ATP-dependent calcium transport in rat parotid basolateral membrane vesicles is modulated by membrane potential

Characterization of nitric oxide synthase in the rat parotid gland

Nitric oxide (NO) acts as an inter- and intracellular signalling molecule of various cells such as vascular endothelium, macrophages, and neurones. NO is produced by nitric oxide synthase (NOS) from L-arginine. Here the characteristics of NOS in the rat parotid gland were investigated. Approximately 74% of total activity of NOS was present in the cytosolic fraction. For full activation of the NOS in the cytosolic fraction, tetrahydroxybiopterin, NADPH, Ca(2+) and calmodulin were needed as cofactors, because the activity was clearly reduced in the absence of tetrahydroxybiopterin, NADPH, or Ca(2+), or in the absence of calmodulin and presence of trifluoperazine, a calmodulin antagonist, in the reaction mixture. The partially purified NOS activity was completely abolished in the absence of calmodulin or Ca(2+), and activated by them in a dose-dependent manner; EC(50) for calmodulin and Ca(2+) were 10 and 340 nM, respectively. The K(m) for L-arginine was 1.57 microM. Immunoblot analysis revealed that a 165-kDa protein band in the rat parotid gland cytosolic fraction cross-reacted with a rabbit polyclonal antibody against human brain NOS. These results suggest that NOS of the rat parotid gland is a neuronal isoform and that its activity is regulated by physiological concentrations of calmodulin and Ca(2+).

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.

A pilot study on the effect of radiation on calmodulin in rat submandibular salivary glands

Xerostomia and loss of salivary gland secretion is one of the most common complications of the radiation treatment of head-and-neck malignancies. The secretory mechanism in the salivary glands can be modulated by the concentration of intracellular Ca2+. Calmodulin is a calcium-binding protein that is widely distributed in nature and is involved in regulating intracellular calcium. In this study the effect of radiation on the concentration of calmodulin in rat salivary glands was investigated. Fourteen rats were divided into three groups: R1 (n = 4) and R2 (n = 5) received a single dose of 15 Gy and group C (n = 5) received no radiation. R1 and R2 animals were killed at weeks 2 and 10 post-irradiation, respectively. The submandibular glands were removed, homogenized and their total calmodulin was determined. The mean calmodulin concentrations were 6.4+/-1.1 microg/gland for controls, 14.1+/-3.7 microg/gland for R1 and 68.2+/-14.4 microg/gland for R2. Kruskal-Wallis ANOVA revealed a significant increase in the concentration of calmodulin following irradiation (p = 0.003). The relationship between this increase and the loss of salivary gland function is not yet clear.

Effects of calcium and calmodulin on the binding of rat parotid secretion granules to the plasma membrane

Since numerous studies suggest that Ca++ and calmodulin may modulate the fusion of secretion granules to the plasma membrane which takes place in exocytosis, we have examined the role of calcium and calmodulin in the binding of isolated parotid secretion granules to plasma membrane vesicles. 125I-labeled inside-out plasma membrane vesicles were incubated with secretion granules, the mixture was layered over 20% sucrose, the gradient was centrifuged, and the amount of 125I in the granule pellet was determined. Addition of Ca++ (20 nM to 10 microM) produced a concentration-dependent increase in the binding of 125I-labeled plasma membrane vesicles to the secretion granules, reaching a maximum value at 10 microM free Ca++; half-maximal binding occurred at 400 nM. Neither right-side-out parotid plasma membrane vesicles nor inside-out pancreatic islet plasma membrane vesicles bound to granules in the presence of 1 microM Ca++. Calmodulin produced a concentration-dependent increase in binding above that of Ca++ alone, and this effect was inhibited by the calmodulin antagonists, trifluoperazine and calmidazolium. Incubation of secretion granules with octadecylrhodamine B (R18)-loaded inside-out plasma membrane vesicles and 2 microM Ca++ caused de-quenching of fluorescence, indicating that the lipids in the granule membrane and the plasma membrane had intermixed. Added calmodulin increased the fluorescence two-fold above that with Ca++ alone. These results suggest that Ca++ and calmodulin may play a role in parotid gland exocytosis by modulating the interaction between the secretion granules and plasma membrane.

