Pulsatile intracellular calcium release does not depend on fluctuations in inositol trisphosphate concentration

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)

Receptor-activated cytoplasmic Ca2+ oscillations in pancreatic acinar cells: generation and spreading of Ca2+ signals

Receptor-activated cytoplasmic Ca2+ oscillations have been investigated using both single cell microfluorometry and voltage-clamp recording of Ca(2+)-dependent Cl- current in single internally perfused acinar cells. In these cells there is direct experimental evidence showing that the ACh-evoked [Ca2+]i fluctuations are due to an inositol trisphosphate-induced small steady Ca2+ release which in turn evokes repetitive Ca2+ spikes via a caffeine-sensitive Ca(2+)-induced Ca2+ release process. There is indirect evidence suggesting that receptor-activation in addition to generating the Ca2+ releasing messenger, inositol trisphosphate, also produces another regulator involved in the control of Ca2+ signal spreading. Intracellular inositol trisphosphate or Ca2+ infusion produce short duration repetitive spikes confined to the cytoplasmic area close to the plasma membrane, but these signals can be made to progress throughout the cell by addition of caffeine or by receptor activation.

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.

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.

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.

Interaction of caffeine-, IP3- and vanadate-sensitive Ca2+ pools in acinar cells of the exocrine pancreas

Previous studies have shown the existence of functionally distinguishable inositol 1,4,5-trisphosphate- (IP3) sensitive and IP3-insensitive nonmitochondrial intracellular Ca2+ pools in acinar cells of the exocrine pancreas. For further characterization of Ca2+ pools, endoplasmic reticulum (ER) membrane vesicles were separated by Percoll gradient centrifugation which allowed us to distinguish five discrete fractions designated P1 to P5 from the top to the bottom of the gradient. Measuring Ca2+ uptake and Ca2+ release with a Ca2+ electrode, we could differentiate three nonmitochondrial intracellular Ca2+ pools: (i) an IP3-sensitive Ca2+ pool (IsCaP), vanadate- and caffeine-insensitive, (ii) a caffeine-sensitive Ca2+ pool (CasCaP), vanadate- and IP3-insensitive, and (iii) a vanadate-sensitive Ca2+ pool (VasCaP), neither IP3- nor caffeine-sensitive, into which Ca2+ uptake is mediated via a Ca2+ ATPase sensitive to vanadate at 10(-4) mol/liter. A fourth Ca2+ pool is neither IP3- nor caffeine- or vanadate-sensitive. Percoll fraction P1 contained essentially the IsCaP, CasCaP and VasCaP and was mainly used for studies on Ca2+ uptake and Ca2+ release. When membrane vesicles were incubated in the presence of caffeine (2 x 10(-2) mol/liter), Ca2+ uptake up to the steady state [Ca2+] did not appear to be altered as compared to the control Ca2+ uptake. However, in control vesicles spontaneous Ca2+ release occurred after the steady state had been reached, whereas caffeine-pretreated vesicles did not spontaneously release Ca2+. Addition of IP3 at steady state [Ca2+] induced similar Ca2+ release followed by Ca2+ reuptake in both caffeine-pretreated and control vesicles. However, when caffeine was acutely added at steady state, Ca2+ was released from all Ca2+ pools including the IsCaP. Following Ca2+ reuptake after IP3 had been added, a second addition of IP3 to control vesicles induced further but smaller Ca2+ release, and a third addition resulted in a steady Ca2+ efflux by which all Ca2+ that had been taken up was released. This steady Ca2+ release started at a Ca2+ concentration between 5.5-8 x 10(-7) mol/liter and could also be induced by the IP3 analogue inositol 1,4,5-trisphosphorothioate (IPS3) or by addition of Ca2+ itself. Ruthenium red (10(-5) mol/liter) inhibited both caffeine-induced as well as Ca2(+)-induced but not IP3-induced Ca2+ release. Heparin (100 micrograms/ml) inhibited IP3- but not caffeine-induced Ca2+ release. The data indicate the presence of at least three separate Ca2+ pools in pancreatic acinar cells: the IsCaP, CasCaP and VasCaP. During Ca2+ uptake these Ca2+ pools appear to be separate.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Cellular harmonic information transfer through a tissue tensegrity-matrix system

