Agonist-induced activation of Na+/H+ exchange in rat parotid acinar cells is dependent on calcium but not on protein kinase C

Ca2+-CaMKKβ pathway is required for adiponectin-induced secretion in rat submandibular gland

Adiponectin functions as a promoter of saliva secretion in rat submandibular gland via activation of adenosine monophosphate-activated protein kinase (AMPK) and increased paracellular permeability. Ca²⁺ mobilization is the primary signal for fluid secretion in salivary acinar cells. However, whether intracellular Ca²⁺ mobilization is involved in adiponectin-induced salivary secretion is unknown. Here, we found that full-length adiponectin (fAd) increased intracellular Ca²⁺ and saliva secretion in submandibular glands. Pre-perfusion with ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) combined with thapsigargin (TG), an endoplasmic reticulum Ca²⁺-ATPase inhibitor, abolished fAd-induced salivary secretion, AMPK phosphorylation, and enlarged tight junction (TJ) width. Furthermore, in cultured SMG-C6 cells, co-pretreatment with EGTA and TG suppressed fAd-decreased transepithelial electrical resistance and increased 4-kDa FITC-dextran flux responses. Moreover, fAd increased phosphorylation of calcium/calmodulin-dependent protein kinase (CaMKKβ), a major kinase that is activated by elevated levels of intracellular Ca²⁺, but not liver kinase B1 phosphorylation. Pre-perfusion of the isolated gland with STO-609, an inhibitor of CaMKKβ, abolished fAd-induced salivary secretion, AMPK activation, and enlarged TJ width. CaMKKβ shRNA suppressed, whereas CaMKKβ re-expression rescued fAd-increased paracellular permeability. Taken together, these results indicate that adiponectin induced Ca²⁺ modulation in rat submandibular gland acinar cells. Ca²⁺-CaMKKβ pathway is required for adiponectin-induced secretion through mediating AMPK activation and increase in paracellular permeability in rat submandibular glands.

Calcium signalling in salivary gland physiology and dysfunction

Studies over the past four decades have established that Ca(2+) is a critical factor in control of salivary gland function and have led to identification of the critical components of this process. The major ion transport mechanisms and ion channels that are involved in fluid secretion have also been established. The key event in activation of fluid secretion is an increase in [Ca(2+) ]i triggered by inositol 1,4,5-trisphosphate (IP3 )-induced release of Ca(2+) from ER via the IP3 receptor (IP3 R). IP3 Rs determine the site of initiation and the pattern of the [Ca(2+) ]i signal in the cell. However, Ca(2+) entry into the cell is required to sustain the elevation of [Ca(2+) ]i and fluid secretion and is mediated by the store-operated Ca(2+) entry (SOCE) mechanism. Orai1, TRPC1, TRPC3 and STIM1 have been identified as critical components of SOCE in these cells. Cells finely tune the generation and amplification of [Ca(2+) ]i signals for regulation of cell function. An important emerging area is the concept that unregulated [Ca(2+) ]i signals in cells can directly cause cell damage, dysfunction and disease. Alternatively, aberrant [Ca(2+) ]i signals can also amplify and increase the rates of cell damage. Such defects in Ca(2+) signalling have been described in salivary glands in conjunction with radiation-induced loss of salivary gland function as well as in the salivary defects associated with the autoimmune exocrinopathy Sjögren's syndrome. Such defects have been associated with altered function or expression of key Ca(2+) signalling components, such as STIM proteins and TRP channels. These studies offer new avenues for examining the mechanisms underlying the disease and development of novel clinical targets and therapeutic strategies.

Ca²⁺ signaling and regulation of fluid secretion in salivary gland acinar cells

Neurotransmitter stimulation of plasma membrane receptors stimulates salivary gland fluid secretion via a complex process that is determined by coordinated temporal and spatial regulation of several Ca(2+) signaling processes as well as ion flux systems. Studies over the past four decades have demonstrated that Ca(2+) is a critical factor in the control of salivary gland function. Importantly, critical components of this process have now been identified, including plasma membrane receptors, calcium channels, and regulatory proteins. The key event in activation of fluid secretion is an increase in intracellular [Ca(2+)] ([Ca(2+)]i) triggered by IP3-induced release of Ca(2+) from ER via the IP3R. This increase regulates the ion fluxes required to drive vectorial fluid secretion. IP3Rs determine the site of initiation and the pattern of [Ca(2+)]i signal in the cell. However, Ca(2+) entry into the cell is required to sustain the elevation of [Ca(2+)]i and fluid secretion. This Ca(2+) influx pathway, store-operated calcium influx pathway (SOCE), has been studied in great detail and the regulatory mechanisms as well as key molecular components have now been identified. Orai1, TRPC1, and STIM1 are critical components of SOCE and among these, Ca(2+) entry via TRPC1 is a major determinant of fluid secretion. The receptor-evoked Ca(2+) signal in salivary gland acinar cells is unique in that it starts at the apical pole and then rapidly increases across the cell. The basis for the polarized Ca(2+) signal can be ascribed to the polarized arrangement of the Ca(2+) channels, transporters, and signaling proteins. Distinct localization of these proteins in the cell suggests compartmentalization of Ca(2+) signals during regulation of fluid secretion. This chapter will discuss new concepts and findings regarding the polarization and control of Ca(2+) signals in the regulation of fluid secretion.

