Characterization of Cytosolic Calcium Oscillations Induced by Phenylephrine and Vasopressin in Single Fura-2-loaded Hepatocytes

Suppression of agonist induced Ca2+ oscillations in cultured hepatocytes by nafenopin: possible involvement of protein kinase C

The alpha 1-agonist phenylephrine (5 microM) induces an increase in the free cytosolic Ca2+ concentration, followed by repetitive transients of the cytoplasmic Ca2+ concentration, in single Fura-2 loaded hepatocytes. The tumor promoting, hypolipidemic drug nafenopin suppressed the cellular Ca2+ response to phenylephrine. The effect of nafenopin on the Ca2+ increase and Ca2+ oscillations was largely prevented by the specific protein kinase C inhibitor Gö 6976. This finding suggests involvement of protein kinase C in the action of nafenopin on phenylephrine induced Ca2+ mobilization.

Hepatocyte growth factor-induced calcium waves in hepatocytes as revealed with rapid scanning confocal microscopy

Cytosolic Ca2+ transients induced by hepatocyte growth factor (HGF) were imaged in primary cultured rat hepatocytes using newly developed rapid scanning confocal microscopes and indo-1. HGF (40 ng/ml) increased cytosolic free Ca2+ concentration ([Ca2+]i) in about 60% of hepatocytes, in 45% of which the increases were oscillatory. In each of the oscillatory hepatocytes, the repetitive increases in [Ca2+]i originated from a specific same region adjacent to the cell membrane and propagated across the cell like waves. Phenylephrine (10 microM) also induced Ca2+ waves. The locus where HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves were originated was the same, and there was a correlation in the peak height between HGF-induced Ca2+ waves and phenylephrine-induced Ca2+ waves in each cell, although the mechanisms of inositol 1,4,5-trisphosphate (ins(1,4,5)P3) formation induced by HGF should be different from those by phenylephrine. On the other hand, there was no correlation between sensitivity of each cell to HGF and that to phenylephrine which were measured as latent periods prior to Ca2+ rises after an addition of the agonists. These results suggested the following: the spatial patterns of Ca2+ waves were decided by a common mechanism, probably not the propagation of ins(1,4,5)P3 but the distribution of ins(1,4,5)P3-sensitive Ca2+ pools; sensitivities of each cell to the agonists did not mainly depend on the common mechanism.

Effects of Hg2+ on cytosolic Ca2+ in isolated skate hepatocytes

Hg2+ is an environmental pollutant that adversely affects a range of cellular functions, including those that regulate free cytosolic Ca2+ (Ca(i)2+). To investigate the mechanism of Hg(2+)-induced Ca(i)2+ signaling, we examined the effects of Hg2+ on Ca(i)2+ in isolated skate hepatocytes, and developed a method to assess cytosolic Hg2+ (Hgi2+) in these cells as well. At lower concentrations (1-5 microM), Hg2+ induced little detectable change in Ca(i)2+. At higher concentrations (10 microM-1 mM), Hg2+ induced a dose-dependent, progressive increase in Ca(i)2+, which occurred even in Ca(2+)-free medium. Pretreatment of hepatocytes with the membrane-impermeant Hg2+ chelator glutathione (GSH) blocked the Hg(2+)-induced Ca(i)2+ increase, whereas addition of GSH after exposure to Hg2+ slowed but did not prevent further increases in Ca(i)2+. Pretreatment with the membrane-permeant Hg2+ chelator dithiothreitol (DTT) also blocked Hg(2+)-induced increases in Ca(i)2+. Unlike GSH, however, addition of DTT after Hg2+ significantly decreased Ca(i)2+, returning it to near-baseline levels. Thapsigargin induced a sustained increase in Ca(i)2+, but subsequent addition of Hg2+ resulted in a further, progressive Ca(i)2+ increase. We also describe the use of the fluorescent dye BTC-5N to measure Hgi2+, and with it found that Hgi2+ reaches nanomolar levels within minutes of extracellular application, but that these measurable levels of Hgi2+ do not precede elevations in Ca(i)2+. Hg2+ did not irreversibly damage the hepatocytes over this time period (< 5 min), as determined both by propidium iodide permeability and light microscopic appearance. Together, these findings suggest: (i) Hg2+ increases Ca(i)2+ in skate hepatocytes; (ii) Hg2+ must enter the hepatocytes for this Ca(i)2+ increase to occur; (iii) this increase is mediated by release of Ca2+ from endogenous stores that are distinct from the thapsigargin-sensitive Ca2+ stores; and (iv) this increase occurs in association with measureable levels of Hg2+ in the cytosol. Adverse cellular effects of Hg2+ may be mediated by changes in Ca(i)2+ that result from intracellular accumulation of this toxic metal.

Decoding of cytosolic calcium oscillations in the mitochondria

Frequency-modulated oscillations of cytosolic Ca2+ ([Ca2+]c) are believed to be important in signal transduction, but it has been difficult to correlate [Ca2+]c oscillations directly with the activity of Ca(2+)-regulated targets. We have studied the control of Ca(2+)-sensitive mitochondrial dehydrogenases (CSMDHs) by monitoring mitochondrial Ca2+ ([Ca2+]m) and the redox state of flavoproteins and pyridine nucleotides simultaneously with [Ca2+]c in single hepatocytes. Oscillations of [Ca2+]c induced by IP3-dependent hormones were efficiently transmitted to the mitochondria as [Ca2+]m oscillations. Each [Ca2+]m spike was sufficient to cause a maximal transient activation of the CSMDHs and [Ca2+]m oscillations at frequencies above 0.5 per minute caused a sustained activation of mitochondrial metabolism. By contrast, sustained [Ca2+]c increases yielded only transient CSMDH activation, and slow or partial [Ca2+]c elevations were ineffective in increasing [Ca2+]m or stimulating CSMDHs. We conclude that the mitochondria are tuned to oscillating [Ca2+]c signals, the frequency of which can control the CSMDHs over the full range of potential activities.

