Characteristics and regulation of a muscarinically activated K current in HSG-PA cells

Calcium antagonists cause dry mouth by inhibiting resting saliva secretion

Ca2+ antagonists cause dry mouth by inhibiting saliva secretion. The present study was undertaken to elucidate the mechanism by which Ca2+ antagonists cause dry mouth. Since the intracellular Ca2+ concentration ([Ca2+]i) is closely related to saliva secretion, [Ca2+]i was measured with a video-imaging analysis system by using human submandibular gland (HSG) cells as the material. The Ca2+ antagonist, nifedipine, inhibited the elevation in [Ca2+]i induced by 1-10 microM carbachol (CCh), but had no inhibitory effect on that induced by 30 and 100 microM CCh. The other kinds of Ca2+ antagonists, verapamil (10 microM), diltiazem (10 microM), and the inorganic Ca2+ channel blocker, CdCl2 (50 microM), also inhibited the [Ca2+]i elevation induced by 10 microM CCh. The Ca2+ channel activator, Bay K 8644 (5 microM), significantly enhanced the CCh (10 microM)-induced [Ca2+]i elevation. Endothelin-1 and norepinephrine also increased the CCh (10 microM)-induced [Ca2+]i elevation. SKF-96365 reversed the enhancement of the CCh (10 microM)-induced [Ca2+]i elevation caused by AlF4- and phenylephrine. The phospholipase Cbeta (PLCbeta) inhibitor, U-73122 (5 microM), significantly inhibited the [Ca2+]i elevation induced by 100 microM CCh compared with that induced by 10 microM CCh, while the PLCbeta activator, m-3M3FBS (20 microM), significantly increased the [Ca2+]i elevation induced by 100 microM CCh compared with that induced by 10 microM CCh. We therefore conclude that non-selective cation and voltage-dependent Ca2+ channels are involved in resting salivation and that Ca2+ antagonists depress H2O secretion by blocking the Ca2+ channels and thereby cause dry mouth.

Calcium receptor message, expression and function decrease in differentiating keratinocytes

Calcium-sensing receptor (CaSR) expression and function were studied in proliferating and differentiating cultured human gingival keratinocytes (HGKs). CaSR mRNA and protein were present in proliferating HGKs cultured in 0.03 mM [Ca(2+)] and decreased in cells induced to differentiate by culturing in 1.2 mM [Ca(2+)] for 2 days. CaSR protein was also detected in gingival tissue. Exposure to 10 mM extracellular [Ca(2+)] activated two sequential whole-cell currents. The first was a small, transient calcium release activated calcium current I(CRAC)-like current with an inwardly rectifying I-V curve. The second current was larger with a linear I-V curve. Both currents were significantly decreased in differentiating cells. Neither neomycin nor gadolinium induced changes in whole cell currents nor in intracellular [Ca(2+)], but neomycin inhibited the late large current. Extracellular Ca(2+) increased intracellular [Ca(2+)] of proliferating HGKs in a dose-dependent fashion. Comparison of the time-courses of the whole-cell currents and the intracellular [Ca(2+)] responses indicated both induced currents supported a Ca(2+) influx. Extracellular [Mg(2+)] changes did not affect intracellular [Ca(2+)]. La(3+) and 2-APB inhibited the whole cell current and intracellular [Ca(2+)] changes. The results indicate that the CaSR signaling response likely plays a major role in initiating Ca(2+) induced differentiation responses in HGKs.

Sequential activation of store-operated currents in human gingival keratinocytes

Calcium ion store-activated currents in undifferentiated human gingival keratinocytes were measured with the whole cell patch clamp and fura techniques. Thapsigargin or intracellular inositol 1,4,5-trisphosphate and BAPTA rapidly induced an early transient current with I(CRAC) (calcium release activated calcium ion current) characteristics, and several later, larger sustained currents that depended on the mode of store depletion. Thapsigargin activated two currents within minutes of I(CRAC) activation. The first was a nonspecific cation current, I(NSC). A second conducted Na+ and Cs+, and was partially inhibited by thapsigargin (INa1). Dialysis with inositol 1,4,5-trisphosphate and BAPTA induced a later current that also conducted Na+ and Cs+, but was inhibited by extracellular calcium ion (INa2), with properties consistent with an epithelial Na+ channel current in some cells, and a calcium ion-insensitive Na+ current (INa3). Comparison of thapsigargin-evoked current changes with fura-2/AM results from separate cells indicated that both the I(CRAC) and the later, larger calcium ion conducting currents contributed to changes in intracellular calcium ion concentration, and likely play important parts in calcium ion signaling in undifferentiated keratinocytes.

