Oscillations of cytosolic sodium during calcium oscillations in exocrine acinar cells

A technique for quantifying intracellular free sodium ion using a microplate reader in combination with sodium-binding benzofuran isophthalate and probenecid in cultured neonatal rat cardiomyocytes

Background

Intracellular sodium ([Na⁺]_i) kinetics are involved in cardiac diseases including ischemia, heart failure, and hypertrophy. Because [Na⁺]_i plays a crucial role in modulating the electrical and contractile activity in the heart, quantifying [Na⁺]_i is of great interest. Using fluorescent microscopy with sodium-binding benzofuran isophthalate (SBFI) is the most commonly used method for measuring [Na⁺]_i. However, one limitation associated with this technique is that the test cannot simultaneously evaluate the effects of several types or various concentrations of compounds on [Na⁺]_i. Moreover, there are few reports on the long-term effects of compounds on [Na⁺]_i in cultured cells, although rapid changes in [Na⁺]_i during a period of seconds or several minutes have been widely discussed.

Findings

We established a novel technique for quantifying [Na⁺]_i in cultured neonatal rat cardiomyocytes attached to a 96-well plate using a microplate reader in combination with SBFI and probenecid. We showed that probenecid is indispensable for the accurate measurement because it prevents dye leakage from the cells. We further confirmed the reliability of this system by quantifying the effects of ouabain, which is known to transiently alter [Na⁺]_i. To illustrate the utility of the new method, we also examined the chronic effects of aldosterone on [Na⁺]_i in cultured cardiomyocytes.

Conclusions

Our technique can rapidly measure [Na⁺]_i with accuracy and sensitivity comparable to the traditional microscopy based method. The results demonstrated that this 96-well plate based measurement has merits, especially for screening test of compounds regulating [Na⁺]_i, and is useful to elucidate the mechanisms and consequences of altered [Na⁺]_i handling in cardiomyocytes.

Frequency spectrum of transepithelial potential difference reveals transport-related oscillations

How epithelia transport fluid is a fundamental issue that is unresolved. Explanations offered include molecular engines, local transcellular osmosis, local paracellular osmosis, and paracellular fluid transport. On the basis of experimental and theoretical work done on corneal endothelium, a fluid transporting epithelium, we suggest electroosmotic coupling at the level of the intercellular junctions driven by the transendothelial electrical potential difference as an explanation of paracellular fluid transport. We collect frequency spectra of that potential difference in real-time. For what we believe is the first time for any epithelium, we report that, unexpectedly, the potential difference displays oscillations at many characteristic frequencies. We also show that on both stimulating cell activity and inhibiting ion transport mechanisms, there are corresponding changes in the oscillations amplitudes that mirror changes known previously in rates of fluid transport. We believe these findings provide a novel tool to study the kinetics of electrogenic elements such as channels and transporters, which from this evidence would give rise to current oscillations with characteristic periods going from 150 ms to 8 s.

A mathematical model of fluid secretion from a parotid acinar cell

Salivary fluid secretion is crucial for preventing problems such as dryness of mouth, difficulty with mastication and swallowing, as well as oral pain and dental cavities. Fluid flow is driven primarily by the transepithelial movement of chloride and sodium ions into the parotid acinus lumen. The activation of Cl(-) channels is calcium dependent, with the average elevated calcium concentration during calcium oscillations increasing the conductance of the channels, leading to an outflow of Cl(-). The accumulation of NaCl in the lumen drives water flow by osmosis. We construct a mathematical model of the calcium concentration oscillations and couple this to a model for Cl(-) efflux. We also construct a model governing fluid flow in an isolated parotid acinar cell, which includes a description of the rate of change of intracellular ion concentrations, cell volume, membrane potential and water flow rate. We find that [Ca(2+)] oscillations lead to oscillations in fluid flow, and that the rate of fluid flow is regulated by the average calcium concentration and not the frequency of the oscillations.

Aquaporin expression and cell volume regulation in the SV40 immortalized rat submandibular acinar cell line

The amount of aquaporins present and the cellular ability to perform regulatory volume changes are likely to be important for fluid secretions from exocrine glands. In this work these phenomena were studied in an SV40 immortalized rat submandibular acinar cell line. The regulatory cell volume characteristics have not previously been determined in these cells. Cell volume regulation following hyposmotic exposure and aquaporin induction was examined with Coulter counter methodology, radioactive efflux studies, fura-2 fluorescence, and polymerase chain reaction and Western blot techniques. Cell volume regulation was inhibited by the K(+) channel antagonists quinine and BaCl(2) and the Cl(-) channel blocker 5-nitro-2-(3-phenypropylamino)benzoic acid. A concomitant increase in cellular (3)H-taurine release and Ca(2+) concentration was also observed. Chelation of both intra- and extracellular Ca(2+) with EGTA and the Ca(2+) ionophore A23187 did not, however, affect cell volume regulation. Aquaporin 5 (AQP5) mRNA and protein levels were upregulated in hyperosmotic conditions and downregulated upon return to isosmotic solutions, but were reduced by the mitogen-activated ERK-activating kinase (MEK) inhibitor U0126. A 24-h MEK inhibition also diminished hyposmotically induced cell swelling and cell volume regulation. In conclusion, it was determined that regulatory volume changes in this immortalized cell line are due to KCl and taurine efflux. In conditions that increased AQP5 levels, the cells showed a faster cell swelling and a more complete volume recovery following hyposmotic exposure. This response could be overturned by MEK inhibition.

Alteration by cadmium of rat submandibular gland secretory function and the role of the l-arginine/nitric oxide pathway

The effects of cadmium, l-arginine (nitric oxide precursor) and N(omega)-nitro-l-arginine methyl ester (l -NAME) as a nitric oxide synthesis inhibitor and cotreatment of them on rat submandibular secretory function were studied. Pure submandibular saliva was collected intraorally by micro polyethylene cannula from anaesthetized rats using pilocarpine as secretagogue. Fourteen days treatment with 10 mg l(-1)cadmium as cadmium chloride in drinking water caused significant alterations on salivary function. Salivary flow rate, total protein concentration and amylase activity of saliva were decreased while secretion of calcium was increased by cadmium. Two weeks treatment of rats with l -arginine (2.25%) in drinking water caused an increase in submandibular gland weight. Flow rate was reduced by l-NAME. The total protein concentration of saliva was increased by l-arginine while decreased by l-NAME. Calcium concentration of saliva was reduced by l-arginine and increased by l-NAME. Cotreatment of cadmium with l-arginine prevented cadmium-induced reduction of flow rate while l-NAME cotreatment potentiated cadmium-induced reduction of flow rate. l-arginine showed a preventive effect on cadmium-induced decrease of protein concentration and reached control levels. l-arginine potentiated cadmium-induced increase of saliva calcium concentration. It is confirmed that nitric oxide (NO) has a role in salivary gland function. It is also concluded that cadmium inhibitory effects on salivary gland function are modulated by the NO system as it is observed that the cadmium inhibitory effect on submandibular gland function is diminished by l-arginine and extended by l-NAME. Considering the properties of cadmium substitution for calcium in many intracellular events, different types of alterations can be discussed.

