Electrical resistance of rabbit submaxillary main duct: a tight epithelium with leaky cell membranes

The structure of tight junctions in mouse submandibular gland

Salivary gland cells are joined by junctional complexes consisting of a tight junction (TJ), zonula adherens and one or more desmosomes. TJs regulate paracellular permeability, maintain separate apical and basolateral membrane domains, and serve as signaling centers. We examined TJs of mouse submandibular glands (SMG) in thin sections and freeze-fracture replicas. TJs between acinar cells and between intercalated duct cells had 2-6 parallel strands on the protoplasmic fracture face, with occasional branches, interconnections and free ends, and corresponding grooves on the extracellular face. Granular duct cell TJs had 2-30 strands, a depth of <or=0.5 microm, and occasional loops extending further basally. Where 3 or 4 cells met, the TJs extended basally <or=1 microm and consisted of 2 parallel boundary strands into which the apical strands inserted. Quantitative analyses showed significant differences in TJ complexity, measured by fractal geometry, and strand number of acinar compared to granular duct cells, and a greater number of strands in male compared to female granular ducts. Pilocarpine stimulation increased TJ strand number in female acinar cells, and increased complexity of male granular duct cell TJs. As the salivary gland water channel aquaporin 5 (AQP5) has been proposed to functionally interact with TJs to regulate salivary fluid composition, we also studied glands from AQP5 knock-out mice. In males lacking AQP5, granular duct TJs were more complex than those of wild-type mice, and exhibited more strands following pilocarpine stimulation. The results demonstrate specific gender, cell type and genetic differences in TJ structure and response to stimulation.

Distribution of tight junction proteins in adult human salivary glands

Tight junctions (TJs) are an essential structure of fluid-secreting cells, such as those in salivary glands. Three major families of integral membrane proteins have been identified as components of the TJ: claudins, occludin, and junctional adhesion molecules (JAMs), plus the cytosolic protein zonula occludens (ZO). We have been working to develop an orally implantable artificial salivary gland that would be suitable for treating patients lacking salivary parenchymal tissue. To date, little is known about the distribution of TJ proteins in adult human salivary cells and thus what key molecular components might be desirable for the cellular component of an artificial salivary gland device. Therefore, the aim of this study was to determine the distribution of TJ proteins in human salivary glands. Salivary gland samples were obtained from 10 patients. Frozen and formalin-fixed paraffin-embedded sections were stained using IHC methods. Claudin-1 was expressed in ductal, endothelial, and approximately 25% of serous cells. Claudins-2, -3, and -4 and JAM-A were expressed in both ductal and acinar cells, whereas claudin-5 was expressed only in endothelial cells. Occludin and ZO-1 were expressed in acinar, ductal, and endothelial cells. These results provide new information on TJ proteins in two major human salivary glands and should serve as a reference for future studies to assess the presence of appropriate TJ proteins in a tissue-engineered human salivary gland.

Characterization of murine autologous salivary gland graft cells: a model for use with an artificial salivary gland

The purpose of this study was to examine the growth and key functional abilities of primary cultures of salivary epithelial cells toward developing an artificial salivary gland. Cultures of epithelial cells originating from submandibular glands of BALB/c mice were established. Parenchymal cells were isolated by a Percoll gradient technique and thereafter seeded on irradiated NIH 3T3 fibroblasts serving as a feeder layer. The isolated cells were termed autologous salivary gland epithelial (ASGE) cells and could be cultivated for at least five passages (time limit of experiments). ASGE cells presented the typical organizational behavior of epithelial cells and electron microscopy, as well as immunostaining for cytokeratins, confirmed their epithelial origin. Furthermore, measurements of transepithelial resistance and water permeability indicated the ability of the ASGE cells to form a functional epithelial barrier. This study suggests that primary salivary epithelial cells can be obtained that exhibit critical characteristics needed for use with an artificial secretory device.