Identification and regulation of K+ and Cl- channels in human parotid acinar cells

The properties of K+ channels in these cells were studied using patch-clamp methods. Two channels, with conductances of 165+/-13 pS (n=6) and 30+/-1 pS (n=3), were identified in single-channel experiments. In cell-attached patches the reversal potentials were -67+/-8 and -74+/-2 mV for the large and small conductance channel, respectively, suggesting that both channels are K+-selective. The large conductance channel was also shown to be K+-selective in inside-out patches. The open probability (P(o)) of this channel was increased at depolarizing potentials and by increasing intracellular Ca2+ concentration ([Ca2+]i). These properties suggest that the large conductance channel is a 'maxi' Ca2+-activated K+ channel (BK(Ca)). The small conductance channel was not observed in inside-out patches. Carbachol (CCh; 10(-5) M) activated the BK(Ca) channel, but not the small conductance channel, in cell-attached patches. CCh also caused a dose-dependent increase in [Ca2+]i measured by fura-2 in microspectrofluorimetric studies, with a half-maximal response at approximately 3x10(-6) M. Neither isoproterenol (10(-5) M) nor substance P (10(-6) M) affected K+-channel activity or [Ca2+]i. In whole-cell experiments, CCh caused an increase in outward current. Charybdotoxin (10(-7) M), a BK(Ca) blocker, inhibited a large component of the CCh-induced current. A large component of the charybdotoxin-insensitive current may be carried by Ca2+-activated Cl- channels, which were also observed in human parotid acinar cells. The results indicate that BK(Ca) channels make a significant contribution to the whole-cell conductance in human parotid acinar cells.

Using HSV-thymidine kinase for safety in an allogeneic salivary graft cell line

Extreme salivary hypofunction is a result of tissue damage caused by irradiation therapy for cancer in the head and neck region. Unfortunately, there is no currently satisfactory treatment for this condition that affects up to 40,000 people in the United States every year. As a novel approach to managing this problem, we are attempting to develop an orally implantable, fluid-secreting device (an artificial salivary gland). We are using the well-studied HSG salivary cell line as a potential allogeneic graft cell for this device. One drawback of using a cell line is the potential for malignant transformation. If such an untoward response occurred, the device could be removed. However, in the event that any HSG cells escaped, we wished to provide additional patient protection. Accordingly, we have engineered HSG cells with a hybrid adeno-retroviral vector, AdLTR.CMV-tk, to express the herpes simplex virus thymidine kinase (HSV-tk) suicide gene as a novel safety factor. Cells were grown on plastic plates or on poly-L-lactic acid disks and then transduced with different multiplicities of infection (MOIs) of the hybrid vector. Thereafter, various concentrations of ganciclovir (GCV) were added, and cell viability was tested. Transduced HSG cells expressed HSV-tk and were sensitive to GCV treatment. Maximal effects were seen at a MOI of 10 with 50 microM of GCV, achieving 95% cell killing on the poly-L-lactic acid substrate. These results suggest that engineering the expression of a suicide gene in an allogeneic graft cell may provide additional safety for use in an artificial salivary gland device.

Prospects for re-engineering salivary glands

In the last decade, two areas of biomedical research--gene therapy and tissue engineering--have especially captured the imagination of the public. Both areas offer the potential for the treatment of clinical conditions that now are considered impossible or extremely difficult to manage by conventional therapeutic measures. Gene therapy has made remarkable scientific progress in the laboratory, but has yet to realize its enormous clinical promise. Tissue engineering studies have led to some tangible clinical breakthroughs, but the routine replacement of whole internal organs is still well into the future. This report will examine the applications of gene therapy and tissue engineering to salivary glands, with a focus on the repair of irreversible gland damage.