Ca²⁺ signaling and fluid secretion by secretory cells of the airway epithelium

Cytoplasmic Ca(2+) is a master regulator of airway physiology; it controls fluid, mucus, and antimicrobial peptide secretion, ciliary beating, and smooth muscle contraction. The focus of this review is on the role of cytoplasmic Ca(2+) in fluid secretion by airway exocrine secretory cells. Airway submucosal gland serous acinar cells are the primary fluid secreting cell type of the cartilaginous conducting airways, and this review summarizes the current state of knowledge of the molecular mechanisms of serous cell ion transport, with an emphasis on their regulation by intracellular Ca(2+). Many neurotransmitters that regulate secretion from serous acinar cells utilize Ca(2+) as a second messenger. Changes in intracellular Ca(2+) concentration regulate the activities of ion transporters and channels involved in transepithelial ion transport and fluid secretion, including Ca(2+)-activated K(+) channels and Cl(-) channels. We also review evidence of interactions of Ca(2+) signaling with other signaling pathways (cAMP, NO) that impinge upon different ion transport pathways, including the cAMP/PKA-activated cystic fibrosis (CF) transmembrane conductance regulator (CFTR) anion channel. A better understanding of Ca(2+) signaling and its targets in airway fluid secretion may identify novel strategies to intervene in airway diseases, for example to enhance fluid secretion in CF airways.

Cevimeline-induced monophasic salivation from the mouse submandibular gland: decreased Na+ content in saliva results from specific and early activation of Na+/H+ exchange

Cevimeline and pilocarpine are muscarinic agonists used clinically to treat dry mouth. In this study, we explored fluid secretion from mouse submandibular glands to determine the mechanism of cevimeline, pilocarpine, and an experimentally used agent carbachol. Cevimeline evoked almost the same amount of secretion at concentrations from 30 μM to 1 mM. Pilocarpine also induced secretion at a concentration as low as 1 μM and was the most powerful secretagogue at 10 μM. Secretion was induced by carbachol at 0.1 μM, with maximum secretion at 1.0 μM. Cevimeline induced monophasic secretion at all concentrations tested, whereas higher concentrations of pilocarpine and carbachol induced secretion with variable kinetics, i.e., an initial transient high flow rate, followed by decreased secretion after 2 to 3 min. In the presence of an epithelial Na(+) channel blocker, amiloride, neither carbachol nor pilocarpine affected the Na(+) level of secreted saliva; however, it significantly increased the Na(+) content of cevimeline-induced saliva. The intracellular Ca(2+) response of acinar cells was almost identical among all three agents, although recovery after drug removal was slower for cevimeline and pilocarpine. A profound decrease in intracellular pH was observed during pilocarpine and carbachol treatment, whereas intracellular acidification induced by cevimeline was only seen in the presence of a Na(+)/H(+) exchange inhibitor. When external HCO(3)(-) was removed, cevimeline-induced saliva significantly decreased. These findings suggest that cevimeline specifically activates Na(+)/H(+) exchange and may promote Na(+) reabsorption by stabilizing epithelial sodium channel activity.

HCO3(-) secretion by murine nasal submucosal gland serous acinar cells during Ca2+-stimulated fluid secretion

Airway submucosal glands contribute to airway surface liquid (ASL) composition and volume, both important for lung mucociliary clearance. Serous acini generate most of the fluid secreted by glands, but the molecular mechanisms remain poorly characterized. We previously described cholinergic-regulated fluid secretion driven by Ca²⁺-activated Cl⁻ secretion in primary murine serous acinar cells revealed by simultaneous differential interference contrast (DIC) and fluorescence microscopy. Here, we evaluated whether Ca²⁺-activated Cl⁻ secretion was accompanied by secretion of HCO₃⁻, possibly a critical ASL component, by simultaneous measurements of intracellular pH (pH_i) and cell volume. Resting pH_i was 7.17 ± 0.01 in physiological medium (5% CO₂–25 mM HCO₃⁻). During carbachol (CCh) stimulation, pH_i fell transiently by 0.08 ± 0.01 U concomitantly with a fall in Cl⁻ content revealed by cell shrinkage, reflecting Cl⁻ secretion. A subsequent alkalinization elevated pH_i to above resting levels until agonist removal, whereupon it returned to prestimulation values. In nominally CO₂–HCO₃⁻-free media, the CCh-induced acidification was reduced, whereas the alkalinization remained intact. Elimination of driving forces for conductive HCO₃⁻ efflux by ion substitution or exposure to the Cl⁻ channel inhibitor niflumic acid (100 μM) strongly inhibited agonist-induced acidification by >80% and >70%, respectively. The Na⁺/H⁺ exchanger (NHE) inhibitor dimethylamiloride (DMA) increased the magnitude (greater than twofold) and duration of the CCh-induced acidification. Gene expression profiling suggested that serous cells express NHE isoforms 1–4 and 6–9, but pharmacological sensitivities demonstrated that alkalinization observed during both CCh stimulation and pH_i recovery from agonist-induced acidification was primarily due to NHE1, localized to the basolateral membrane. These results suggest that serous acinar cells secrete HCO₃⁻ during Ca²⁺-evoked fluid secretion by a mechanism that involves the apical membrane secretory Cl⁻ channel, with HCO₃⁻ secretion sustained by activation of NHE1 in the basolateral membrane. In addition, other Na⁺-dependent pH_i regulatory mechanisms exist, as evidenced by stronger inhibition of alkalinization in Na⁺-free media.

