A regulatory calcium-binding site for calcium channel in isolated rat hepatocytes

Changes in Calcium Homeostasis and Gene Expression Implicated in Epilepsy in Hippocampi of Mice Overexpressing ORAI1

Previously, we showed that the overexpression of ORAI1 calcium channel in neurons of murine brain led to spontaneous occurrence of seizure-like events in aged animals of transgenic line FVB/NJ-Tg(ORAI1)Ibd (Nencki Institute of Experimental Biology). We aimed to identify the mechanism that is responsible for this phenomenon. Using a modified Ca²⁺-addback assay in the CA1 region of acute hippocampal slices and FURA-2 acetomethyl ester (AM) Ca²⁺ indicator, we found that overexpression of ORAI1 in neurons led to altered Ca²⁺ response. Next, by RNA sequencing (RNAseq) we identified a set of genes, whose expression was changed in our transgenic animals. These data were validated using customized real-time PCR assays and digital droplet PCR (ddPCR) ddPCR. Using real-time PCR, up-regulation of hairy and enhancer of split-5 (Hes-5) gene and down-regulation of aristaless related homeobox (Arx), doublecortin-like kinase 1 (Dclk1), and cyclin-dependent kinase-like 5 (Cdkl5, also known as serine/threonine kinase 9 (Stk9)) genes were found. Digital droplet PCR (ddPCR) analysis revealed down-regulation of Arx. In humans, ARX, DCLK1, and CDLK5 were shown to be mutated in some rare epilepsy-associated disorders. We conclude that the occurrence of seizure-like events in aged mice overexpressing ORAI1 might be due to the down-regulation of Arx, and possibly of Cdkl5 and Dclk1 genes.

Inositol trisphosphate and calcium mobilization

Calcium-mobilizing agonists act by stimulating the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) to inositol 1,4,5-trisphosphate and diacylglycerol (DG). In response to such agonists cells also produce inositol 1,3,4-trisphosphate but this isomer is unlikely to influence calcium mobilization. Application of inositol 1,4,5-trisphosphate (Ins1,4,5P3) to permeabilized cells results in a rapid release of calcium from the endoplasmic reticulum. Structure-activity studies reveal that the vicinal phosphates on the 4- and 5-positions are essential for releasing calcium whereas the phosphate on the opposite side enhances the affinity of Ins1,4,5P3 for its putative receptor. The flow of calcium across the endoplasmic reticulum appears to be electrogenic and requires an opposite flow of potassium to neutralize charge movements. Diacylglycerol, acting through protein kinase C, does not play a direct role in calcium signalling but it does modulate various aspects of the InsP3/Ca2+ pathway. The DG/protein kinase C pathway can influence both the formation and hydrolysis of PtdIns4,5P2 and can alter the responsiveness of various processes to the action of calcium. The Ins1,4,5P3/Ca2+ signal pathway functions throughout the life history of cells to regulate such diverse activities as egg maturation and fertilization, growth, secretion, metabolism, neural activity, and perhaps excitation-contraction coupling in skeletal muscle.

Mechanisms of capacitative calcium entry

Capacitative Ca2+ entry involves the regulation of plasma membrane Ca2+ channels by the filling state of intracellular Ca2+ stores in the endoplasmic reticulum (ER). Several theories have been advanced regarding the mechanism by which the stores communicate with the plasma membrane. One such mechanism, supported by recent findings, is conformational coupling: inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) receptors in the ER may sense the fall in Ca2+ levels through Ca2+-binding sites on their lumenal domains, and convey this conformational information directly by physically interacting with Ca2+ channels in the plasma membrane. In support of this idea, in some cell types, store-operated channels in excised membrane patches appear to depend on the presence of both Ins(1,4,5)P3 and Ins(1,4,5)P3 receptors for activity; in addition, inhibitors of Ins(1,4,5)P3 production that either block phospholipase C or inhibit phosphatidylinositol 4-kinase can block capacitative Ca2+ entry. However, the electrophysiological current underlying capacitative Ca2+ entry is not blocked by an Ins(1,4,5)P3 receptor antagonist, and the blocking effects of a phospholipase C inhibitor are not reversed by the intracellular application of Ins(1,4,5)P3. Furthermore, cells whose Ins(1,4,5)P3 receptor genes have been disrupted can nevertheless maintain their capability to activate capacitative Ca2+ entry channels in response to store depletion. A tentative conclusion is that multiple mechanisms for signaling capacitative Ca2+ entry may exist, and involve conformational coupling in some cell types and perhaps a diffusible signal in others.

