquid nitrogen. RNA was prepared and pooled into cut and uncut groups. Synthesis of cRNA was performed according to the GeneChip technical manual. Microarray analysis was performed on a U34A Rat Array (Affymetrix). This array has approximately 8,800 gene probe sets, approximately 6,600 known genes and approximately 2,200 estimated sequence transcripts.

RESULTS

Dramatic results were found during GeneChip microarray expression analysis. A total of 126 candidate genes were noted to be altered based on the magnitude of expression change using rigorous statistical criteria, including 47 that were down-regulated and 79 that were up-regulated. Among the many significant changes seen 1 dominant class of genes was the submandibular rat genes. Submandibular rat 1 (SMR1) was down-regulated 82.5 fold. Other genes in this family were down-regulated 226 and 90 times. This result was confirmed by reverse transcriptase-polymerase chain reaction and Western blot analyses. These assays verified decreases in SMR1 at multiple time points after surgery.

CONCLUSIONS

Impressive and previously unrecognized genetic changes are being intensely investigated as they are being unmasked by GeneChip technology. We have identified and begun the investigation of 1 interesting family of genes, namely submandibular gland proteins. The role of SMR as a clinically relevant change in penile and/or urethral function following cavernous nerve injury is speculative.

Regulation of phospholipase D by muscarinic receptors in rat submandibular ductal cells

The muscarinic agonist carbachol stimulated phospholipase D (PLD) in rat submandibular gland (RSMG) ductal cells in a time and concentration-dependent manner. This effect was inhibited by chelation of extracellular calcium with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). PLD could also be activated by epinephrine and AlF(4)(-), two polyphosphoinositide-specific phospholipase C (PPI-PLC) activators, and by the phorbol ester o-tetradecanoylphorbol 13-acetate (TPA) which activates protein kinase C (PKC). Ionomycin and thapsigargin only slightly increased PLD activity. Ortho-vanadate, a tyrosine phosphatase inhibitor, also stimulated PLD activity. Both carbachol and o-vanadate increased the formation of inositol phosphates and the tyrosine phosphorylation of at least two proteins (55-60 and 120 kDa). Calphostin C (a PKC inhibitor), U73122 (a PPI-PLC inhibitor) and genistein (a tyrosine kinase inhibitor) blocked the activation of PLD, of PLC and the phosphorylation of tyrosyl residues in response to carbachol and vanadate. Taken together, these results suggest that rat submandibular gland ductal cells express a calcium-dependent PLD activity. This enzyme is regulated by carbachol via a PLC-PKC-tyrosine kinase pathway.

Mechanisms underlying the structural alterations of the rat submandibular gland induced by Tityus serrulatus tityustoxin

As the mechanisms underlying the structural changes induced in rat submandibular glands by Tityus serrulatus tityustoxin have not been reported, the present study was undertaken to investigate the participation of adrenergic and muscarinic cholinergic receptors in these alterations. Most of the stimulatory effects of the toxin are observed in the secretory cells of the acini and granular convoluted tubules (GCT). We evaluated the ability of the toxin to induce morphological changes in acinar and GCT cells after adrenoreceptor and cholino receptor blockage. The influence of tityustoxin-induced adrenal discharge on the acinar and GCT cells was also investigated after bilateral adrenalectomy. We show that the intense cytoplasmic vacuolation of the acinar cells induced by tityustoxin was prevented by prazosin (alpha(1) adenoreceptor blockade) and atropine (muscarinic cholinoreceptor blockade). The decrease of GCT cell granules following tityustoxin injection was completely blocked by prasozin and partially by propranolol. These results indicate that acinar vacuolation, degranulation of GCT cells, reduction of GCT diameter and height of its epithelium depends on tityustoxin induced adrenergic and cholinergic mechanisms. In contrast, tityustoxin induced acinar cell degranulation was not modified by atropine, prasozin or propranolol (beta(1)-beta(2) adenoreceptor blockade). Thus, acinar degranulation seems to be due to a direct action of tityustoxin on of the rat submandibular glands. The degranulation of the GCT cells and the acinar vacuolation was also prevented by bilateral adrenalectomy, suggesting that these effects are mostly due to catecholamines released from the adrenal glands.

