Channels and transporters in salivary glands

According to the two-stage hypothesis, primary saliva, a NaCl-rich plasma-like isotonic fluid is secreted by salivary acinar cells and its ionic composition becomes modified in the duct system. The ducts secrete K(+) and HCO (3) (-) and reabsorb Na(+) and Cl(-) without any water movement, thus establishing a hypotonic final saliva. Salivary secretion depends on the coordinated action of several channels and transporters localized in the apical and basolateral membrane of acinar and duct cells. Early functional studies in perfused glands, followed by the molecular cloning of several transport proteins and the subsequent analysis of mutant mice, have greatly contributed to our understanding of salivary fluid and the electrolyte secretion process. With a few exceptions, most of the key channels and transporters involved in salivary secretion have now been identified and characterized. However, the picture that has emerged from all these studies is one of a complex molecular network characterized by redundancy for several transport proteins, compensatory mechanisms, and adaptive changes in health and disease. Current research is directed to the molecular interactions between the determinants and the ways in which they are regulated by extracellular signals and intracellular mediators. This review focuses on the functionally and molecularly best-characterized channels and transporters that are considered to be involved in transepithelial fluid and electrolyte transport in salivary glands.

Apical maxi-K (KCa1.1) channels mediate K+ secretion by the mouse submandibular exocrine gland

The exocrine salivary glands of mammals secrete K+ by an unknown pathway that has been associated with HCO3(-) efflux. However, the present studies found that K+ secretion in the mouse submandibular gland did not require HCO3(-), demonstrating that neither K+/HCO3(-) cotransport nor K+/H+ exchange mechanisms were involved. Because HCO3(-) did not appear to participate in this process, we tested whether a K channel is required. Indeed, K+ secretion was inhibited >75% in mice with a null mutation in the maxi-K, Ca2+-activated K channel (KCa1.1) but was unchanged in mice lacking the intermediate-conductance IKCa1 channel (KCa3.1). Moreover, paxilline, a specific maxi-K channel blocker, dramatically reduced the K+ concentration in submandibular saliva. The K+ concentration of saliva is well known to be flow rate dependent, the K+ concentration increasing as the flow decreases. The flow rate dependence of K+ secretion was nearly eliminated in KCa1.1 null mice, suggesting an important role for KCa1.1 channels in this process as well. Importantly, a maxi-K-like current had not been previously detected in duct cells, the theoretical site of K+ secretion, but we found that KCa1.1 channels localized to the apical membranes of both striated and excretory duct cells, but not granular duct cells, using immunohistochemistry. Consistent with this latter observation, maxi-K currents were not detected in granular duct cells. Taken together, these results demonstrate that the secretion of K+ requires and is likely mediated by KCa1.1 potassium channels localized to the apical membranes of striated and excretory duct cells in the mouse submandibular exocrine gland.

Hypertonic shrinking but not hypotonic swelling increases sodium concentration in rat brain synaptosomes

Neurotransmitter release is dependent on both calcium and sodium influx. Hypotonic swelling and hypertonic shrinking of neurons evokes calcium-independent exocytosis of neurotransmitters into the synaptic cleft. To date, there are not too much data available on relationship between extracellular osmolarity and sodium concentration in presynaptic endings. In the present study we investigated the effects of hypotonic swelling and hypertonic shrinking on sodium levels, as measured using fluorescent dyes SBFI-AM and Sodium Green in rat brain synaptosomes. Reduction of incubation medium osmolarity from 310 to 230 mOsm did not raise the intrasynaptosomal sodium concentration. An increase of osmolarity from 310 to 810 mOsm is accompanied by a dose-dependent elevation of sodium concentration from 8.1+/-0.5 to 46.5+/-2.8mM, respectively. This effect was insensitive to several channel inhibitors such as: tetrodotoxin, an inhibitor of voltage-gated sodium channels, bumetanide, an inhibitor of Na(+)/K(+)/2Cl(-) cotransport, gadolinium, an inhibitor of nonselective mechanosensitive channels, ruthenium red, an inhibitor of transient receptor potential channel and amiloride, an inhibitor of epithelial sodium channel/degenerin. Additionally, using the fluorescent dye BCECF-AM, we have shown that hypertonic shrinking caused a dose-dependent acidification of intrasynaptosomal cytosol, which suggests that the Na(+)/H(+) exchanger is not involved in the effect of increased osmolarity on cytosolic sodium levels. The increase in intrasynaptosomal sodium concentrations following increases in osmolarity is probably due to sodium influx through another sodium channels.