Evidence for norepinephrine-activated Ca2+ permeable channels in guinea-pig hepatocytes using a patch clamp technique

To determine whether the hepatocyte plasma membrane possesses a Ca2+ channel. we applied a patch clamp technique to isolated guinea-pig hepatocytes. In a cell-attached configuration, using an internal pipette solution of 110 mM BaCl2 or CaCl2, we observed sporadic inward single channel currents (Po = 0.004 +/- 0.002, n = 6) at various membrane potentials. The unit amplitude was 0.60 +/- 0.15 pA (n = 6) at resting membrane potential. The single channel conductance was 20.4 +/- 4.6 pS (n = 6) and this channel showed no rectification and no voltage dependence. Bay K 8644, a dihydropyridine Ca2+ channel activator, did not affect this channel activity. Although norepinephrine in the pipette solution did not activate this channel, its external application increased channel activity. These observations suggest that guinea-pig hepatocytes possess Ca2+ permeable channels that differ from the voltage-operated Ca2+ channels found in excitable cells and that such channels are responsible for the agonist-stimulated Ca2+ entry in hepatocytes.

Hepatotoxicity of ethanol: protective effect of calcium channel blockers in isolated hepatocytes

This study examines the effects of three calcium channel blockers (verapamil, nifedipine and diltiazem) on isolated rat hepatocytes exposed to ethanol. In the first part of our study, hepatocytes were incubated with increasing concentrations of ethanol (100, 300, 500, 1000 mM) for varying times. Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) release were measured to evaluate the cytotoxic effects of ethanol. The concentration of 300 mM and time of incubation of 45 min were chosen for cytoprotection experiments in which calcium channel blockers, at two different concentrations, were added to the medium 30 min prior to the addition of ethanol. ALT, AST and LDH release as well as lipid peroxidation and cellular reduced glutathione (GSH) were measured. Nifedipine and verapamil (25 microM) reduced ALT, AST and LDH activities. The highest dose of diltiazem (50 microM) was more effective than the lowest one (25 microM). Ethanol caused a significant depletion of cellular GSH content as well as a moderate enhancement of lipid peroxidation. While none of the three calcium channel blockers was able to restore the decrease in GSH levels, diltiazem (25 microM) and nifedipine (50 microM) showed the greatest effect, significantly reducing lipid peroxidation.

Hormone-regulated Ca2+ channel in rat hepatocytes revealed by whole cell patch clamp

An inward current responsible for hormone regulated Ca2+ entry has been identified in cultured rat hepatocytes using whole cell patch clamp. Addition of 20 nM vasopressin or of 100 microM ATP induced the inward current, which could be observed more clearly after blocking an outward K+ current. This large outward K+ current, which appeared after addition of vasopressin or ATP, could be blocked either by replacing K+ with Cs+ in the external medium and in the pipette solution, or by simply including 0.5 microM apamin in the K(+)-containing external medium. The outward current appears to be carried by a Ca2+ activated K+ channel. In the presence of apamin, hepatocytes pretreated with vasopressin in a Ca(2+)-free media reveal an inward current on addition of external Ca2+ (5 mM). The current could also be elicited by addition of vasopressin when cells are preincubated in the presence of 5 mM external Ca2+. No current is seen on addition of Ca2+ in the absence of vasopressin. Initially, the inward current was ca 200-300 pA at -60 mV, but it declined rapidly over 3 min to ca 20 pA. The current approached zero, as an asymptote at positive potential, and appeared to be somewhat inwardly rectifying. Additions of 5 mM Mn2+ or 5 mM Ba2+ in place of Ca2+ produced little or no current. An inhibitor of ER Ca(2+)-ATPase, thapsigargin, could also trigger the cascade of events leading to plasma membrane conductance of Ca2+. The data suggest that hormone-stimulated Ca2+ entry into hepatocytes is mediated by a Ca(2+)-release activated channel highly specific for Ca2+. This is the first demonstration of such a channel in hepatocytes, though similar ones have been described in mast cells, in vascular endothelial cells and T-lymphocytes.