Cells and intracellular elements are capable of vibrating in a dynamic manner with complex harmonics, the frequency of which can now be measured and analyzed in a quantitative manner by Fourier analysis. Cellular events such as changes in shape, membrane ruffling, motility, and signal transduction occur within spatial and temporal harmonics that have potential regulatory importance. These vibrations can be altered by growth factors and the process of carcinogenesis. It is important to understand the mechanism by which this vibrational information is transferred directly throughout the cell. From these observations we propose that vibrational information is transferred through a tissue tensegrity-matrix which acts as a coupled harmonic oscillator operating as a signal transucing system from the cell periphery to the nucleus and ultimately to the DNA. The vibrational interactions occur through a tissue matrix system consisting of the nuclear matrix, the cytoskeleton, and the extracellular matrix that is poised to couple the biologic oscillations of the cell from the peripheral membrane to the DNA through a tensegrity-matrix structure. Tensegrity has been defined as a structural system composed of discontinuous compression elements connected by continuous tension cables, which interact in a dynamic fashion. A tensegrity tissue matrix system allows for specific transfer of information through the cell by direct transmission of vibrational chemomechanical energy through harmonic wave motion.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.

Calcium oscillations and morphological transformations in single cultured gastric parietal cells

Calcium is an important regulator of cellular activities including HCl secretion by parietal cells. With cholinergic agonists, a role for calcium is established; however, with histamine, at least two signaling pathways may be involved including calcium and adenosine 3',5'-cyclic monophosphate (cAMP). Because chelation of medium and/or cellular calcium has pronounced inhibitory effects on cholinergic but lesser effects on histamine-stimulated acid secretory responses in cell populations, the calcium pathway may not be of central importance for HCl secretion regulated by histamine. We have used digitized video imaging of fura-2 fluorescence ratios and cellular morphology to determine more precisely the relationship between cellular calcium signaling mechanisms and acid secretion in single cultured rabbit parietal cells. Calcium signaling patterns were found to exhibit striking differences with histamine as compared with the cholinergic agonist carbachol. Maximal doses of histamine initiated repetitive oscillations in intracellular calcium ([Ca2+]i) in approximately 50% of cells, whereas the maximal carbachol response was characterized by a typical initial spike followed by a sustained elevation in [Ca2+]i. Oscillations in response to carbachol were detected only at doses below the half-maximal concentration for initiation of acid secretion. Correlation of gradual expansion of acidic vacuoles with increases in [Ca2+]i in the same cells indicated that approximately 20% of cells increased acid secretory-related activities in response to histamine with no detectable rise in [Ca2+]i. These data suggest two possibilities: 1) a rise in [Ca2+]i is not necessary for histamine-stimulated HCl secretion, or 2) heterogeneous receptor-coupling mechanisms exist in parietal cell populations with either calcium or cAMP mechanisms predominating in different subpopulations. The ability to assess simultaneously acid secretory-related responses and calcium signaling patterns allows, for the first time, correlation of "physiological" and biochemical responses in single parietal cells. This methodology is expected to provide new insight into second messenger control mechanisms that are not possible either in cell populations or acutely isolated parietal cells that do not exhibit morphological transformations detectable at the light microscope level.

Cytosolic Ca2+ gradients triggering unidirectional fluid secretion from exocrine pancreas

Exocrine gland cells secrete Cl(-)-rich fluid when stimulated by neurotransmitters or hormones. This is generally ascribed to a rise in cytosolic Ca2+ concentration ([Ca2+]i), which leads to activation of Ca2(+)-dependent ion channels. A precise understanding of Cl- secretion from these cells has been hampered by a lack of knowledge about the spatial distribution of the Ca2+ signal and of the Ca2(+)-dependent ion channels in the secreting epithelial cells. We have now used the whole-cell patch-clamp method and digital imaging of [Ca2+]i to examine the response of rat pancreatic acinar cells to acetylcholine. We found a polarization of [Ca2+]i elevation and ion channel activation, and suggest that this comprises a novel 'push-pull' mechanism for unidirectional Cl- secretion. This mechanism would represent a role for cytosolic Ca2+ gradients in cellular function. The cytosolic [Ca2+]i gradients and oscillations of many other cells could have similar roles.