Fluorescence lifetime microscopy of the sodium indicator sodium-binding benzofuran isophthalate in HeLa cells

The behavior of the sodium indicator sodium-binding benzofuran isophthalate (SBFI) is investigated in HeLa cells by time-resolved fluorescence microscopy. The fluorescence relaxation of SBFI in HeLa cells can be described by a triexponential for intracellular sodium concentration ([Na(+)](i)) between 0 and 90 mM. Changes in [Na(+)](i) affect neither the fluorescence relaxation times (0.21, 0. 60, and 2.7 ns) nor the average decay time (2.2 ns). The preexponential factor of the shortest decay time is negative. However, the ratio of the fluorescence excitation signal at 340 nm to that at 380 nm increases with [Na(+)](i). To elucidate the behavior of SBFI in cells, experiments are performed on SBFI in buffer at various concentrations of sodium, potassium, and bovine serum albumin (BSA) and at various viscosities. The fluorescence decay is triexponential only in the presence of BSA. The relaxation times are independent of [Na(+)] and [BSA]. The preexponential factor of the shortest decay time is negative from a certain [BSA] on, which depends on [Na(+)]. The data indicate that interactions with intracellular components rather than microviscosity influence the SBFI behavior in cells. A model is suggested in which the fluorescence intensities are mainly determined by the signals from the Na(+) subsetSBFI and SBFI subsetprotein complexes.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

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

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

[Ca2+]i determines the effects of protein kinases A and C on activity of rat renal Na+,K+-ATPase

1. It is well established that the activity of Na+,K+-ATPase (NKA) is regulated by protein kinases A (PKA) and C (PKC), but results on their effects have been conflicting. The aim of this study was to examine if this is ascribed to the intracellular concentration of Ca2+ ([Ca2+]i). 2. Rat renal NKA was stably expressed in COS cells (green monkey kidney cells). Increases in [Ca2+]i were achieved with the Ca2+ ionophore A23187 and verified by direct measurements of [Ca2+]i using fura-2 AM as an indicator. The activity of NKA was measured as ouabain-sensitive 86Rb+ uptake and the state of phosphorylation of NKA was monitored with two site-directed phosphorylation state-specific antibodies. 3. Activation of PKA with forskolin decreased NKA activity by 45.5 +/- 8.9 % at low [Ca2+]i (120 nM) and increased it by 40.5 +/- 6.4 % at high [Ca2+]i (420 nM). The change in NKA activity by forskolin correlated with the level of increase in [Ca2+]i. 4. The effect of 1-oleoyl-2-acetoyl-sn-glycerol (OAG), a specific PKC activator, on the activity of NKA was also Ca2+ dependent, being inhibitory when [Ca2+]i was low (29.3 +/- 3.6 % decrease at 120 nM Ca2+) and stimulatory when [Ca2+]i was high (36.6 +/- 10.1 % increase at 420 nM Ca2+). 5. The alpha subunit of NKA was phosphorylated under both low and high [Ca2+]i conditions upon PKA or PKC activation. PKA phosphorylates Ser943. PKC phosphorylates Ser23. 6. To see if the observed effects on NKA activity are secondary to changes in Na+ entry, we measured NKA hydrolytic activity using permeabilized membranes isolated from cells under controlled Na+ conditions. A decreased activity at low [Ca2+]i and no change in activity at high [Ca2+]i were observed following forskolin or OAG treatment. 7. Purified NKA from rat renal cortex was phosphorylated and inhibited by PKC. This phosphorylation-associated inhibition of NKA was neither affected by Ca2+ nor by calmodulin, tested alone or together. 8. We conclude that effect of PKA/PKC on NKA activity is dependent on [Ca2+]i. This Ca2+ dependence may provide an explanation for the diversity of responses of NKA to activation of either PKA or PKC.

Effects of depolarization evoked Na+ influx on intracellular Na+ concentration at neurosecretory nerve endings

Electrophysiological measurements of voltage-dependent Na+ influx using patch-clamp methodology were combined with optical monitoring of the free intracellular Na+ concentration in isolated rat neurohypophysial nerve endings to determine the relationship between Na+ influx generated by repetitive stimulation and change in [Na+]i. Application of step depolarizations under voltage-clamp-evoked tetrodotoxin-sensitive inward currents that were dependent upon extracellular Na+ and that exhibited rapid activation and inactivation properties. These characteristics substantiated the evoked current as a voltage-dependent Na+ current. Application of stimulus trains consisting of step depolarizations that mimick in frequency and duration those of action potentials were found to result in increases in [Na+]i. The induced change in [Na+]i was found to be related to the frequency and period of stimulation. Changes in [Na+]i were greatest at frequencies of 40 Hz and gave maximal changes with 30 s of continuous stimulation of approximately 2.4 mM. Sodium influx expressed as a molar quantity resulted in a nearly directly proportional increase in [Na+]i during the initial period of stimulation at low Na+ loads. When expressed as a charge density (pC/microm2) Na+ influx was found to increase with smaller diameter nerve endings as did the rate of change in [Na+]i in response to applied repetitive step depolarizations. Repetitive step depolarizations which simulate impulse activity that invade neuroendocrine nerve endings in vivo in response to physiological demand for hormone secretion resulted in an increased [Na+]i. It is postulated that this increased [Na+]i may provide a modulatory influence on the secretory response indirectly via alteration of intracellular calcium regulation or, perhaps, via a direct action on the secretory mechanism.

Functional significance of cell volume regulatory mechanisms

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

Alteration of rat submandibulary gland secretion of protein, calcium and N-acetyl-beta-D-glucosaminidase activity by lead

1. Effects of various doses of long-term lead treatment (0.01%, 0.04% and 0.05%) on rat submandibular saliva were investigated in this study. 2. Both submandibular ducts were cannulated intraorally with polyethylene tubes and saliva was collected from anesthetized lead treated and control rats using pilocarpine as secretagogue. 3. Saliva protein concentration was found to be reduced in lead (0.04%)- and (0.05%)-treated groups. 4. Saliva calcium concentration had a significant reduction only in the lead (0.05%)-treated group. 5. The secretion of the lysosomal enzyme, N-acetyl-beta-D-glucosaminidase (NAG) in saliva decreased significantly in the lead (0.04%)- and (0.05)-treated groups. Specific activity of the enzyme showed an increase in these treated groups. 6. Good correlations were found between saliva protein concentration and NAG activity, saliva protein and calcium concentrations and calcium concentration and NAG activity. 7. There was a correlation between blood and submandibular saliva lead levels, and the saliva/blood ratio was approximately constant for all treated groups. 8. With respect to the ability of lead to substitute for calcium in several intracellular regulatory events, explanation for these alterations in submandibular saliva composition can be made.