Arginine vasopressin-induced potentiation of unitary L-type Ca2+ channel current in guinea pig ventricular myocytes

The effects of arginine vasopressin (AVP) on L-type Ca2+ channels were studied by recording single-channel activity from cell-attached patches on isolated guinea pig ventricular myocytes, with 100 mmol/L Ba2+ used as the charge carrier. Bath application of AVP (100 nmol/L) reversibly increased channel open probability by a factor of 2.92 +/- 1.43 (n = 15) because of the increased number of channel openings and increased open times. AVP did not change the amplitudes of single-channel currents (1.17 +/- 0.10 pA in the control condition and 1.12 +/- 0.11 pA after AVP, at +20 mV; n = 6). In our experimental conditions, in which myocytes were bathed in Ca(2+)-free high-potassium solutions, AVP-induced potentiation was observed without changes in [Ca2+]i measured by fura 2 fluorescence signals (estimated [Ca2+]i, approximately 80 nmol/L). The AVP-induced increase in channel open probability was abolished by OPC-21268 (8 mumol/L), a specific blocker of V1 receptor, but not by a V2 blocker, OPC-31260 (5 mumol/L). AVP-induced potentiation was also suppressed by a broad-spectrum protein kinase inhibitor, H7 (100 mumol/L, bath application), but not by H89 (1 mumol/L), a blocker with high specificity to protein kinase A. AVP application after the treatment by phorbol ester (phorbol 12-myristate 13-acetate, 100 nmol/L for 1 hour) failed to potentiate the channel activity. These results raised the possibility that protein kinase C might be involved during signal transduction. Our results provide direct evidence that AVP potentiates cardiac L-type Ca2+ currents via V1 receptor stimulation.

Inhibition of the increase of intrahepatic Ca2+ by diltiazem in rats with liver ischemia

The effects of a continuous infusion of a calcium entry blocker, 1, 5-benzothiazepine derivative (diltiazem), on ischemic liver cell damage were studied using quantitative 45Ca-autoradiographic and liquid scintillation techniques. The drug was administered to male Wistar rats as a continuous infusion for 3 h, beginning 30 min before ischemia. Autoradiographic studies showed that 45Ca accumulated in the liver lobuli after 1 h of liver ischemia and 3 h of reperfusion, but the level of 45Ca accumulation was significantly lower in drug-treated rats than in untreated animals. In addition, liquid scintillation studies showed significant differences in the intrahepatic 45Ca contents. These results suggest that diltiazem may inhibit the rise of intracellular Ca2+ due to the flow of extracellular Ca2+ into the cytosol, and may protect the ischemic liver from damage.

Calcium signalling in platelets and other nonexcitable cells

By virtue of their biological simplicity and widespread availability, platelets frequently have been used as a model system to study signal transduction. Such studies have revealed that changes in intracellular free calcium concentration are central to platelet functioning. The following article reviews current concepts of platelet structure and function, with particular emphasis on the mechanisms involved in platelet Ca2+ signalling.

Changes in Ca2+ mobilization in platelets from stroke-prone spontaneously hypertensive rats

Intracellular free Ca2+, [Ca2+]i, levels were measured in platelets from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) using fura-2AM. In the presence of extracellular Ca2+ (1 mM), [Ca2+]i levels in unstimulated platelets of 2- and 9-month-old SHRSP were both significantly higher than those of the age matched WKY. In the absence of extracellular Ca2+, the levels in platelets from 9-month-old SHRSP were also higher than any other groups examined. Receptor-linked Ca2+ influxes of old SHRSP were smaller when thrombin or collagen was given to the platelets. Phorbol 12-myristate 13-acetate (TPA) enhanced more prominently the Ca2+ influx into old SHRSP platelets than into old WKY platelets. These results strongly suggest that the Ca2+ permeability across plasma membrane is increased in young as well as old SHRSP platelets, where the resting [Ca2+]i level is highly sustained because of an impaired Ca2+ uptake mechanism and possible enhancement of protein kinase C activity.