Membrane-specific regulation of Cl- channels by purinergic receptors in rat submandibular gland acinar and duct cells

Measurement of [Cl-]i and the Cl- current in the rat salivary submandibular gland (SMG) acinar and duct cells was used to evaluate the role of Cl- channels in the regulation of [Cl-]i during purinergic stimulation. Under resting conditions [Cl-]i averaged 56 +/- 8 and 26 +/- 7 mM in acinar and duct cells, respectively. In both cells, stimulation with 1 mM ATP resulted in Cl- efflux and subsequent influx. Inhibition of NaKCl2 cotransport had no effect on [Cl-]i changes in duct cells and inhibited only about 50% of Cl- uptake in acinar cells. Accordingly, low levels of expression of NaKCl2 cotransporter protein were found in duct cells. Acinar cells expressed high levels of the cotransporter. Measurement of Cl- current under selective conditions revealed that acinar and duct cells express at least five distinct Cl- channels; a ClCO-like, volume-sensitive, inward rectifying, Ca2+-activated and CFTR-like Cl- currents. ATP acting on both cell types activated at least two channels, the Ca2+-activated Cl- channel and a Ca2+-independent glibenclamide-sensitive Cl--current, possibly cystic fibrosis transmembrane regulator (CFTR). Of the many nucleotides tested only 2'-3'-benzoylbenzoyl (Bz)-ATP and UTP activated Cl- channels in SMG cells. Despite their relative potency in increasing [Ca2+]i, BzATP in both SMG cell types largely activated the Ca2+-independent, glibenclamide-sensitive Cl- current, whereas UTP activated only the Ca2+-dependent Cl- current. We interpret this to suggest that BzATP and UTP largely activate Cl- channels residing in the membrane expressing the receptor for the active nucleotide. The present studies reveal a potentially new mechanism for transcellular Cl- transport in a CFTR-expressing tissue, the SMG. Coordinated action of the P2z (luminal) and P2u (basolateral) receptors can mediate part of the transcellular Cl- transport by acinar and duct cells to determine the final electrolyte composition of salivary fluid.

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.

Characterization of the Cl- conductance in the granular duct cells of mouse mandibular glands

We have previously shown that mouse mandibular granular ducts contain a hyperpolarization-activated Cl- conductance. We now show that the instantaneous current/voltage (I/V) relation of this Cl- conductance is inwardly rectifying with a slope conductance of 15.4 +/- 1.8 nS (n = 4) at negative potentials and of 6.7 +/- 0.9 nS (n = 4) at positive potentials. Thus, the inward rectification seen in the steady-state I/V relation is due, not only to voltage activation of the Cl- conductance, but also to the intrinsic conductance properties of the channel. We show further that the ductal Cl- conductance is not activated by including ATP (10 mmol/l) in the pipette solution. Finally, we show that the conductance is not blocked by the addition of any of the following compounds to the extracellular solution: anthracene-9-carboxylate (A9C, 1 mmol/l), diphenylamine-2-carboxylate (DPC, 1 mmol/l), 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 mumol/l), 4,4'-diisothiocyanato-stilbene-2,2'-disulphonate (DIDS, 100 mumol/l), indanyloxyacetic acid (IAA-94, 100 mumol/l), verapamil (100 mumol/l), glibenclamide (100 mumol/l) and Ba2+ (5 mmol/l). The properties of the ductal Cl- conductance most nearly resemble those of the ClC-2 channel. Both channel types have instantaneous I/V relations that are slightly inwardly rectifying, are activated by hyperpolarization with a time-course in the order of hundreds of milliseconds, have a selectivity sequence of Br- > Cl- > I-, and are insensitive to DIDS. The only identified difference between the two is that the ClC-2 channel is 50% blocked both by DPC and A9C (1 mmol/l), whereas the ductal Cl- conductance is insensitive to these compounds.