Simultaneous measurements of Ca2+ in the intracellular stores and the cytosol of hepatocytes during hormone-induced Ca2+ oscillations

Simultaneous Ca2+ measurements in the cytosol and intracellular stores (IS) of rat hepatocytes were performed using two Ca(2+)-sensitive probes (Fluo-3 and Mag-fura-2), and combined whole-cell patch clamp and fluorescence microscopy. A steady-state Ca2+ concentration of approximately 630 microM was estimated in the IS. alpha 1-Adrenergic stimulation induced periodic elevations of cytosolic Ca2+ and parallel synchronized transient declines in the IS. Subsequent application of the intracellular Ca(2+)-pump inhibitor thapsigargin resulted in a release of Ca2+ from the IS to reach a level of Ca2+ depletion much lower than the lowest transient decline observed during the oscillations.

Calcium ion-dependent signalling and mitochondrial dysfunction: mitochondrial calcium uptake during hormonal stimulation in intact liver cells and its implication for the mitochondrial permeability transition

Hormones that elevate cytosolic Ca2+ concentrations ([Ca2+]cyt) often use Ca2+ as a messenger to activate intramitochondrial metabolic processes. However, the mitochondrial Ca2+ level also regulates the activation of the mitochondrial permeability transition (MPT), a process that involves the assembly of a high conductance proteinaceous pore across the inner and outer membrane. Studies on intact liver cells indicate that the MPT is a critical step in the cell killing induced by anoxia or respiratory inhibitors. In this study, we used freshly isolated hepatocytes to investigate to what extent the elevation of [Ca2+]cyt by vasopressin or other agonists causes Ca2+ accumulation in the mitochondria and how this treatment affects the mitochondrial susceptibility to undergo the MPT. Hepatocytes were incubated with vasopressin, glucagon, or with thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump) prior to permeabilization with digitonin. Mitochondrial Ca2+ accumulation was determined by following the ionomycin-induced Ca2+ release in permeabilized cells and mitochondrial swelling was studied by following cyclosporin A-sensitive light scattering changes induced by phenyl-arsenoxide and rotenone. The results indicate that agents that elevate [Ca2+]cyt cause a significant Ca2+ accumulation in the mitochondria. Excessive Ca2+ accumulation (> 10-fold increase over basal levels) was obtained with the combination of vasopressin and glucagon or with incubations containing thapsigargin. These conditions were also associated with a marked increase in rotenone-induced mitochondrial swelling. However, the more modest increase in mitochondrial Ca2+ content after treating cells with vasopressin alone did not enhance the swelling response; instead, vasopressin suppressed mitochondrial swelling compared to control incubations. Vasopressin also partly suppressed the swelling associated with thapsigargin treatment, although it did not significantly affect the Ca2+ accumulation under these conditions. This effect of vasopressin was mimicked by phorbol ester, suggesting a role for protein kinase C. The data indicate that mitochondrial Ca2+ accumulation following elevation of elevation of [Ca2+]cyt enhances the susceptibility for activation of the MPT, a response that may increase cell injury during anoxia or in response to other challenges. However, hormones also activate protective responses in the cell that suppress the MPT.

Coordination of Ca2+ signaling by intercellular propagation of Ca2+ waves in the intact liver

Activation of the inositol lipid signaling system results in cytosolic Ca2+ oscillations and intra- and intercellular Ca2+ waves in many isolated cell preparations. However, this form of temporal and spatial organization of signaling has not been demonstrated in intact tissues. Digital imaging fluorescence microscopy was used to monitor Ca2+ at the cellular and subcellular level in intact perfused rat liver loaded with fluorescent Ca2+ indicators. Perfusion with low doses of vasopressin induced oscillations of hepatocyte Ca2+ that were coordinated across entire lobules of the liver by propagation of Ca2+ waves along the hepatic plates. At the subcellular level these periodic Ca2+ waves initiated from the sinusoidal domain of cells within the periportal region and propagated radially across cell-cell contacts into the pericentral region, or until terminated by annihilation collision with other Ca2+ wave fronts. With increasing agonist dose, the frequency but not the amplitude of the Ca2+ waves increased. Intracellular Ca2+ wave rates were constant, but transcellular signal propagation was determined by agonist dose, giving rise to a dose-dependent increase in the rate at which Ca2+ waves spread through the liver. At high vasopressin doses, a single Ca2+ wave was observed and the direction of Ca2+ wave propagation was reversed, initiating in the pericentral region and spreading to the periportal region. It is concluded that intercellular Ca2+ waves may provide a mechanism to coordinate responses across the functional units of the liver.

Inositol trisphosphate receptor mediated spatiotemporal calcium signalling

Spatiotemporal Ca2+ signalling in the cytoplasm is currently understood as an excitation phenomenon by analogy with electrical excitation in the plasma membrane. In many cell types, Ca2+ waves and Ca2+ oscillations are mediated by inositol 1,4,5-trisphosphate (IP3) receptor/Ca2+ channels in the endoplasmic reticulum membrane, with positive feedback between cytosolic Ca2+ and IP3-induced Ca2+ release creating a regenerative process. Remarkable advances have been made in the past year in the analysis of subcellular Ca2+ microdomains using confocal microscopy and of Ca2+ influx pathways that are functionally coupled to IP3-induced Ca2+ release. Ca2+ signals can be conveyed into the nucleus and mitochondria. Ca2+ entry from outside the cell allows repetitive Ca2+ release by providing Ca2+ to refill the endoplasmic reticulum stores, thus giving rise to frequency-encoded Ca2+ signals.

Intracellular Ca2+ oscillations and membrane potential fluctuations in intact human T lymphocytes: role of K+ channels in Ca2+ signaling

In intact human T lymphocytes, voltage-gated K+ [K(V)] channels and Ca(2+)-activated K+ [K(Ca)] channels have been recorded using the patch clamp technique in the cell-attached configuration. The reversal potential of the voltage-gated current with high K+ solution in the pipette gives a measure for the cell membrane potential (VM). The open probability of the K(Ca) channels gives a measure for intracellular Ca2+ concentration ([Ca2+]i). By simultaneous recording of both types of K+ channels, the interaction of VM and [Ca2+]i in T lymphocytes was investigated. It was demonstrated that VM fluctuates under resting conditions in a 20 mV range around an average value of -60 mV. In response to T cell receptor stimulation by PHA, rises in [Ca2+]i occur, which vary between cells from transient or sustained elevations to Ca2+ oscillations, in parallel with amplification of the hyperpolarizing deflections of VM. The correlation between VM and [Ca2+]i suggests that Ca2+ oscillations are modulated by positive feedback between Ca2+ influx, [Ca2+]i and VM mediated by K(Ca) channels and by intrinsic VM fluctuations caused by negative feedback between VM and the K(V) channel. Differences in the ratio between K(Ca) and K(V) channel numbers can account for the variability in Ca2+ responses between cells. The results predict periodic K(V) channel activity at rest and alternating K(V) and K(Ca) channel activity during Ca2+ signaling, which was consistent with subsequent observations.