Regulation of fluid and electrolyte secretion in salivary gland acinar cells

The secretion of fluid and electrolytes by salivary gland acinar cells requires the coordinated regulation of multiple water and ion transporter and channel proteins. Notably, all the key transporter and channel proteins in this process appear to be activated, or are up-regulated, by an increase in the intracellular Ca2+ concentration ([Ca2+]i). Consequently, salivation occurs in response to agonists that generate an increase in [Ca2+]i. The mechanisms that act to modulate these increases in [Ca2+]i obviously influence the secretion of salivary fluid. Such modulation may involve effects on mechanisms of both Ca2+ release and Ca2+ entry and the resulting spatial and temporal aspects of the [Ca2+]i signal, as well as interactions with other signaling pathways in the cells. The molecular cloning of many of the transporter and regulatory molecules involved in fluid and electrolyte secretion has yielded a better understanding of this process at the cellular level. The subsequent characterization of mice with null mutations in many of these genes has demonstrated the physiological roles of individual proteins. This review focuses on recent developments in determining the molecular identification of the proteins that regulate the fluid secretion process.

Critical role for NHE1 in intracellular pH regulation in pancreatic acinar cells

The primary function of pancreatic acinar cells is to secrete digestive enzymes together with a NaCl-rich primary fluid which is later greatly supplemented and modified by the pancreatic duct. A Na+/H+ exchanger(s) [NHE(s)] is proposed to be integral in the process of fluid secretion both in terms of the transcellular flux of Na+ and intracellular pH (pHi) regulation. Multiple NHE isoforms have been identified in pancreatic tissue, but little is known about their individual functions in acinar cells. The Na+/H+ exchange inhibitor 5-(N-ethyl-N-isopropyl) amiloride completely blocked pHi recovery after an NH4Cl-induced acid challenge, confirming a general role for NHE in pHi regulation. The targeted disruption of the Nhe1 gene also completely abolished pHi recovery from an acid load in pancreatic acini in both HCO3--containing and HCO3--free solutions. In contrast, the disruption of either Nhe2 or Nhe3 had no effect on pHi recovery. In addition, NHE1 activity was upregulated in response to muscarinic stimulation in wild-type mice but not in NHE1-deficient mice. Fluctuations in pHi could potentially have major effects on Ca2+ signaling following secretagogue stimulation; however, the targeted disruption of Nhe1 was found to have no significant effect on intracellular Ca2+ homeostasis. These data demonstrate that NHE1 is the major regulator of pHi in both resting and muscarinic agonist-stimulated pancreatic acinar cells.

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

What can transgenic and gene-targeted mouse models teach us about salivary gland physiology?

Thousands of genetically modified mice have been developed since the first reports of stable expression of recombinant DNA in this species nearly 20 years ago. This mammalian model system has revolutionized the study of whole-animal, organ, and cell physiology. Transgenic and gene-targeted mice have been widely used to characterize salivary-gland-specific expression and to identify genes associated with tumorigenesis. Moreover, several of these mouse lines have proved to be useful models of salivary gland disease related to impaired immunology, i.e., Sjögren's syndrome, and disease states associated with pathogens. Despite the availability of genetically modified mice, few investigators have taken advantage of this resource to better their understanding of salivary gland function as it relates to the production of saliva. In this article, we describe the methods used to generate transgenic and gene-targeted mice and provide an overview of the advantages of and potential difficulties with these models. Finally, using these mouse models, we discuss the advances made in our understanding of the salivary gland secretion process.

