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

Oscillations and waves of cytosolic calcium: insights from theoretical models

Oscillations in cytosolic Ca2+ occur in a wide variety of cells, either spontaneously or as a result of external stimulation. This process is often accompanied by intracellular Ca2+ waves. A number of theoretical models have been proposed to account for the periodic generation and spatial propagation of Ca2+ signals. These models are reviewed and their predictions compared with experimental observations. Models for Ca2+ oscillations can be distinguished according to whether or not they rely on the concomitant, periodic variation in inositol 1,4,5-trisphosphate. Such a variation, however, is not required in models based on Ca(2+)-induced Ca2+ release. When Ca2+ diffusion is incorporated into these models, propagating waves of cytosolic Ca2+ arise, with profiles and rates comparable to those seen in the experiments.

[Modeling in biology. Structured analysis of intracellular calcium oscillations in electrically non-excitable cells]

In this paper a systematic approach to the mathematical modeling of intracellular Ca2+ oscillations is introduced. After a structured analysis a stochastic model of the system is derived which is numerically tractable by means of a stochastic simulation. A critical discussion of theoretical models for Ca2+ oscillations reveals that not all of the proposed mechanisms are consistent with experimental data. In addition, a model for oscillatory calcium waves is presented. Uncovering these mechanisms facilitates the design of anti-mitotic drugs interfering with Ca2+ metabolism.

Cytosolic calcium oscillations induced by hepatocyte growth factor (HGF) in single fura-2-loaded cultured hepatocytes: effects of extracellular calcium and protein kinase C

Hepatocyte growth factor (HGF) induced the periodic fluctuations of cytosolic calcium concentration ([Ca2+]i) in primary cultured rat hepatocytes, which were dependent on extracellular calcium. The HGF-induced [Ca2+]i oscillations were suppressed by the pretreatment with phorbol 12-myristate 13-acetate (PMA). Administration of PMA during oscillations also caused their blockade, but the subsequent addition of protein kinase C (PKC) inhibitor H-7 reversed the inhibitory effects of PMA, thereby resulting in the resumption of the oscillatory responses. Moreover, the prior exposure to H-7 caused apparent increases in [Ca2+]i spike peaks elicited by HGF. These results suggest a negative modulation via PKC in HGF-induced repetitive [Ca2+]i transients. The absence of HGF-induced oscillations after the thapsigargin treatment indicates that the agonist-sensitive intracellular Ca2+ pool plays a crucial role in the [Ca2+] oscillations.

Intracellular calcium as a second messenger for human hepatocyte growth factor in hepatocytes

Human hepatocyte growth factor is a newly discovered substance that stimulates DNA synthesis in vitro. In this study, we examined intracellular Ca2+ movement as one of the second messengers for human hepatocyte growth factor in primary-cultured hepatocytes. The addition of hHGF induced Ca2+ oscillation, but the frequency of oscillations varied from cell to cell. We also saw marked intercellular heterogeneity in the initial latent period for the Ca2+ response; the mean latent period was rather longer than those seen with phenylephrine and vasopressin. This difference in the initial latent period may be due to the difference in the pathways of Ca2+ elevation. Duration of culture determined the number of human hepatocyte growth factor-responsive cells; their number peaked at 2 to 5 hours of confluent culture, whereas the peak was earlier in a low-density culture. These changes in responsiveness during culture can be explained by the cell cycle-dependent sensitivity to human hepatocyte growth factor of hepatocytes. The Ca2+ response to human hepatocyte growth factor was dose dependent; 10(-10) mol/L hHGF gave the highest Ca2+ response, similar to the dose-response curve of DNA synthesis. We even observed the Ca2+ response in the Ca(2+)-free buffer, so the increase in Ca2+ was considered due to release from intracellular Ca2+ stores. These results suggest that human hepatocyte growth factor causes the intracellular Ca2+ elevation in the early stage of the cell cycle and that it plays important roles in the signal transduction systems for human hepatocyte growth factor and the proliferation of hepatocytes.

Protein phosphorylation driven by intracellular calcium oscillations: a kinetic analysis

Given the ubiquitous nature of signal-induced Ca2+ oscillations, the question arises as to how cellular responses are affected by repetitive Ca2+ spikes. Among these responses, we focus on those involving protein phosphorylation. We examine, by numerical simulations of a theoretical model, the situation where a protein is phosphorylated by a Ca(2+)-activated kinase and dephosphorylated by a phosphatase. This reversible phosphorylation system is coupled to a mechanism generating cytosolic Ca2+ oscillations; for definiteness, this oscillatory mechanism is based on the process of Ca(2+)-induced Ca2+ release. The analysis shows that the average fraction of phosphorylated protein increases with the frequency of repetitive Ca2+ spikes; the latter frequency generally rises with the extent of external stimulation. Protein phosphorylation therefore provides a mechanism for the encoding of the external stimulation in terms of the frequency of signal-induced Ca2+ oscillations. Such a frequency encoding requires precise kinetic conditions on the Michaelis-Menten constants of the kinase and phosphatase, their maximal rates, and the degree of cooperativity in kinase activation by Ca2+. In particular, the most efficient encoding of Ca2+ oscillations based on protein phosphorylation occurs in conditions of zero-order ultrasensitivity, when the kinase and phosphatase are saturated by their protein substrate. The kinetic analysis uncovers a wide variety of temporal patterns of phosphorylation that could be driven by signal-induced Ca2+ oscillations.