Reversible regulation of P2Y(2) nucleotide receptor expression in the duct-ligated rat submandibular gland

Ligation of the main excretory duct of the rat submandibular gland (SMG) produces a pronounced atrophy that is reversed upon ligature removal. Based on previous studies by our group and others suggesting that P2Y(2) nucleotide receptors are upregulated in response to tissue damage, we hypothesized that P2Y(2) receptor activity and mRNA levels would increase after duct ligation and return to control levels after ligature removal. Our results support this hypothesis. Intracellular Ca(2+) mobilization in response to the P2Y(2) receptor agonist UTP in SMG cells was increased significantly after ligation periods of 1.5 to 7 days, whereas no significant response was observed in the contralateral, nonligated gland. P2Y(2) receptor mRNA, as measured by semiquantitative RT-PCR, increased about 15-fold after 3 days of ligation. These increases reverted to control levels by 14 days after ligature removal. In situ hybridization revealed that the changes in P2Y(2) receptor mRNA abundance occurred mostly in acinar cells, which also were more adversely affected by ligation, including an increase in the appearance of apoptotic bodies. These findings support the idea that P2Y(2) receptor upregulation may be an important component of the response to injury in SMG and that recovery of normal physiological function may signal a decreased requirement for P2Y(2) receptors.

The growth and morphological behavior of salivary epithelial cells on matrix protein-coated biodegradable substrata

The purpose of this study was to examine the growth and morphology of a salivary epithelial cell line (HSG) in vitro on several biodegradable substrata as an important step toward developing an artificial salivary gland. The substrates examined were poly-L-lactic acid (PLLA), polyglycolic acid (PGA), and two co-polymers, 85% and 50% PLGA, respectively. The substrates were formed into 20- to 25-mm disks, and the cells were seeded directly onto the polymers or onto polymers coated with specific extracellular matrix proteins. The two copolymer substrates became friable over time in aqueous media and proved not useful for these experiments. The purified matrix proteins examined included fibronectin (FN), laminin (LN), collagen I, collagen IV, and gelatin. In the absence of preadsorbed proteins, HSG cells did not attach to the polymer disks. The cells, in general, behaved similarly on both PLLA and PGA, although optimal results were obtained consistently in PLLA. On FN-coated PLLA disks, HSG cells were able to form a uniform monolayer, which was dependent on time and FN concentration. Coating of disks with LN, collagen I, and gelatin also promoted monolayer growth. This study defines the conditions necessary for establishing a monolayer organization of salivary epithelial cells with rapid proliferation on a biodegradable substrate useful for tissue engineering.

Salivary gland P2 nucleotide receptors

The effects of ATP on salivary glands have been recognized since 1982. Functional and pharmacological studies of the P2 nucleotide receptors that mediate the effects of ATP and other extracellular nucleotides have been supported by the cloning of receptor cDNAs, by the expression of the receptor proteins, and by the identification in salivary gland cells of multiple P2 receptor subtypes. Currently, there is evidence obtained from pharmacological and molecular biology approaches for the expression in salivary gland of two P2X ligand-gated ion channels, P2Z/P2X7 and P2X4, and two P2Y G protein-coupled receptors, P2Y1 and P2Y2. Activation of each of these receptor subtypes increases intracellular Ca2+, a second messenger with a key role in the regulation of salivary gland secretion. Through Ca2+ regulation and other mechanisms, P2 receptors appear to regulate salivary cell volume, ion and protein secretion, and increased permeability to small molecules that may be involved in cytotoxicity. Some localization of the various salivary P2 receptor subtypes to specific cells and membrane subdomains has been reported, along with evidence for the co-expression of multiple P2 receptor subtypes within specific salivary acinar or duct cells. However, additional studies in vivo and with intact organ preparations are required to define clearly the roles the various P2 receptor subtypes play in salivary gland physiology and pathology. Opportunities for eventual utilization of these receptors as pharmacotherapeutic targets in diseases involving salivary gland dysfunction appear promising.