Targeted disruption of the Nhe1 gene prevents muscarinic agonist-induced up-regulation of Na(+)/H(+) exchange in mouse parotid acinar cells

The onset of salivary gland fluid secretion in response to muscarinic stimulation is accompanied by up-regulation of Na(+)/H(+) exchanger (NHE) activity. Although multiple NHE isoforms (NHE1, NHE2, and NHE3) have been identified in salivary glands, little is known about their specific function(s) in resting and secreting acinar cells. Mice with targeted disruptions of the Nhe1, Nhe2, and Nhe3 genes were used to investigate the contribution of these proteins to the stimulation-induced up-regulation of NHE activity in mouse parotid acinar cells. The lack of NHE1, but not NHE2 or NHE3, prevented intracellular pH recovery from an acid load in resting acinar cells, in acini stimulated to secrete with the muscarinic agonist carbachol, and in acini shrunken by hypertonic addition of sucrose. In HCO(3)(-)-containing solution, the rate of intracellular pH recovery from a muscarinic agonist-stimulated acid load was significantly inhibited in acinar cells from mice lacking NHE1, but not in cells from NHE2- or NHE3-deficient mice. These data demonstrate that NHE1 is the major regulator of intracellular pH in both resting and muscarinic agonist-stimulated acinar cells and suggest that up-regulation of NHE1 activity has an important role in modulating saliva production in vivo.

Na(+)-H(+) exchange in salivary secretory cells is controlled by an intracellular Na(+) receptor

It recently has been shown that epithelial Na(+) channels are controlled by a receptor for intracellular Na(+), a G protein (G(o)), and a ubiquitin-protein ligase (Nedd4). Furthermore, mutations in the epithelial Na(+) channel that underlie the autosomal dominant form of hypertension known as Liddle's syndrome inhibit feedback control of Na(+) channels by intracellular Na(+). Because all epithelia, including those such as secretory epithelia, which do not express Na(+) channels, need to maintain a stable cytosolic Na(+) concentration ([Na(+)](i)) despite fluctuating rates of transepithelial Na(+) transport, these discoveries raise the question of whether other Na(+) transporting systems in epithelia also may be regulated by this feedback pathway. Here we show in mouse mandibular secretory (endpiece) cells that the Na(+)-H(+) exchanger, NHE1, which provides a major pathway for Na(+) transport in salivary secretory cells, is inhibited by raised [Na(+)](i) acting via a Na(+) receptor and G(o). This inhibition involves ubiquitination, but does not involve the ubiquitin protein ligase, Nedd4. We conclude that control of membrane transport systems by intracellular Na(+) receptors may provide a general mechanism for regulating intracellular Na(+) concentration.

Expression of multiple Na+/H+ exchanger isoforms in rat parotid acinar and ductal cells

Several members of the Na+/H+ exchanger gene family (NHE1, NHE2, NHE3, and NHE4) with unique functional properties have been cloned from rat epithelial tissues. The present study examined the molecular and pharmacological properties of Na+/H+ exchange in rat parotid salivary gland cells. In acinar cells superfused with a physiological salt solution (145 mM Na+), Na+/H+ exchanger activity was inhibited by low concentrations of the amiloride derivative ethylisopropyl amiloride (EIPA; IC50 = 0.014 +/- 0.005 microM), suggesting the expression of amiloride-sensitive isoforms NHE1 and/or NHE2. Semiquantitative RT-PCR confirmed that NHE1 transcripts are most abundant in this cell type. In contrast, the intermediate sensitivity of ductal cells to EIPA indicated that inhibitor-sensitive and -resistant Na+/H+ exchanger isoforms are coexpressed. Ductal cells were about one order of magnitude more resistant to EIPA (IC50 = 0.754 +/- 0.104 microM) than cell lines expressing NHE1 or NHE2 (IC50 = 0.076 +/- 0.013 or 0.055 +/- 0.015 microM, respectively). Conversely, ductal cells were nearly one order of magnitude more sensitive to EIPA than a cell line expressing the NHE3 isoform (IC50 = 6.25 +/- 1.89 microM). Semiquantitative RT-PCR demonstrated that both NHE1 and NHE3 transcripts are expressed in ducts. NHE1 was immunolocalized to the basolateral membranes of acinar and ductal cells, whereas NHE3 was exclusively seen in the apical membrane of ductal cells. Immunoblotting, immunolocalization, and semiquantitative RT-PCR experiments failed to detect NHE2 expression in either cell type. Taken together, our results demonstrate that NHE1 is the dominant functional Na+/H+ exchanger in the plasma membrane of rat parotid acinar cells, whereas NHE1 and NHE3 act in concert to regulate the intracellular pH of ductal cells.