Thrombin-induced calcium oscillation in human platelets and MEG-01, a megakaryoblastic leukemia cell line

Digital imaging microscopy revealed that human platelets show periodic intracellular Ca++ elevation in response to 0.01 U/ml thrombin. MEG-01, a megakaryoblastic leukemia cell line, also responded with oscillatory intracellular Ca++ elevation (0.7-1 times/min) to thrombin (0.001-0.003U/ml). Ca++ transients appears to be fused with higher thrombin doses. With extracellular Ca++ concentrations of 0.1 mM or less, Ca++ oscillation could not be elicited, or even when present, it disappeared after a few spikings of [Ca++]i. Extracellular Ca++ concentrations of 0.3 mM or more were required to facilitate ongoing Ca++ oscillation, suggesting an important role of Ca++ influx for Ca++ oscillation.

Structured Biological Modelling: a method for the analysis and simulation of biological systems applied to oscillatory intracellular calcium waves

In biology signal and information processing networks are widely known. Due to their inherent complexity and non-linear dynamics the time evolution of these systems can not be predicted by simple plausibility arguments. Fortunately, the power of modern computers allows the simulation of complex biological models. Therefore the problem becomes reduced to the question of how to develop a consistent mathematical model which comprises the essentials of the real biological system. As an interface between the phenomenological description and a computer simulation of the system the proposed method of Structured Biological Modelling (SBM) uses top-down levelled dataflow diagrams. They serve as a powerful tool for the analysis and the mathematical description of the system in terms of a stochastic formulation. The stochastic treatment, regarding the time evolution of the system as a stochastic process governed by a master equation, circumvents most difficulties arising from high dimensional and non-linear systems. As an application of SBM we develop a stochastic computer model of intracellular oscillatory Ca2+ waves in non-excitable cells. As demonstrated on this example, SBM can be used for the design of computer experiments which under certain conditions can be used as cheap and harmless counterparts to the usual time-consuming biological experiments.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

Cytoplasmic Ca2+ signals evoked by activation of cholecystokinin receptors: Ca(2+)-dependent current recording in internally perfused pancreatic acinar cells

The effects on the cytosolic Ca2+ concentration of activating cholecystokinin receptors on single mouse pancreatic acinar cells have been investigated using patch-clamp whole-cell recording of Ca(2+)-dependent Cl- current. We used the nonsulphated octapeptide of cholecystokinin (CCK8-NS) since the effects of even high concentrations were rapidly reversible which was not the case for the sulphated octapeptide. A submaximal concentration of CCK8-NS (10 nM) evoked a current response consisting of short-lasting (a few seconds) spikes, and some of these spikes were seen to trigger larger and longer (about half a minute) current pulses. At a higher concentration (100 nM) CCK8-NS evoked smooth and sustained responses. The effect of CCK8-NS was almost abolished when the internal perfusion solution contained a high concentration of the Ca2+ chelator EGTA (5 mM). The responses evoked by CCK8-NS were independent of the presence of Ca2+ in the external solution at least for the first 5 min of stimulation. Internal perfusion with GTP-gamma-S markedly potentiated the effect of CCK8-NS or at a higher concentration itself induced responses very similar to those normally evoked by CCK8-NS. Caffeine added to the external solution at a low concentration (0.2-1 mM) enhanced weak CCK8-NS responses, whereas high caffeine concentrations always inhibited the CCK8-NS-evoked responses. These inhibitory caffeine effects were quickly reversible. Forskolin evoked a similar inhibitory effect. Intracellular heparin (200 micrograms/ml) infusion markedly inhibited the response to CCK8-NS stimulation. We conclude that the primary effect of activating CCK receptors is to induce inositoltrisphosphate (IP3) production.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ACTH and angiotensin II on cytosolic calcium in cultured adrenal glomerulosa cells. Role of cAMP production in the ACTH effect