Protective effect of various calcium antagonists against an experimentally induced calcium overload in isolated hepatocytes

The effect of the hepatotoxic substance diamidinothionaphthene (98/202) on cytosolic, mitochondrial and extra-mitochondrial calcium distribution was measured in isolated rat hepatocytes. The drastic disturbance of the intracellular calcium homeostasis caused by this substance (increase of the cytosolic and mitochondrial calcium contents and depletion of extra-mitochondrial calcium stores, which at last lead to cell death) gave rise to an investigation of the possible cytoprotective effect of calcium antagonists of various chemical classes: verapamil, diltiazem, and nifedipine on isolated hepatocytes. Our results show that all three calcium antagonists prevented cell death caused by 98/202. The 98/202-induced increase of cytosolic and mitochondrial calcium content was inhibited by all three calcium antagonists. However, only verapamil was able to inhibit the depletion of extra-mitochondrial calcium stores. Since 98/202-induced cell death occurs only in the presence of extracellular calcium, it is concluded that calcium antagonists are also able to inhibit the influx of extracellular calcium in liver cells, which leads to a calcium overload of the cytosol and mitochondria. The various ways of interfering with the calcium homeostasis of liver cells qualifies the hepatotoxic substance 98/202 as a suitable in vitro hepatotoxicity model for testing the hepatoprotective effect of different calcium antagonists.

Characterization of the 1,25-dihydroxycholecalciferol-stimulated calcium influx pathway in CaCo-2 cells

The present studies were conducted to investigate the mechanisms underlying the 1,25-dihydroxycholecalciferol (1,25(OH)2D3)-induced increase in intracellular Ca2+ ([Ca2+]i) in individual CaCo-2 cells. In the presence of 2 mM Ca2+, 1,25(OH)2D3-induced a rapid transient rise in [Ca2+]i in Fura-2-loaded cells in a concentration-dependent manner, which decreased, but did not return to baseline levels. In Ca(2+)-free buffer, this hormone still induced a transient rise in [Ca2+]i, although of lower magnitude, but [Ca2+]i then subsequently fell to baseline. In addition, 1,25(OH)2D3 also rapidly induced 45Ca uptake by these cells, indicating that the sustained rise in [Ca2+]i was due to Ca2+ entry. In Mn(2+)-containing solutions, 1,25(OH)2D3 increased the rate of Mn2+ influx which was temporally preceded by an increase in [Ca2+]i. The sustained rise in [Ca2+]i was inhibited in the presence of external La3+ (0.5 mM). 1,25(OH)2D3 did not increase Ba2+ entry into the cells. Moreover, neither high external K+ (75 mM), nor the addition of Bay K 8644 (1 microM), an L-type, voltage-dependent Ca2+ channel agonist, alone or in combination, were found to increase [Ca2+]i. 1,25(OH)2D3 did, however, increase intracellular Na+ in the absence, but not in the presence of 2 mM Ca2+, as assessed by the sodium-sensitive dye, sodium-binding benzofuran isophthalate. These data, therefore, indicate that CaCo-2 cells do not express L-type, voltage-dependent Ca2+ channels. 1,25(OH)2D3 does appear to activate a La(3+)-inhibitable, cation influx pathway in CaCo-2 cells.

[Calcium and liver]

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

Adrenergic modulation of Ca2+ homeostasis in isolated fish hepatocytes

Isolated hepatocytes of the American eel (Anguilla rostrata LeSueur) and brown bullhead (Ictaluras nebulosus) have been used to characterize the effects of alpha- and beta-adrenergic agonists (epinephrine, phenylephrine, isoproterenol) and antagonists (phentolamine, propranolol) on calcium flux (influx, efflux) and cytosolic free calcium concentrations ([Ca2+]i). Bullhead hepatocytes have higher influx but lower efflux of Ca2+ than eel hepatocytes, which may relate to the primary source for changes in [Ca2+]i. Adrenergic agonists did not affect influx, but significantly enhanced efflux in eel hepatocytes (not bullhead) and [Ca2+]i in both species. Increases in efflux and in [Ca2+]i were blocked by alpha-antagonists (phentolamine) but not beta-antagonists (propranolol) when present in 100-fold excess of the agonist. Isoproterenol had no significant effect on either parameter tested. This study supports our hypothesis that liver cell Ca2+ homeostasis is modulated by alpha-adrenoceptor-linked pathways in these two fish species as has been previously demonstrated for the rat.