Receptor-activated cytoplasmic Ca2+ spiking mediated by inositol trisphosphate is due to Ca2(+)-induced Ca2+ release

Receptor-mediated inositol 1,4,5-trisphosphate (Ins-(1,4,5)P3) generation evokes fluctuations in the cytoplasmic Ca2+ concentration ([Ca2+]i). Intracellular Ca2+ infusion into single mouse pancreatic acinar cells mimicks the effect of external acetylcholine (ACh) or internal Ins(1,4,5)P3 application by evoking repetitive Ca2+ release monitored by Ca2(+)-activated Cl- current. Intracellular infusion of the Ins(1,4,5)P3 receptor antagonist heparin fails to inhibit Ca2+ spiking caused by Ca2+ infusion, but blocks ACh- and Ins(1,4,5)P3-evoked Ca2+ oscillations. Caffeine (1 mM), a potentiator of Ca2(+)-induced Ca2+ release, evokes Ca2+ spiking during subthreshold intracellular Ca2+ infusion. These results indicate that ACh-evoked Ca2+ oscillations are due to pulses of Ca2+ release through a caffeine-sensitive channel triggered by a small steady Ins(1,4,5)P3-evoked Ca2+ flow.

Effects of alanine on insulin-secreting cells: patch-clamp and single cell intracellular Ca2+ measurements

The effects of alanine, glucose and tolbutamide on insulin-secreting cells (RINm5F) have been investigated using patch-clamp and single cell intracellular Ca2+ measurements. When directly challenged with the amino acid L-alanine (2-10 mM) the cells underwent a sharp depolarization, which led to the generation of Ca2+ spike potentials and an increase in [Ca2+]i. The L-alanine-induced depolarization was associated with a net inward membrane current but no measurable change in the resistance of the cell. The latter effect was found to be in contrast to the actions of glucose (5-10 mM) and tolbutamide (100 microM), both of which depolarized cells and raised [Ca2+]i by an increase in the input resistance of the cell membrane, due to the closure of ATP-sensitive potassium channels. In the complete absence of external Na+ (by replacement with 140 mM NMDG+), L-alanine had no effects on either the membrane potential or [Ca2+]i. Similarly, replacing Na+ with NMDG+ in the continued presence of the amino acid resulted in a repolarization of the membrane and an attenuation of the L-alanine-induced rise in [Ca2+]i. The Na+ channel blocker TTX (1-2 microM) had no effects on the alanine-evoked electrical activity. Exchange of the L-form of the amino acid with the D-stereoisomer had similar actions to those of removing external Na+, since D-alanine had no effects on the membrane potential or [Ca2+]i. The actions of L-alanine were also found to be mimicked by the N-methylated amino acid analogue methylamino isobutyric acid (MeAIB) (2-10 mM), suggesting that the A-type electrogenic amino acid cotransport system operates in the RINm5F insulin-secreting cell line.

Computer simulation of a cytosolic calcium oscillator

A new interpretation of existing data permits us to define a model capable of accounting for agonist-induced Ca2+ oscillations in the cytosol of electrically non-excitable cells. The model only requires one Ca2+ store, which contains Ca2+ channels controlled by inositol 1,4,5-trisphosphate and Ca2+. Computer simulations may generate different experimentally observed patterns of Ca2+ oscillations.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Latency correlates with period in a model for signal-induced Ca2+ oscillations based on Ca2(+)-induced Ca2+ release

Oscillations in cytosolic Ca2+ develop in a variety of cells after an induction phase, called latency, the duration of which depends on the magnitude of external stimulation. Experiments in hepatocytes indicate that the period and latency of Ca2+ oscillations both decrease as the level of the stimulus increases. We analyze the correlation between period and latency in a model recently proposed for signal-induced Ca2+ oscillations. We show that the linear relationship between period and latency observed in the experiments arises naturally in this model as a result of the mechanism of Ca2(+)-induced Ca2+ release on which it is based.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

The effects of ryanodine and caffeine on Ca-activated current in guinea-pig ventricular myocytes