We have used microspectrofluorometry and video imaging techniques in order to study and compare the changes in intracellular calcium concentrations [( Ca2+]i) of individual Fura-2 loaded glomerulosa cells cultured for three days and stimulated either with angiotensin II (AT), K+, or adrenocorticotropin (ACTH). As previously demonstrated for freshly isolated cells, K+ ion induces an immediate increase in [Ca2+]i, although AT induces a biphasic response, characterized by an initial transient spike, followed by a sustained plateau. In this study, we demonstrate, for the first time, that ACTH is able to induce a [Ca2+]i increase in cultured glomerulosa cells from rat and bovine sources. Moreover, it is clear that the pattern of [Ca2+]i increase elicited by ACTH is different from that observed with AT. In most cases, addition of ACTH leads to a slow increase in [Ca2+]i after a long latency period ranging from 10-15 min, which could be correlated to cAMP time-production. The present results show that: (a) in the absence of extracellular Ca2+, ACTH does not increase [Ca2+]i; (b) the response develops slowly and cases immediately after [Ca2+]e depletion or addition of calcium channel blockers, such as nifedipine or omega-conotoxin; (c) the addition of the calcium channel agonist Bay K 8644 enhances the ACTH response; (d) the cAMP analog, 8-Br-cAMP, induces an increase in [Ca2+]i similar to that observed with ACTH, which is also dependent of the presence of calcium in the extracellular medium; (e) time-production of ACTH-induced cAMP follows quite well the increase in [Ca2+]i; (f) Bay K 8644 also enhances the 8-Br-cAMP induced increase in [Ca2+]i; and (g) ACTH-induced Cai response is inhibited by the specific protein kinase A blocker, HA1004. These observations, combined with previous results obtained on the effects of ACTH on calcium currents and action potentials, suggest that the [Ca2+]i increase induced by ACTH results from a calcium influx through dihydropyridine and omega-conotoxin sensitive calcium channels, which need to be phosphorylated by cAMP for full activation. The use of video-imaging techniques has allowed us to examine the spatial distribution of changes in [Ca2+]i in single cells. The ability to simultaneously record images of a number of cells confirm the heterogeneity of cellular responses, and corroborate results obtained through photocounting only. Our results indicate that ACTH initially increases [Ca2+]i locally beneath the cell membrane and throughout the cell thereafter, whereas angiotensin II elicits a more prominent effect in certain regions of the cell and eventually extends to the entire cell surface.

A distinct 2,5-di-(tert-butyl)-1,4-benzohydroquinone-sensitive calcium store in bovine adrenal chromaffin cells

The fluorescent Ca2+ indicator Fura 2 was used to monitor Ca2+ release induced by the Ins(1,4,5)P3-mobilizing agonist angiotensin II (Ag II), caffeine and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tuBHQ), in intact bovine adrenal chromaffin cells. Under low external Ca2+ conditions, tuBHQ, Ag II and caffeine elicited Ca2+ rises, indicating Ca2+ release from internal stores. Prior addition of Ag II had no noticeable effect on the extent of release of Ca2+ induced by tuBHQ. Stimulation of the cells with tuBHQ before either Ag II or caffeine, similarly had no effect on Ca2+ released by these two agonists. It was concluded, therefore, that there is a third intracellular Ca2+ store in bovine adrenal chromaffin cells, distinct and non-overlapping, from those sensitive to caffeine or Ins(1,4,5)P3-mobilizing agonists.

Spontaneous calcium release from inositol trisphosphate-sensitive calcium stores

Inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger to mobilize Ca2+ from intracellular reservoirs. The release mechanism displays all-or-none characteristics, that may account for other observations that the InsP3-induced mobilization of Ca2+ is quantal. Quantal release may depend on the sensitivity of the InsP3 receptor being regulated by the Ca2+ concentration in the lumen of the endoplasmic reticulum. We report here that the InsP3-sensitive store in hepatocytes discharges spontaneously when overloaded with Ca2+. The release, which is blocked by heparin, is preceded by an increasing sensitivity of the InsP3 receptor to endogenous InsP3, and is promoted by those sulphydryl reagents (oxidized glutathione and thimerosal) that induce Ca2+ oscillations in intact cells (ref. 8, and T. A. Rooney, D. C. Renard, E. J. Sass and A. P. Thomas, manuscript in preparation). This novel process could have a role in generating both Ca2+ oscillations and Ca2+ waves.