Ethanol modulates the VR-1 variant amiloride-insensitive salt taste receptor. II. Effect on chorda tympani salt responses

The effect of ethanol on the amiloride- and benzamil (Bz)-insensitive salt taste receptor was investigated by direct measurement of intracellular Na⁺ activity ([Na⁺]_i) using fluorescence imaging in polarized fungiform taste receptor cells (TRCs) and by chorda tympani (CT) taste nerve recordings. CT responses to KCl and NaCl were recorded in Sprague-Dawley rats, and in wild-type (WT) and vanilloid receptor-1 (VR-1) knockout mice (KO). CT responses were monitored in the presence of Bz, a specific blocker of the epithelial Na⁺ channel (ENaC). CT responses were also recorded in the presence of agonists (resiniferatoxin and elevated temperature) and antagonists (capsazepine and SB-366791) of VR-1 that similarly modulate the Bz-insensitive VR-1 variant salt taste receptor. In the absence of mineral salts, ethanol induced a transient decrease in TRC volume and elicited only transient phasic CT responses. In the presence of mineral salts, ethanol increased the apical cation flux in TRCs without a change in volume, increased transepithelial electrical resistance across the tongue, and elicited CT responses that were similar to salt responses, consisting of both a phasic component and a sustained tonic component. At concentrations <50%, ethanol enhanced responses to KCl and NaCl, while at ethanol concentrations >50%, those CT responses were inhibited. Resiniferatoxin and elevated temperature increased the sensitivity of the CT response to ethanol in salt-containing media, and SB-366791 inhibited the effect of ethanol, resiniferatoxin, and elevated temperature on the CT responses to mineral salts. VR-1 KO mice demonstrated no Bz-insensitive CT response to NaCl and no sensitivity to ethanol. We conclude that ethanol increases salt taste sensitivity by its direct action on the Bz-insensitive VR-1 variant salt taste receptor.

Ethanol modulates the VR-1 variant amiloride-insensitive salt taste receptor. I. Effect on TRC volume and Na+ flux

The effect of ethanol on the amiloride- and benzamil (Bz)-insensitive salt taste receptor was investigated by the measurement of intracellular Na(+) activity ([Na(+)](i)) in polarized rat fungiform taste receptor cells (TRCs) using fluorescence imaging and by chorda tympani (CT) taste nerve recordings. CT responses were monitored during lingual stimulation with ethanol solutions containing NaCl or KCl. CT responses were recorded in the presence of Bz (a specific blocker of the epithelial Na(+) channel [ENaC]) or the vanilloid receptor-1 (VR-1) antagonists capsazepine or SB-366791, which also block the Bz-insensitive salt taste receptor, a VR-1 variant. CT responses were recorded at 23 degrees C or 42 degrees C (a temperature at which the VR-1 variant salt taste receptor activity is maximally enhanced). In the absence of permeable cations, ethanol induced a transient decrease in TRC volume, and stimulating the tongue with ethanol solutions without added salt elicited only transient phasic CT responses that were insensitive to elevated temperature or SB-366791. Preshrinking TRCs in vivo with hypertonic mannitol (0.5 M) attenuated the magnitude of the phasic CT response, indicating that in the absence of mineral salts, transient phasic CT responses are related to the ethanol-induced osmotic shrinkage of TRCs. In the presence of mineral salts, ethanol increased the Bz-insensitive apical cation flux in TRCs without a change in cell volume, increased transepithelial electrical resistance across the tongue, and elicited CT responses that were similar to salt responses, consisting of both a transient phasic component and a sustained tonic component. Ethanol increased the Bz-insensitive NaCl CT response. This effect was further enhanced by elevating the temperature from 23 degrees C to 42 degrees C, and was blocked by SB-366791. We conclude that in the presence of mineral salts, ethanol modulates the Bz-insensitive VR-1 variant salt taste receptor.