Regulation of calcium in salivary gland secretion

Neurotransmitter-regulation of fluid secretion in the salivary glands is achieved by a coordinated sequence of intracellular signaling events, including the activation of membrane receptors, generation of the intracellular second messenger, inositol 1,4,5, trisphosphate, internal Ca2+ release, and Ca2+ influx. The resulting increase in cytosolic [Ca2+] ([Ca2+]i) regulates a number of ion transporters, e.g., Ca2+-activated K+ channel, Na+/K+/2Cl- co-transporter in the basolateral membrane, and the Ca2+-activated Cl- channel in the luminal membrane, which are intricately involved in fluid secretion. Thus, regulation of [Ca2+]i is central to the regulation of salivary acinar cell function and is achieved by the concerted activities of several ion channels and Ca2+-pumps localized in various cellular membranes. Ca2+ pumps, present in the endoplasmic reticulum and the plasma membrane, serve to remove Ca2+ from the cytosol. Ca2+ channels present in the endoplasmic reticulum and the plasma membrane facilitate rapid influx of Ca2+ into the cytosol from the internal Ca2+ stores and from the external medium, respectively. It is well-established that prolonged fluid secretion is regulated via a sustained elevation in [Ca2+]i that is primarily achieved by the influx of Ca2+ into the cell from the external medium. This Ca2+ influx occurs via a putative plasma-membrane-store-operated Ca2+ channel which has not yet been identified in any non-excitable cell type. Understanding the molecular nature of this Ca2+ influx mechanism is critical to our understanding of Ca2+ signaling in salivary gland cells. This review focuses on the various active and passive Ca2+ transport mechanisms in salivary gland cells--their localization, regulation, and role in neurotransmitter-regulation of fluid secretion. In addition to a historical perspective of Ca2+ signaling, recent findings and challenging problems facing this field are highlighted.

ATP-dependent activation of K(Ca) and ROMK-type K(ATP) channels in human submandibular gland ductal cells

[Ca(2+)](i) and membrane current were measured in human submandibular gland ductal (HSG) cells to determine the regulation of salivary cell function by ATP. 1-10 microM ATP activated internal Ca(2+) release, outward Ca(2+)-dependent K(+) channel (K(Ca)), and inward store-operated Ca(2+) current (I(SOC)). The subsequent addition of 100 microM ATP activated an inwardly rectifying K(+) current, without increasing [Ca(2+)](i). The K(+) current was also stimulated by ATP in cells treated with thapsigargin in a Ca(2+)-free medium and was blocked by glibenclamide and tolbutamide, but not by charybdotoxin. This suggests the involvement of a Ca(2+)-independent, sulfonylurea-sensitive K(+) channel (K(ATP)). UTP mimicked the low [ATP] effects, while benzoyl-ATP activated internal Ca(2+) release, a Ca(2+) influx pathway, and K(Ca). Thus, ATP acts via P(2U) (P2Y(2)) and P(2Z) (P2X(7)) receptors to increase [Ca(2+)](i) and activate K(Ca), but not K(ATP). Importantly, (i) ROMK1 and the cystic fibrosis transmembrane regulator protein (but not SUR1, SUR2A, or SUR2B) and (ii) cAMP-stimulated Cl(-) and K(+) currents were detected in HSG cells. These data demonstrate for the first time that a ROMK-type K(ATP) channel is present in salivary gland duct cells that is regulated by extracellular ATP and possibly by the cystic fibrosis transmembrane regulator. This reveals a potentially novel mechanism for K(+) secretion in these cells.