Calcium entry in rabbit corneal epithelial cells: evidence for a nonvoltage dependent pathway

We performed experiments to elucidate the calcium influx pathways in freshly dispersed rabbit corneal epithelial cells. Three possible pathways were considered: voltage-gated Ca++ channels, Na+/Ca++ exchange, and nonvoltage-dependent Ca(++)-permeable channels. Whole cell inward currents carrying either Ca++ or Ba++ were not detected using voltage clamp techniques. We also used imaging technology and the Ca(++)-sensitive ratiometric dye fura 2 to measure changes in intracellular Ca++ concentration ([Ca]i). Bath perfusion with NaCl Ringer's solution containing the calcium channel agonist Bay-K-8644 (1 microM), or Ni++ (40 microM), a blocker of many voltage-dependent calcium channels, did not affect [Ca++]i. Membrane depolarization with a KCl Ringer's bath solution resulted in a decrease in [Ca++]i. These results are inconsistent with the presence of voltage gated Ca++ channels. Nonvoltage gated Ca++ entry, on the other hand, would be reduced by membrane depolarization and enhanced by membrane hyperpolarization. Agents which hyperpolarize via stimulation of K+ current, such as flufenamic acid, resulted in an increase in ratio intensity. The cells were found to be permeable to Mn++ and bath perfusion with 5 mM Ni++ decreased [Ca++]i suggesting that the Ca++ conductance was blocked. These results are most consistent with a nonvoltage gated Ca++ influx pathway. Finally, replacing extracellular Na+ with Li+ resulted in an increase in [Ca++]i if the cells were first Na(+)-loaded using the Na+ ionophore monensin and ouabain, a Na(+)-K(+)-ATPase inhibitor. These results suggest that Na+/Ca++ exchange may also regulate [Ca++]i in this cell type.

Involvement of carboxyl groups in the divalent cation permeability of rat parotid gland basolateral plasma membrane

Divalent cation permeability of rat parotid gland basolateral plasma membranes was examined in dispersed parotid acini (by Ca2+ or Mn2+ entry) and in isolated basolateral plasma membrane vesicles (BLMV, by 45Ca2+ influx). Mn2+ entry (fura2 quenching) was about 1.6 fold higher in internal Ca2+ pool-depleted acini (Ca(2+)-depl acini) than in unstimulated cells. Mn2+ entry into Ca(2+)-depl acini was increased at external pH > 7.4 and decreased at pH < 7.4. Pretreatment of Ca(2+)-depl acini with the relatively hydrophobic carboxylic group reagent, N,N'-dicyclohexylcarbodiimide (DCCD, 50 microM for 30 min) resulted in the inhibition of Mn2+ entry into Ca(2+)-depl acini to unstimulated levels. Another hydrophobic carboxyl group reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) and the relatively hydrophilic carboxyl group reagents, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide (CMCD) did not affect Mn2+ entry. Similar to the effects in intact acini, Ca2+ influx into BLMV was decreased when the external pH was lowered below 7.4. Also DCCD (5 mM, 30 min), but not EEDQ, decreased (40%) Ca2+ influx in BLMV. However, unlike in acini, the hydrophilic reagents, EDC, EAC, and CMCD decreased Ca2+ permeability in BLMV and the effects were nonadditive with the decrease induced by DCCD. The aggregate effects of carboxyl group reagents on the Ca2+ and Mn2+ permeability in BLMV and intact acini, respectively, suggest that a critical carboxyl group (most likely accessible from the cytoplasmic side of the plasma membrane) is involved in divalent cation flux in rat parotid acinar cells.

Membrane potential regulates Ca2+ uptake and inositol phosphate generation in rat sublingual mucous acini