Binding of human factor VIIa to tissue factor induces cytosolic Ca2+ signals in J82 cells, transfected COS-1 cells, Madin-Darby canine kidney cells and in human endothelial cells induced to synthesize tissue factor

Tissue factor (TF) is the most potent trigger of blood clotting known. It activates factor VII (FVII) thereby initiating a cascade of proteolytic reactions resulting in thrombin production. The cloning of TF revealed its structural characteristics to be those of a receptor related to the class 2 cytokine receptor superfamily, but until now no intracellular signal has been discovered related to binding of the ligand (FVIIa) to the putative receptor. We have studied possible intracellular signaling effects of the FVIIa-TF interaction by measuring cytosolic free Ca2+ in single fura-2-loaded cells and found that 200 nM FVIIa caused Ca2+ transients in about 30% of human umbilical vein endothelial cells treated with interleukin-1 beta to express TF, compared to below 5% in uninduced cells. A gradual increase of the basal Ca2+ level was also caused by binding of FVIIa. In the human bladder carcinoma cell line J82, which has a high constitutive TF activity, similar results were found. An antibody neutralizing TF activity decreased the response rate to control levels. COS-1 cells which do not make TF did not respond to FVIIa as opposed to COS-1 cells expressing TF after transfection with a human TF cDNA construct. The canine kidney cell line MDCK, a constitutive TF producer, responded especially well; up to 100% of the cells examined showed Ca2+ oscillations which were dose dependent with regard to frequency, latency, maximal amplitude, and recruitment of responding cells. The frequency was reduced by inhibition of Ca2+ influx with 100 microM LaCl3. In confluent MDCK cells the Ca2+ oscillations were synchronous, constituting the first evidence of a synchronous cytosolic Ca2+ oscillator generated by global application of agonist. Thus, TF mediates a cytosolic Ca2+ signal upon interaction with its ligand FVIIa, thereby suggesting a more complex biological role for TF.

Allyl alcohol cytotoxicity in isolated rat hepatocytes: mechanism of cell death does not involve an early rise in cytosolic free calcium

We examined the effect of a toxic concentration of allyl alcohol (0.5 mM) on intracellular calcium concentrations in isolated rat hepatocytes. An increase in phosphorylase a activity was evident in the hepatocytes after 30 min of incubation with allyl alcohol, suggesting that the toxicant may produce an early rise in cytosolic free calcium. The increase in phosphorylase a activity was not reversed by the addition of dithiothreitol (DTT), a sulfhydryl compound that reverses the events that initiate cell killing by allyl alcohol. When intracellular calcium concentrations were measured directly, using fura-2 as the calcium indicator, there was no effect of allyl alcohol on cytosolic free calcium during the first 60 min of exposure, a critical period for development of irreversible damage. Incubation with allyl alcohol did not interfere with the measurement of intracellular calcium. The increases in cytosolic free calcium produced by phenylephrine or ATP were similar to those reported by others and not affected by the presence of allyl alcohol. The results from this study demonstrate that increased cytosolic free calcium is not essential for allyl alcohol-induced cytotoxicity to isolated rat hepatocytes.

Slow oscillations of free intracellular calcium ion concentration in human fibroblasts responding to mechanical stretch

Calcium transients in single, human gingival fibroblasts were studied after mechanical stretching of flexible culture substrates. A model system was developed to reproducibly stretch and rapidly (<1 sec) refocus cells in the same focal plane so that changes in the concentration of free intracellular calcium ions ([Ca2+]i) were monitored without delay. Attached cells were grown on flexible bottom Petriperm dishes, loaded with fura-2/AM, and stretched by 1% or 2.8% of substrate area. The stretch caused no significant cell detachment or membrane lesions. A 1% stretch induce no calcium response, but a 2.8% stretch stimulated an initial calcium transient and the subsequent generation of [Ca2+]i oscillations of up to 2,000 sec. At 1% stretch, there was no calcium response. Cell shape and plating time were important determinants in the calcium response to mechanical stimulation: the responder cells were small and round without long processes. Major calcium transients were inhibited completely by 5 mM EGTA or by 10 microM gadolinium ions, by 50 microM nifedipine, or 250 microM verapamil, suggesting an influx of calcium through stretch-activated (SA) channels and L-type calcium channels. Depolarization by high KCl (144 mM) in the extracellular medium enhanced the amplitude of calcium transients by 54%. Calcium oscillations were not inhibited by preincubation with thapsigargin, caffeine, cholera toxin, staurosporine or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), indicating that IP3 sensitive pools, IP3 insensitive pools, GS alpha subunits, and protein kinase C, respectively, were not involved in the generation of calcium oscillations. Pretreatment with genistein, a specific tyrosine kinase inhibitor or cytochalasin D, an inhibitor of actin polymerization, or pertussis toxin, an inhibitor of Gi alpha and G(o) alpha subunits, completely abolished calcium transients and oscillations. These results indicate that Ca2+ flux due to mechanical stretching is likely mediated through SA ion channels and is dependent on tyrosine kinases, pertussis toxin-sensitive subunits of G-proteins, and actin filaments.