Evidence for receptor-mediated calcium entry and refilling of intracellular calcium stores in FRTL-5 rat thyroid cells

The aim of the present study was to investigate the relationship between agonist-induced changes in intracellular free Ca2+ ([Ca2+]i) and the refilling of intracellular Ca2+ stores in Fura 2-loaded thyroid FRTL-5 cells. Stimulating the cells with ATP induced a dose-dependent increase in ([Ca2+]i). The ATP-induced increase in [Ca2+]i was dependent on both release of sequestered intracellular Ca2+ as well as influx of extracellular Ca2+. Addition of Ni2+ prior to ATP blunted the component of the ATP-induced increase in [Ca2+]i dependent on influx of Ca2+. In cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ induced a rapid increase in [Ca2+]i; this increase was inhibited by Ni2+. In addition, the ATP-induced influx of 45Ca2+ was blocked by Ni2+. Stimulating the cells with noradrenaline (NA) also induced release of sequestered Ca2+ and an influx of extracellular Ca2+. When cells were stimulated first with NA, a subsequent addition of ATP induced a blunted increase in [Ca2+]i. If the action of NA was terminated by addition of prazosin, and ATP was then added, the increase in [Ca2+]i was restored to control levels. Addition of Ni2+ prior to prazosin inhibited the restoration of the ATP response. In the presence of extracellular Mn2+, ATP stimulated quenching of Fura 2 fluorescence. The quenching was probably due to influx of Mn2+, as it was blocked by Ni2+. The results thus suggested that stimulating release of sequestered Ca2+ in FRTL-5 cells was followed by influx of extracellular Ca2+ and rapid refilling of intracellular Ca2+ stores.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Cytosolic calcium transients differ between porcine coronary arterial and aortic smooth muscle cells in primary culture

Using quin 2 microfluorometry of porcine vascular smooth muscle cells in primary culture at 25 degrees C, we investigated differences in cytosolic calcium transients between epicardial coronary artery and aorta. Both in coronary arterial and aortic smooth muscle cells, histamine induced transient and dose-dependent elevations of cytosolic calcium concentrations, with a similar time course and EC50 (coronary artery, 1.4 x 10(-7) M; aorta, 1.8 x 10(7) M). However, a transient and dose-dependent elevation of cytosolic calcium concentrations was induced by norepinephrine in aortic smooth muscle cells (EC50 = 2.5 x 10(-7) M) but not in coronary arterial smooth muscle cells. Isoproterenol, which produced no change in cytosolic calcium concentrations in aortic vascular smooth muscle cells, significantly and dose dependently decreased concentrations of calcium in coronary arterial smooth muscle cells (EC50 = 1.5 x 10(7) M). Dibutyryl cAMP decreased the concentration of cytosolic calcium both in the coronary arterial and aortic vascular smooth muscle cells with a similar time course and EC50 (coronary artery, 9.8 x 10(-6) M; aorta, 1.1 x 10(-5) M). Intracellular concentration of cAMP was increased in response to isoproterenol, as determined with radioimmunoassay of the coronary arterial smooth muscle cells but not in the aortic cells. Thus, the characteristics of receptors on the sarcolemma may play a key role in the regulation of responsiveness of vascular smooth muscle cells to various vasoactive substances. Aortic smooth muscle cells are alpha-receptor dominant, and activation results in a transient elevation of cytosolic calcium concentrations. The epicardial coronary arterial smooth muscle cells are beta-receptor dominant, and activation results in an increase in cAMP and a reduction of cytosolic calcium concentrations. These results may account for the poor contraction, or relaxation, of epicardial coronary artery induced by sympathetic stimulation and exogenously applied catecholamines.