Expression of phosphodiesterase 3 in rat submandibular gland

The expression of phosphodiesterase (PDE) 3 isoforms was investigated in extracts of rat submandibular gland by reverse transcription-polymerase chain reaction (RT-PCR) and the PCR fragments were then sequenced. PDE3 activity was detected in gland homogenates; about 90% of the activity was in the supernatant fraction and about 10% in the particulate fraction. PDE3A and 3B mRNA was detected by RT-PCR in RNA from the gland. The nucleotide sequences of the fragments were identical to those of rat PDE3A and 3B. The results indicate that two PDE3 isoforms are present in rat submandibular gland and may regulate an important cAMP pool.

Characterization of the ca2 + response mediated by activation of beta-adrenoceptors in rat submandibular ducts

The Ca2+ signaling mediated by activation of beta-adrenoceptors was studied in a purified preparation of ducts from rat submandibular glands. At concentrations above 1 nM, isoproterenol (ISO) caused a small but significant increase in cytosolic Ca2+ concentration ([Ca2+]i). The ISO-induced increase in [Ca2+]i was completely inhibited by the beta-adrenoceptor antagonist propranolol but not by the alpha-adrenoceptor antagonist phentolamine. Forskolin was able to mimic the Ca2+ response to ISO. These results suggest that the ISO-induced increase in [Ca2+]i in rat submandibular ducts is mediated by an accumulation of cAMP resulting from activation of beta-adrenoceptors. In the absence of extracellular Ca2+, ISO or forskolin caused a transient increase in [Ca2+]i, indicating Ca2+ mobilization from intracellular Ca2+ stores. Further, stimulation with ISO failed to mobilize Ca2+ after the depletion of intracellular Ca2+ stores by phenylephrine or carbachol, suggesting that the cAMP-mediated increase in [Ca2+]i is due to a Ca2+ release from inositol trisphosphate (IP3)-sensitive Ca2+ stores. As ISO did not stimulate a detectable production of IP3, the cAMP-mediated Ca2+ mobilization may be evoked by a mechanism different from activation of phosphoinositide hydrolysis.

Pertussis toxin inhibits cholecystokinin- and epidermal growth factor-induced mitogen-activated protein kinase activation by disinhibition of the cAMP signaling pathway and inhibition of c-Raf-1

Pertussis toxin (PTx), which inactivates G(i/o) type G proteins, is widely used to investigate the involvement of G(i/o) proteins in signal transduction. Activation of extracellular-regulated kinases 1 and 2 (ERK1/2) by G protein-coupled receptors has been described to occur either through a PTx-insensitive pathway involving activation of phospholipase C and protein kinase C (PKC), or through a PTx-sensitive pathway involving G(i)betagamma-mediated activation of Src. Cholecystokinin (CCK) activates ERK1/2 by a PKC-dependent, and thus presumably PTx-insensitive, pathway. However, CCK has recently been shown to induce activation of G(i) proteins in addition to G(q/11). In the present study, PTx partially inhibited CCK-induced ERK1/2 activation in pancreatic AR42J cells, although activation of phospholipase C was not reduced. PTx also inhibited ERK1/2 activation in response to the PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF) as well as activation of c-Raf-1 by EGF and CCK. In contrast, PTx, CCK, and EGF had only minor effects on A-Raf and B-Raf activity. Forskolin, a direct activator of adenylyl cyclase, inhibited CCK- and EGF-induced activation of c-Raf-1 and ERK1/2 in a manner similar to that of PTx. In PTx-treated cells, the cAMP content was increased and forskolin did not further inhibit CCK- and EGF-induced activation of c-Raf-1 or ERK1/2. In conclusion, the present study shows that PTx-sensitivity of receptor-induced ERK1/2 activation could be a consequence of disinhibition of the adenylyl cyclase signaling pathway, which in turn causes inhibition of c-Raf-1 activation rather than indicating involvement of a PTx-sensitive G protein in this signaling pathway.

The type 8 adenylyl cyclase is critical for Ca2+ stimulation of cAMP accumulation in mouse parotid acini