Effects of protein kinase C and cytosolic Ca2+ on exocytosis in the isolated perfused rat liver

Both protein kinase C and cytosolic Ca2+ are involved in the regulation of exocytosis in a number of cell types. However, the relative importance of each of these for apical exocytosis in the hepatocyte is unknown. To investigate this, we studied the effects of protein kinase C and Ca2+ agonists on horseradish peroxidase excretion in the isolated perfused rat liver. Vasopressin increased both horseradish peroxidase concentration and net horseradish peroxidase excretion in bile, and these effects were abolished by the protein kinase C inhibitor H-7. The protein kinase C activator phorbol dibutyrate also increased both net excretion and the concentration of biliary horseradish peroxidase. In contrast, the Ca2+ ionophore A23187 and the Ca2+ mobilizing agent 2,5'-di(tertbutyl)-1,4-benzohydroquinone both had minimal effects on horseradish peroxidase concentration and inhibited the rate of horseradish peroxidase excretion. These results suggest that protein kinase C stimulates apical exocytosis in the hepatocyte, whereas increased Cai2+ per se does not influence exocytosis and inhibits excretion only transiently by reducing bile flow.

Propagation of cytosolic calcium waves into the nuclei of hepatocytes

The temporal and spatial organization of [Ca2+] changes within the nucleus of Fura-2 loaded hepatocytes maintained in primary culture has been investigated. Vasopressin stimulation induced oscillatory waves of cytosolic free [Ca2+] increase, which propagated freely through the nuclear region. Based on the amplitude of the Fura-2 signals from this region, the morphology of the hepatocyte nucleus and the rapid penetration of the nucleus by injected Fura-2, it can be concluded that the nuclear Ca2+ responses reflect changes occurring within the nucleoplasm. Intranuclear Ca2+ increases occurred as waves that appear to be directed by the Ca2+ waves passing through the surrounding cytoplasm. The apparent velocity of Ca2+ waves was higher in the nucleoplasm than in the cytoplasm (19.5 +/- 2.9 versus 11.0 +/- 1.1 microns/s). The nucleoplasm does not contain vesicular Ca2+ stores that might be released by Ins(1,4,5)P3. However, the nuclear envelope functions as a Ca2+ store that is sensitive to mobilization by Ins(1,4,5)P3. We conclude that the [Ca2+] in the nucleoplasm of the hepatocyte is close to equilibrium with the cytosolic [Ca2+] and that oscillatory waves of cytosolic [Ca2+] are closely paralleled by similar [Ca2+] changes in the nucleoplasm. The nuclear envelope is a component of the intracellular Ins(1,4,5)P3-sensitive Ca2+ storage pool and may serve as a reservoir for [Ca2+] elevations within the nucleus.

Characterization of signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes

To investigate the signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes, we measured changes in cytosolic free calcium and intracellular pH levels at the single-cell level using digital imaging fluorescence microscopy of fura-2- or BCECF-loaded hepatocytes in primary culture. Epidermal growth factor induced cytosolic free calcium oscillations consisting of periodic trains of spikes with a latency period of up to several minutes. These calcium responses were inhibited by tyrosine kinase inhibitor genistein (100 mumol/L) and abolished by emptying of intracellular Ca2+ pools with 3 mumol/L thapsigargin, an inhibitor of Ca(2+)-ATPase on the endoplasmic reticulum. Epidermal growth factor (1 nmol/L) induced an intracellular pH increase of 0.12 +/- 0.07 units from the basal level of 7.25 +/- 0.09 units after several minutes of latency. This effect was completely abolished by 1 mmol/L amiloride, an inhibitor of the Na+/H+ exchanger. The epidermal growth factor-induced intracellular pH increase was inhibited by pretreatment of hepatocytes with genistein (100 mumol/L), thapsigargin (3 mumol/L) or calmodulin inhibitor W-7 (25 mumol/L), but not with protein kinase C inhibitor H-7 (50 mumol/L) or with cyclic AMP-dependent kinase inhibitor H-8 (60 mumol/L). Phorbol ester PMA (phorbol 12-myristate 13-acetate), a potent activator of protein kinase C, induced a slight intracellular pH increase significantly smaller than that with epidermal growth factor, whereas this effect was completely blocked by pretreatment with H-7, indicating that PMA-induced intracellular pH increase is mediated by protein kinase C pathways, unlike epidermal growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

A membrane potential model with counterions for cytosolic calcium oscillations

An initial model has been proposed to describe a mechanism for cytosolic calcium oscillations [Jafri MS. Vajda S. Pasik P. Gillo B. (1992) A membrane model for cytosolic calcium oscillations: a study using Xenopus oocytes. Biophys. J., 63, 235-246]. In this paper we extend our original model to include the effects of counterion movement into the ER in response to calcium release. This produces smoother oscillations over a wider parameter range. We have lowered the endoplasmic reticulum (ER) intraluminal free calcium concentration and shown that the oscillations can occur at lower ER membrane potentials, consistent with physiological values. The improved model is then tested with two representative paradigms that are currently under investigation by many researchers. The model predicts that the reduction of the ER calcium pump (Ca-ATPase) rate can cause the termination of cytosolic calcium oscillations in an active cell, and induce oscillations in a resting cell. This result is consistent with experiments with thapsigargin, a Ca-ATPase activity inhibitor. In addition, we simulate the latency period for the response to the application of agonist and offer a plausible explanation for it. Our mathematical model is currently the only model that formulates the contributions of calcium binding proteins, ER membrane potential, ER counterion movements, and distinct calcium pump populations, and describes their effects on cytosolic calcium oscillations.

Quantitative analysis of cytosolic free calcium oscillations in neutrophils by mathematical modelling

Mathematical models are often used to elucidate mechanisms behind cytosolic Ca2+ oscillations. We have evaluated the use of mathematical modelling to analyse and quantify Ca2+ signal patterns, in single, adherent human neutrophils (PMN) after stimulation by the bacterial peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). The cells were loaded with Fura-2 and fluctuations in cytosolic Ca2+ recorded with a video based digital imaging system. A new indirect intracellular calibration method was introduced to avoid the uncertainty in obtaining an equilibrium between the extracellular and intracellular calcium concentrations. Two different approaches to mathematical modelling were used. First, we applied a sensitivity analysis with a two-pool model by assuming an optimal situation using reliable a priori estimates of all structural parameters (e.g. Hill coefficients and dissociation constants). We found that the a priori estimates of the other 5 more variable parameters must lie within the range of 25-400% of the postulated true parameter values to be reliable in a parameter estimation method. Small changes (less than 5%) in those variable parameter values induced very different types of signal patterns which may have some relevance in evaluating a possible functional significance to the oscillatory signals. Second, we employed a one-pool, non oscillatory model integrated with a power spectrum method as a tool to quantify the dose dependency between fMLP (1-1000 nM) and parameters describing the biphasic process of calcium signalling and parameters describing only the oscillatory components. We conclude that the frequency of the observed oscillations assembled around one characteristic frequency independent of fMLP concentration, and sinusoidal oscillations were observed most frequently in PMN stimulated to a moderate peak [Ca2+]i level.