Calcium-calmodulin modulation of the activity of Na+/H+ exchanger in human platelets

This study aimed at establishing the role of calmodulin in regulating pHi of human platelets under acid loads and in stimulated states. The response of human platelets to thrombin was an initial drop of pHi followed by a recovery with a significant increase above the pre-stimulation level in control experiments and a recovery to initial values in platelets maintained in the presence of 19 mmol/l TFP (trifluoperazine = 2 trifluoromethyl-10 [3'-(1 methyl-4-piperazinyl) propyl] phenothiazine). The change in pHi after 8 min was 0.130 +/- 0.030 in the control and 0.001 +/- 0.011 pH units in TFP (P < 0.05). The initial velocity of recovery from an acid load was reduced to 56.7 +/- 6% of the control (n = 6, P < 0.05) with 50 mumol/l W7 (N-(6 aminohexyl)-5-chloro-l-naphthalene sulphonamide), and to 29.7 +/- 4.3% of the control (n = 8, P < 0.05) with 19 mumol/l TFP. The initial velocity of recovery was significantly greater in recalcified platelets than in the preparations kept in the nominal absence of extracellular calcium (1.08 +/- 0.12 vs 0.66 +/- 0.12 pH units per min, P < 0.05). Lower concentration of TFP had an inhibitory effect only in the presence of calcium. The velocities of recovery reached similar values at higher TFP concentration. The significant interaction between Ca2+ and TFP concentrations indicates that the Ca-calmodulin complex, rather than an unspecified direct action of TFP, is responsible for the modulation of the Na+/H+ exchanger. These findings indicate that calcium-calmodulin participates in both the recovery of pH after an acid load and the increase of pHi in stimulated states of human platelets.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Plasma membrane Na+/H+ exchanger isoforms (NHE-1, -2, and -3) are differentially responsive to second messenger agonists of the protein kinase A and C pathways

Na+/H+ exchanger (NHE) activity is regulated by several types of receptors directly coupled to distinct classes (i.e. Gs, Gi, Gq, and G12) of heterotrimeric (alpha beta gamma) GTP-binding proteins (G proteins), which, upon activation, modulate production of various second messengers (e.g. cAMP, cGMP, diacylglycerol, inositol trisphosphate, and Ca2+). Recently, four isoforms of the rat Na+/H+ exchanger were identified by molecular cloning. To examine their intrinsic responsiveness to G protein and second messenger stimulation, three of these isoforms, NHE-1, -2, and -3, were stably expressed in mutant Chinese hamster ovary cells devoid of endogenous NHE activity (AP-1 cells). Incubation of cells with either AIF4-, a general agonist of G proteins, or cholera toxin, a selective activator of G alpha s that stimulates adenylate cyclase, accelerated the rates of amiloride-inhibitable 22Na+ influx mediated by NHE-1 and -2, whereas they inhibited that by NHE-3. Similarly, short term treatment with phorbol 12-myristate 13-acetate, which mimics diacylglycerol activation of protein kinase C (PKC), or with agents (i.e. forskolin, 8-(4-chlorophenylthio)-cAMP, and isobutylmethylxanthine) that lead to activation of cAMP-dependent protein kinase (PKA) also stimulated transport by NHE-1 and NHE-2 but depressed that by NHE-3. The effects of phorbol 12-myristate 13-acetate were blocked by depleting cells of PKC or by inhibiting PKC using chelerythrine chloride, confirming a role for PKC in modulating NHE isoform activities. Likewise, the PKA antagonist, H-89, attenuated the effects of elevated cAMPi on NHE-1, -2, and -3, further demonstrating the regulation by PKA. Unlike cAMPi, elevation of cGMPi by treatment with dibutyryl-cGMP or 8-bromo-cGMP had no influence on NHE isoform activities, thereby excluding the possibility of a role for cGMP-dependent protein kinase in these cells. These data support the concept that the NHE isoforms are differentially responsive to agonists of the PKA and PKC pathways.

Interaction of amiloride with rat parotid muscarinic and alpha-adrenergic receptors

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.

Responses of salivary acinar cells to intracellular alkalinization

Responses of rat submandibular acini to intracellular alkalinization were investigated. Intracellular alkalinization was induced by addition of NH4Cl or methyl amines, or by prepulse with Na butyrate. Only partial recovery occurred following Na butyrate prepulse or methylated amine addition, but full recovery was observed following addition of NH4Cl. The latter recovery was DIDS and dimethylamiloride-insensitive but was inhibited by bumetanide or high [K+] and stimulated in Na(+)-free buffer and by ouabain. Acetylcholine stimulated recovery from NH4Cl- or Na butyrate pre-pulse-induced alkalinization and reduced the extent of alkalinization induced by methylated amines. Acetylcholine-stimulated recovery from NH4Cl-induced alkalinization was mimicked by substance P or ionomycin and was partially Ca(2+)-dependent. This stimulated recovery was bumetanide-insensitive but was partially sensitive to charybdotoxin. Taken together, these data indicate that in unstimulated cells, recovery from alkalinization induced by NH4Cl occurs by bumetanide-sensitive transport of the NH4+ ion, that DIDS-inhibitable anion transport contributes little to this recovery, and that acetylcholine and other Ca(2+)-elevating agents accelerate recovery from NH4Cl-induced alkaline challenge by a mechanism insensitive to bumetanide, DIDS, ouabain, and dimethylamiloride but sensitive to extracellular Ca2+ and to charybdotoxin. Partial recovery from alkaline challenge can also occur in the absence of NH4+ ions, and acetylcholine also stimulates this mode of recovery. Together, these data suggest that these cells have little intrinsic ability to recover from intracellular alkalinization and that the NH4+ ion may be a surrogate for K+ in at least two ion transport pathways.