Agonist-induced frequency modulation of Ca2+ oscillations in salt gland secretory cells

Oscillations in intracellular calcium concentration ([Ca2+]i) induced by the acetylcholine analogue carbachol (CCh) were characterized by microspectrofluorimetry of fura-2 in single secretory cells from the avian salt gland. The frequency of oscillations increased in graded fashion with [CCh] between 25 nM (2.7 +/- 0.6 min-1) and 250 nM (11.8 +/- 1.4 min-1), whereas the amplitude of the spikes was independent of [CCh]. An interperiod return to prestimulatory [Ca2+]i was generally seen only at very low (25 nM) CCh. Between 50 and 250 nM CCh, oscillations were associated with sustained elevated [Ca2+]i levels. The amplitude of the oscillatory spikes was found not to exceed that of initial spikes arising from prestimulatory [Ca2+]i, despite the dose-dependent [effective concentration at 50% (EC50) = 200 nM CCh] sustained rise in [Ca2+]i. At 1 microM CCh, oscillations gave way to a maximal sustained increase in [Ca2+]i. Reduction of [Ca2+]o to 1.5 microM during an oscillatory train or blockage of Ca2+ influx with Ni+ resulted in a reduction in sustained Ca2+i levels and in frequency, but not amplitude, of oscillations. A relationship between the sustained partial rise in [Ca2+]i derived from Ca2+ influx and the oscillatory frequency at a given [CCh] was further indicated by the lower frequency (P less than 0.01) of the early spikes in a train when interspike [Ca2+]i initially returned to near-basal levels. In some cells, oscillations were slow enough (less than 2 min-1) to resolve an interperiod of elevated baseline [Ca2+]i, showing that the latter can occur independent of the repetitive Ca2+ spikes. (ABSTRACT TRUNCATED AT 250 WORDS)

Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate

Intercellular Ca2+ signaling in primary cultures of glial cells was investigated with digital fluorescence video imaging. Mechanical stimulation of a single cell induced a wave of increased [Ca2+]i that was communicated to surrounding cells. This was followed by asynchronous Ca2+ oscillations in some cells. Similar communicated Ca2+ responses occurred in the absence of extracellular Ca2+, despite an initial decrease in [Ca2+]i in the stimulated cell. Mechanical stimulation in the presence of glutamate induced a typical communicated Ca2+ wave through cells undergoing asynchronous Ca2+ oscillations in response to glutamate. The coexistence of communicated Ca2+ waves and asynchronous Ca2+ oscillations suggests distinct mechanisms for intra- and intercellular Ca2+ signaling. This intercellular signaling may coordinate cooperative glial function.

Characterisation of distinct inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive calcium stores in digitonin-permeabilised adrenal chromaffin cells

The effect of inositol 1,4,5-trisphosphate [Ins-(1,4,5)P3] and caffeine on Ca2+ release from digitonin-permeabilised bovine adrenal chromaffin cells was examined by using the Ca2+ indicator fura-2 to monitor [Ca2+]. Permeabilised cells accumulated Ca2+ in the presence of ATP and addition of either Ins(1,4,5)P3 or caffeine released 17% or 40-50%, respectively, of the accumulated Ca2+, indicated by sustained rises in [Ca2+] in the cell suspension. Prior addition of Ins(1,4,5)P3 had no effect on the magnitude of the response to a subsequent addition of caffeine. The response to Ins(1,4,5)P3 was prevented by prior addition of caffeine or CaCl2, indicating that the Ins(1,4,5)P3 response was blocked by elevated [Ca2+]. The responses were essentially identical in the presence of the proton ionophore carbonyl cyanide m-chlorophenylhydrazone, indicating that the Ca2+ release was not from mitochondria or secretory granules and that a proton gradient was not required for Ca2+ accumulation into the Ins(1,4,5)P3- or caffeine-sensitive stores. Ca2+ release from the caffeine-sensitive store was selectively blocked by ryanodine. The Ins(1,4,5)P3-sensitive store was emptied by thapsigargin, which had no effect on caffeine responses. These data suggest that permeabilised chromaffin cells possess two distinct nonoverlapping Ca2+ stores sensitive to either Ins(1,4,5)P3 or caffeine and support previous conclusions that these stores possess different Ca2(+)-ATPases.

Cholinergic stimulation produces oscillations of cytosolic Ca2+ in a secretory epithelial cell line, T84

The effects of carbamylcholine (carbachol) on intracellular Ca2+ concentration ([Ca2+]c) of T84 cells were examined using the fluorescent Ca2+ indicator fura-2 and microfluorometric techniques. In single isolated cells, carbachol (100 microM) caused a rapid increase in [Ca2+]c of 184 +/- 15 nM (SE, n = 44) from a resting value of 56 +/- 7 nM. This initial transient was followed by a series of oscillations in 68% of the cells. Atropine (10 microM) blocked this response. Removal of bath Ca2+ did not inhibit the rise in [Ca2+]c or oscillations, but the response duration was shortened in 47% of the cells. The amplitude and latency of the initial Ca2+ rise, frequency of oscillations, and number of responding cells varied with the agonist concentration. We have previously shown that carbachol induces an oscillating K+ conductance in T84 cells [D. Devor, S. Simasko, and M. Duffey. Am. J. Physiol. 258 (Cell Physiol. 27): C318-C326, 1990]. Simultaneous measurement of membrane K+ current and fura-2 fluorescence in the same cell demonstrated a correlation between the rise in [Ca2+]c and increase in K+ current. These results show that a rise in [Ca2+]c and oscillations is likely to underlie the membrane K+ current responses to carbachol in T84 cells. Responses from a single cell within a subconfluent monolayer were different from those of isolated cells. In cells of a monolayer the initial [Ca2+]c rise (111 +/- 8 nM; n = 41) was followed by a decline to a stable plateau, and oscillations were not seen. Removal of bath Ca2+ both reduced the initial transient and eliminated the plateau phase of the response. These results suggest that cell-to-cell contact or differentiation during monolayer formation influences the Ca2+ handling mechanisms of T84 cells.