Cellular and subcellular calcium signaling in gastrointestinal epithelium

Ca2+ is a critical second messenger in virtually all cell types, including the various epithelial cell types within the digestive system. When measured in cell populations, Ca2+ signals usually appear as a single transient or prolonged elevation. In individual epithelial cells, signaling patterns often vary from cell to cell and may contain more complex features such as Ca2+ oscillations. Subcellular Ca2+ signals show a further level of complexity, such as Ca2+ waves, and may relate to the polarized structure and function of epithelial cells. The approaches to detect cytosolic Ca2+ signals, the patterns and mechanisms of Ca2+ signaling, and the role of such signals in regulating the function of polarized epithelium within the gastrointestinal tract, pancreas, and liver are reviewed in this report.

Transitions of latency time and oscillation phase on parameter surfaces from models of intracellular calcium ion dynamics

The dynamics of two classical elementary compartmental models stimulating intracellular calcium ion oscillatory behavior are examined in terms of parameter surfaces. It has been found that, along certain lines of instability on surfaces defined by model parameters, the highly non-linear nature of these models produces sharp transitions in the latency time which determines the phase of oscillations once they commence. This sensitivity to initial conditions in deterministic models, along with the stochastic variance inevitably present in actual biological systems, illustrates how two seemingly identical cells activated by identical synchronous stimulation can exhibit oscillatory responses which are out of phase with respect to each other.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Calcium as a permissive factor but not an initiation factor in DNA synthesis induction in cultured rat hepatocytes by the peroxisome proliferator ciprofibrate

The non-genotoxic hepatocarcinogen and peroxisome proliferating agent, ciprofibrate, is a liver mitogen both in vivo and in cultured adult rat hepatocytes, but the mechanisms of its mitogenicity have not been elucidated. We previously observed that ciprofibrate rapidly increased hepatocyte free intracellular Ca2+ concentration ([Ca2+]i), suggesting that this effect may play a role in the initiation of DNA synthesis. In the present study, we have identified a relationship between Ca2+ and the stimulation of hepatocyte DNA synthesis by ciprofibrate. Exposure of cultured adult rat hepatocytes to ciprofibrate (200 microM) for 48 hr increased DNA synthesis by approximately 2-fold, and this response was attenuated in a Ca(2+)-deficient medium and by the Ca2+ channel blockers nicardipine and verapamil. To examine the relationship between the stimulation of hepatocyte DNA synthesis and increases in [Ca2+]i by ciprofibrate, the intracellular Ca2+ chelator 5,5'-dimethyl-1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid (dimethyl-BAPTA) was employed. Pretreatment of hepatocytes with dimethyl-BAPTA blocked ciprofibrate-induced [Ca2+]i increase, but did not block ciprofibrate-induced hepatocyte DNA synthesis. Dimethyl-BAPTA was only effective in reducing ciprofibrate-induced DNA synthesis when present during the latter 24 hr of a 48-hr culture period. These data suggest that the early mobilization of hepatocyte [Ca2+]i by ciprofibrate does not play an initiating role in the induction of hepatocyte DNA synthesis but rather may operate as a permissive factor for the entry of ciprofibrate-treated adult rat hepatocytes into S-phase.

Intracellular calcium waves generated by Ins(1,4,5)P3-dependent mechanisms

Cellular oscillations of cytosolic free Ca2+ ([Ca2+]i) have been observed in many cell types in response to cell surface receptor agonists acting through inositol 1,4,5-trisphosphate (InsP3). In a number of cases where appropriate spatial and temporal resolution have been used to examine these [Ca2+]i oscillations, they have been found to be organized as repetitive waves of Ca2+ increase that propagate through the cytosol of individual cells. In some cases Ca2+ waves also occur as a single pass through stimulated cells. This review discusses the factors underlying the spatial organization of [Ca2+]i signals in the form of Ca2+ waves. In addition, potential mechanisms for the initiation and subsequent propagation of these Ca2+ waves are described.

Microdomains with high Ca2+ close to IP3-sensitive channels that are sensed by neighboring mitochondria

Microdomains of high intracellular calcium ion concentration, [Ca2+]i, have been hypothesized to occur in living cells exposed to stimuli that generate inositol 1,4,5-trisphosphate (IP3). Mitochondrially targeted recombinant aequorin was used to show that IP3-induced Ca2+ mobilization from intracellular stores caused increases of mitochondrial Ca2+ concentration, [Ca2+]m, the speed and amplitude of which are not accounted for by the relatively small increases in mean [Ca2+]i. A similar response was obtained by the addition of IP3 to permeabilized cells but not by perfusion of cells with Ca2+ at concentrations similar to those measured in intact cells. It is concluded that in vivo, domains of high [Ca2+]i are transiently generated close to IP3-gated channels and sensed by nearby mitochondria; this may provide an efficient mechanism for optimizing mitochondrial activity upon cell stimulation.