Modulation of rat chorda tympani NaCl responses and intracellular Na+ activity in polarized taste receptor cells by pH

Mixture interactions between sour and salt taste modalities were investigated in rats by direct measurement of intracellular pH (pH(i)) and Na(+) activity ([Na(+)](i)) in polarized fungiform taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the lingual surface with NaCl solutions adjusted to pHs ranging between 2.0 and 10.3 increased the magnitude of NaCl CT responses linearly with increasing external pH (pH(o)). At pH 7.0, the epithelial sodium channel (ENaC) blocker, benzamil, decreased NaCl CT responses and inhibited further changes in CT responses induced by varying pH(o) to 2.0 or 10.3. At constant pH(o), buffering NaCl solutions with potassium acetate/acetic acid (KA/AA) or HCO(3)(-)/CO(2) inhibited NaCl CT responses relative to CT responses obtained with NaCl solutions buffered with HEPES. The carbonic anhydrase blockers, MK-507 and MK-417, attenuated the inhibition of NaCl CT responses in HCO(3)(-)/CO(2) buffer, suggesting a regulatory role for pH(i). In polarized TRCs step changes in apical pH(o) from 10.3 to 2.0 induced a linear decrease in pH(i) that remained within the physiological range (slope = 0.035; r(2) = 0.98). At constant pH(o), perfusing the apical membrane with Ringer's solutions buffered with KA/AA or HCO(3)(-)/CO(2) decreased resting TRC pH(i), and MK-507 or MK-417 attenuated the decrease in pH(i) in TRCs perfused with HCO(3)(-)/CO(2) buffer. In parallel experiments, TRC [Na(+)](i) decreased with (a) a decrease in apical pH, (b) exposing the apical membrane to amiloride or benzamil, (c) removal of apical Na(+), and (d) acid loading the cells with NH(4)Cl or sodium acetate at constant pH(o). Diethylpyrocarbonate and Zn(2+), modification reagents for histidine residues in proteins, attenuated the CO(2)-induced inhibition of NaCl CT responses and the pH(i)-induced inhibition of apical Na(+) influx in TRCs. We conclude that TRC pH(i) regulates Na(+)-influx through amiloride-sensitive apical ENaCs and hence modulates NaCl CT responses in acid/salt mixtures.

Immunolocalization of anion exchanger AE2 and Na(+)-HCO(-)(3) cotransporter in rat parotid and submandibular glands

Salivary glands secrete K(+) and HCO(-)(3) and reabsorb Na(+) and Cl(-), but the identity of transporters involved in HCO(-)(3) transport remains unclear. We investigated localization of Cl(-)/HCO(-)(3) exchanger isoform AE2 and of Na(+)-HCO(-)(3) cotransporter (NBC) in rat parotid gland (PAR) and submandibular gland (SMG) by immunoblot and immunocytochemical techniques. Immunoblotting of PAR and SMG plasma membranes with specific antibodies against mouse kidney AE2 and rat kidney NBC revealed protein bands at approximately 160 and 180 kDa for AE2 and approximately 130 kDa for NBC, as expected for the AE2 full-length protein and consistent with the apparent molecular mass of NBC in several tissues other than kidney. Immunostaining of fixed PAR and SMG tissue sections revealed specific basolateral staining of PAR acinar cells for AE2 and NBC, but in SMG acinar cells only basolateral AE2 labeling was observed. No AE2 expression was detected in any ducts. Striated, intralobular, and main duct cells of both glands showed NBC expression predominantly at basolateral membranes, with some cells being apically stained. In SMG duct cells, NBC staining exhibited a gradient of distribution from basolateral localization in more proximal parts of the ductal tree to apical localization toward distal parts of the ductal tree. Both immunoblotting signals and immunostaining were abolished in preabsorption experiments with the respective antigens. Thus the mechanisms of fluid and anion secretion in salivary acinar cells may be different between PAR and SMG, and, because NBC was detected in acinar and duct cells, it may play a more important role in transport of HCO(-)(3) by rat salivary duct cells than previously believed.

5-HT3 receptor-channels coupled with Na+ influx in human T cells: role in T cell activation

The study was conducted on a human (Jurkat) T cell line, loaded with a Na+ fluorescent probe, SBFI/AM. Serotonin and an agonist of 5-HT3 receptor-channels, 2-methyl-5HT, evoked Na+ influx, whereas the agonists of other serotonergic receptor subtypes, i.e., 5-HT1A and 5-HT1B receptors, failed to induce Na+ influx in these cells. By using 3H-BRL43694, an agonist of 5-HT3 receptor-channels, we characterized 5-HT3 lymphocyte receptors which exhibited a density (Bmax) of 300 +/- 20 fmol/10(6) cells and a Kd of 30 nM in Jurkat T cells. The T-cell 5-HT3 receptor-channel is not regulated either by the protein kinase C or by the free intracellular calcium concentrations as the agents known to activate the PKC and to induce increases in intracellular free calcium concentrations failed to influence the free intracellular Na+ concentrations, [Na+]i, in these cells. Furthermore, an increase in [Na+]i, induced by 2-methyl-5HT, via 5-HT3 receptor-channels seems to stimulate T-cell activation by facilitating the progression of T cells from S to G2/M phase of the cell cycle.