Regulation of KCa current by store-operated Ca2+ influx depends on internal Ca2+ release in HSG cells

This study examines the Ca2+ influx-dependent regulation of the Ca2+-activated K+ channel (KCa) in human submandibular gland (HSG) cells. Carbachol (CCh) induced sustained increases in the KCa current and cytosolic Ca2+ concentration ([Ca2+]i), which were prevented by loading cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA). Removal of extracellular Ca2+ and addition of La3+ or Gd3+, but not Zn2+, inhibited the increases in KCa current and [Ca2+]i. Ca2+ influx during refill (i.e., addition of Ca2+ to cells treated with CCh and then atropine in Ca2+-free medium) failed to evoke increases in the KCa current but achieved internal Ca2+ store refill. When refill was prevented by thapsigargin, Ca2+ readdition induced rapid activation of KCa. These data provide further evidence that intracellular Ca2+ accumulation provides tight buffering of [Ca2+]i at the site of Ca2+ influx (H. Mogami, K. Nakano, A. V. Tepikin, and O. H. Petersen. Cell 88: 49-55, 1997). We suggest that the Ca2+ influx-dependent regulation of the sustained KCa current in CCh-stimulated HSG cells is mediated by the uptake of Ca2+ into the internal Ca2+ store and release via the inositol 1,4,5-trisphosphate-sensitive channel.

Mycoplasma orale infection affects K+ and Cl- currents in the HSG salivary gland cell line

The relations between K+ channel and Cl- channel currents and mycoplasma infection status were studied longitudinally in HSG cells, a human submandibular gland cell line. The K+ channel currents were disrupted by the occurrence of mycoplasma infection: muscarinic activation of K+ channels and K+ channel expression as estimated by ionomycin- or hypotonically induced K+ current responses were all decreased. Similar decreases in ionomycin- and hypotonically induced responses were observed for Cl- channels, but only the latter decrease was statistically significant. Also, Cl- currents could be elicited more frequently than K+ currents (63% of cases versus 0%) in infected cells when tested by exposure to hypotonic media, indicating that mycoplasma infection affects K+ channels relatively more than Cl- channels. These changes occurred in the originally infected cells, were ameliorated when the infection was cleared with sparfloxacin, and recurred when the cells were reinfected. Such changes would be expected to result in hyposecretion of salivary fluid if they occurred in vivo.

Hypotonically activated chloride current in HSG cells

Hypotonically induced changes in whole-cell currents and in cell volume were studied in the HSG cloned cell line using the whole-cell, patch clamp and Coulter counter techniques, respectively. Exposures to 10 to 50% hypotonic solutions induced dose-dependent increases in whole-cell conductances when measured using K+ and Cl- containing solutions. An outward current detected at 0 mV, corresponded to a K+ current which was transiently activated, (usually preceding activation of an inward current and had several characteristics in common with a Ca(2+)-activated K+ current we previously described in these cells. The hypotonically induced inward current had characteristics of a Cl- current. This current was inhibited by NPPB (5-nitro-2-(3-phenyl-propylamino)-benzoate) and SITS (4-acetamido-4'-isothiocyanostilbene), and its reversal potentials corresponded to the Cl- equilibrium potentials at high and low external Cl- concentrations. The induced current inactivated at voltages greater than +80 mV, and the I-V curve was outwardly rectifying. The current was unaffected by addition of BAPTA or removal of GTP from the patch pipette, but was inhibited by removal of ATP or by the presence of extracellular arachidonic acid, quinacrine, nordihydroguairetic acid, and cytochalasin D. Moreover, exposure of HSG cells to hypotonic media caused them to swell and then to undergo a regulatory volume decrease (RVD) response. Neither NPPB, SITS or quinine acting alone could inhibit RVD, but NPPB and quinine together totally inhibited RVD. These properties, plus the magnitudes of the induced currents, indicate that the hypotonically induced K+ and Cl- currents may underlie the RVD response. Cytochalasin D also blocked the RVD response, indicating that intact cytoskeletal F-actin may be required for activation of the present currents. Hence, our results indicate that hypotonic stress activates K+ and Cl- conductances in these cells, and that the activation pathway for the K+ conductance apparently involves [Ca2+], while the activation pathway for the Cl- conductance does not involve [Ca2+] nor lipoxygenase metabolism, but does require intact cytoskeletal F-actin.