Temporal patterns of alpha 1-receptor stimulation regulate amplitude and frequency of calcium transients

The pulsatile release of neurotransmitters and many hormones might encode specific biological information according to temporal pattern. We tested this hypothesis by applying pulsed alpha 1-adrenoceptor stimulation to single aequorin-injected hepatocytes. The amplitude of free Ca2+ transients induced by rapid phenylephrine pulses (20-s interpulse interval) and continuous stimulation was similar (approximately 640 nM) but increased to approximately 1,000 nM as the interpulse interval was increased to 120 s. The same overall response was maintained despite a 13-fold reduction in average phenylephrine concentration. Some regimes of pulsed phenylephrine stimulation could give a higher frequency of pulsed phenylephrine stimulation could give a higher frequency of free calcium oscillations than continuous stimulation, or more rapid stimulation when some agonist pulses failed to elicit a free Ca2+ transient. For the same average phenylephrine concentration (0.3-0.6 microM), pulsed regimes could result in significantly higher frequencies and integrated responses than constant application. The lags between phenylephrine pulses and free Ca2+ transients reduced as the period between pulses increased. The amplitude and lag data are consistent with a refractory period of 18 s and a recovery phase with a time constant of approximately 100 s, perhaps corresponding to dephosphorylation of alpha 1-adrenoceptors phosphorylated by protein kinase C during each free Ca2+ transient.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

[Calcium and liver]

Cells expand energy to lower the concentration of free calcium in the cytosol ([Ca2+]i) to a very low level. Extracellular Ca2+ entering via channels situated in the plasma membrane is expelled into the extracellular medium by a Ca(2+)-Mg(2+)-ATPase or by Na(+)-Ca2+ exchangers. The Ca2+ that enters the cell is sequestered, once inside the cytosol, by a Ca(2+)-Mg(2+)-ATPase, which concentrates Ca2+ in specialized domains of the endoplasmic reticulum. The nucleus and the mitochondria also concentrate Ca2+, but less efficiently. The stimulation of numerous receptors by hormones, growth factors and neurotransmitters coupled to GTP-binding proteins provokes a rapid increase in [Ca2+]i by mobilizing Ca2+ from intra- and extracellular compartments. Membrane coupling is ensured by the activation of a phospholipase C-beta, which hydrolyses a doubly phosphorylated phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The inositol (1,4,5)-trisphosphate (InsP3) consequently formed binds to a receptor consisting in 4 homologous of 250 kDa each. The InsP3 receptor has been localized to a specialized region, rich in Ca2+, of the endoplasmic reticulum. The receptor has been purified and its sequence obtained. Reincorporated into planar bilayers, it displays the properties of a channel. In the cell, opening of the InsP3 receptor-channel provokes the release of the Ca2+ accumulated within the endoplasmic reticulum. Analyzing the kinetics of channel opening by the methods of rapid mixing, rapid filtration or flash photolysis of caged InsP3 has revealed that InsP3 opens the channel within a very short time, probably less than 30 msec. The InsP3 receptor-channel is autoregenerative. With the sustained stimulation of a Ca2+ influx the release of Ca2+ leads to an augmentation of [Ca2+]i, which is responsible for triggering cellular responses. The complexity of Ca2+ signals produced by stimulated cells has been revealed by studies in which highly effective techniques have been used to detect Ca2+ ions in the cytosol, such as bioluminescent proteins, fluorescent indicators or ionic currents sensitive to Ca2+. It appears that variations in [Ca2+]i induced by stimulation consist of oscillations of which the frequency, but not the amplitude, depends on the concentration of the hormone. Moreover, by summing the images picked up with a video recorder, it has been possible to demonstrate the changes in [Ca2+]i at the subcellular level and the waves of Ca2+ in stimulated cells.

One-pool model for Ca2+ oscillations involving Ca2+ and inositol 1,4,5-trisphosphate as co-agonists for Ca2+ release

Experimental observations indicate that Ca(2+)-induced Ca2+ release (CICR) may underlie Ca2+ oscillations in a variety of cells. In its original version, a theoretical model for signal-induced Ca2+ oscillations based on CICR assumed the existence of two types of pools, one sensitive to inositol 1,4,5-trisphosphate (IP3) and the other one sensitive to Ca2+. Recent experiments indicate that Ca2+ channels may sometimes be sensitive to both IP3 and Ca2+. Such a regulation may be viewed as Ca(2+)-sensitized IP3-induced Ca2+ release or, alternatively, as a form of IP3-sensitized CICR. We show that sustained oscillations can still occur in a one-pool model, provided that the same Ca2+ channels are sensitive to both Ca2+ and IP3 behaving as co-agonists. This model and the two-pool model based on CICR both account for a number of experimental observations but differ in some respects. Thus, while in the two-pool model the latency and period of Ca2+ oscillations are of the same order of magnitude and correlate in a roughly linear manner, latency in the one-pool model is always brief and remains much shorter than the period of oscillations. Moreover, the first Ca2+ spike is much larger than the following ones in the one-pool model. These distinctive properties might provide an explanation for the differences in Ca2+ oscillations observed in various cell types.

Mechanisms of intercellular calcium signaling in glial cells studied with dantrolene and thapsigargin

Mechanical stimulation of a single cell in a primary mixed glial cell culture induced a wave of increased intracellular calcium concentration ([Ca2+]i) that was communicated to surrounding cells. Following propagation of the Ca2+ wave, many cells showed asynchronous oscillations in [Ca2+]i. Dantrolene sodium (10 microM) inhibited the increase in [Ca2+]i associated with this Ca2+ wave by 60-80%, and prevented subsequent Ca2+ oscillations. Despite the markedly decreased magnitude of the increase in [Ca2+]i, the rate of propagation and the extent of communication of the Ca2+ wave were similar to those prior to the addition of dantrolene. Thapsigargin (10 nM to 1 microM) induced an initial increase in [Ca2+]i ranging from 100 nM to 500 nM in all cells that was followed by a recovery of [Ca2+]i to near resting levels in most cells. Transient exposure to thapsigargin for 2 min irreversibly blocked communication of Ca2+ wave from the stimulated cell to adjacent cells. Glutamate (50 microM) induced an initial increase in [Ca2+]i in most cells that was followed by sustained oscillations in [Ca2+]i in some cells. Dantrolene (10 microM) inhibited this initial [Ca2+]i increase caused by glutamate by 65-90% and abolished subsequent oscillations. Thapsigargin (10 nM to 1 micron) abolished the response to glutamate in over 99% of cells. These results suggest that while both dantrolene and thapsigargin inhibit intracellular Ca2+ release, only thapsigargin affects the mechanism that mediates intercellular communication of Ca2+ waves. These findings are consistent with the hypothesis that inositol trisphosphate (IP3) mediates the propagation of Ca2+ waves whereas Ca(2+)-induced Ca2+ release amplifies Ca2+ waves and generates subsequent Ca2+ oscillations.