Capacitative Ca(2+) entry stimulates cAMP synthesis in mouse parotid acini, suggesting that one of the Ca(2+)-sensitive adenylyl cyclases (AC1 or AC8) may play an important role in the regulation of parotid function (Watson, E. L., Wu, Z., Jacobson, K. L., Storm, D. R., Singh, J. C., and Ott, S. M. (1998) Am. J. Physiol. 274, C557-C565). To evaluate the role of AC1 and AC8 in Ca(2+) stimulation of cAMP synthesis in parotid cells, acini were isolated from AC1 mutant (AC1-KO) and AC8 mutant (AC8-KO) mice and analyzed for Ca(2+) stimulation of intracellular cAMP levels. Although Ca(2+) stimulation of intracellular cAMP levels in acini from AC1-KO mice was indistinguishable from wild type mice, acini from AC8-KO mice showed no Ca(2+)-stimulated cAMP accumulation. This indicates that AC8, but not AC1, plays a major role in coupling Ca(2+) signals to cAMP synthesis in parotid acini. Interestingly, treatment of acini from AC8-KO mice with agents, i.e. carbachol and thapsigargin that increase intracellular Ca(2+), lowered cAMP levels. This decrease was dependent upon Ca(2+) influx and independent of phosphodiesterase activation. Immunoblot analysis revealed that AC5/6 and AC3 are expressed in parotid glands. Inhibition of calmodulin (CaM) kinase II with KN-62, or inclusion of the CaM inhibitor, calmidazolium, did not prevent agonist-induced inhibition of stimulated cAMP accumulation. In vitro studies revealed that Ca(2+), independently of CaM, inhibited isoproterenol-stimulated AC. Data suggest that agonist augmentation of stimulated cAMP levels is due to activation of AC8 in mouse parotid acini, and strongly support a role for AC5/6 in the inhibition of stimulated cAMP levels.

Chloride channels and salivary gland function

Fluid and electrolyte transport is driven by transepithelial Cl- movement. The opening of Cl- channels in the apical membrane of salivary gland acinar cells initiates the fluid secretion process, whereas the activation of Cl- channels in both the apical and the basolateral membranes of ductal cells is thought to be necessary for NaCl re-absorption. Saliva formation can be evoked by sympathetic and parasympathetic stimulation. The composition and flow rate vary greatly, depending on the type of stimulation. As many as five classes of Cl- channels with distinct gating mechanisms have been identified in salivary cells. One of these Cl- channels is activated by intracellular Ca2+, while another is gated by cAMP. An increase in the intracellular free Ca2+ concentration is the dominant mechanism triggering fluid secretion from acinar cells, while cAMP may be required for efficient NaCl re-absorption in many ductal cells. In addition to cAMP- and Ca(2+)-gated Cl- channels, agonist-induced changes in membrane potential and cell volume activate different Cl- channels that likely play a role in modulating fluid and electrolyte movement. In this review, the properties of the different types of Cl- currents expressed in salivary gland cells are described, and functions are proposed based on the unique properties of these channels.

Rodent submandibular gland peptide hormones and other biologically active peptides

The cervical sympathetic trunk-submandibular gland neuroendocrine axis plays an integral role in physiological adaptations and contributes to the maintenance of systemic homeostasis, particularly under the 'stress conditions' seen with tissue damage, inflammation, and aggressive behavior. The variety of polypeptides, whose release from acinar and ductal cells is under sympathetic nervous system control, offers coordinated and progressive levels of endocrine communication. Proteolytic enzymes (e.g. the kallikreins and furin maturases) are involved in the conversion of inactive precursors (e. g. Pro-EGF and SMR1) into biologically active molecules (e.g. EGF, SMR1-pentapeptide), which act on local or distant targets and thereby modulate the homeostatic process.

Alternate coupling of receptors to Gs and Gi in pancreatic and submandibular gland cells

Many Gs-coupled receptors can activate both cAMP and Ca2+ signaling pathways. Three mechanisms for dual activation have been proposed. One is receptor coupling to both Gs and G15 (a Gq class heterotrimeric G protein) to initiate independent signaling cascades that elevate intracellular levels of cAMP and Ca+2, respectively. The other two mechanisms involve cAMP-dependent protein kinase-mediated activation of phospholipase Cbeta either directly or by switching receptor coupling from Gs to Gi. These mechanisms were primarily inferred from studies with transfected cell lines. In native cells we found that two Gs-coupled receptors (the vasoactive intestinal peptide and beta-adrenergic receptors) in pancreatic acinar and submandibular gland duct cells, respectively, evoke a Ca2+ signal by a mechanism involving both Gs and Gi. This inference was based on the inhibitory action of antibodies specific for Galphas, Galphai, and phosphatidylinositol 4,5-bisphosphate, pertussis toxin, RGS4, a fragment of beta-adrenergic receptor kinase and inhibitors of cAMP-dependent protein kinase. By contrast, Ca2+ signaling evoked by Gs-coupled receptor agonists was not blocked by Gq class-specific antibodies and was unaffected in Galpha15 -/- knockout mice. We conclude that sequential activation of Gs and Gi, mediated by cAMP-dependent protein kinase, may represent a general mechanism in native cells for dual stimulation of signaling pathways by Gs-coupled receptors.