Cystic fibrosis transmembrane conductance regulator regulates luminal Cl-/HCO3- exchange in mouse submandibular and pancreatic ducts

We have demonstrated previously the regulation of Cl-/HCO3- exchange activity by the cystic fibrosis transmembrane conductance regulator (CFTR) in model systems of cells stably or transiently transfected with CFTR (Lee, M. G., Wigley, W. C., Zeng, W., Noel, L. E., Marino, C. R., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 3414-3421). In the present work we examine the significance of this regulation in cells naturally expressing CFTR. These include the human colonic T84 cell line and the mouse submandibular gland and pancreatic ducts, tissues that express high levels of CFTR in the luminal membrane. As in heterologous expression systems, stimulation of T84 cells with forskolin increased the Cl-/HCO3- exchange activity independently of CFTR Cl- channel activity. Freshly isolated submandibular gland ducts from wild type mice showed variable Cl-/HCO3- exchange activity. Measurement of [Cl-]i revealed that this was largely the result of variable steady-state [Cl-]i. Membrane depolarization with 5 mM Ba2+ or 100 mM K+ increased and stabilized [Cl-]i. Under depolarized conditions wild type and DeltaF/DeltaF mice had comparable basal Cl-/HCO3- exchange activity. Notably, stimulation with forskolin increased Cl-/HCO3- exchange activity in submandibular gland ducts from wild type but not DeltaF/DeltaF mice. Microperfusion of the main pancreatic duct showed Cl-/HCO3- exchange activity in both the basolateral and luminal membranes. Stimulation of ducts from wild type animals with forskolin had no effect on basolateral but markedly stimulated luminal Cl-/HCO3- exchange activity. By contrast, forskolin had no effect on either basolateral or luminal Cl-/HCO3- exchange activity of ducts from DeltaF/DeltaF animals. We conclude that CFTR regulates luminal Cl-/HCO3- exchange activity in CFTR-expressing cells, and we discuss the possible physiological significance of these findings regarding cystic fibrosis.

All three WW domains of murine Nedd4 are involved in the regulation of epithelial sodium channels by intracellular Na+

The amiloride-sensitive epithelial sodium channel (ENaC) plays a critical role in fluid and electrolyte homeostasis and consists of alpha, beta, and gamma subunits. The carboxyl terminus of each ENaC subunit contains a PPxY motif which is necessary for interaction with the WW domains of the ubiquitin-protein ligase, Nedd4. Disruption of this interaction, as in Liddle's syndrome where mutations delete or alter the PY motif of either the beta or gamma subunits, results in increased ENaC activity. We have recently shown using the whole-cell patch clamp technique that Nedd4 mediates the ubiquitin-dependent down-regulation of Na+ channel activity in response to increased intracellular Na+. In this paper, we demonstrate that WW domains 2 and 3 bind alpha-, beta-, and gamma-ENaC with varying degrees of affinity, whereas WW domain 1 does not bind to any of the subunits. We further show using whole-cell patch clamp techniques that Nedd4-mediated down-regulation of ENaC in mouse mandibular duct cells involves binding of the WW domains of Nedd4 to three distinct sites. We propose that Nedd4-mediated down-regulation of Na+ channels involves the binding of WW domains 2 and 3 to the Na+ channel and of WW domain 1 to an unknown associated protein.

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.