Intracellular Cl- concentration in striated intralobular ducts from rabbit mandibular salivary glands

Intralobular striated ducts have been isolated from rabbit mandibular salivary glands and maintained in primary culture for up to 2 days. Such ducts were loaded with the Cl(-)-sensitive fluorescent dye N-(ethoxycarbonylmethyl)-(6-methoxyquinolinium bromide) (MQAE) and intracellular Cl- concentration ([Cl-]i) monitored using a fluorescence microscope. Intracellular Cl- could be rapidly and reversibly emptied from striated duct cells by replacing Cl- in the superfusing solution with NO(3)-. [Cl-]i could be lowered by removal of external Na+, exposure to 10 microM amiloride or to 10 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). Both amiloride and DIDS were able to inhibit the recovery of [Cl-]i after an initial exposure to Na(+)- or Cl(-)-free solution. The amiloride derivatives, benzamil (2 microM) and N-isobutyl-N-methylamiloride (MIBA), (10 microM) also lowered [Cl-]i by similar amounts as 10 microM amiloride. Varying external K+ concentration ([K+]o) also affected [Cl-]i. Increasing [K+]o increased [Cl-]i, but decreasing [K+]o did not decrease [Cl-]i. Instead, [Cl-]i was also increased when [K+]o was lowered below the control value. Bumetanide (0.1 mM) lowered [Cl-]i by only a small amount, while ouabain (1 mM) had no significant effect on [Cl-]i. These data are consistent with current models of electrolyte transport in salivary ducts which include Cl- channels, Na+ channels, and Na+/H+ exchangers in the apical membrane. The effects of low [K+]o can be interpreted in terms of a K(+)-dependent exit mechanism for Cl-.

Sidedness of action of loop diuretics and ouabain on nonsensory cells of utricle: a micro-Ussing chamber for inner ear tissues

It is known that nonsensory tissues of the utricle produce a lumen-positive transepithelial electrical potential difference (VT). This potential has been shown previously to be inhibited by ouabain and bumetanide applied to the bathing medium in vitro. In order to more fully characterize the origin of this potential we mounted the utricle as a flat sheet in a newly designed Ussing chamber and measured the VT and transepithelial resistance (RT) while perfusing the endolymphatic and perilymphatic surfaces independently with identical solutions. The aperture of the chamber was 1.5 X 10(-4) cm2. VT averaged 5.6 +/- 0.46 mV and RT was 24.0 +/- 2.43 omega X cm2 (n = 45). Ouabain and loop diuretics of the furosemide family were found to inhibit the VT only from the serosal side. The KI for ouabain was 7.63 X 10(-5) M. The loop diuretics tested inhibited the VT in the same order as in other tissues known to contain a Na/2 Cl/K cotransporter (KI: 2-benzylamino-4-cyclohexylsulfonyl-5-sulfamoylbenzolsulfonate++ + (BCSB), 1.72 X 10(-7) M; bumetanide, 1.10 X 10(-6) M; piretanide, 5.67 X 10(-6) M; furosemide, 4.14 X 10(-5) M). It is concluded that this tissue produces a lumen-positive VT (i) in the absence of a transepithelial chemical gradient; the generation of which is dependent upon the activity of (ii) Na,K-ATPase and (iii) a Na/2 Cl/K cotransporter; (iv) in the basolateral membranes of the nonsensory cells; (v) which is not depressed by luminal application of inhibitors of these transporters.

Voltage sensitive, high-conductance chloride channels in the luminal membrane of cultured pulmonary alveolar (type II) cells

The properties of ion channels in patches from the luminal membrane of cultured monolayers of type II alveolar epithelial cells from adult rat lung have been studied. In excised patches, prepared as described to yield the "outside-out" configuration (with cesium ions in the bathing solutions to suppress currents arising from potassium channels), we observed anion-selective channels with a conductance of 350-400 pS, and burst lengths lasting seconds. When patches were bathed in solutions with equal chloride concentrations, channels opened and closed spontaneously at membrane voltages close to zero, but tended to close when the potential was shifted by +/- 10 mV, particularly in the negative direction. Other anions could pass through these channels with a permeability sequence of I(1.5) greater than Br(1.02) = greater than Cl(1.00) greater than NO3(0.9) greater than gluconate(0.6) but there was a very low permeability for sodium (PNa/PCl = 0.015 +/- 0.006). Calcium ions reduced channel conductance. The channels probably have a role to play in salt absorption by this epithelium, but insufficient information is available about how sodium crosses it for the role of the anion channels in transport to be defined with confidence.