Extracellular Mg2+ regulates the intracellular Na+ concentration in rat sublingual acini

The intracellular free Na+ concentration ([Na+]i) increases during muscarinic stimulation in salivary acinar cells. The present study examined in rat sublingual acini the role of extracellular Mg2+ in the regulation of the stimulated [Na+]i increase using the fluorescent sodium indicator benzofuran isophthalate (SBFI). The muscarinic induced rise in [Na+]i was approximately 4-fold greater in the absence of extracellular Mg2+. When Na+ efflux was blocked by the Na+,K+-ATPase inhibitor ouabain, the stimulated [Na+]i increase was comparable to that seen in an Mg2+-free medium. Moreover, ouabain did not add further to the stimulated [Na+]i increase in an Mg2+-free medium suggesting that removal of extracellular Mg2+ may inhibit the Na+ pump. In agreement with this assumption, ouabain-sensitive Na+ efflux and rubidium uptake were reduced by extracellular Mg2+ depletion. Our results suggest that extracellular Mg2+ may regulate [Na+]i in sublingual salivary acinar cells by modulating Na+ pump activity.

Na+-dependent release of Mg2+ from an intracellular pool in rat sublingual mucous acini

Muscarinic stimulation induces release of Mg2+ from an intracellular pool in rat sublingual mucous acini (Zhang, G. H., and Melvin, J. E. (1992) J. Biol. Chem. 267, 20721-20727). In the present study we examined the interdependence of Mg2+ mobilization on intracellular Na+ and Ca2+ by monitoring the intracellular free concentrations of Na+ ([Na+]i), Mg2+ ([Mg2+]i), and Ca2+ ([Ca2+]i) using ion-sensitive fluorescent indicators. Gramicidin increased the intracellular concentrations of all three ions. Comparable to agonist-stimulated mobilization of Mg2+, the gramicidin-induced [Mg2+]i increase was independent of extracellular Mg2+ indicating release of Mg2+ from an intracellular pool. Clamping the [Ca2+]i near 30 nM with the Ca2+-selective chelator BAPTA failed to alter the [Na+]i or [Mg2+]i increases generated by gramicidin. In contrast, depletion of intracellular Na+ markedly suppressed the muscarinic-stimulated [Mg2+]i increase, whereas the [Ca2+]i increase was similar to that seen in physiological extracellular Na+. These results revealed that intracellular Mg2+ mobilization did not directly relate to the [Ca2+]i, but required an increase in [Na+]i. Consistent with this hypothesis, increasing [Na+]i by activating Na+ influx via the Na+/H+ exchanger also increased the [Mg2+]i. The Na+/Mg2+ exchange inhibitor quinidine suppressed both the gramicidin- and muscarinic-induced discharge of internal Mg2+. These results suggest that release of Mg2+ from an intracellular pool is mediated by a Na+-dependent Mg2+ transport mechanism in salivary acinar cells.

Na-K-ATPase in lacrimal gland acinar cell endosomal system: correcting a case of mistaken identity

Na-K-ATPase is associated with a variety of membrane populations in lacrimal acinar cells. Acinus-like structures formed by rabbit acinar cells in primary culture were incubated with horseradish peroxidase (HRP) to label basolateral and endosomal membranes and then analyzed by electron microscopy cytochemistry with the 3-3'-diaminobenzidine reaction or by fractionation and measurement of marker catalytic activities or immunoreactivities. HRP adsorbed to basolateral membranes at 4 degrees C. Fractionation showed it associated with low-density membranes enriched in acid phosphatase and TGN38 but containing only minor amounts of Na-K-ATPase. Cells internalized HRP to cytoplasmic vesicles, Golgi structures, and lysosomes at 37 degrees C. The major endosomal compartment revealed by fractionation coincided with major peaks of Na-K-ATPase and Rab6 and secondary peaks of galactosyltransferase and gamma-adaptin. Carbachol (10 microM) increased lysosomal and Golgi labeling. Thus most of the Na-K-ATPase is located in the basolateral membrane-oriented endosomal system, concentrated in a compartment possibly related to the trans-Golgi network. Constitutive and stimulation-accelerated traffic to and from this compartment may serve several exocrine cell functions.

Na+,K(+)-ATPase activity in the cochlear lateral wall of the gerbil

We examined Na+,K(+)-ATPase activity in four regions of the gerbil cochlear lateral wall by measuring intracellular Na+ concentration ([Na+]i). The application of 1 mM ouabain and removal of external K+ caused a most striking increase in [Na+]i of the suprastrial fibrocytes, followed by the stria vascularis and the type II fibrocytes, but no detectable response in the type I fibrocytes. These results may imply the potency of the Na+,K(+)-ATPase activity. The high values of the resting [Na+]i in these cells under perilymph-like solution returned to the [Na+]i within physiological values after exposure to low-Na+ conditions similar to endolymph. Our results are interpreted in the light of recent morphological findings with regard to the roles in the secretion of K+ into endolymph of several cell types within the lateral wall.

Spatial domains of Ca2+ signaling in secretory epithelial cells

Secretory epithelial cells are found in exocrine organs such as the pancreas and are also found in the lining of the lungs and gut. One important regulator of cell function in epithelial cells is the concentration of cytosolic Ca2+. The study of Ca2+ signaling in these cells has a long history and recent work has now identified, at the molecular level, key components in the Ca2+ signaling cascade. Furthermore, advances in fluorescent imaging techniques has enabled a detailed insight into the subcellular distribution of the agonist-evoked [Ca2+]i signal. A number of spatially different [Ca2+]i responses have been identified. Firstly, global [Ca2+]i signals are observed in response to high agonist concentrations. Secondly, at lower agonist concentrations trains of local [Ca2+]i spikes, restricted to the secretory pole region of pancreatic acinar cells, have been identified. Finally, these local [Ca2+]i spikes have now been further devolved into microdomains of [Ca2+]i elevation. The [Ca2+]i signal within a single microdomain has been shown to be the crucial trigger in the regulation of the ion channels important in fluid secretion.