Cytosolic Ca2+ oscillations in REF52 fibroblasts: Ca(2+)-stimulated IP3 production or voltage-dependent Ca2+ channels as key positive feedback elements

Oscillations in cytosolic free calcium concentrations ([Ca2+]i) can be elicited in REF52 fibroblasts by three different modes of stimulation. We have previously demonstrated that [Ca2+]i oscillations result when these cells are simultaneously depolarized and stimulated with a hormone linked to phosphoinositide breakdown. Further evidence is now presented that such oscillations are linked to fluctuations in the concentration of IP3 and the Ca2+ content of an IP3-sensitive Ca2+ store. [Ca2+]i oscillations can also be generated in REF52 cells either by direct stimulation of G-proteins with GTP gamma S or AlF4- or by destabilizing the membrane potential and opening voltage-dependent calcium channels. This report compares the different types of oscillations and their mechanisms.

Spatial and temporal organization of calcium signalling in hepatocytes

Treatment of hepatocytes with agonists which act via the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), results in increases of cytosolic free Ca2+ [( Ca2+]i) which are manifest as a series of discrete [Ca2+]i transients or oscillations. With increasing agonist dose [Ca2+]i oscillation frequency increases and the initial latent period decreases, but the amplitude of the [Ca2+]i oscillations remains constant. Studies of these [Ca2+]i oscillations at the subcellular level have indicated that the [Ca2+]i changes do not occur synchronously throughout the cell, but initiate at a specific subcellular domain, adjacent to a region of the plasma membrane, and then propagate through the cell as a [Ca2+]i wave. For a given ceil, the locus of [Ca2+]i wave initiation is constant for every oscillation in a series and is also identical when the cell is sequentially stimulated with different agonists or when the phospholipase C-linked G protein is activated directly using AIF4-. The kinetics of the [Ca2+]i waves indicate that a Ca(2+)-activated mechanism is involved in propagating the oscillatory [Ca2+]i increases throughout the cell, and the data appear to be most consistent with a process of Ca(2+)-induced Ca2+ release. It is proposed that the ability to propagate [Ca2+]i oscillations into regions of the cell distal to the region in which the signal transduction apparatus is localized could serve an important function in allowing all parts of the cell to respond to the stimulus.

Signal-induced Ca2+ oscillations: properties of a model based on Ca(2+)-induced Ca2+ release

We consider a simple, minimal model for signal-induced Ca2+ oscillations based on Ca(2+)-induced Ca2+ release. The model takes into account the existence of two pools of intracellular Ca2+, namely, one sensitive to inositol 1,4,5 trisphosphate (InsP3) whose synthesis is elicited by the stimulus, and one insensitive to InsP3. The discharge of the latter pool into the cytosol is activated by cytosolic Ca2+. Oscillations in cytosolic Ca2+ arise in this model either spontaneously or in an appropriate range of external stimulation; these oscillations do not require the concomitant, periodic variation of InsP3. The following properties of the model are reviewed and compared with experimental observations: (a) Control of the frequency of Ca2+ oscillations by the external stimulus or extracellular Ca2+; (b) correlation of latency with period of Ca2+ oscillations obtained at different levels of stimulation; (c) effect of a transient increase in InsP3; (d) phase shift and transient suppression of Ca2+ oscillations by Ca2+ pulses, and (e) propagation of Ca2+ waves. It is shown that on all these counts the model provides a simple, unified explanation for a number of experimental observations in a variety of cell types. The model based on Ca(2+)-induced Ca2+ release can be extended to incorporate variations in the level of InsP3 as well as desensitization of the InsP3 receptor; besides accounting for the phenomena described by the minimal model, the extended model might also account for the occurrence of complex Ca2+ oscillations.