Characterization of adrenoceptors involved in the electrogenic chloride secretion by cultured rat epididymal epithelium

1. Short-circuit current (SCC) technique was used to study the adrenoceptors involved in the electrogenic chloride secretion by cultured cauda epididymal epithelium of rats. Stimulation of the epithelium with noradrenaline (primarily beta 1-adrenoceptor selective agonist), salbutamol (beta 2-adrenoceptor selective agonist) and adrenaline (non-selective beta-adrenoceptor agonist) led to a rise in SCC. At a low chart-speed (2 mm min-1), the response profile to these agonists consisted of a peak followed by a sustained response considerably higher than the basal SCC. 2. The EC50s (doses of agonist producing 50% maximum response) of noradrenaline, salbutamol and adrenaline were 300, 115 and 10 nM respectively. Pretreating the tissues with 1 microM atenolol (beta 1-selective antagonist) and 10 microM butoxamine (beta 2-selective antagonist) shifted the dose-response curves of noradrenaline (shifted EC50 = 4000 nM) and salbutamol (shifted EC50 = 1050 nM) to the right. Atenolol (1 microM) and butoxamine (10 microM) shifted the dose-response curve of adrenaline to the right with new EC50s of 30 nM and 115 nM, respectively. 3. The rapidly rising phase of the SCC response to noradrenaline and adrenaline observed at low chart-speed consisted of a brief and transient retraction followed by a rebound increase in SCC. At a high chart-speed (1 mm s-1), the retraction and rebound phenomenon manifested as a fast initial spike which could be blocked by phentolamine (non-specific alpha-adrenoceptor antagonist) in a dose-dependent fashion. Similar initial spikes were observed when the tissues were stimulated with phenylephrine (alpha-selective agonist) but not with isoprenaline (non-selective beta-agonist) or forskolin (activator of adenylate cyclase). The response of the initial spike triggered by noradrenaline was dose-dependent and the EC50 was 2000 nM.4. The present study showed that the electrogenic chloride secretion by rat epididymis could be stimulated by (alphaxi-, beta131- and beta2-adrenoceptor agonists. The al-mediated response had a faster onset and more transient action than the 3-counterpart. It is postulated that epididymal chloride secretion might be regulated by neural (noradrenaline-mediated) and humoral (adrenaline-mediated) controls and that the stimulus-secretion coupling mechanisms might involve both Ca2+(alpha-mediated response) and adenosine 3':5'-cyclic monophosphate (beta-mediated response) as intracellular second messengers.

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.

Bradykinin and muscarine induce Ca(2+)-dependent oscillations of membrane potential in rat glioma cells indicating a rhythmic Ca2+ release from internal stores: thapsigargin and 2,5-di(tert-butyl)-1, 4-benzohydroquinone deplete InsP3-sensitive Ca2+ stores in glioma and in neuroblastoma-glioma hybrid cells

Continuous superfusion of rat glioma cells with medium containing bradykinin (from 0.2 nM) induced a transient hyperpolarization followed by regular hyperpolarizing oscillations of the membrane potential. Similar repetitive hyperpolarizing oscillations were caused by extracellularly applied bradykinin or muscarine or by intracellularly injected GTP-gamma-S. The frequency of the oscillations was 1 per minute at bradykinin concentrations ranging from 0.2 nM to 2 microM, but the amplitude and duration increased with rising peptide concentration. The muscarine-induced oscillations were blocked by atropine. In the presence of extracellular Ca2+, the substances thapsigargin, 2,5-di(tert-butyl)-1,4-benzohydroquinone (tBuBHQ), and ionomycin reversibly suppressed the bradykinin-induced oscillations. Thapsigargin and tBuBHA, which are known to block the Ca2+ ATPase of endoplasmic reticulum, caused a transient rise in cytosolic Ca2+ activity, monitored with Fura-2, in suspensions of rat glioma cells or of mouse neuroblastoma-rat glioma hybrid cells. After a transient Ca2+ rise caused by thapsigargin, tBuBHQ, or ionomycin, the Ca2+ response to bradykinin which is known to be due to release of Ca2+ from internal stores was suppressed. This indicates that thapsigargin and tBuBHQ deplete internal Ca2+ stores as already seen previously for ionomycin. Thus, the inhibition of the membrane potential oscillations by thapsigargin, tBuBHQ, and ionomycin indicates that the oscillations are associated with activation of InsP3-sensitive Ca2+ stores. In some cells composite oscillation patterns which consisted of two independent oscillations with different amplitudes that overlapped additively were seen. We discuss that this pattern and the concentration dependency of the oscillations could be due to "quantal" Ca2+ release from stores with different inositol 1,4,5-triphosphate sensitivities. Subsidence of the oscillations after omission of extracellular Ca2+ seems to be due to a lack of replenishment of the intracellular stores with Ca2+, which comes from the extracellular compartment.

Functional involvement of α1and α2-adrenoceptors in86Rb efflux from liver slices and lipolysis in guinea-pig isolated adipocytes

1. The application of an alpha 1-adrenoceptor agonist, amidephrine, to guinea-pig liver slices increases glucose release and 86Rb efflux. Since prazosin was more potent than yohimbine in inhibiting both responses, alpha 1-adrenoceptors seem to be involved in the effects evoked by the agonist. 2. Clonidine (an alpha 2-adrenoceptor agonist) at doses unable to activate liver glycogenolysis increased 86Rb release and potentiated isoprenaline in promoting 86Rb efflux. Since yohimbine antagonized clonidine in promoting 86Rb efflux, alpha 2-adrenoceptors also seem to control plasmalemmal permeability to 86Rb. 3. The liver slice responses resulting from alpha 1- and alpha 2-adrenoceptor stimulation required extracellular calcium. Calcium absence or the administration of D-600 attenuated the effects of amidephrine on glucose release and 86Rb outflow and Ca2+ excess re-established both responses. D-600 and apamin blocked clonidine-induced 86Rb efflux, suggesting that alpha 2-adrenoceptor stimulation activates calcium dependent K+ channels. 4. alpha 2-adrenoceptors do not appear to mediate antilipolytic effects in guinea-pig fat cells.