Regulation of pHi in Saos-2 cells by thrombin: roles of proteolytic activity and cytosolic calcium transients

Some, if not all, of the cellular actions of alpha-thrombin are now believed to be mediated by proteolytic cleavage of the cell surface thrombin receptor to yield a tethered ligand that initiates signal transduction via the receptor. We have investigated the actions of alpha-thrombin on the regulation of cytosolic free Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) in human osteoblast-like Saos-2 cells. After acidification with nigericin, thrombin induced an acute increase of [Ca2+]i and a rise in pHi. The action of thrombin on pHi was dependent on activation of the Na(+)-H+ antiporter. Thrombin elicited parallel concentration-dependent increases in both [Ca2+]i and pHi, and the rise in [Ca2+]i was a prerequisite for the increase in pHi. Preincubation of thrombin with the active site proteolytic inhibitor, BOC-D-Phe-L-Pro-D,L-Lys-CF3, prevented the alkalinization response to thrombin but had little or no effect on the thrombin-induced rise in [Ca2+]i. Hirudin, a natural inhibitor of thrombin, acts by tight binding to several discrete regions on the thrombin molecule. Preincubation of thrombin with hirudin completely blocked the rise in both [Ca2+]i and pHi. These results demonstrate that the thrombin-induced rise in [Ca2+]i alone is not sufficient to cause alkalinization in Saos-2 cells. More importantly, our findings reveal that not all of the cellular actions of thrombin can be explained by proteolytic cleavage of the thrombin receptor and suggest that different domains on the thrombin molecule may be required for eliciting signals that raise [Ca2+]i and pHi in Saos-2 cells.

Characterization of the TRH-induced activation of Na+/H(+)-exchange in pituitary GH4C1 cells

In the present study in GH4C1 cells, the dependence of TRH-induced activation of Na+/H(+)-exchange on extracellular Na+ and Ca2+ was examined. Furthermore, the effects of both extracellular and intracellular H+ on Na+/H(+)-exchange were investigated. The buffering capacity was 63 +/- 11.8 mM (pH unit)-1 at basal intracellular pH (pHi) of 7.02 +/- 0.02. The initial rate of alkalinization in cells acidified with nigericin increased with increasing concentrations of extracellular Na+ according to simple Michaelis-Menten kinetics. The apparent Km-value for Na+ was 53 +/- 17.5 mM and the Vmax value was 28 +/- 4.5 mM H+/min. Addition of Na+ together with TRH increased Vmax to 56 +/- 6.4 mM H+/min (P < 0.05), while no difference was observed in Km. Decreasing extracellular pH (pHo) decreased the rate of alkalinization of acid-loaded cells, despite a large inward Na+ gradient. Furthermore, a decrease in pHi was necessary to obtain activation of Na+/H+ exchange. At pHi-values close to basal pHi no activation of Na+/H(+)-exchange was obtained. In addition, the results showed that extracellular Ca2+ was necessary for TRH-induced activation of Na+/H+ exchange. Blocking influx of extracellular Ca2+ with Ni2+ abolished the effect of TRH, suggesting that the TRH-induced activation of Na+/H(+)-exchange in GH4C1 cells is dependent on influx of extracellular Ca2+.

Regulation of cytoplasmic pH in rat sublingual mucous acini at rest and during muscarinic stimulation

The regulation of intracellular pH (pHi) in rat sublingual mucous acini was monitored using dual-wavelength microfluorometry of the pH-sensitive dye BCECF (2',7'-biscarboxyethyl-5(6)-carboxyfluorescein). Acini attached to coverslips and continuously superfused with HCO3(-)-containing medium (25 mM NaHCO3/5% CO2; pH 7.4) have a steady-state pHi of 7.25 +/- 0.02. Acid loading of acinar cells using the NH4+/NH3 prepulse technique resulted in a Na(+)-dependent, MIBA-inhibitable (5-(N-methyl-N-isobutyl) amiloride, Ki approximately 0.42 microM) pHi recovery, the kinetics of which were not influenced by the absence of extracellular Cl-. The rate and magnitude of the pHi recovery were dependent on the extracellular Na+ concentration, indicating that Na+/H+ exchange plays a critical role in maintaining pHi above the pH predicted for electrochemical equilibrium. When the NH4+/NH3 concentration was varied, the rate of pHi recovery was enhanced as the extent of the intracellular acidification increased, demonstrating that the activity of the Na+/H+ exchanger is regulated by the concentration of intracellular protons. Switching BCECF-loaded acini to a Cl(-)-free medium did not significantly alter resting pHi, suggesting the absence of Cl-/HCO3- exchange activity. Muscarinic stimulation resulted in a rapid and sustained cytosolic acidification (t 1/2 < 30 sec; 0.16 +/- 0.02 pH unit), the magnitude of which was amplified greater than two-fold in the presence of MIBA (0.37 +/- 0.05 pH unit) or in the absence of extracellular Na+ (0.34 +/- 0.03 pH unit). The agonist-induced intracellular acidification was blunted in HCO3(-)-free media and was inhibited by DPC (diphenylamine-2-carboxylate), an anion channel blocker. In contrast, the acidification was not influenced by removal of extracellular Cl-. The Ca2+ ionophore, ionomycin, mimicked the effects of stimulation, whereas preloading acini with BAPTA (bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid) to chelate intracellular Ca2+ blocked the agonist-induced cytoplasmic acidification. The above results indicate that during muscarinic stimulation an intracellular acidification occurs which: (i) is partially buffered by increased Na+/H+ exchange activity; (ii) is most likely mediated by HCO3- efflux via an anion channel; and (iii) requires an increase in cytosolic free [Ca2+].