Purines, a new class of agonists in salivary glands?

The response of rat submandibular glands to extracellular purines was tested. In crude cellular suspensions, ATP increased the [Ca2+]i mostly by promoting uptake of extracellular calcium. ATP caused the pHi to drop, a response blocked by chloride channel inhibitors. ATP also inhibited the basal and isoproterenol-stimulated activity of the Na+ -K+ -2Cl-cotransporter. These effects were reproduced by benzoyl-ATP, an agonist of ionotropic purinoceptors. In pure ductal suspensions, ATP activated a metabotropic P2Y1 purinergic receptor coupled to phospholipase C and opened a non-specific cation channel coupled to a P2X7 receptor. Activation of these receptors stimulated a Ca2+ -dependent and a Ca2+ -independent phospholipase A2, the latter resulting in kallikrein secretion. We conclude that purinergic agonists can modulate the activity of both acinar and ductal phases of secretion. Activation of metabotropic receptors coupled to phospholipase C could lead to responses resembling those to muscarinic or adrenergic agonists. Activation of ionotropic receptors could stimulate new intracellular responses also involved in secretory function.

Effects of crude venom, tityustoxin and toxin Ts-gamma from Tityus serrulatus scorpion on secretion and structure of the rat submandibular gland

Changes in the rat submandibular glands after intravenous injections of Tityus serrulatus scorpion venom, tityustoxin or toxin Ts-gamma were studied histologically and morphometrically. The acini and the granular convoluted tubules presented the most prominent changes. The following variables were measured: (a) relative volume occupied by the glandular structures; (b) diameter of the granular convoluted tubules and thickness of their epithelium; (c) diameter of the acini. The cytoplasm of the acinar cells was extensively occupied by large confluent vacuoles and had a reduced number of secretory granules after intravenous injections of venom or toxins. The morphological changes caused by toxin Ts-gamma were greater than those evoked by tityustoxin or crude venom injections. In spite of the changes in acinar cells, acinar diameter showed no significant alterations after venom or toxin injections. Reduction of diameter and depletion of the cytoplasmic secretory granules were observed in the granular convoluted tubules 2 h after intravenous injections of crude venom, or after 1 h with tityustoxin or toxin Ts-gamma. The intravenous injection of crude venom did not induce any visible change in the granular convoluted tubules after 1 h. These structural changes could explain the concomitant intense sialagogue effect elicited by crude venom, tityustoxin and toxin Ts-gamma. The sialagogue effect induced by toxin Ts-gamma was larger than those induced by crude venom or tityustoXin.

Study on the activation of phospholipases A2 by purinergic agonists in rat submandibular ductal cells

Extracellular ATP and benzoyl-ATP (Bz-ATP) increased the release of [3H]arachidonic acid ([3H]AA) from prelabeled rat submandibular gland (RSMG) ductal cells respectively two- and threefold. Both agonists also increased the release of [3H]AA from acini but at a lower level (+50% and +100% respectively). Carbachol had no significant effect on either cellular population. In ductal cells phorbol myristate acetate, an activator of protein kinase C, slightly increased the basal release of [3H]AA but did not affect the release of [3H]AA in response to ATP. Staurosporine, an inhibitor of protein kinases, inhibited the response to the purines. The removal of calcium from the extracellular medium decreased the response to ATP and Bz-ATP. Only barium could partly substitute for calcium to restore the purinergic response. Zinc inhibited the release of [3H]AA. Permeabilization of the cells with streptolysin O (SLO) activated the calcium-independent phospholipase A2 activity (iPLA2). The iPLA2, not the calcium-dependent PLA2 (cPLA2), released [3H]oleic acid ([3H]OA) from RSMG ductal cells. It is concluded that RSMG ducts have a higher PLA2 activity when compared to acini. This activity is accounted for by iPLA2 and cPLA2. Both enzymes are activated by P2X agonists by a staurosporine-sensitive mechanism. Cells permeabilized with SLO or membranes from Escherichia coli as a substrate are not good models to study the regulation of these enzymes. In intact RSMG ductal cells the two activities can be distinguished by rather specific inhibitors, by different ionic conditions and also by the fatty acid used to label the cells.