Alteration of tight junctional permeability in the rat parotid gland after isoproterenol stimulation

The permeability of junctional complexes to ultrastructural tracers of different molecular weight and the freeze-fracture appearance of junctional structure were investigated in the resting and stimulated rat parotid gland. Tracers were administered retrogradely via the main excretory duct, and allowed to flow by gravity (16 mmHg) into the gland for 15-60 min. Secretion was induced in some animals by intraperitoneal injection of isoproterenol. In resting glands, the tracers microperoxidase , cytochrome c, myoglobin, tyrosinase (subunits), and hemoglobin were restricted to the luminal space of the acini and ducts. In glands stimulated 1-4 h before tracer administration, reaction product for microperoxidase , cytochrome c, myoglobin, and tyrosinase was found in the intercellular and interstitial spaces, whereas hemoglobin was usually retained in the lumina. In contrast, horseradish peroxidase and lactoperoxidase appeared to penetrate the tight junctions and reaction product was localized in the extracellular spaces in both resting and stimulated glands. Diffuse cytoplasmic staining for horseradish peroxidase and lactoperoxidase was frequently observed in acinar and duct cells. The distribution of horseradish peroxidase was similar in both Sprague-Dawley and Wistar-Furth rats, and at concentrations of 0.1-10 mg/ml in the tracer solution. Freeze-fracture replicas of stimulated acinar cells revealed an increased irregularity of the tight junction meshwork, but no obvious gaps or discontinuities were observed. These findings indicate that (a) tight junctions in the resting rat parotid gland are impermeable to tracers of molecular weight greater than or equal to 1,900; (b) stimulation with isoproterenol results in a transient increase in junctional permeability allowing passage of tracers of molecular weight less than or equal to 34,500; (c) junctional permeability cannot be directly correlated with junctional structure; and (d) the behavior of horseradish peroxidase and lactoperoxidase in the rat parotid gland is inconsistent with their molecular weights. Cell membrane damage due to the enzymatic activity or binding of these two tracers may account for the observed distribution.

Distribution of three hexose derivatives across the pancreatic epithelium: paracellular shunts or cellular passage?

It has been proposed that the pancreatic epithelium is permeable to three presumably passively distributed non-electrolytes, namely sucrose, inulin and mannitol, via paracellular shunts, and that the increased flux of sucrose and inulin seen during augmented digestive enzyme secretion is due to an increase in the permeability of these shunts. The present study considers this hypothesis by comparing the permeability of the epithelium to three different hexose derivatives, mannitol, inositol and 3-O-methyl-glucose, in both the unstimulated state and after the augmentation of protein secretion with a cholinergic drug. The epithelium was found to be more permeable to mannitol than to either inositol or 3-O-methyl-glucose. In the unstimulated state, the concentration of mannitol in ductal fluid at the steady state was approx. 54% of its concentration in the interstitium, as compared to 12% for inositol and 8% for 3-O-methyl-glucose. Cholinergic stimulation substantially increased the concentration of inositol and 3-O-methyl-glucose in secretion, but did not increase that of mannitol. The increase in the concentration of inositol occurred in the absence of an increase in its rate of transepithelial movement. Taken together, the results suggest that: (1) there is a substantial passage of mannitol through the cells of the epithelial layer, and (2) the increased concentration of inositol and 3-O-methyl-glucose in ductal fluid that occurs with stimulation is due to an increase in their efflux from secretory cells.

Pancreatic epithelium is permeable to sucrose and inulin across secretory cells

The pancreatic epithelium is permeable to both sucrose and inulin and becomes more permeable when protein secretion by the gland is stimulated. Because these molecules are not thought to enter cells, it has been assumed that their movement across the epithelium from interstitial to ductal fluid, as well as the increase in that flux that is observed during augmented protein secretion, is due to their passage through paracellular shunts. In the present experiments we have considered the alternative possibility that sucrose and inulin travel through the cells of the secretory epithelium instead of, or in addition to, their passage through paracellular shunts. The data support this view and suggest that the pancreas is unusually permeable to water soluble molecules of substantial size by means of a transcellular pathway.