Ca2+ oscillations induced by histamine H1 receptor stimulation in HeLa cells: Fura-2 and patch clamp analysis

The response of HeLa cells to histamine H1 receptor stimulation is characterized by periodic increases in cytosolic free Ca2+ concentration. The mechanisms underlying this oscillatory behaviour are not well understood. Fura-2 and patch clamp experiments carried out on HeLa cells have previously shown: (a) that Ca2+ oscillations are not initially dependent on the presence of external Ca2+, that external Ca2+ is required to maintain the oscillatory activity; (b) that a depolarization of the cell membrane leads to an inhibition of Ca2+ oscillations during the external Ca2+ dependent phase of the process; and (c) that Ca2+ oscillations can be abolished during this latter phase by the exogenous addition of Ca2+ channel blocking agents, such as Co2+ or La3+. The contribution of the inositol phosphate pathway to Ca2+ oscillations was more recently investigated in whole cell experiments performed with patch pipettes containing IP3 or the non-hydrolysable GTP analogue GTP-gamma S. Clear periodic current fluctuations were recorded using both patch pipette solutions. Assuming that the intracellular IP3 level remained constant under these conditions, these findings provide direct evidence that the Ca2+ oscillations in HeLa cells do not arise from a periodic production of IP3. The effect of the internal and external cell pH on the oscillatory process was also investigated in Fura-2 and patch clamp experiments. It was found that an increase in intracellular pH from 7.4 to 7.7 during the external Ca2+ dependent phase of the histamine stimulation abolishes the appearance of Ca2+ spikes whereas, a cellular acidification to pH 7.2 maintains or stimulates the Ca2+ oscillatory activity. The former effect was observed in the absence of Ca2+ in the bathing medium, indicating that the inhibitory action of alkaline pH was not related to a reduced Ca2+ entry. An increase in extracellular pH from 7.3 to 9.0 in contrast elicited an intracellular Ca2+ accumulation which resulted in most cases in an inhibition of the oscillatory process. This effect was dependent on external Ca2+ and was observed in alkaline internal pH conditions (pH 7.7). These observations suggest: (a) that the net Ca2+ influx in HeLa cells is strongly dependent on the cell internal and external pH; and (b) that the magnitude of this Ca2+ influx controls to a large extent the oscillation frequency. Finally, an inhibition of the histamine induced Ca2+ oscillatory activity was observed following the addition of the Ca(2+)-induced Ca(2+)-release (CICR) inhibitor adenine to the external medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoplasmic calcium oscillations: a two pool model

Cytosolic calcium oscillations induced by a wide range of agonists, particularly those which stimulate phosphoinositide metabolism, are the result of a periodic release of stored calcium. The formation of inositol 1,4,5 trisphosphate (Ins(1,4,5)P3) seems to play an important role because it can initiate this periodic behaviour when injected or perfused into a variety of cells. A two pool model has been developed to explain how Ins(1,4, 5)P3 sets up these calcium oscillations. It is proposed that Ins(1,4,5)P3 acts through its specific receptor to create a constant influx of primer calcium (Ca2+p) made up of calcium released from the Ins(1,4,5)P3-sensitive pool (ISCS) together with an influx of external calcium. This Ca2+p fails to significantly elevate cytosolic calcium because it is rapidly sequestered by the Ins(1,4,5)P3-insensitive (IICS) stores of calcium distributed throughout the cytosol. Once the latter have filled, they are triggered to release their stored calcium through a process of calcium-induced calcium release to give a typical calcium spike (Ca2+s). In many cells, each Ca2+s begins at a discrete initiation site from which it then spreads through the cell as a wave. The two pool model can account for such waves if it is assumed that calcium released from one IICS diffused across to excite its neighbours thereby setting up a self-propagating wave based on calcium-induced calcium release.

A model for intracellular calcium signaling and the coordinate regulation of hormone biosynthesis, receptors and secretion

A two-state model for the stimulus-induced nongraded response of a single cell is formulated. Individual metestrus gonadotropes stimulated with LHRH operate as a simple switch: either on or off. At a given concentration of stimulus some gonadotropes switch on, while others do not switch on, secretion. The probability of a gonadotrope being in the secretory state is enhanced with each increment of LHRH concentration. Individual gonadotropes in a secretory state are envisioned to decrease their number of LHRH receptors and to switch off LH biosynthesis. On the other hand, individual gonadotropes that are not in a secretory state are thought to increase their number of LHRH receptors and to switch on LH biosynthesis. The group of individuals in the population that have thresholds falling in the range of a given stimulus initiate secretion. And, the group of individuals in the population that have thresholds that fall above the range of a given stimulus do not initiate secretion. More remarkable is evidence that the cells that are protected from hormone secretion nevertheless respond with a set of intracellular signals and this provides a new perspective of how they switch on hormone biosynthesis and up-regulate the LHRH receptors. These changes are envisioned to reduce the threshold of an individual cell and accordingly to enhance the probability that the cell responds in the secretory state with the next stimulus. This scheme would appear to lead to automatic cycles of secretion and biosynthesis since an individual cell can occupy only one of two states at any time and occupancy of either state promotes change to the other. This may provide a solution to the problem of how an endocrine gland might reconcile differences in the time-course of hormone secretion which occurs rapidly and hormone biosynthesis that requires a longer period of time. Parenthetically, the model may also be adapted to the case where the vast majority of individuals in the population are generally subthreshold in relation to the physiological stimulus: such an adaption leads to interesting ways of viewing the mammalian reproductive cycle and the regulation of the preovulatory LH surge. A two-state model of the internal Ca2+ store is outlined here to stimulate thought on how the intracellular signals of each binary state may switch a variety of cellular responses either on or off. The model provides a new perspective on the coordinate regulation of hormone biosynthesis, receptors, and secretion that may be useful in the final reconciliation of population studies with insights about individual cells.