Effects of ammonium chloride on membrane currents of acinar cells dispersed from the rat parotid gland

In acinar cells freshly dispersed from rat parotid glands, the effects of ammonium chloride (NH4Cl) on membrane currents were studied using the whole-cell clamp method. When membrane currents were recorded with command pulses to 0 mV, applied at 2-s intervals from a holding potential of -70 mV, NH4Cl (5-20 mM) transiently decreased outward currents and then slowly increased both outward and inward currents. After reaching a peak in about 40-50 s, both outward and inward currents gradually decreased in the presence of NH4Cl and, on its wash-out, the currents returned to the control level. Butyrate (5-20 mM) had little effect on the resting membrane currents, but markedly inhibited the response to NH4Cl. Tetraethylammonium (5 mM) strongly reduced both the resting and NH4Cl-induced outward currents, whereas it slightly potentiated the NH4Cl-induced inward current without affecting the membrane current at the holding potential. Without ATP in the patch pipettes, carbachol-induced membrane currents were relatively resistant to Ca2+ removal from the external medium, but NH4Cl-induced currents were quickly abolished in the absence of Ca2+. We conclude that intracellular alkalinization with NH4Cl increases Ca2+ influx and activates Ca(2+)-dependent outward K+ and inward Cl- currents.

Cellular distribution of gastric chief cell protein kinase C activity: differential effects of diacylglycerol, phorbol esters, carbachol, and cholecystokinin

Stimulation of chief cells with carbachol or cholecystokinin (CCK) results in the production of inositol trisphosphate (IP3) and diacylglycerol (DAG). Although IP3 increases cell calcium concentration, thereby stimulating pepsinogen secretion, the role of DAG and its target, protein kinase C (PKC), is less clear. To examine the relation between the cellular distribution of PKC activity and pepsinogen secretion, we determined PKC activity in cytosolic and membrane fractions from dispersed chief cells from guinea pig stomach. To validate our assay, we studied the actions of the phorbol ester PMA. PMA caused a rapid, dose-dependent, 6-fold increase in pepsinogen secretion and membrane-associated PKC activity. Similarly, dose-response curves for pepsinogen secretion and the increase in membrane-associated PKC activity induced by a membrane-permeant DAG (1-oleoyl-2-acetylglycerol) were superimposable. In contrast, CCK (0.1 nM to 1.0 microM) and carbachol (0.1 microM to 1.0 mM) caused a 4-fold increase in pepsinogen secretion, but did not alter the distribution of PKC activity. These results indicate that in gastric chief cells, PMA- and DAG-induced pepsinogen secretion is accompanied by increased membrane-associated PKC activity. However, the cellular distribution of PKC activity is not altered by CCK or carbachol.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Changes in pHi associated with activation of ion secretion in avian nasal salt gland cells

The fluorescent pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)- carboxyfluorescein (BCECF) was used to determine changes in intracellular pH (pHi) associated with activation of secretion in isolated cells from the salt-secreting avian nasal gland. A correction procedure overcoming artifacts due to BCECF leakage is described. Resting pHi averaged 7.15 +/- 0.03 and was unaffected by the nominal removal of medium HCO3- or by the addition of the anion-exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) but was significantly reduced by amiloride (7.07 +/- 0.02). Muscarinic activation of secretion resulted in a rapid intracellular acidification that was compensated by mechanisms which raised pHi to restore approximately resting levels within 5 min. The principal mechanism involved was amiloride-sensitive and independent of any sustained intracellular Ca2+ concentration change. Recovery of pHi was also aided by HCO3(-)-dependent and DIDS-sensitive mechanisms not seen in the resting cell. The direction of the latter was pHi-dependent, with DIDS further decreasing pHi in acidified cells and increasing pHi in alkalinized cells. This suggests that the DIDS-sensitive pathways are activated under conditions where pHi has been shifted away from resting levels in either direction and act primarily to restore resting pHi.