1. Action potentials from guinea-pig single ventricular myocytes were interrupted by application of a 300 ms voltage clamp to -40 mV in order to evoke the Ca-activated tail current which is thought to be carried by Na:Ca exchange. Stimulation frequency was 1 Hz and temperature 36 degrees C. 2. The actions of ryanodine (1 microM and 10 microM) and caffeine (1 mM and 10 mM) on Ca-activated tail currents were investigated. 3. Exposure to 10 mM caffeine and ryanodine reduced tail currents associated with very abbreviated (12 ms duration) action potentials and greatly reduced the difference between first and steady-state tail currents at this action potential duration. These observations were interpreted in terms of suppression of Ca release from the sarcoplasmic reticulum (SR) stores. 4. Tail current decay during the voltage clamp is thought to reflect the fall in [Ca]i which accompanies muscle relaxation. Current decay is dependent on Ca extrusion via Na:Ca exchange and on Ca accumulation by the SR stores. Time constants of tail current decay were seen to decrease with increasing action potential duration. This relationship was not affected by 1 mM caffeine or 1 microM ryanodine. Ryanodine at 10 microM and 10 mM caffeine abolished this relationship and increased the time constants of current decay. An increase in the time constant of tail current decay was thought to reflect a reduction in the rate of Ca accumulation by the sarcoplasmic reticulum. 5. The actions of caffeine and ryanodine on the Ca-activated tail currents are consistent with a dose-dependent leakage of Ca from the SR Ca stores. The Ca-activated tail current appears to be a useful tool in the study of Ca homeostasis.

Spatial distribution of intracellular, free Ca2+ in isolated rat parotid acini

The spatial distribution of intracellular, free Ca2+ ([Ca2+]i) in rat parotid acini was measured by imaging fura-2 fluorescence from individual acinar cells by means of a digital imaging microscope. Upon cholinergic stimulation in a Krebs-Ringer bicarbonate buffer at (37 degrees C), [Ca2+]i increased synchronously at both the basolateral and luminal membranes as well as in all cells of the secretory endpiece, reaching peak [Ca2+]i levels 1 s after stimulation. Atropine addition caused a rapid down-regulation of [Ca2+]i, which, however, never reached prestimulatory levels. When acini were stimulated in a medium containing 5 nM Ca2+, the Ca2+ mobilization arising from internal pools caused an increase in [Ca2+]i predominantly near the basolateral area, where the endoplasmic reticulum is located, and standing Ca2+ gradients were observed for up to 10 s. A mathematical model is developed to simulate the time courses of the Ca2+ profiles through the cytoplasm using estimated values of the Ca2+ diffusion coefficients and the cytosolic Ca2+ buffering capacity. It is concluded that under physiological conditions, the Ca2+ release from the endoplasmic reticulum is responsible for the activation of the basolaterally located K+ channels. Furthermore, Ca2+ influx from the interstitium is responsible for much of the rise in [Ca2+]i near the luminal membranes, where the Cl- channels are supposed to be located.

Inositol trisphosphate analogues induce different oscillatory patterns in Xenopus oocytes

Agonists that utilize the calcium-mobilizing second messenger inositol(1,4,5)trisphosphate Ins(1,4,5)P3 usually generate oscillations in intracellular calcium. Such oscillations, based on the periodic release of calcium from the endoplasmic reticulum, can also be induced by injecting cells with Ins(1,4,5)P3. The mechanism responsible for oscillatory activity was studied in Xenopus oocytes by injecting them with different inositol trisphosphates. The plasma membrane of Xenopus oocytes has calcium-dependent chloride channels that open in response to calcium, leading to membrane depolarization. Oscillations in calcium were thus monitored by recording membrane potential. The naturally occurring Ins(1,4,5)P3 produced a large initial transient followed by a single transient or a burst of oscillations. By contrast, two analogues (Ins(2,4,5)P3 and Ins(1,4,5)P(S)3) produced a different oscillatory pattern made up of a short burst of sharp transients. Ins(1,3,4,5)P4 had no effect when injected by itself, and it also failed to modify the oscillatory responses to either Ins(2,4,5)P3 or Ins(1,4,5)P(S)3. Both analogues failed to induce a response when injected immediately after the initial Ins(1,4,5)P3-induced response, indicating that they act on the same intracellular pool of calcium. The existence of different oscillatory patterns suggests that there may be different mechanisms for setting up calcium oscillations. The Ins(2,4,5)P3 and Ins(1,4,5)P(S)3 analogues may initiate oscillations through a negative feedback mechanism whereby calcium inhibits its own release. The two-pool model is the most likely mechanism to describe the Ins(1,4,5)P3-induced oscillations.