Regulation of changes in cytosolic Ca2+ and Na+ concentrations in rat submandibular gland acini exposed to carbachol and ATP

The relationship between cytosolic concentrations of Ca2+ (Ca2i) and Na+ (Na+i) were studied in preparations of rat submandibular and pancreatic acini loaded with the Ca(2+)-sensitive dye Fura-2 or the Na(+)-sensitive dye SBFI. Pancreatic acini showed no changes in Na+i during either transient or persistent changes in Ca2+i. Increases in Ca2+i produced by exposure of submandibular gland acini to carbachol, a muscarinic cholinergic agonist, were followed by an increase in Na+i after a delay of 5-10 s. When Ca2+ stores were mobilized without Ca2+ influx Na+i also increased, but in acini loaded with BAPTA, a nonfluorescent Ca2+ chelator, the transient increase in Ca2+ caused by mobilization of stored Ca2+ was virtually abolished, as was the increase in Na+i. In the presence of inomycin, increases in Ca2+i were followed by increases in Na+i. Ca(2+)-dependent increases in Na+i were abolished in Na(+)-free buffer and by the presence of furosemide, a blocker of Na(+)-K(+)-2Cl- cotransport. In other studies, extracellular ATP (ATPo) produced an increase in Ca2+i and Na+i. The steady-state increase in Ca(i)2+ was reduced by increasing extracellular Na+ concentrations (Na+o in dose-dependent fashion (IC50 = 16.4 +/- 4.7 mM Na+). Likewise, increasing Na+o reduced ATPo-stimulated 45Ca2+ uptake at steady state (IC50 = 15.8 +/- 9.2 mM Na+). Changing Na+o had no effect on carbachol-stimulated increases in Ca2+i. We conclude that, in rat submandibular gland acini, ATPo promotes an increase in Ca2+i and Na+i via a common influx pathway and that, under physiologic conditions, Na+ significantly limits the ATPo-stimulated increase in Ca2+i. In the presence of carbachol, however, Na+i rises in Ca2+i-dependent fashion in submandibular gland acini via stimulation of Na(+)-K(+)-2Cl- cotransport.

Cytosolic Na+ controls and epithelial Na+ channel via the Go guanine nucleotide-binding regulatory protein

In tight Na+-absorbing epithelial cells, the fate of Na+ entry through amiloride-sensitive apical membrane Na+ channels is matched to basolateral Na+ extrusion so that cell Na+ concentration and volume remain steady. Control of this process by regulation of apical Na+ channels has been attributed to changes in cytosolic Ca2+ concentration or pH, secondary to changes in cytosolic Na+ concentration, although cytosolic Cl- seems also to be involved. Using mouse mandibular gland duct cells, we now demonstrate that increasing cytosolic Na+ concentration inhibits apical Na+ channels independent of changes in cytosolic Ca2+, pH, or Cl-, and the effect is blocked by GDP-beta-S, pertussis toxin, and antibodies against the alpha-subunits of guanine nucleotide-binding regulatory proteins (Go). In contrast, the inhibitory effect of cytosolic anions is blocked by antibodies to inhibitory guanine nucleotide-binding regulatory proteins (Gi1/Gi2. It thus appears that apical Na+ channels are regulated by Go and Gi proteins, the activities of which are controlled, respectively, by cytosolic Na+ and Cl-.

Potential-dependent block of human delayed rectifier K+ channels by internal Na+

The delayed rectifier K+ currents in differentiated human SH-SY5Y neuroblastoma cells were characterized with tight-seal recording techniques. Activation and inactivation parameters were measured. At high positive potentials, the current showed a marked rectification, causing a region of negative slope conductance in the current vs. potential curve. The rectification depended markedly on the pipette Na+ concentration. Without Na+, no rectification was observed, whereas with high Na+ (20-60 mM), a marked rectification was always observed. Tail current measurements showed a fast ( < 400 microseconds) block of K+ currents in the presence of internal Na+. With 60 mM Na+ in the pipette 8% of the K+ current was blocked at 0 mV, 27% at +20 mV, and 82% at +100 mV. Similar degrees of block were often seen with 30 mM Na+ in the pipette. The submembrane Na+ concentration in intact cells was estimated, on the basis of the reversal of Na+ current, to be approximately 15 mM. Single-channel K+ currents, in the cell-attached configuration, showed a conductance of approximately 20 pS at 40-60 mV above rest but showed rectification at high potentials.

Agonist-specific regulation of [Na+]i in pancreatic acinar cells

In a companion paper (Zhao, H., and S. Muallem. 1995), we describe the relationship between the major Na+,K+, and Cl- transporters in resting pancreatic acinar cells. The present study evaluated the role of the different transporters in regulating [Na+]i and electrolyte secretion during agonist stimulation. Cell stimulation increased [Na+]i and 86Rb influx in an agonist-specific manner. Ca(2+)-mobilizing agonists, such as carbachol and cholecystokinin, activated Na+ influx by a tetraethylammonium-sensitive channel and the Na+/H+ exchanger to rapidly increase [Na+]i from approximately 11.7 mM to between 34 and 39 mM. As a consequence, the NaK2Cl cotransporter was largely inhibited and the activity of the Na+ pump increased to mediate most of the 86Rb(K+) uptake into the cells. Secretin, which increases cAMP, activated the NaK2Cl cotransporter and the Na+/H+ exchanger to slowly increase [Na+]i from approximately 11.7 mM to an average of 24.6 mM. Accordingly, secretin increased total 86Rb uptake more than the Ca(2+)-mobilizing agonists and the apparent coupling between the NaK2Cl cotransport and the Na+ pump. All the effects of secretin could be attributed to an increase in cAMP, since forskolin affected [Na+]i and 86Rb fluxes similar to secretin. The signaling pathways mediating the effects of the Ca(2+)-mobilizing agonists were less clear. Although an increase in [Ca2+]i was required, it was not sufficient to account for the effect of the agonists. Activation of protein kinase C stimulated the NaK2Cl cotransporter to increase [Na+]i and 86Rb fluxes without preventing the inhibition of the cotransporter by Ca(2+)-mobilizing agonists. The effects of the agonists were not mediated by changes in cell volume, since cell swelling and shrinkage did not reproduce the effect of the agonists on [Na+]i and 86Rb fluxes. The overall findings of the relationships between the various Na+,K+, and Cl- transporters in resting and stimulated pancreatic acinar cells are discussed in terms of possible models of fluid and electrolyte secretion by these cells.

Intracellular Na+ measurements using sodium green tetraacetate with flow cytometry

Intracellular sodium concentration ([Na+]i) can be measured using SBFI with image analysis or spectrofluorometry. However, SBFI cannot be used with most flow cytometry systems, due to its requirement for dual excitation in the UV range. Recently a new sodium indicator, Sodium Green, was developed which can be used with flow cytometry to measure [Na+]i. Adequate staining of cells with Sodium Green is dependent upon both dye and cell concentration, time and temperature of loading, use of the non-ionic detergent Pluronic, and the cell line used. Both Sodium Green and SBFI give similar calibration curves using the ionophore gramicidin. Sodium Green has a small pH dependence, but this should not affect most applications. Since a fluorescence ratio technique cannot be used with Sodium Green, we used a ratio of dye fluorescence to forward angle light scatter to normalize for cell size variations in the population. The basal [Na+]i obtained using the Sodium Green technique in CHO ISI cells was 19 mM, in reasonable agreement with previous studies using SBFI in other cells.