PKA induces Ca2+ release and enhances ciliary beat frequency in a Ca2+-dependent and -independent manner

The intent of this work was to evaluate the role of cAMP in regulation of ciliary activity in frog mucociliary epithelium and to examine the possibility of cross talk between the cAMP- and Ca2+-dependent pathways in that regulation. Forskolin and dibutyryl cAMP induced strong transient intracellular Ca2+ concentration ([Ca2+]i) elevation and strong ciliary beat frequency enhancement with prolonged stabilization at an elevated plateau. The response was not affected by reduction of extracellular Ca2+ concentration. The elevation in [Ca2+]i was canceled by pretreatment with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, and a phospholipase C inhibitor, U-73122. Under those experimental conditions, forskolin raised the beat frequency to a moderately elevated plateau, whereas the initial strong rise in frequency was completely abolished. All effects were canceled by H-89, a selective protein kinase A (PKA) inhibitor. The results suggest a dual role for PKA in ciliary regulation. PKA releases Ca2+ from intracellular stores, strongly activating ciliary beating, and, concurrently, produces moderate prolonged enhancement of the beat frequency by a Ca2+-independent mechanism.

Apical vesicles bearing inositol 1,4,5-trisphosphate receptors in the Ca2+ initiation site of ductal epithelium of submandibular gland

In polarized epithelial cells, agonists trigger Ca2+ waves and oscillations. These patterns may be caused by the compartmentalization of inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pools into specific regions. We have investigated the relationship between the distribution of IP3 receptors (IP3Rs) and the spatiotemporal pattern of Ca2+ signaling in the duct cells of the rat submandibular gland (SMG). Using immunofluorescence, although labeling was somewhat heterogeneous, the IP3Rs were colocalized to the apical pole of the duct cells. Immunoelectron microscopy identified small apical vesicles bearing IP3R2 in some types of duct cells. Real-time confocal imaging of intact ducts demonstrated that, after carbachol stimulation, an initial Ca2+ spike occurred in the apical region. Subsequently, repetitive Ca2+ spikes spread from the apical to the middle cytoplasm. These apical Ca2+ initiation sites were found only in some "pioneer cells," rather than in all duct cells. We performed both Ca2+ imaging and immunofluorescence on the same ducts and detected the strongest immunosignals of IP3R2 in the Ca2+ initiation sites of the pioneer cells. The subcellular localization and expression level of IP3Rs correlated strongly with the spatiotemporal nature of the intracellular Ca2+ signal and distinct Ca2+ responses among the rat SMG duct cells.

Activation of the Na+-K+(NH4+)-2Cl(-)- cotransporter from rat submandibular glands in response to VIP

A cellular suspension from rat submandibular glands was prepared with collagenase. The intracellular pH (pHi) was estimated with 2',7'-bis-(2-carboxy-ethyl)-5(6)-carboxyfluorescein (BCECF). After exposure to NH4Cl, the pHi transiently increased (diffusion of NH3) and then dropped (influx of NH4+). Isoproterenol increased 2.5-fold the rate of NH4+ influx; bumetanide, an inhibitor of the Na+-K+-2Cl(-)-cotransporter blocked the response to isoproterenol, confirming that the beta-adrenergic agonist stimulated the cotransporter. Forskolin (1 micromol/L) mimicked the response to isoproterenol. VIP (1 nmol/L(-1) micromol/L) also increased the activity of the cotransporter. Cyclic AMP rather than calcium was the mediator of this activation since 1) carbachol which increased the [Ca2+]i fivefold increased the uptake of NH4+ by only 50%; 2) only high concentrations of VIP significantly increased the [Ca2+]i; 3) incubation in the presence of EGTA had no effect on the response to VIP; 4) low concentrations (nmol/L) of the neuropeptide increased the intracellular level of cAMP; and 5) the stimulation of the cotransporter by VIP, forskolin, and isoproterenol was inhibited by H8, an inhibitor of cAMP-dependent protein kinase. It is concluded that the Na+-K+-2Cl(-)-cotransporter of rat submandibular glands is activated by isoproterenol, forskolin, and neuropeptides of the VIP family by a mechanism involving cAMP-dependent processes. The activation of the cotransporter by VIP could partly explain the potentiating effect of VIP on the response to sialagogues like substance P or muscarinic agonists.