In salivary acinar cells, muscarinic-induced fluid secretion is associated with a 1,4,5-IP3 induced increase in the cytosolic free Ca2+ concentration ([Ca2+]i), which in turn activates Ca(2+)-dependent K+ and Cl- channels that modulate the membrane potential. In the present study the influence of the membrane potential on [Ca2+]i and inositol phosphates was monitored in rat sublingual mucous acini. Depolarization induced by switching from 5.8 mM extracellular K+ ([K+]e) to 116 mM [K+]e resulted in a transient increase in the [Ca2+]i measured using the Ca2+ sensitive fluorescent indicator Fura-2. This initial rapid (t1/2 approximately 5 s) increase (approximately 3-fold) in [Ca2+]i was dependent on extracellular Ca2+, insensitive to nifedipine, and followed by establishment of a 'new' resting [Ca2+]i, approximately 35% higher than the level in physiological [K+]e. Depolarization also induced a significant rise in the resting cellular inositol trisphosphate (IP3) and inositol tetrakisphosphate (IP4) contents, but not 1,4,5-IP3 content. Stimulation with 10 microM carbachol (CCh, a muscarinic agonist) produced a biphasic increase in [Ca2+]i, the initial transient phase due to mobilization of Ca2+ from an intracellular pool, and a sustained phase mediated by an influx of Ca2+. Membrane depolarization had no effect on the initial phase, while, the sustained increase in [Ca2+]i was eliminated. The CCh-enhanced quench of the Fura-2 signal by Mn2+ (an index for divalent cation entry) was reversibly inhibited by depolarization. The enhanced Mn2+ uptake induced by inhibiting microsomal Ca(2+)-ATPase with thapsigargin was similarly inhibited by membrane depolarization, consistent with the effect of depolarization primarily acting on the Ca2+ entry pathway and not on receptor coupling. Depolarization did not alter the initial CCh-induced increases in IP3, IP4 or 1,4,5-IP3 content, or the sustained increase in 1,4,5-IP3, whereas, depolarization significantly blunted (> 70%) the sustained, CCh-induced generation of IP3 and IP4. The membrane potential, therefore, appears to modulate Ca2+ activated fluid secretion by controlling the driving force for Ca2+ entry via a depletion-activated Ca2+ entry pathway. Inositol phosphate metabolism is also influenced by the membrane potential, but this effect apparently plays a minor role in regulating [Ca2+]i since 1,4,5-IP3 levels were unchanged by depolarization.

Ca2+ permeability of rat parotid gland basolateral plasma membrane vesicles is modulated by membrane potential and extravesicular [Ca2+]

This study examines the Ca2+ permeability of basolateral plasma membrane vesicles (BLMVs) isolated from the rat parotid gland by monitoring the rate of 45Ca2+ efflux from actively-loaded (via the Ca(2+)-ATPase) inside-out BLMVs. Ca2+ efflux from BLMVs into a K(+)-gluconate medium which hyperpolarizes the cytoplasmic side (i.e. outside) of the inside-out BLMVs resulted in a faster rate of Ca2+ efflux compared with a control medium containing N-methyl-D-glucamine (NMDG)-gluconate. Conversely, Ca2+ efflux into a medium which depolarizes the cytoplasmic side of the BLMVs (NMDG-chloride) resulted in slower rates of efflux compared with those observed with the control medium. This increased rate of 45Ca2+ efflux from the hyperpolarized BLMV was inhibited by 1 mM Ni2+, yielding a rate of efflux similar to the rate observed in depolarized BLMVs. The rate of Ca2+ efflux from BLMVs was affected by [Ca2+]o ([Ca2+] on the extravesicular, cytoplasmic side of the vesicle). When [Ca2+]o was kept > 200 nM during efflux, the rate of Ca2+ efflux from both hyper- and depolarized BLMVs was slow and relatively unresponsive to changes in [Ca2+]o, despite sizeable changes in the Ca2+ gradient across the BLMV. However, when [Ca2+]o was lowered < 200 nM, there was an abrupt increase in the rate of Ca2+ efflux from both hyper- and depolarized BLMVs. Additionally, when [Ca2+] was < 200 nM, the rate of Ca2+ efflux appeared to be more sensitive to driving force changes. These data suggest that Ca2+ permeability across the rat parotid gland basolateral plasma membrane is modulated by membrane potential and [Ca2+] on the cytoplasmic side.