A non-selective cation current activated via the multifunctional Ca(2+)-calmodulin-dependent protein kinase in human epithelial cells

1. Activation of macroscopic membrane currents by intracellular calcium ([Ca2+]i) signalling pathways was examined in human T84 epithelial cells, a model secretory cell line. 2. Elevation of [Ca2+]i by either the calcium ionophore A23187 (1 microM) or the cholinergic agonist carbachol, led to the transient activation of both a chloride and cation current in single voltage clamped cells. The channels underlying the cation conductance were found to be equally permeable to external Na+, K+ and Cs+, but impermeable to the large organic cations tetraethylammonium and N-methyl-D-glucamine (NMDG). These observations indicate that the cation channels are non-selective with respect to monovalent cations. 3. Persistent activation of both the chloride and non-selective cation currents by [Ca2+]i was observed following inhibition of cellular phosphatase activity by the phosphatase inhibitor microcystin LR or the ATP analogue ATP gamma S. This finding strongly suggests the presence of a phosphorylation event in the calcium-dependent activation pathway for both currents. 4. Intracellular dialysis with peptide inhibitors of the multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) blocked the activation of both the chloride and cation conductances by elevated [Ca2+]i. Dialysis with an inactive control peptide had no effect on the activation of either current. CaM kinase thus appears to be critically involved in the calcium-dependent activation of both the chloride and cation currents in these cells. 5. Associated with the whole-cell cation conductance were macroscopic tail currents observed at the chloride reversal potential. The distinct kinetic properties of these tail currents were used as a biophysical 'signature' of the whole-cell conductance. 6. In excised, inside-out membrane patches, [Ca2+]i activated single cation channel activity. These channels had a mean conductance of 20 pS, were impermeable to NMDG, and their mean open probability increased at positive membrane potentials. The properties of these single channel events thus closely resemble those reported previously for calcium-activated cation channels in epithelia. 7. Using a novel 'tail current' voltage clamp protocol in excised membrane patches, we observed that ensemble averages of single cation channel events reproduced the behaviour and kinetic properties of the macroscopic tail currents of the calcium-activated cation conductance. This finding provides evidence that the observed single channel events probably underlie the macroscopic cation current recorded from intact cells. 8. The results from this study demonstrate that CaM kinase mediates the calcium-dependent activation of both a chloride and a non-selective cation current in human T84 epithelial cells. Using single channel recordings, we believe we have identified the corresponding whole-cell current for the 20-40 pS calcium-activated cation channel activity reported previously in epithelia and other cell preparations. Physiologically, a calcium-activated inward cation current would allow sodium influx in association with calcium-dependent electrolyte and protein secretion. Thus CaM kinase-dependent activation of cation channels may serve as a co-ordinated influx pathway to balance the efflux and influx of osmotically active solutes as part of an overall cell volume regulatory mechanism.

The influence of thapsigargin on Na,K-ATPase activity in cultured nonpigmented ciliary epithelial cells

Experiments were conducted to test the influence of thapsigargin on the NaK-ATPase activity of cultured cells (ODM2) derived from human nonpigmented ciliary epithelium. The rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was diminished in cells that had been pretreated with thapsigargin then permeabilized. Following 20 min exposure of intact cells to thapsigargin, the cells were permeabilized with digitonin and the rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was measured immediately in a calcium-free buffer. In permeabilized cells that had been pretreated with 1 microM thapsigargin for 20 min, the rate of ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was reduced by 38%. Pretreatment with lesser concentrations of thapsigargin caused smaller changes of Na,K-ATPase activity. The decrease of Na,K-ATPase activity was the same whether or not calmodulin antagonists W7 or trifluoperazine were present during the thapsigargin pretreatment period. This inhibitory effect upon the Na,K-ATPase may serve to limit the extent of sodium pump activation that takes place in intact cells when thapsigargin causes sodium pump stimulation by a mechanism that appears to involve changes in cytoplasmic ion levels when potassium channels open.

Cell volume and shape oscillations in rat type-II somatotrophs at hypotonic conditions

The size and shape of growth hormone (GH)-producing rat type-II somatotrophs was studied during osmotic manipulation. When somatotrophs were exposed to large osmotic stress (200 and 225 mOsm), the peak projected cell area (PCA) was 132.9% +/- 12.6% and 116.8% +/- 2.8% (P < 0.01) and triggered a regulatory volume decrease (RVD) to avoid lysis. At lower osmotic stress (250 mOsm), the rate of swelling was slower, and the volume reached a steady state at 109.4% +/- 2.4% (P < 0.05) and was without RVD. At 275 and 287 mOsm, the swelling was delayed [PCA peak at 3-4 min; 105.8% +/- 1.5% (P < 0.05) and 104.2% +/- 1.7%] and then showed repeated synchronized cycles of swelling and shrink-age (P < 0.05). The data suggest that somatotrophs may have more than one mechanism for volume regulation. One mechanism is for large swelling (classic RVD response), whereas the other represents more physiological mechanisms for regulating the cell volume within a more limited geometry range. For low osmotic stress (250-287 mOsm), the somatotrophs became less spherical during swelling and, thus, were without membrane dilation. Therefore, this type of volume regulation must work independently from membrane stress. Related volume regulation mechanisms may underlie the previously observed volume fluctuations in somatotrophs seen during secretory stimulation with GH-releasing hormone.

On the relation of calcium channel blockers to rat parotid and submandibular glands function in vivo

1. The effects of nifedipine, verapamil and diltiazem on rat parotid and submandibular glands function were studied. 2. Nifedipine (5 mg/kg), verapamil (5 mg/kg) and diltiazem (10 mg/kg) were injected intraperitoneally 15 min before saliva collection. 3. Animals were anesthetized with 50 mg/kg of sodium pentobarbital and 8 mg/kg of pilocarpine was used as secretagogue. 4. Submandibular saliva was analyzed for flow rate, protein and calcium concentrations; and parotid saliva for calcium and amylase contents. 5. In treated groups, flow rate and calcium of submandibular saliva were significantly lower than controls. Parotid calcium in the nifedipine group was decreased and in verapamil and diltiazem groups was increased. Parotid amylase was significantly decreased in both the nifedipine and diltiazem groups. 6. It is concluded that a blockade of calcium channels in salivary glands acinar cells by CCBs causes some alterations in salivary secretions.