Intracellular aluminium inhibits acetylcholine- and caffeine-evoked Ca2+ mobilization

The effect of intracellular aluminium on Ca2+ signalling in single internally perfused mouse pancreatic acinar cells was investigated by measurement of the Ca2(+)-dependent Cl- current using the patch-clamp whole-cell recording configuration. Acetylcholine (ACh) normally evoked a pulsatile Ca2(+)-dependent Cl- current, but when AlCl3 (1 mM) was present in the internal perfusion solution the ACh responses were virtually absent. When aluminium was acutely infused into the internal perfusion solution, the ACh-evoked Ca2+ signals and also the caffeine-evoked responses quickly disappeared, but the Ca2+ ionophore, ionomycin (100 nM), could still induce a large increase in the Cl- current. It is concluded that intracellular aluminium can abolish receptor-activated intracellular Ca2+ release probably by inhibition of Ca2(+)-induced Ca2+ release.

Inhibitors of protein kinase C prolong the falling phase of each free-calcium transient in a hormone-stimulated hepatocyte

Many cells generate oscillations in cytoplasmic free Ca2+ concentration ('free Ca') when stimulated with Ca-mobilizing hormones. The frequency of repetitive free-Ca transients in a rat hepatocyte is a function of hormone concentration and can be depressed by phorbol esters. We show here that the protein kinase C (PKC) inhibitors staurosporine and sphingosine can reverse the effects of phorbol dibutyrate on the frequency of free-Ca transients induced by phenylephrine or vasopressin. An important feature of the hepatocyte free-Ca oscillator is that the transient's time course, particularly the rate of fall of free Ca from peak to resting, depends on the species of agonist, and is measurably different for phenylephrine, vasopressin, angiotensin II or ATP. We show here that the rate of fall of free Ca in transients induced by phenylephrine or vasopressin is markedly decreased after treatment of the cells with a PKC inhibitor. A receptor-controlled oscillator model is discussed, in which PKC provides negative feedback during the falling phase of free-Ca transients.

Cytosolic free calcium spiking affected by intracellular pH change

The characteristics underlying cytosolic free calcium oscillation were evaluated by superfused dual wave-length microspectrofluorometry of fura-2-loaded single acinar cells from rat pancreas. Application of a physiological concentration of cholecystokinin octapeptide (CCK) (20 pM) induced a small basal increase in cytosolic free calcium concentration ([Ca2+]i) averaging 34 nM above the prestimulation level (69 nM) with superimposed repetitive Ca2+ spike oscillation. The oscillation amplitude averaged 121 nM above the basal increase in [Ca2+]i and occurred at a frequency of one pulse every 49 s. Although extracellular Ca2+ was required for maintenance of high frequency and amplitude of the spikes with increase in basal [Ca2+]i, the primary source utilized for oscillation was intracellular. The threshold of the peak [Ca2+]i amplitude for causing synchronized and same-sized oscillations was less than 300 nM. The [Ca2+]i oscillation was sensitive to intracellular pH (pHi) change. This is shown by the fact that the large pHi shift toward acidification (delta pHi decrease, 0.95) led to a basal increase in [Ca2+]i to the spike peak level with inhibiting Ca2+ oscillation. The pHi shift toward alkalinization (delta pHi increase, 0.33) led to a basal decrease in [Ca2+]i to the prestimulation level, possibly due to reuptake of Ca2+ into the Ca2+ stores, with inhibiting Ca2+ oscillation. Whereas extracellular pH (pHo) change had only minimal effects on Ca2+ oscillation (and/or Ca2+ release from intracellular stores), the extra-Ca2+ entry process, which was induced by higher concentrations of CCK, was totally inhibited by decreasing pHo from 7.4 to 6.5. Thus the major regulatory sites by which H+ affects Ca2+ oscillation are accessible from the intracellular space.

Cytoplasmic Ca2+ oscillations evoked by acetylcholine or intracellular infusion of inositol trisphosphate or Ca2+ can be inhibited by internal Ca2+

In single internally perfused mouse pancreatic acinar cells, changes in the free intracellular Ca2+ concentration ([Ca2+]i) were monitored by measuring the Ca2(+)-dependent transmembrane Cl- current under voltage-clamp conditions. Cytoplasmic Ca2+ oscillations were induced by external acetylcholine (ACh) application, internal infusion of inositol (1,4,5) trisphosphate or its non-metabolizable analogue inositol trisphosphorothioate or by intracellular Ca2+ infusion. Such [Ca2+]i oscillations could be rapidly inhibited by external application of the Ca2+ ionophore ionomycin (10-100 nM). Cytoplasmic Ca2+ oscillations could also be evoked by external caffeine (1 mM) application when the internal perfusion solution did not contain any Ca2+ chelator. In such cases intracellular Ca2+ infusion transiently abolished the [Ca2+]i oscillations. We conclude that although Ca2(+)-induced Ca2+ release is the cause of the ACh-evoked [Ca2+]i oscillations, there is also a negative feed-back since Ca2+ can inhibit Ca2+ release initiated by Ca2+.