Cytoplasmic pH influences cytoplasmic calcium in MC3T3-E1 osteoblast cells

We found that the cytoplasmic concentration of calcium (Cai) of MC3T3-E1 osteoblasts was influenced by the type of pH buffer we used in the perfusing medium, suggesting that intracellular pH (pHi) might influence Cai. To study this effect, the Cai and pHi were monitored as we applied various experimental conditions known to change pHi. Exposure to NH4Cl caused a transient increase in both pHi and Cai without a change in extracellular pH (pHo). Decreasing pHo and pHi by lowering the bicarbonate concentration of the medium decreased Cai, and increasing pHi by the removal of 5% CO2 increased Cai. Clamping pHi to known values with 10 microM nigericin, a potassium proton ionophore, also influenced Cai: acid pHi lowered Cai, whereas alkaline pHi increased it. The rise in Cai appears to be very sensitive to the extracellular concentration of calcium, suggesting the existence of a pH-sensitive calcium influx mechanism. We conclude that physiologic changes in pH could modulate Cai by controlling the influx of calcium ions and could change the time course of the Cai transient associated with hormonal activation.

pHi controls cytoplasmic calcium in rat parotid cells

The goal of this investigation was to determine if cytoplasmic pH (pHi) modulated the basal level of the concentration of calcium ions in the cytoplasm (Cai) in rat parotid cells. We investigated the effects of various experimental manipulations on both pHi and Cai as measured with BCECF and the calcium photoprotein aequorin, respectively. We found that various experimental manipulations that increased pHi, such as exposure of the cells to NH4Cl, a decrease of the partial pressure of CO2 or an increase in extracellular pH in the presence of nigericin invariably increased Cai. Moreover, experimental manipulations which lowered Cai, such as a reduction of extracellular [NaHCO3] or the removal of loaded NH4 invariably decreased Cai. Thus pHi and Cai are directly related in parotid cells. Since recent studies have shown that Cai directly influences pHi, we suggest that Cai-handling and pHi-handling are tightly linked in parotid cells.

Membrane electrostatics

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium efflux across the plasma membrane of rat parotid acinar cells is unaffected by receptor activation or by the microsomal calcium ATPase inhibitor, thapsigargin

The rate of Ca2+ extrusion across the plasma membrane of rat parotid acinar cells was determined by measuring the decay of the intracellular calcium concentration, [Ca2+]i, following the addition of EGTA to agonist stimulated cells. In the presence of extracellular Ca2+, the muscarinic cholinergic receptor agonist, methacholine, rapidly increased [Ca2+]i (peaking within 5 s), which then decreased to a higher steady state level. This elevated steady state level was dependent on extracellular Ca2+ concentration. Likewise, thapsigargin, a non-phorbol ester tumor promoter that does not increase inositol phosphates, gradually increased [Ca2+]i, peaking within 1 min and then declining to a new elevated plateau level which was also dependent on extracellular Ca2+. [Ca2+]i, elevated by methacholine or thapsigargin, was rapidly decreased by the addition of EGTA by a process the kinetics of which depended on the value of [Ca2+]i before the addition of EGTA. That is, [Ca2+]i increased as a function of the extracellular Ca2+ concentration and also the apparent half-time for Ca2+ extrusion following the addition of EGTA to cells was increased as the [Ca2+]i increased. This presumably reflects the saturable nature of the Ca2+ extrusion mechanism. The steady state [Ca2+]i in cells stimulated with methacholine or thapsigargin in nominally Ca2+ free medium was similar to the steady state [Ca2+]i in unstimulated cells in normal, Ca2(+)-containing medium. Under these similar [Ca2+]i conditions, stimulated and unstimulated cells showed a similar time course of decay upon addition of EGTA. In addition, neither methacholine nor phorbol myristate acetate decreased the sustained elevation of [Ca2+]i induced by ionomycin.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional reconstitution of prokaryote and eukaryote membrane proteins

A new strategy for the functional reconstitution of membrane proteins is described. This approach introduces a new class of protein stabilizing agents--osmolytes--whose presence at high concentration (10-20%) during detergent solubilization prevents the inactivations that normally occur when proteins are extracted from natural membranes. Osmolytes that act in this way include compounds such as glycerol and higher polyols (erythritol, xylitol, sorbitol), sugars (glucose, trehalose), and certain amino acids (glycine, proline, betaine). The beneficial effects of osmolytes are documented by reconstitution of a variety of prokaryote and eukaryote membrane proteins, including several proton- and calcium-motive ATPases, cation- and anion-linked solute carriers (symport and antiport), and a membrane-bound hydrolase from endoplasmic reticulum. In all cases, the presence of 20% glycerol or other osmolyte during detergent solubilization led to 10-fold or more increased specific activity in proteoliposomes. These positive effects did not depend on use of any specific detergent for protein solubilization, nor on any particular method of reconstitution, but for convenience most of the work reported here has used octylglucoside as the solubilizing agent, followed by detergent-dilution to form proteoliposomes. The overall approach outlined by these experiments is simple and flexible. It is now feasible to use reconstitution as an analytical tool to study the biochemical and physiological properties of membrane proteins.