Membrane-limited expression and regulation of Na+-H+ exchanger isoforms by P2 receptors in the rat submandibular gland duct

1. Cell-specific reverse transcriptase-polymerase chain reaction (RT-PCR), immunolocalization and microspectrofluorometry were used to identify and localize the Na+-H+ exchanger (NHE) isoforms expressed in the submandibular gland (SMG) acinar and duct cells and their regulation by basolateral and luminal P2 receptors in the duct. 2. The molecular and immunofluorescence analysis showed that SMG acinar and duct cells expressed NHE1 in the basolateral membrane (BLM). Duct cells also expressed NHE2 and NHE3 in the luminal membrane (LM). 3. Expression of NHE3 was unequivocally established by the absence of staining in SMG from NHE3 knockout mice. NHE3 was expressed in the LM and in subluminal regions of the duct. 4. Measurement of the inhibition of NHE activity by the amiloride analogue HOE 694 (HOE) suggested expression of NHE1-like activity in the BLM and NHE2-like activity in the LM of the SMG duct. Several acute and chronic treatments tested failed to activate NHE activity with low affinity for HOE as expected for NHE3. Hence, the physiological function and role of NHE3 in the SMG duct is not clear at present. 5. Activation of P2 receptors resulted in activation of an NHE-independent, luminal H+ transport pathway that markedly and rapidly acidified the cells. This pathway could be blocked by luminal but not basolateral Ba2+. 6. Stimulation of P2U receptors expressed in the BLM activated largely NHE1-like activity, and stimulation of P2Z receptors expressed in the LM activated largely NHE2-like activity. 7. The interrelation between basolateral and luminal NHE activities and their respective regulation by P2U and P2Z receptors can be used to co-ordinate membrane transport events in the LM and BLM during active Na+ reabsorption by the SMG duct.

Fluorescence imaging of Na+ influx via P2X receptors in cochlear hair cells

The adenosine 5'-triphosphate (ATP)-activated membrane conductance, mediated by P2X receptors, was examined in isolated guinea-pig cochlear inner and outer hair cells. Photo-activated release of caged-ATP elicted a 30-ms latency inwardly rectifying non-selective cation conductance, blocked by the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10-100 microM), consistent with the direct activation of ATP-gated ion channels. A K(Ca) conductance in the inner hair cells (IHC), activated by the entry of Ca2+ through the ATP-gated ion channels, was blocked by including 10 mM 1,2-his(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the internal solution. Real-time confocal slit-scanning fluorescence imaging of Na+ influx through the ATP-gated ion channels was performed using the dye Sodium Green with simultaneous whole-cell recording of membrane currents. The Na+ entry was localized to the endolymphatic surface, with the increase in [Na+]i detected within approximately 200 ms of the onset of the inward current response. Within 600 ms Na+ had diffused throughout the cell cytoplasm with the exception of the subnuclear region of the outer hair cells. Correlation of voltage-clamp measurements of Na+ entry with regional increases in Na+-induced fluorescence demonstrated ATP-induced increases in intracellular Na+ in excess of 45 mM within 4 s. These data provide direct evidence for the Na+ permeability of the ATP-gated ion channels as well as independent evidence for the localization of P2X receptors at the endolymphatic surface of the sensory hair cells. The localization of the ATP-gated ion channels to the apical surface of the hair cells supports an ATP-mediated modulation of 'silent' K+ current across the cochlear partition which could regulate hearing sensitivity by controlling the transcellular driving force for both mechanoelectrical and electromechanical transduction in hair cells.

Immunolocalization of vacuolar-type H+-ATPase in rat submandibular gland and adaptive changes induced by acid-base disturbances

Using antibodies against the 31-kD and 70-kD subunits of vacuolar type H+-ATPase (V-ATPase) and light microscopic immunocytochemistry, we have demonstrated the presence of this V-ATPase in rat submandibular gland. We have also investigated the adaptive changes of this transporter during acid-base disturbances such as acute and chronic metabolic acidosis or alkalosis. Our results show intracellularly distributed V-ATPase in striated, granular, and main excretory duct cells in controls, but no V-ATPase immunoreaction in acinar cells. Both acute and chronic metabolic acidosis caused a shift in V-ATPase away from diffuse distribution towards apical localization in striated and granular duct cells, suggesting that a V-ATPase could be involved in the regulation of acid-base homeostasis. In contrast, during acidosis the main excretory duct cells showed no changes in the V-ATPase distribution compared to controls. With acute and chronic metabolic alkalosis, no changes in the V-ATPase distribution occurred. (J Histochem Cytochem 46:91-100, 1998)

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.