Single-channel and Fura-2 analysis of internal Ca2+ oscillations in HeLa cells: contribution of the receptor-evoked Ca2+ influx and effect of internal pH

Patch-clamp and Fura-2 experiments were performed in order to investigate the calcium oscillations due to H1 receptor stimulation in HeLa cells. The cytosolic calcium fluctuations occurring directly at the plasma membrane inner face were detected by measuring the activity of calcium-dependent potassium channels. This method also allowed measurement of changes in intracellular potential using as indicator the amplitude of the channel current jump. The average internal calcium concentration was obtained from Fura-2 experiments carried out at either the single-cell level or from a small population of cells in monolayer. The results indicate that the internal calcium oscillations in HeLa cells arise from a biphasic process with an initial phase independent of the presence of external calcium. External calcium was found, however, to become essential once the regular oscillatory process has been established. Removing external calcium after this initial phase produced a rapid decay in the burst frequency and eventually a complete abolition of the oscillations. In addition, the calcium oscillations occurring during the external-calcium-dependent phase could be blocked by calcium entry blockers such as Co2+ or La3+, or abolished by perfusing the external medium with a high-K+ solution. Experiments were also performed in which the cell internal pH (pHi) was changed by removing the external bicarbonate or by adding NH4Cl to the bathing solution. The results obtained under these conditions indicate that an increase in internal pH abolishes selectively the appearance of calcium spikes without increasing the basal calcium level, while a cellular acidification maintains or stimulates the calcium oscillatory process. It was also observed that the inhibitory effect of alkaline pH was independent of external calcium, and that calcium oscillations could always be seen at alkaline pH during the initial phase of histamine stimulation. On the basis of these results, it is proposed that the internal calcium oscillations in HeLa cells depend on the release of calcium from internal pools, which are reloaded via a pH-dependent mechanism. Part of the calcium sequestration occurring during the oscillatory process would be carried out, however, by pH-insensitive calcium compartments.

Feedback inhibition by calcium limits the release of calcium by inositol trisphosphate in Limulus ventral photoreceptors

Injection of inositol 1,4,5 trisphosphate (InsP3) into Limulus ventral photoreceptors elevates the concentration of intracellular calcium ions and as a consequence depolarizes the photoreceptor. This InsP3-induced elevation can be inhibited by a prior injection of calcium or InsP3 delivered 1 s earlier. Recovery from this inhibition has a half-time of between 1.5 and 5 s at 20 degrees C. Calcium released by InsP3 therefore inhibits further release of calcium from InsP3-sensitive calcium stores. This feedback inhibition may protect the calcium stores from depletion during prolonged bright illumination. Feedback inhibition, rather than periodic depletion of calcium stores, may also underlie the oscillatory bursts of InsP3-induced calcium release that have been observed in many cell types.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Cholinergic-induced oscillating transepithelial short-circuit current in cultured human sweat duct cells

Human sweat duct cells in primary culture were investigated by voltage-clamp technique. Stimulation with the muscarinic agonist, metacholine (MCh), produced an abrupt transient rise followed by sustained regular oscillations in the transepithelial short-circuit current (Iscc), which in these cells is carried by a mucosal amiloride-sensitive Na+ influx, secondary to a Ca2(+)-activated, voltage-dependent, large K+ shunt across the serosal membrane. The time of latency, the initial transient phase, and the sustained oscillating phase of the MCh-induced Iscc response were demonstrated to be differently affected by changes in temperature, agonist concentration and external Ca2+ supply. From these results a model is proposed for the MCh-induced signal transduction in cultured sweat duct cells, involving a primary intracellular oscillatory Ca2+ mobilization, activated by IPP, sustained by a temperature-regulated external Ca2+ supply, and counter-regulated by cytosolic Ca2+.