Na+ transport processes in isolated guinea pig nasal gland acinar cells

In the dispersed acinar cells of the submucosal nasal gland in the guinea pig, intracellular Na+ concentration ([Na+]i) was measured with a microfluorimetric imaging method and the cytosolic indicator dye, sodium-binding benzofuran isophthalate, under HCO3(-)-free conditions. In the unstimulated condition, the [Na+]i was averaged to 12.8 +/- 5.2 mM. Addition of 100 microM ouabain or removal of external K+ caused an increase in [Na+]i. Replacement of external Cl- with NO3- or addition of 0.5 mM furosemide reversibly decreased the [Na+]i. The recovery process from the reduced [Na+]i was inhibited by removal of either K+ or Cl- in the bath solution. These findings indicate the presence of a continuous influx of Na+ coupled with K+ and Cl- movement. Application of acetylcholine (ACh, 1 microM) caused an increase in [Na+]i by about 15-20 mM, which was completely inhibited by addition of 10 microM atropine. Increased cytosolic Na+ induced by ACh was extruded by the Na(+)-K+ pump. Removal of external Cl- and addition of 50 microM dimethylamiloride inhibited ACh-induced increase in [Na+]i by about 66% and 19%, respectively. In both unstimulated and stimulated state, Na(+)-K+ pump, Na-K-Cl cotransport, and Na(+)-H+ exchange play a critical role in maintaining intracellular electrolyte environment and in controlling a continuous secretion of nasal fluids.

HCO3(-)-dependent ACh-activated Na+ influx in sheep parotid secretory endpieces

In the present study we used the Na)+)-sensitive fluorescent dye SBFI and optical measurement of endpiece volume to investigate the transport of Na+ in sheep parotid secretory cells. Sheep parotid endpiece cells bathed in a HCO3(-)-free Cl(-)-rich solution had a resting intracellular Na+ concentration ([Na+]i) of 17 +/- 2 mmol/l (n = 39). Exposure of the cells to a 2-min pulse of acetylcholine (ACh) (3 x 10(-7) mol/l) in a HCO3(-)-free bathing solution produced no change in [Na+]i or in cell volume. Changing from a Cl(-)-containing HCO(3-)-free bath solution to a Cl- solution containing 25 mmol/l HCO3- caused the endpieces to swell by 8 +/- 2% (n = 11) and the [Na+]i to increase by 10 +/- 2 mmol/l (n = 14). Subsequent exposure of the cells to ACh led to shrinkage of the cells by 12 +/- 2% from the volume in the HCO3(-)-containing solution prior to ACh exposure, with the maximum decrease occurring after 29 +/- 7 s (n = 9). This shrinkage was accompanied by a rapid and transient increase in [Na+]i, the [Na+]i reaching a peak at 70 +/- 5 mmol/l above the unstimulated level (n = 9). Substitution of gluconate for Cl- did not significantly alter the effects of HCO3- on unstimulated [Na+]i or endpiece volume, nor did it significantly inhibit the effects of ACh on these two parameters when HCO3- was present.(ABSTRACT TRUNCATED AT 250 WORDS)

Thapsigargin induces cytoplasmic free Ca2+ oscillations in mouse oocytes

The mechanisms of calcium signalling in mammalian oocytes during maturation and fertilization are controversial. In this study we measured intracellular free Ca2+ concentrations ([Ca2+]i) with the photoprotein aequorin microinjected into immature mouse oocytes. Immature mouse oocytes typically produced [Ca2+]i responses to muscarinic acetylcholine (ACh) stimulation with two types of component. The first component consisted of a broad transient rise in [Ca2+]i lasting about 1 min. The second component consisted of pulsatile oscillations which could occur before, during or after the broad transient, but typically occurred on the rising phase of the broad transient, with a duration of about 5 s. Removal of external Ca2+ ([Ca2+]o) abolished the Ca2+ responses to ACh. Exposure of oocytes to the specific microsomal Ca(2+)-ATPase inhibitors thapsigargin (TG) and cyclopiazonic acid unexpectedly produced sustained oscillations in [Ca2+]i which were sensitive to the concentration of Ca2+ in the external milieu. The frequency of these oscillations was slow, and ceased, sometimes after several cycles, when Ca2+o was removed. Raised [Ca2+]o significantly increased the frequency in cells oscillating to TG and stimulated nonoscillating cells to begin oscillating. The majority of responsive oocytes which did not produce oscillations to ACh alone (70%), did so after TG treatment. Detailed data analysis indicated that these oscillations were identical to those generated by TG alone, with a similar sensitivity to changes in [Ca2+]o. Exposure of oocytes to ryanodine did not inhibit oscillatory behaviour. These results suggest that immature mouse oocytes possess a store which is insensitive to both TG and ryanodine and is capable of supporting [Ca2+]i oscillations.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Intercellular signaling in neuronal-glial networks

Glial cells have recently been found to exhibit electrophysiological and metabolic responses to many neurotransmitters and neuromodulators. These findings have focused attention on the possibility that active signaling between neurons and glia could represent an important form of intercellular communication within the brain. Since glial and neuronal networks are both physically and metabolically interlinked, such intercellular signaling may represent a mechanism for inducing collective changes in the cellular physiology of neuronal and glial cell populations. Within the nervous tissue of both vertebrate and invertebrate organisms, glial cells are known to secrete extracellular signal molecules, modulate carbohydrate metabolism, and control the volume and ionic composition of extracellular space. In this paper, the roles that cytoplasmic [Ca2+] transients may play in regulating these glial cell functions are reviewed. Mechanisms by which intracellular Ca oscillations and intercellular Ca waves may be generated in neurotransmitter-stimulated glial cells are also discussed. In addition, it is proposed that rhythmic glial cell contractions and shape changes, which have been observed for many decades, are linked to Ca-induced secretion of ions, water, and neuroactive compounds. These activities represent mechanisms by which Ca-induced changes in glial cell physiology could potentially alter the excitability of neuronal networks.

Quantitative estimation of the area of luminal and basolateral membranes of rat parotid acinar cells: some physiological applications

Knowledge of luminal and basolateral acinar cell membrane areas of the secretory endpieces is a prerequisite for a detailed quantitative analysis of the ion transport involved in secretion of the primary saliva. In the present study, these areas were estimated in rat parotid acinar cells using standard stereological methods. A total of 480 micrographs--obtained by random sampling from eight glands from four rats--were analysed at a final magnification of 40000x. Expressed per unit cell volume, the area of the luminal acinar cell membrane was: 0.125 micron 2.micron-3 (SEM = 0.027 micron 2.micron-3, n = 4 animals) and the area of the basolateral membrane was: 1.54 microns 2.micron-3 (SEM = 0.085 micron 2.micron-3, n = 4 animals). These figures make it possible to perform a synthesis based upon different categories of experimental data, e.g. on ion fluxes, membrane potentials and single-channel conductances. Thus, we have estimated the density of open, low-conductance Cl- channels in the luminal membrane--which are not readily accessible for direct, patch-clamp analysis--to be approximately 18 channels per microns 2 in the stimulated state.