Evidence for the modulation of cell calcium by epinephrine in fish hepatocytes

The effect of epinephrine (10(-7) M) on cytosolic free-Ca2+ concentration ([Ca2+]i) and its dependency on external Ca2+ were studied in fura-2-loaded hepatocytes isolated from three teleost fish species: American eel, brown bullhead, and rainbow trout. Basal [Ca2+]i was similar in eel and trout hepatocytes (79.6 +/- 14.6 and 75.7 +/- 17.4 nM, respectively) but was significantly higher in bullhead cells (184 +/- 23 nM). Epinephrine-induced [Ca2+]i oscillations were observed only in eel hepatocytes. These oscillations, which presented variable patterns among individual cells, also developed in the absence of external Ca2+, although their amplitude progressively declined to eventually vanish under such conditions. In bullhead hepatocytes, epinephrine induced a biphasic [Ca2+]i response, with an initial transient rise followed by a sustained component; this response was virtually abolished in the absence of extracellular Ca2+. The peak [Ca2+]i achieved (433.5 +/- 135.6 nM) was more than two times that of eel cells (184.3 +/- 30 nM) but represented a similar percent increase above control [Ca2+]i for both species. Rainbow trout hepatocytes, contrary to eel and bullhead cells, demonstrated little epinephrine sensitivity, with less than 20% of the cells responding. These data clearly point to significant species differences both in terms of epinephrine-induced changes in [Ca2+]i and in the dependence of these transients on external Ca2+. Thus the eel response relies primarily on intracellular stores, whereas the bullhead response principally involves enhanced influx of Ca2+ from the extracellular milieu. Furthermore, the similarity of these responses with those reported for mammalian hepatocytes strongly suggests that an alpha-adrenoceptor/Ca2+ transduction system is involved in at least eel and bullhead hepatocytes.

Role of Ca2+ channel blockers in insulin secretion resulting from α1- and β-adrenoceptor stimulation in the rabbit

In conscious fasted rabbits the i.v. infusion of amidephrine (alpha 1-agonist) or isoprenaline (beta-agonist) induced an increase in plasma levels of immunoreactive insulin. The alpha 1-mediated response was suppressed in animals pretreated with calcium channel blockers (verapamil and elgodipine). The potentiated insulin secretory response in rabbits exposed to dual (alpha 1 + beta) adrenoceptor stimulation was also prevented by verapamil and elgodipine. These results suggest that extracellular calcium is required to mediate the effects of amidephrine on insulin secretion and to support potentiation.

Transient but not oscillating component of the calcium mobilizing response to gonadotropin-releasing hormone depends on calcium influx in pituitary gonadotrophs

Gonadotropin-releasing hormone (GnRH)-stimulated changes in the cytosolic free Ca2+ concentration ([Ca2+]i) were studied in gonadotrophs cultured from 3-week ovariectomized rat pituitaries. One animal was used per cell preparation. [Ca2+]i was monitored in individual gonadotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short stimulation with GnRH evoked a complex concentration-dependent Ca2+ response in individual gonadotrophs. 0.1-1 nM GnRH triggered a series of sinusoidal-like [Ca2+]i oscillations superimposed upon a modest slow [Ca2+]i rise--the oscillating response mode--while 10-100 nM GnRH caused a biphasic increase in [Ca2+]i consisting of a monophasic transient and oscillations--the transient/oscillating response mode. Despite the consistency of Ca2+ responses, an inter-preparation heterogeneity of [Ca2+]i oscillations frequency was noticed. Moreover, we observed that, within a given cell preparation, the frequency of [Ca2+]i oscillations was independent of GnRH concentration whereas both peak [Ca2+]i and area under the [Ca2+]i versus time curve were concentration-dependent. Thus, in gonadotrophs, the presence of the GnRH signal would lead to [Ca2+]i oscillations, while the amplitude of the [Ca2+]i responses would code for the concentration of agonist. Both transient and oscillating components of GnRH responses depended on releasing activity of Ca(2+)-sequestering pools in as much as GnRH responses were unaffected by brief removal of external Ca2+, but suppressed by chelating intracellular free Ca2+ with BAPTA. However, prolonged exposure to a Ca(2+)-free medium suppressed the transient component while leaving the oscillating component unaffected. We therefore propose that gonadotrophs employ Ca(2+)-sequestering pools, whose maintenance depends on a slow Ca(2+)-entry, to give an amplitude-coded Ca2+ rise in response to a short GnRH stimulation.

Epidermal growth factor induces dose-dependent calcium oscillations in single fura-2-loaded hepatocytes

Digital imaging fluorescence microscopy has been used to investigate epidermal growth factor-induced calcium responses of fura-2-loaded hepatocytes in primary culture at the single-cell level. Epidermal growth factor induced oscillations in cytosolic free calcium ([Ca2+]i) consisting of a periodic train of spikes unlike the monophasic elevation in cell suspensions reported previously. In this study, 79% of the cells in the microscopic field responded to 0.1 nmol/L epidermal growth factor, and 78% of the responsive cells displayed oscillations. However, the frequency of oscillations differed considerably from cell to cell. [Ca2+]i measurement in a cell population was simulated using these data, but only a slightly biphasic pattern was obtained, indicating the significance of single-cell measurement of [Ca2+]i. Because considerable heterogeneity existed in the sensitivity to epidermal growth factor between the cells, single hepatocytes were stimulated sequentially with increasing concentrations of epidermal growth factor to investigate the dose dependence of the oscillations. The frequency of the oscillations increased with increasing epidermal growth factor concentration, but the amplitude was similar for all concentrations, suggesting the existence of frequency-encoded information even in the pathway through tyrosine kinase for epidermal growth factor signaling. The pattern of the oscillations with epidermal growth factor, especially the latency, was considerably different from that with phenylephrine, which is known to use the phosphatidylinositol pathway, possibly because of the difference in the pathway toward phosphatidylinositol turnover between these agonists.(ABSTRACT TRUNCATED AT 250 WORDS)