Effect of amiloride on electrolyte transport parameters of the main duct of the rabbit mandibular salivary gland

We have studied the response of the rabbit mandibular main duct perfused in vitro to luminally administered amiloride. The half-maximal inhibitory concentrations (KI) when the duct was bathed in Cl solutions were: for net Na+ transport, 3 X 10(-6) mol l-1; for transepithelial potential difference, 6 X 10(-6) mol l-1; and for transepithelial conductance, 3 X 10(-7) mol l-1. Substitution of the impermeant SO2-(4) anion for Cl- changed the KI for conductance to 3 X 10(-6) mol l-1. Within Cl- -containing media, the time course of the amiloride effect on potential difference showed an early rapid fall of 10 mV with a half-time 2 s, followed by a slower depolarization of 9 mV, and the conductance change followed the slower component of the potential change. In SO2-(4)-containing media, the potential difference and conductance changes followed time courses similar to one another. Finally, experiments on the effect of serosal applications of ouabain revealed that, although, in general, ouabain reduced resistance, it caused an increase in resistance in those ducts where the initial resistance was low. We conclude that: i) luminal Na+ transport occurs via amiloride-sensitive, conductive Na+ channels; ii) the Cl- conductance is the major determinant of transepithelial conductance; iii) the first phase of the potential response is due to blocking of the Na+ conductive channels, whilst the slow phase reflects secondary inhibition of an electrogenic Na+ pump; and iv) duct resistance changes are secondary to alterations in intracellular Cl- concentration.

Occluding junctions in cultured epithelial monolayers

When MDCK cells are cultured in monolayers, they synthesize, assemble, and seal occluding junctions that limit the paracellular route. These processes may be impaired by inhibitors of the protein synthesis but not by inhibitors of the synthesis of RNA. Once established, the occluding junctions confer to the monolayer an overall electrical resistance of 80-600 omega . cm2. At the microscopical level, the resistance of individual junctions have large variations along the perimeter of a given cell. This agrees with the images of freeze-fracture electron microscopy where the network of the junction varies abruptly from 1 to 10 strands. The junctions are impermeable to macromolecular tracers, have a 9 to 1 Na+/Cl- discrimination, and a cation selectivity following the order: K+ greater than Na+ greater than Rb+ greater than Cs+ greater than Li+. Sealing requires extracellular Ca2+, but the junctions open when the concentration of Ca2+ in the cytoplasm increases. The structural components of the cytoskeleton (microtubules and microfilaments) seem to be involved in the junctional events as revealed by staining with immunofluorescent specific antibodies. If the cells are treated with cytochalasin B, actin microfilaments disorganize, the junctions open, and the electrical resistance across the monolayers falls. The resealing of the tight junction is inhibited by this drug.

Influence of aldosterone on epithelial leakiness of rat proximal tubules. Microconductivity Measurements*

Oil blocked superficial segments of late proximal tubules were perfused with salt free isosmolal mannitol solution. Time course of total net ion influx into the lumen after sudden stop of perfusion was monitored by continuous measurement of the conductivity of tubular fluid inside the lumen. Double barreled microelectrodes with high coupling resistance (Rc = 350-450 kOhm) were used as microconductivity probes. The observed time course was interpreted in terms of a two compartment system. Time constants for equilibration were 5.9 plus or minus 2.8 (SD) s in control rats, 14.4 plus or minus 8.0 s in adrenalectomized rats and 5.5 plus or minus 2.3 s in adrenalectomized rats substituted with 100 microgram . kg-1 . d-1 aldosterone for 3 days. The observed effect of aldosterone on the equilibration time constant indicates that this hormone increases the leakiness of proximal tabular epithelium for passive ion influx into the lumen.