Anoxia induces an increase in intracellular sodium in rat central neurons in vitro

Following our previous observations that anoxia induces a drop in extracellular Na+ in the brain slice and that removal of extracellular Na+ prevents the anoxia-induced morphological changes in dissociated hippocampal neurons, we hypothesized that intracellular Na+ increases during anoxia in isolated neurons. Using the fluorophore Sodium Green in freshly dissociated rat CA1 neurons, and SBFI in cultured cortical neurons, we found that 10 min of anoxia caused an increase in Nai+ in both types of cells, with a latency of about 2 min. In CA1 neurons, fluorescence increased by an average of 20.34% (n = 8). The mean baseline Nai+ level (determined using SBFI) was 25 +/- 2.5 mM, which increased to about an average of 52 +/- 3 mM after 3-4 min. These and our previous results strongly suggest that Na(+)-mediated events are involved in anoxia-induced nerve injury.

Na+ transport pathways in secretory acinar cells: membrane cross talk mediated by [Cl-]i

Fluid secretion by epithelial cells can be modulated by agents that activate Cl- channels in the apical membrane. To sustain secretion, Cl- influx across the basolateral membrane must also be accelerated. To examine the cellular mechanisms that couple Cl- efflux across the apical membrane to Na(+)-coupled Cl- entry across the basolateral membrane, we employed optical imaging techniques, utilizing single rat salivary acinar cells. Na+ influx was negligible in resting cells but was rapidly increased by carbachol due to activation of a Na(+)-H+ exchanger, a Na(+)-K(+)-2Cl- cotransporter, and, most likely, a nonselective cation channel. Receptor stimulation was not necessary, since elevation of intracellular Ca2+ concentration ([Ca2+]i) by thapsigargin activated the Na+ transporters at equivalent rates. Cell acidification, activation of protein kinase C, cell shrinkage, and other events associated with the rise of [Ca2+]i had little effect on Na+ transport in resting cells. Nevertheless, stimulation of cells in a medium that prevented normal Ca(2+)-induced cell shrinkage prevented activation of all three transport pathways. The block of the activation was not overcome by osmotic shrinkage but was relieved when [Cl-]i was allowed to fall, including conditions in which [Cl-]i fell in the absence of cell shrinkage. Activation of a Na(+)-H+ exchanger, Na(+)-K(+)-2Cl- cotransporter, and nonselective cation channel therefore exhibits a requirement for agonist-induced fall in [Cl-]i. Low [Cl-]i may create a permissive environment for Ca(2+)-dependent activation of multiple Na(+)-transport pathways, providing a mechanism for cross talk that coordinates transport activities of the apical and basolateral membranes in secretory epithelial cells.

Oscillating activity of a Ca(2+)-sensitive K+ channel. A prerequisite for migration of transformed Madin-Darby canine kidney focus cells

Migration plays an important role in the formation of tumor metastases. Nonetheless, little is known about electrophysiological phenomena accompanying or underlying migration. Previously, we had shown that in migrating alkali-transformed Madin-Darby canine kidney focus (MDCK-F) cells a Ca(2+)-sensitive 53-pS K+ channel underlies oscillations of the cell membrane potential. The present study defines the role this channel plays in migration of MDCK-F cells. We monitored migration of individual MDCK-F cells by video imaging techniques. Under control conditions, MDCK-F cells migrated at a rate of 0.90 +/- 0.03 microns/min (n = 201). Application of K+ channel blockers (1 and 5 mmol/liter Ba2+, 5 mmol/liter tetraethylammonium, 100 mumol/liter 4-aminopyridine, 5 nmol/liter charybdotoxin) caused marked inhibition of migration, pointing to the importance of K+ channels in migration. Using patch-clamp techniques, we demonstrated the sensitivity of the Ca(2+)-sensitive 53-pS K+ channel to these blockers. Blockade of this K+ channel and inhibition of migration were closely correlated, indicating the necessity of oscillating K+ channel activity for migration. Migration of MDCK-F cells was also inhibited by furosemide or bumetanide, blockers of the Na+/K+/2Cl- cotransporter. We present a model for migration in which oscillations of cell volume play a central role. Whenever they are impaired, migration is inhibited.

An insulin-sensitive cation channel controls [Na+]i via [Ca2+]o-regulated Na+ and Ca2+ entry

The insulin-stimulated cation channel previously identified in patch-clamped muscle preparations is here shown to be responsible for bulk Na+ entry into the cell. The mainly Na+ current of the channel was shown to be accompanied by an inhibitory Ca2+ component responsible for oscillations. Here, using quantitative fluorescence imaging of Fura-2- and SBFI-loaded soleus muscle, we measure changes in [Na+]i and [Ca2+]i related to channel function. Insulin increased [Na+]i and [Ca+]i in a transient spike of < 1-min duration. There was a momentary dip in [Na+]i related to inhibition of the channel by the Ca2+ spike, and changes in external Ca2+ were shown to alter [Na+]i via the cation channel, all effects being blocked by the specific channel inhibitor mu-conotoxin, but not by tetrodotoxin. The [Ca2+]i spike could also be induced by 8-bromo cyclic-guanosine 5'-monophosphate, an analogue of the channel-activator cyclic-guanosine 5'-monophosphate (cGMP). In addition it was noted that insulin reduced the [Ca2+]i rise upon subsequent muscle depolarization by a factor of 3.5. Insulin could be substituted with phorbol ester for the same effect and HA1004, a protein kinase inhibitor, blocked the reduction.

Properties of the fluorescent sodium indicator "SBFI" in rat and rabbit cardiac myocytes

INTRODUCTION

Although some properties of the fluorescent sodium indicator "SBFI" are known, there is no accepted method by which the SBFI signal might be calibrated for intracellular Na (Nai) in cardiac cells. The first aim of this study was to characterize the loading and compartmentalization of this indicator in single cardiac myocytes. The second aim was, from experimental observation, to develop a rational calibration method for SBFI. The third aim was to use this Na indicator to study the dependence of tonic contraction on Nai in voltage-clamped ventricular myocytes.

METHODS AND RESULTS

SBFI was incorporated into myocytes by incubating with the acetoxymethyl ester (10 microM) for 2 hours. This led to an intracellular concentration of SBFI free acid of 122 +/- 17 microM. We considered a number of issues with respect to compartmentalization of indicator and, under our conditions, we found the majority (71%) of indicator was situated in the cytoplasm. Therefore, SBFI indicates mainly changes of cytoplasmic Na. Calibration of the indicator for Nai was performed by equilibrating internal and external Na. We investigated the conditions required for optimum transmembrane Na equilibration and found it necessary to use a calibration solution free of both Ca and magnesium (Mg). The Na ionophores gramicidin D and monensin were both required, and it was also necessary to inhibit the Na/K pump for optimum Na equilibration. Using these conditions, the Nai concentration in quiescent rat ventricular myocytes was 10.9 +/- 0.74 mM (mean +/- SEM, n = 40; equivalent to an Na activity of 8.3 mM). The concentration of Nai was significantly lower in quiescent rabbit myocytes: 3.8 +/- 0.23 mM (n = 24; equivalent to an Na activity of 2.9 mM). In voltage-clamped rabbit myocytes, inhibition of the Na/K pump caused a rise of Nai; there were also marked effects on the tonic shortening elicited by ramp depolarizations. As Nai rose, the magnitude of tonic shortening increased and its voltage dependence also changed.