Modelling receptor-controlled intracellular calcium oscillators

This paper presents mathematical models for the hepatocyte calcium oscillator which follow the concepts in a class of informal models developed to account for the striking dependence on the receptor type of several features of the calcium oscillations, in particular the shape and duration of the free calcium transients. The essence of these models is that the transients should be timed by a build-up of activated GTP-binding proteins, which, combined with positive feedback processes and perhaps with cooperative effects, leads to a sudden activation of phospholipase C (PLC), followed by negative feedback processes which switch off the calcium rise and lead to a fall in free calcium back to resting levels. These models predict pulsatile oscillations in inositol (1,4,5)P3 as well as in free calcium. We show that receptor-controlled intracellular calcium oscillators involving an unknown positive feedback pathway onto PLC and negative feedback from protein kinase C (PKC) onto G-proteins and receptors, or negative feedback by stimulation of GTPase activity can simulate many of the features of observed intracellular calcium oscillations. These oscillators exhibit a dependence of frequency on agonist concentration and a dependence of transient duration on receptor and G-protein type. We also show that a PLC-dependent GTPase activating factor (GAF) could provide explanations for some otherwise puzzling features of intracellular calcium oscillations.

The hepatocyte calcium oscillator

Hepatocytes stimulated with calcium-mobilising agonists generate free Ca transients whose frequency is modulated by hormone concentration. Importantly, the time-course of individual free Ca transients is independent of agonist dose but does change with agonist species. A receptor-controlled model in which protein kinase C provides negative feedback directed against the different receptors, or receptor-specific G proteins, has been proposed in order to explain the agonist-specificity of the falling phase of the free Ca spikes. Here we show further evidence, from mixing of hormones and from the effects of elevated cAMP, of receptor-specific information within the spikes.

Antidiuretic hormone acts via V1 receptors on intracellular calcium in the isolated perfused rabbit cortical thick ascending limb

The effect of antidiuretic hormone [( Arg]vasopressin, ADH) on intracellular calcium activity [Ca2+]i of isolated perfused rabbit cortical thick ascending limb (cTAL) segments was investigated with the calcium fluorescent dye fura-2. The fluorescence emission ratio at 500-530 nm (R) was monitored as a measure of [Ca2+]i after excitation at 335 nm and 380 nm. In addition the transepithelial potential difference (PDte) and transepithelial resistance (Rte) of the tubule were measured simultaneously. After addition of ADH (1-4 nmol/l) to the basolateral side of the cTAL R increased rapidly, but transiently, from 0.84 +/- 0.05 to 1.36 +/- 0.08 (n = 46). Subsequently, within 7-12 min R fell to control values even in the continued presence of ADH. The increase in R evoked by the ADH application corresponded to a rise of [Ca2+]i from a basal level of 155 +/- 23 nmol/l [Ca2+]i up to 429 +/- 53 nmol/l [Ca2+]i at the peak of the transient, as estimated by intra- or extracellular calibration procedures. The electrical parameters (PDte and Rte) of the tubules were not changed by ADH. The ADH-induced Ca2+ transient was dependent on the presence of Ca2+ on the basolateral side, whereas luminal Ca2+ had no effect. d(CH2)5[Tyr(Me)2]2,Arg8vasopressin, a V1 antagonist (Manning compound, 10 nmol/l), blocked the ADH effect on [Ca2+]i completely (n = 5). The V2 agonist 1-desamino-[D-Arg8]vasopressin (10 nmol/l, n = 4), and the cAMP analogues, dibutyryl-cAMP (400 mumol/l, n = 4), 8-(4-chlorophenylthio)-cAMP (100 mumol/l, n = 1) or 8-bromo-cAMP (200 mumol/l, n = 4) had no influence on [Ca2+]i. The ADH-induced [Ca2+]i increase was not sensitive to the calcium-channel blockers nifedipine and verapamil (100 mumol/l, n = 4). We conclude that ADH acts via V1 receptors to increase cytosolic calcium activity transiently in rabbit cortical thick ascending limb segments, possibly by an initial Ca2+ release from intracellular stores and by further Ca2+ influx through Ca2+ channels in the basolateral membrane. These channels are insensitive to L-type Ca2+ channel blockers, e.g. nifedipine and verapamil.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Generation of calcium oscillations in fibroblasts by positive feedback between calcium and IP3