Characterization and localization of P2 receptors in rat submandibular gland acinar and duct cells

[Ca2+]i and the Cl- current were measured in isolated submandibular gland acinar and duct cells to characterize and localize the purinergic receptors expressed in these cells. In both cell types 2'-3'-benzoylbenzoyl (Bz)-ATP and ATP increased [Ca2+]i mainly by activation of Ca2+ influx. UTP had only minimal effect on [Ca2+]i at concentrations between 0.1 and 1 mM. However, a whole cell current recording showed that all nucleotides effectively activated Cl- currents. Inhibition of signal transduction through G proteins by guanyl-5'-beta-thiophosphate revealed that the effect of ATP on Cl- current was mediated in part by activation of a G protein-coupled and in part by a G protein-independent receptor. BzATP activated exclusively the G protein-independent portion, whereas UTP activated only the G protein-dependent portion of the Cl- current. Measurement of [Ca2+]i in the microperfused duct showed that ATP stimulated a [Ca2+]i increase when applied to the luminal or the basolateral sides. BzATP increased [Ca2+]i only when applied to the luminal side, whereas UTP at 100 microM increased -Ca2+-i only when applied to the basolateral side. The combined results suggest that duct and possibly acinar cells express P2z receptors in the luminal and P2u receptors in the basolateral membrane.

Beta-adrenergic-induced calcium efflux in rat parotid gland. Is mitochondrial Na+/Ca2+ exchange involved?

The effects of diltiazem were examined on 45Ca efflux in rat parotid glands. First, we showed that mitochondrial Na+/Ca2+ exchange occurs in rat parotid glands and that diltiazem inhibited the mitochondrial Na(+)-dependent calcium efflux. We also confirmed that in rat parotid gland, diltiazem did not modify calcium movements at plasma membrane level. Secondly, we tested the effects of diltiazem on pieces of parotid glands. Diltiazem alone induced 45Ca efflux from parotid lobules. When the effect of diltiazem was tested on isoproterenol-induced 45Ca efflux, the effects of the two drugs were less than additive. By comparison, diltiazem did not modify carbachol induced 45Ca efflux. Diltiazem was able to induce calcium efflux from an intracellular calcium pool, which is not the IP3 sensitive one. These data support the previous hypothesis that isoproterenol and carbachol do not mobilize the same calcium pool. Although we did not precisely determine the calcium pool sensitive to beta-adrenergic stimulation, we cannot rule out the hypothesis that mitochondria would be that store.

Elevation of intracellular cAMP by noradrenaline and vasoactive intestinal peptide in striated ducts isolated from the rabbit mandibular salivary gland

Salivary gland intralobular ducts are responsible for the modification of the electrolyte composition of the primary fluid secreted by the acini. However, the intracellular messengers that regulate this and other intralobular duct cell processes have not been fully characterized. To investigate the possibility that cAMP-mobilizing agonists may be involved, intralobular (striated) ducts were isolated from the rabbit mandibular salivary gland by tissue dissociation and microdissection and maintained in tissue culture overnight. Individual duct fragments were stimulated with the secretory agonists noradrenaline, vasoactive intestinal peptide (VIP) and substance P and their cAMP content measured by acetylated radioimmunoassay. Both noradrenaline and VIP elevated intracellular cAMP content concentration dependently, but substance P did not. The response to noradrenaline was blocked by the beta-adrenoceptor antagonist propranolol, but not by the alpha-adrenoceptor antagonist prazosin. Application of the VIP analogue [D-p-Cl-Phe6, Leu17]-VIP decreased the VIP-induced cAMP response. These results demonstrate that striated intralobular duct cells possess beta-adrenoceptors and peptidergic receptors that are coupled to adenylate cyclase and activated by noradrenaline and VIP, respectively. By elevating ductal cAMP content, these agonists may regulate both the electrolyte content of the primary saliva and the secretion of protein(s) from the ducts.

Beta-adrenergic receptor-mediated calcium mobilization in the human Jurkat T cell line

The regulation of Ca2+ mobilization by beta-adrenergic receptor in the human Jurkat T cell line was investigated. Jurkat cells had a single class of beta-adrenergic receptor binding sites. Isoproterenol (ISP) caused the increase in cytosolic free Ca2+ concentration ([Ca2+]i) in a dose-dependent manner. In the absence of extracellular Ca2+, the pretreatment with OKT3, an anti-CD3 antibody, did not affect a transient increase in [Ca2+]i induced by the subsequent addition of ISP, and vice versa. On the other hand, the pretreatment with thapsigargin abolished the response of [Ca2+]i to the subsequent additions of ISP and OKT3. In permeabilized Jurkat cells, the addition of cAMP released Ca2+ from the intracellular Ca2+ pool. Neither nimodipine nor H8, a protein kinase A inhibitor, affected the increase in [Ca2+]i induced by ISP. The results suggest that cAMP accumulated by the activation of beta-adrenergic receptor may directly release Ca2+ from the inositol trisphosphate-insensitive intracellular Ca2+ pool in Jurkat T cells.