Calcium flux mediated by purified inositol 1,4,5-trisphosphate receptor in reconstituted lipid vesicles is allosterically regulated by adenine nucleotides

When incorporated into lipid vesicles, the purified inositol 1,4,5-trisphosphate (IP3) receptor protein mediates 45Ca2+ flux. We observe a potent, selective allosteric regulation by ATP of IP3 actions on Ca2+ flux. The action of ATP is selective for adenine nucleotides with ADP and AMP less potent and GTP inactive. At 1-10 microM, ATP increases maximal IP3-induced flux by 50% with no change in IP3 potency. The enhancing effect of ATP diminishes between 0.1 and 1 mM. Concentration-response curves are steep for both the increasing and the decreasing effects of ATP on IP3 actions, suggesting a physiological regulatory role of ATP in IP3-induced Ca2+ release. Diminishing local ATP concentrations coincident with filling of Ca2+ stores by the Ca2(+)-ATPase may enhance IP3 release of Ca2+, an effect that would decline as ATP returns to physiological levels. ATP regulation of Ca2+ release may also play a role in oscillations of intracellular Ca2+ concentration.

The size of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores depends on inositol 1,4,5-trisphosphate concentration

An explanation of the complex effects of hormones on intracellular Ca2+ requires that the intracellular actions of Ins(1,4,5)P3 and the relationships between intracellular Ca2+ stores are fully understood. We have examined the kinetics of 45Ca2+ efflux from pre-loaded intracellular stores after stimulation with Ins(1,4,5)P3 or the stable phosphorothioate analogue, Ins(1,4,5)P3[S]3, by simultaneous addition of one of them with glucose/hexokinase to rapidly deplete the medium of ATP. Under these conditions, a maximal concentration of either Ins(1,4,5)P3 or Ins(1,4,5)P3[S]3 evoked rapid efflux of about half of the accumulated 45Ca2+, and thereafter the efflux was the same as occurred under control conditions. Submaximal concentrations of Ins(1,4,5)P3 or Ins(1,4,5)P3[S]3 caused a smaller rapid initial efflux of 45Ca2+, after which the efflux was similar whatever the concentration of Ins(1,4,5)P3 or Ins(1,4,5)P3[S]3 present. The failure of submaximal concentrations of Ins(1,4,5)P3 and Ins(1,4,5)P3[S]3 to mobilize fully the Ins(1,4,5)P3-sensitive Ca2+ stores despite prolonged incubation was not due either to inactivation of Ins(1,4,5)P3 or to desensitization of the Ins(1,4,5)P3 receptor. The results suggest that the size of the Ins(1,4,5)P3 sensitive Ca2+ stores depends upon the concentration of Ins(1,4,5)P3.

Histamine-H1-receptor-mediated phosphoinositide hydrolysis, Ca2+ signalling and membrane-potential oscillations in human HeLa carcinoma cells

In human HeLa carcinoma cells, histamine causes a dose-dependent formation of inositol phosphates, production of diacylglycerol and a transient rise in intracellular [Ca2+]. These responses are completely blocked by the H1-receptor antagonist pyrilamine. In streptolysin-O-permeabilized cells, formation of inositol phosphates by histamine is strongly potentiated by guanosine 5'-[gamma-thio]triphosphate and inhibited by guanosine 5'-[beta-thio]diphosphate, suggesting the involvement of a GTP-binding protein. Histamine stimulates the rapid but transient formation of Ins(1,4,5)P3, Ins(1,3,4)P3 and InsP4. InsP accumulates in a much more persistent manner, lasting for at least 30 min. Studies with streptolysin-O-permeabilized cells indicate that InsP accumulation results from dephosphorylation of Ins(1,4,5)P3, rather than direct hydrolysis of PtdIns. The rise in intracellular [Ca2+] is biphasic, with a very fast release of Ca2+ from intracellular stores, that parallels the Ins(1,4,5)P3 time course, followed by a more prolonged phase of Ca2+ influx. In individual cells, histamine causes a rapid initial hyperpolarization of the plasma membrane, which can be mimicked by microinjected Ins(1,4,5)P3. Histamine-induced hyperpolarization is followed by long-lasting oscillations in membrane potential, apparently owing to periodic activation of Ca2+-dependent K+ channels. These membrane-potential oscillations can be mimicked by microinjection of guanosine 5'-[gamma-thio]triphosphate, but are not observed after microinjection of Ins(1,4,5)P3. We conclude that H1-receptors in HeLa cells activate a PtdInsP2-specific phospholipase C through participation of a specific G-protein, resulting in long-lasting oscillations of cytoplasmic free Ca2+.