Intracellular calcium-mediated activation of hepatic Na+/H+ exchange by arginine vasopressin and phenylephrine

The effect of Ca++ mobilizing agonists arginine vasopressin and phenylephrine on Na+/H+ exchange was studied in freshly isolated hepatocytes and isolated perfused rat livers. The activity of Na+/H+ exchange was determined from the rate of H+ efflux, 22Na uptake and pHi recovery. Arginine vasopressin and phenylephrine stimulated H+ efflux and 22Na uptake in isolated rat hepatocytes and increased the rate of pHi recovery from acid-loaded hepatocytes. These effects were inhibited by amiloride. Arginine vasopressin- and phenylephrine-induced increases in H+ efflux were also dependent on extracellular Na+. Arginine vasopressin- and phenylephrine-induced increases in intracellular Ca++ concentration, H+ efflux, 22Na uptake and intracellular pH recovery were decreased in hepatocytes preloaded with the Ca(++)-buffering agent [bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] (MAPTA). Na+/H+ exchange-dependent intracellular pH recovery from cytosolic acidification was stimulated by thapsigargin, which increases intracellular calcium concentration by inhibiting endoplasmic reticulum Ca++ ATPase. Arginine vasopressin- and phenylephrine-induced increases in intracellular pH recovery were not dependent on extracellular Ca++ and were inhibited by calmidazolium, a calmodulin inhibitor. Arginine vasopressin and phenylephrine also increased H+ efflux in the absence but not in the presence of amiloride in perfused rat livers without affecting biliary HCO3- excretion. These results indicate that arginine vasopressin and phenylephrine activate Na+/H+ exchange in rat hepatocytes, an effect mediated in part by intracellular Ca++ and calmodulin kinase. Furthermore, sinusoidal Na+/H+ exchange does not appear to be involved in biliary HCO3- excretion.

Oscillations of cytosolic sodium during calcium oscillations in exocrine acinar cells

In acinar cells from rat salivary glands, cholinergic agonists cause oscillations in cytoplasmic free calcium concentration, which then drive oscillations of cell volume that reflect oscillating cell solute content and fluid secretion. By quantitative fluorescence ratio microscopy of an intracellular indicator dye for sodium, it has now been shown that large amplitude oscillations of sodium concentration were associated with the calcium and cell volume oscillations. Both calcium and sodium oscillations were dependent on the continued presence of calcium in the extracellular medium and were abolished by the specific sodium-potassium adenosine triphosphatase inhibitor ouabain. Thus, calcium oscillations in salivary acinar cells, by modulating the activities of ion transport pathways in the plasma membrane, can cause significant oscillations of monovalent ions that may in turn feed back to regulate calcium oscillations and fluid secretion.

Microfluorimetric imaging study of the mechanism of activation of the Na+/H+ antiport by muscarinic agonist in rat mandibular acinar cells

The mechanism of regulation of intracellular pH (pHi) in dispersed acini from the rat mandibular salivary gland has been studied with a microfluorimetric imaging method and the pH probe 2',7'-bis(2-carboxyethyl)-5(and -6)-carboxyfluorescein. The pHi in the TRIS/HEPES-buffered standard solution was 7.29 +/- 0.01. Addition of 1 mumol/l acetylcholine (ACh) or ionomycin caused a sustained increase in the pHi. These agents decreased pHi in the absence of external Na+ or in the presence of amiloride. The rate of pHi recovery from an acid load after NH+4 prepulse was a linear function of pHi and increased as pHi became more acidic. Addition of ACh shifted the relationship towards a more alkaline pHi range. The increase in pHi induced by ACh or ionomycin was not inhibited by the protein kinase C inhibitors staurosporine (10 nM) and 1-(5-isoquinolinesulfonyl)-1-methylpiperazine (50 mumol/l). Addition of 0.1-1 mumol/l phorbol 12-myristate 13-acetate (TPA) had little effect on pHi within 10 min; however, exposure to TPA for 120 min resulted in a significant rise in pHi. In Ca(2+)-free solution with 50 mumol/l 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate, the ACh-induced rise in both pHi and cytosolic Ca2+ concentration was suppressed. ACh and ionomycin caused an increment of amiloride-sensitive acid output into the extracellular fluid, while 20 mumol/l 1-oleoyl-2-acetylglycerol had little effect on it. It was concluded that (a) stimulation with ACh activated the Na+/H+ antiport in the plasma membrane, (b) ACh also stimulated the intracellular acid production but acid extrusion by the Na+/H+ antiport prevented the cell from intracellular acidification, and (c) the major route of signal transduction for the ACh-induced activation of the Na+/H+ antiport was independent of protein kinase C but was dependent on the rise in cytosolic Ca2+ concentration. The implication of the cytosolic acidification and cell volume change in pHi regulation is discussed.