CONCLUSION

These results confirm the suitability of SBFI for measuring Nai in cardiac cells. Provided that steps are taken to optimize transmembrane Na equilibration, the indicator can be calibrated for Nai. The different Nai of rat and rabbit myocytes has implications for the function of sarcolemmal Na/Ca exchange in each cell type. An Nai of 10.9 mM in rat myocytes gives a calculated reversal potential for the exchange of -35 mV. In comparison, an Nai of 3.8 mM in rabbit myocytes leads to a reversal potential for the exchange +45 mV. Therefore, relatively small changes of Nai can shift the reversal potential of the exchange to values that are substantially more positive or negative than zero. The behavior of voltage-dependent tonic contraction in rabbit myocytes was consistent with the Na/Ca exchange reversal potential being more positive than zero in these cells.

Isosmotic modulation of cell volume and intracellular ion activities during stimulation of single exocrine cells

Stimulation of salivary secretion is associated with a rise of [Ca2+]i in acinar cells. We examined the osmotic and ionic consequences of activation of Ca(2+)-dependent K+ and Cl- channels, by simultaneous optical determinations of cell volume and [Ca2+]i, [Cl-]i or [Na+]i during muscarinic stimulation of single salivary acinar cells, using a differential interference contrast (DIC)-fluorescence microscope. Carbachol caused a rapid rise of [Ca2+]i, as well as a substantial cell shrinkage. Despite variability in the level and kinetics of the subsequent sustained phase of the [Ca2+]i response, cell volume was correlated with [Ca2+]i in all cases. Elevated [Ca2+]i was both necessary and sufficient to cause these changes in cell volume. The proposition that changes in cell volume reflected changes in cell solute content was confirmed by simultaneously measuring [Cl-]i and cell volume. Simultaneous determinations of cell volume and [Na+]i indicated that the initial cell shrinkage was due entirely to K+ and Cl- efflux. Subsequent to the initial shrinkage, [Na+]i rose to high levels, primarily due to activation of Na+/H+ exchange. Thus, modulation of ion transport activities under isosmotic conditions results in substantial changes in cell solute content and cell volume. Subsequent to the early Ca(2+)-induced changes in these parameters, other transporters become active, but it is unclear what signals their activation. Cell swelling by osmotic dilution of the bath resulted in compensatory cell shrinkage (RVD) which was sensitive to K+ and Cl- gradients. Nevertheless, a rise of [Ca2+]i was not necessary for RVD. Osmotic shrinkage and/or cell acidification were insufficient to activate Na+ influx.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Is there a Na+/Ca2+ exchanger in macrophages and in lymphocytes?

In two blood cell types, peritoneal murine macrophages and Jurkat cells (a human T cell line), we have examined whether a Na+/Ca2+ exchange was present and what could be its functional importance. In non-stimulated macrophages, the intracellular Ca2+ concentration, [Ca2+]i, was unchanged when Li+ was substituted for external Na+. However, after stimulation by platelet-activating factor (PAF), the Ca2+ response was larger when the extracellular solution contained Li+ rather than Na+ ions. In stimulated macrophages, the rate of Ca2+ extrusion was smaller in a Li(+)- than in a Na(+)-containing medium. The net electrochemical gradient for ionic movements through the Na+/Ca2+ exchanger, during the course of the response of macrophages to PAF, was determined by combining the measurements of membrane potential (in patch-clamp), of [Ca2+]i (with fura-2), and of the intracellular Na+ concentration (with sodium-binding benzofuran isophthalate). These results show that macrophages possess a Na+/Ca2+ exchange that only functions as a Ca2+ extruder, and this only when [Ca2+]i has been increased, for instance following PAF stimulation. In T lymphocytes, before or after stimulation by an anti-CD3 antibody, no Na+/Ca2+ activity could be detected by measuring either [Ca2+]i, or the rate of Ca2+ extrusion. Even if a Na+/Ca2+ exchanger was present in these cells, its equilibrium potential would be such that it would not allow Ca2+ influx but only Ca2+ extrusion.

Intracellular free sodium concentrations in GH4C1 cells

In the present investigation, intracellular sodium ([Na+]i) levels were determined in GH4C1 cells using the fluorescent probe SBFI. Fluorescence was determined by excitation at 340 nm and 385 nm, and emission was measured at 500 nm. Intracellular free sodium ([Na+]i) was determined by comparing the ratio 340/385 to a calibration curve. The ratio was linear between 10 and 60 mM Na+. Resting [Na+]i in GH4C1 cells was 26 +/- 6.2 mM (mean +/- SD). In cells incubated in Na(+)-free buffer [Na+]i decreased to 3 +/- 3.6 mM. If Na+/K+ ATPase was inhibited by incubating the cells with 1 mM ouabain, [Na+]i increased to 47 +/- 12.8 mM in 15 min. Stimulating the cells with TRH, phorbol myristyl acetate, or thapsigargin had no effect on [Na+]i. Incubating the cells in Ca(2+)-free buffer rapidly increased [Na+]i. The increase was not inhibited by tetrodotoxin. Addition of extracellular Ca2+, nimodipine, or Ni2+ to these cells immediately decreased [Na+]i, whereas Bay K 8644 enhanced the influx of Na+. In cells where [Na+]i was increased the TRH-induced increase in intracellular free calcium ([Ca2+]i) was decreased compared with control cells. Our results suggest that Na+ enters the cells via Ca2+ channels, and [Na+]i may attenuate TRH-induced changes in [Ca2+]i in GH4C1 cells.

Intercellular propagation of calcium waves mediated by inositol trisphosphate

Two types of calcium (Ca2+) signaling-propagating intercellular Ca2+ waves of increasing intracellular Ca2+ concentration ([Ca2+]i) and nonpropagating oscillations in [Ca2+]i-co-exist in a variety of cell types. To investigate this difference in Ca2+ signaling, airway epithelial cells were loaded with heparin, an inositol 1,4,5-triphosphate (IP3) receptor antagonist, by pulsed, high-frequency electroporation. Heparin inhibited propagation of intercellular Ca2+ waves but not oscillations of [Ca2+]i. In heparin-free cells, Ca2+ waves propagated through cells displaying [Ca2+]i oscillations. Depletion of intracellular Ca2+ pools with the Ca2+-pump inhibitor thapsigargin also inhibited the propagation of Ca2+ waves. These studies demonstrate that the release of Ca2+ by IP3 is necessary for the propagation of intercellular Ca2+ waves and suggest that IP3 moves through gap junctions to communicate intercellular Ca2+ waves.