Minimal model for signal-induced Ca2+ oscillations and for their frequency encoding through protein phosphorylation

In a variety of cells, hormonal or neurotransmitter signals elicit a train of intracellular Ca2+ spikes. The analysis of a minimal model based on Ca2(+)-induced Ca2+ release from intracellular stores shows how sustained oscillations of cytosolic Ca2+ may develop as a result of a rise in inositol 1,4,5-trisphosphate (InsP3) triggered by external stimulation. This rise elicits the release of a certain amount of Ca2+ from an InsP3-sensitive intracellular store. The subsequent rise in cytosolic Ca2+ in turn triggers the release of Ca2+ from a second store insensitive to InsP3. In contrast to the model proposed by Meyer and Stryer [Meyer, T. & Stryer, L. (1988) Proc. Natl. Acad. Sci. USA 85, 5051-5055], the present model, which contains only two variables, predicts the occurrence of periodic Ca2+ spikes in the absence of InsP3 oscillations. Such results indicate that repetitive Ca2+ spikes evoked by external stimuli do not necessarily require the concomitant, periodic variation of InsP3. The model is closely related to that proposed by Kuba and Takeshita [Kuba, K. & Takeshita, S. (1981) J. Theor. Biol. 93, 1009-1031] for Ca2+ oscillations in sympathetic neurones, based on Ca2(+)-induced Ca2+ release. We extend their results by showing the minimal conditions in which the latter process gives rise to periodic behavior and take into account the role of the rise in InsP3 caused by external stimulation. The analysis further shows how signal-induced Ca2+ oscillations might be effectively encoded in terms of their frequency through the phosphorylation of a cellular substrate by a protein kinase activated by cytosolic Ca2+.

Ca2(+)-sensitivity of inositol 1,4,5-trisphosphate-mediated Ca2+ release in permeabilized pancreatic acinar cells

Hormonal and phorbol ester pretreatment of pancreatic acinar cells markedly decreases the Ins(1,4,5)P3-induced release of actively stored Ca2+ [Willems, Van Den Broek, Van Os & De Pont (1989) J. Biol. Chem. 264, 9762-9767]. Inhibition occurred at an ambient free Ca2+ concentration of 0.1 microM, suggesting a receptor-mediated increase in Ca2(+)-sensitivity of the Ins(1,4,5)P3-operated Ca2+ channel. To test this hypothesis, the Ca2(+)-dependence of Ins(1,4,5)P3-induced Ca2+ release was investigated. In the presence of 0.2 microM free Ca2+, permeabilized cells accumulated 0.9 nmol of Ca2+/mg of acinar protein in an energy-dependent pool. Uptake into this pool increased 2.2- and 3.3-fold with 1.0 and 2.0 microM free Ca2+ respectively. At 0.2, 1.0 and 2.0 microM free Ca2+, Ins(1,4,5)P3 maximally released 0.53 (56%), 0.90 (44%) and 0.62 (20%) nmol of Ca2+/mg of acinar protein respectively. Corresponding half-maximal stimulatory Ins(1,4,5)P3 concentrations were calculated to be 0.5, 0.6 and 1.4 microM, suggesting that the affinity of Ins(1,4,5)P3 for its receptor decreases beyond 1.0 microM free Ca2+. The possibility that an inhibitory effect of sub-micromolar Ca2+ is being masked by the concomitant increase in size of the releasable store is excluded, since Ca2+ release from cells loaded in the presence of 0.1 or 0.2 microM free Ca2+ and stimulated at higher ambient free Ca2+ was not inhibited below 1.0 microM free Ca2+. At 2.0 and 10.0 microM free Ca2+, Ca2+, Ca2+ release was inhibited by approx. 30% and 75% respectively. The results presented show that hormonal pretreatment does not lead to an increase in Ca2(+)-sensitivity of the release mechanism. Such an increase in Ca2(+)-sensitivity to sub-micromolar Ca2+ is required to explain sub-micromolar oscillatory changes in cytosolic free Ca2+ by a Ca2(+)-dependent negative-feedback mechanism.