ATP-dependent Ca2+ transport in the rat parotid basolateral plasma membrane is regulated by calmodulin

Calmodulin regulation of ATP-dependent Ca2+ transport activity was assessed in inverted basolateral plasma membrane vesicles (BLMV) isolated from rat parotid glands. The initial rate of Ca2+ transport in media containing 100 nM Ca2+ was stimulated by approximately 60% at maximal concentrations (300 nM) of exogenously added calmodulin (CAM). Half-maximal activation was obtained at 50 and 175 nM CAM in KCl and mannitol containing assay media, respectively. In the KCl medium, addition of 300 nM CAM increased the affinity of the BLMV Ca2+ transport activity for Ca2+ from approximately 70 nM, in the absence of added CAM, to approximately 50 nM. Vmax was consistently increased by approximately 20% under these conditions. When BLMV were treated with ethylene glycol bis(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) (200 microM), the affinity of the transporter for Ca2+ decreased by 50% to approximately 150 nM, with no change in Vmax. When CAM was added to the EGTA-treated membranes, Ca2+ transport activity was comparable to that obtained when CAM was added directly to control, untreated BLMV. The CAM antagonists, trifluoperazine (TFP), W-7, and calmidazolium, inhibited Ca2+ transport in the presence of CAM. Half-maximal inhibition of transport was achieved by 12 microM TFP and 20 microM W-7. Calmidazolium (1 microM) inhibited Ca2+ transport by 75%. The inhibitory effects on ATP-dependent Ca2+ transport exerted by these agents were not due to an increase in the passive permeability of the membranes to Ca2+. Furthermore, in the absence of added CAM, the inhibitory effects of these agents on initial Ca2+ transport rate was decreased. The data presented suggest that the Ca2+-dependent interaction of CAM with the ATP-dependent Ca2+ transporter in rat parotid BLMV modifies the kinetic properties of this Ca2+ transporting mechanism.

Modulation of ATP-dependent Ca2+ transport in rat parotid basolateral membrane vesicles by K+ + Cl- flux

In basolateral membrane vesicles (BLMV) isolated from rat parotid glands, the initial rate of ATP-dependent Ca2+ transport, in the presence of KCl, was approx. 2-fold higher than that obtained with mannitol, sucrose or N-methyl-D-glucamine (NMDG)-gluconate. Only NH4+, Rb+, or Br- could effectively substitute for K+ or Cl-, respectively. This KCl activation was concentration dependent, with maximal response by 50 mM KCl. An inwardly directed KCl gradient up to 50 mM KCl had no effect on Ca2+ transport, while equilibration of the vesicles with KCl (greater than 100 mM) increased transport 15-20%. In presence of Cl-, 86Rb+ uptake was 2.5-fold greater than in the presence of gluconate. 0.5 mM furosemide inhibited 86Rb+ flux by approx. 60% in a Cl- medium and by approx. 20% in a gluconate medium. Furosemide also inhibited KCl activation of Ca2+ transport with half maximal inhibition either at 0.4 mM or 0.05 mM, depending on whether 45Ca2+ transport was measured with KCl (150 mM) equilibrium or KCl (150 mM) gradient. In a mannitol containing assay medium, potassium gluconate loaded vesicles had a higher (approx. 25%) rate of Ca2+ transport than mannitol loaded vesicles. Addition of valinomycin (5 microM) to potassium gluconate loaded vesicles further stimulated (approx. 30%) the Ca2+ transport rate. These results suggest that during ATP dependent Ca2+ transport in parotid BLMV, K+ can be recycled by the concerted activities of a K+ and Cl- coupled flux and a K+ conductance.