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.

Effects of calcium antagonists on the erythrocyte membrane

The effects of dihydropyridine compounds nimodipine, nicardipine and NB818 (isopropyl methyl-6-carbamoyloxymethyl-4-(2,3-dichlorophenyl)-1,4-dihydro-2-methyl- 3,5- pyridine-dicarboxylate) on erythrocyte membranes have been studied. These compounds showed protective effects against hypotonic haemolysis, but not against heat-induced haemolysis. An increase in deformability of erythrocytes by these calcium antagonists was observed using a capillary tube centrifugal method. The erythrocytes showed slight stomatocytosis after 30 min of incubation with calcium antagonists, but did not show significant changes in mean corpuscular volume and ATP levels.

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.

The nature and mechanism of activation of the hepatocyte receptor-activated Ca2+ inflow system

Progress in elucidation of the properties of the hepatocyte receptor-activated Ca2+ inflow system (RACIS) has been hampered by difficulties in measuring rates of Ca2+ inflow to hepatocytes. These difficulties have led, for example, to different conclusions about the relationship between the extracellular Ca2+ concentration and the movement of Ca2+ through the RACIS. The hepatocyte RACIS admits Mn2+ and a number of other divalent cations as well as Ca2+. Many of these cations also inhibit the movement of Ca2+ through this system. While the RACIS is inhibited by high concentrations of verapamil and by some other Ca2+ antagonists, it is relatively insensitive to inhibition by organic compounds which inhibit other Ca2+ channels and Ca2+ transporters. There is circumstantial evidence which suggests that the hepatocyte RACIS is an exchange system, possibly one which catalyses Ca(2+)-H+ exchange or the co-transport of Ca2+ and OH-. Other circumstantial evidence suggests that the RACIS is a channel, with some similarities to voltage-operated Ca2+ channels in excitable cells. However, experiments using the patch-clamp technique have not yet detected agonist-stimulated Ca2+ movement across the hepatocyte plasma membrane. The molecular components of the RACIS probably differ from those which facilitate the large inflow of Ca2+ to hepatocytes which occurs in the absence of an agonist. The mechanism by which agonists activate the RACIS has not been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ entry in T cells is activated by emptying the inositol 1,4,5-triphosphate sensitive Ca2+ pool

Using alpha-linolenic acid (ALA), one of several polyunsaturated fatty acids (PUFAs) that have previously been shown to both mobilize intracellular Ca2+ from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pool independently of IP3 production and inhibit Ca2+ influx, the relationship between Ca2+ mobilization from intracellular stores and Ca2+ influx in T cells (JURKAT) was studied. JURKAT cells were treated with 30 microM ALA to deplete the IP3-sensitive Ca2+ pool. When the intracellular free Ca2+ concentration [( Ca2+]i) returned to basal level, fatty acid free bovine serum albumin (BSA) was added to remove extracellular and membrane bound ALA. This resulted in a sustained increase in [Ca2+]i in the absence of inositol phosphates' formation. This sustained increase in [Ca2+]i was insensitive to protein kinase C activation but was inhibited by Ni2+ ions. The extent of Ca2+ influx was found to be correlated to the amount of Ca2+ initially discharged from the IP3-sensitive Ca2+ pool by sub-optimal concentrations of ALA. Ligation of the CD3 complex of the T cell antigen receptor with an anti-CD3 antibody (OKT3) during the sustained [Ca2+]i increased (induced by a sub-optimal concentration of ALA), produced a greater response. No increase in the sustained response was observed when the CD3 complex was activated in cells pretreated with an optimal concentration of ALA. In summary, Ca2+ entry in T cells is activated by emptying of the IP3-sensitive Ca2+ pool which can be dissociated from inositol phosphate production. The rate of Ca2+ influx appears to be closely correlated to the initial discharge of Ca2+ from the IP3-sensitive Ca2+ pool, suggesting that Ca2+ may first enter the depleted pool and then is released into the cytosol.