A wide variety of nonexcitable cells generate repetitive transient increases in cytosolic calcium ion concentration ([Ca2+]i) when stimulated with agonists that engage the phosphoinositide signalling pathway. Current theories regarding the mechanisms of oscillation disagree on whether Ca2+ inhibits or stimulates its own release from internal stores and whether inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DG) also undergo oscillations linked to the Ca2+ spikes. In this study, Ca2+ was found to stimulate its own release in REF52 fibroblasts primed by mitogens plus depolarization. However, unlike Ca2+ release in muscle and nerve cells, this amplification was insensitive to caffeine or ryanodine and required hormone receptor occupancy and functional IP3 receptors. Oscillations in [Ca2+]i were accompanied by oscillations in IP3 concentration but did not require functional protein kinase C. Therefore, the dominant feedback mechanism in this cell type appears to be Ca2+ stimulation of phospholipase C once this enzyme has been activated by hormone receptors.

Organization of intracellular calcium signals generated by inositol lipid-dependent hormones

Recent studies at the single cell level have demonstrated hitherto unsuspected complexities in the organization of intracellular Ca2+ homeostasis in both the temporal and spatial domains. Activation of receptors coupled to the phosphoinositide signalling system has been shown to generate [Ca2+]i oscillations in many cell types. These oscillations display diverse patterns, with variations in oscillation amplitude, latency and frequency which are often tissue and/or agonist dose specific. Furthermore, increases in [Ca2+]i can either occur uniformly or originate from a specific region and propagate throughout the cell in the form of a Ca2+ wave. The significance and underlying mechanisms responsible for these phenomena are discussed.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Calcium sequestration in the liver

Hepatic parenchymal cells maintain intracellular total and cytosolic free Ca2+ levels by: entry of Ca2+ through channels, extrusion of Ca2+ by an outwardly directed Ca2+ pump, and controlled sequestration into intracellular pools. The mechanism of Ca2+ inflow is poorly characterized. The plasma membrane Ca2+ channels seem to share some of the characteristics of Ca2+ channels in excitable cells, but also differ from them. The outwardly directed plasma membrane Ca2(+)-ATPase is a calmodulin independent, P-type enzyme. Ca2+ uptake into the endoplasmic reticulum is due to the activity of a different Ca2(+)-ATPase, which is similar in molecular weight and shares antigenic determinants with the sarcoplasmic reticulum enzyme. In addition, mitochondria and nuclei also take up calcium. The exact mechanism by which Ca2+ is released from intracellular organelles is not well known. Several mechanisms for Ca2+ release from the endoplasmic reticulum were reported, including IP3 and GTP-induced. The most effective identified way of eliciting Ca2+ release from microsomal fraction is by the oxidation of critical -SH groups. This mechanism is likely to be involved in the rise of cytosolic Ca2+ observed in many situations of hepatocellular injury. In addition to being sequestered into subcellular organelles, some of the intracellular Ca2+ is bound to specific Ca2+ binding proteins. Both calmodulin and members of the annexin family were identified in the liver. Stimulation of the liver with gluconeogenic hormones results in increased Ca2+ entry into the cell, the release of Ca2+ from intracellular pools, and an oscillatory increase in free cytosolic Ca2+ levels. Extensive research is still needed for the elucidation of the exact mechanisms by which these events occur.

The caffeine-sensitive Ca2+ store in bovine adrenal chromaffin cells; an examination of its role in triggering secretion and Ca2+ homeostasis

The effect of caffeine on catecholamine secretion and intracellular free Ca2+ concentration [( Ca2+]i) in bovine adrenal chromaffin cells was examined using single fura-2-loaded cells and cell populations. In cell populations caffeine elicited a large (approximately 200 nM) transient rise in [Ca2+]i that was independent of external Ca2+. This rise in [Ca2+]i triggered little secretion. Single cell measurements of [Ca2+]i showed that most cells responded with a large (greater than 200 nM) rise in [Ca2+]i, whereas a minority failed to respond. The latter, whose caffeine-sensitive store was empty, buffered a Ca2+ load induced by a depolarizing stimulus more effectively than those whose store was full. The caffeine-sensitive store in bovine chromaffin cells may be involved in Ca2+ homeostasis rather than in triggering exocytosis.