H+ transporters in the main excretory duct of the mouse mandibular salivary gland

1. We used microspectrofluorimetry with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF) to study the regulation of cytosolic pH (pHi) in the isolated, perfused main excretory duct of the mouse mandibular gland. 2. In nominally HCO3(-)-free solutions, removal of Na+ from the lumen alone caused pHi to decline whereas removing it from the bath alone did not. 3. Readmission of Na+ to the lumen of ducts studied under zero-Na+ conditions caused pHi to recover fully. This recovery was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) with a half-maximum concentration of 0.5 mumol l-1, indicating the presence of an apical Na(+)-H+ exchanger. 4. Readmission of Na+ to the bath of ducts studied under zero-Na+ conditions also caused pHi to recover. This recovery was blocked by 100 mumol l-1 EIPA, indicating the presence of a basolateral Na(+)-H+ exchanger. 5. Measurements of H+ fluxes indicated that the apical Na(+)-H+ exchanger was approximately four times more active than the basolateral Na(+)-H+ exchanger. 6. In three sets of experiments (in the absence of Na+, in the presence of Na+, and in the presence of Na+ plus 100 mumol l-1 EIPA), the effects of changing luminal K+ concentration on pHi were examined. We found no evidence for the presence of K(+)-H+ exchange or Na(+)-coupled K(+)-H+ exchange in the apical membranes of duct cells. 7. pHi recovery under nominally HCO3(-)-free conditions following acidification with an NH4Cl pulse was abolished by removal of Na+ from the bath and luminal solutions, indicating that no Na(+)-independent systems such as H(+)-ATPases were present. 8. A repeat of the above experiments in the presence of 25 mmol l-1 HCO3- plus 5% CO2 did not reveal any additional H+ transport systems. The removal of luminal Cl-, however, caused a small rise in pHi. This latter effect was blocked by 500 mumol l-1 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulphonic acid (H2-DIDS), suggesting that a Cl(-)-HCO3- exchanger in the apical membrane might contribute in a minor way to pHi regulation. 9. We conclude that the predominant H+ transport systems in the mouse mandibular main excretory duct are Na(+)-H+ exchangers in the apical and the basolateral membranes. The model we postulate to account for electrolyte transport across the main duct in the mouse mandibular gland is quite different from that previously developed for the rat duct but is similar to that developed for the rabbit duct. The difference is in concordance with the known ability of the mandibular gland of the rat, but not the rabbit or the mouse, to secrete a HCO3(-)-rich final saliva.

Sodium Green as a potential probe for intracellular sodium imaging based on fluorescence lifetime

We characterized the use of the fluorescent probe Sodium Green for measurements of intracellular free sodium using frequency-domain, phase-modulation fluorometry. The intensity decays were found to be strongly Na+ dependent, with mean lifetime increasing from 1.13 ns in the absence of Na+ to 2.39 ns in the presence of 140 mM Na+. Detailed analysis of the intensity decays in the presence of Na+ and K+ in the concentration range from 0 to 500 mM is provided. Sodium sensing using data measured at a single modulation frequency is described. Phase and modulation data showed high sensitivity to Na+ and substantially lower sensitivity to K+. Additionally, exposure of Sodium Green to intense illumination indicated that Sodium Green is much more photostable than its precursor, fluorescein. These results indicate that lifetime-based measurements with Sodium Green can be used for imaging of intracellular free [Na+] in the range from about 0.5 to 50 mM with high accuracy.

Study of nonspecific cation channel coupled to P2z purinergic receptors using an acid load technique

The intracellular pH (pHi) of rat submandibular cells was measured by 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The cells recovered from ammonium (30 mM) prepulse to their resting pHi within 10 min. Ethylisopropylamiloride (EIPA), an inhibitor of the Na+/H+ exchanger, slows the rate of pHi recovery. ATP (1 mM), in the presence of EIPA, increases the rate of recovery 3.7-fold in the absence or presence of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. The recovery was blocked by the addition of 5 mM Mg2+ or 10 microM Coomassie blue. The response was elicited by 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate but not by ADP, UTP, adenyl (beta-gamma-methylene)-diphosphonate, 2-methylthioadenosine 5'-triphosphate, or muscarinic or beta-adrenergic agonists. The purinergic response was also observed when the cells were acidified by sodium propionate and could not be mimicked by the depolarization of the plasma membrane. Aluminum fluoride did not reproduce the response to ATP, suggesting that the observed response does not involve a high-molecular-weight GTP-binding protein. It is concluded that the activation of P2z receptors, probably by the opening of nonspecific cation channels, increases the permeability to protons in rat submandibular glands.