Ethanol-induced stimulation of phosphoinositide turnover and calcium influx in isolated hepatocytes

Ethanol has been shown to mobilize intracellular calcium in isolated rat hepatocytes by activation of phosphoinositide-specific phospholipase C. However, addition of ethanol to 32P-labeled hepatocytes resulted in a rapid increase in the level of [32P]phosphatidylinositol 4-phosphate over a period of 2 min, concomitant with a small decrease in [32P]phosphatidylinositol 4,5-bisphosphate and an increase in [32P]phosphatidic acid levels. These results indicate that polyphosphatidylinositol metabolism was stimulated by ethanol simultaneously with the activation of phospholipase C. Ethanol also caused a transient increase in the influx of extracellular calcium into quin 2-loaded hepatocytes over a similar period of time. The results demonstrate that ethanol, in common with calcium-mobilizing hormones, directly or indirectly stimulated polyphosphoinositide regeneration and allowed for increased movement of calcium across the hepatocyte plasma membrane.

The effect of nifedipine, a calcium channel blocker, on liver ischemia in dogs

This study was undertaken in order to determine whether the administration of nifedipine, a calcium channel blocker, could protect the liver from ischemic damage and to investigate its effect on the hepatic cellular energy status and cardio-vascular system after 60 minutes of hepatic ischemia in dogs. The ischemia was induced by temporarily clamping the portal vein and hepatic artery. One group of animals (n = 17) received nifedipine (5 micrograms/kg body weight) intravenously 15 minutes before the induction of liver ischemia, which was continued at a dose of 0.2 microgram/kg body weight/min throughout the ischemic period, and for an additional 30 minutes afterwards. Control dogs (n = 16) were not given nifedipine and survival was observed over seven days. The survival rate was 83 per cent in the nifedipine treated animals and 0 per cent in the control animals. Serum glutamic oxaloacetic transaminase levels were greatly increased following ischemia, and they were significantly lowered with the nifedipine treatment. The hepatic energy charge decreased remarkably during the hepatic ischemia, however it increased gradually after declamping but did not returned to its preoperative value in either group until one hour later and then it was higher in the nifedipine treated animals than in the control animals. Cardiac index and portal venous blood flow ratio remained higher in the nifedipine treated animals than in the control animals, after the ischemic period. These results suggest that nifedipine may have a powerful cytoprotective effect and that the period of warm hepatic ischemia could be prolonged with its use.

Effect of extracellular Ca2+ on plasma membrane Ca2+ inflow and cytoplasmic free Ca2+ in isolated hepatocytes

An initial rapid phase and a subsequent slow phase of 45Ca2+ uptake were observed following the addition of 45Ca2+ to Ca2+-deprived hepatocytes. The magnitude of the rapid phase increased 15-fold over the range 0.1-11 mM extracellular Ca2+ (Ca2+o) and was a linear function of [Ca2+]o. The increases in the rate of 45Ca2+ uptake were accompanied by only small increases in the intracellular free Ca2+ concentration. In cells made permeable to Ca2+ by treatment with saponin, the rate of 45Ca2+ uptake (measured at free Ca2+ concentrations equal to those in the cytoplasm of intact cells) increased as the concentration of saponin increased from 1.4 to 2.5 micrograms per mg wet weight cells. Rates of 45Ca2+ uptake by cells permeabilized with an optimal concentration of saponin were comparable with those of intact cells incubated at physiological [Ca2+o], but were substantially lower than those for intact cells incubated at high [Ca2+o]. It is concluded that Ca2+ which enters the hepatocyte across the plasma membrane is rapidly removed by binding and transport to intracellular sites and by the plasma membrane (Ca2+ + Mg2+)-ATPase and the plasma membrane Ca2+ inflow transporter is not readily saturated with Ca2+o.

Fluorescent measurements of intracellular free calcium in isolated toad urinary bladder epithelial cells

Sodium-calcium exchange has been suggested to play a pivotal role in the regulation of cytosolic free calcium (Caf) by epithelial cells. Using isolated epithelial cells from the toad urinary bladder, Caf has been measured using the intracellular Ca-sensitive fluorescent dyes Fura 2 and Quin 2. Dye loading did not alter cell viability as assessed by measurements of ATP and ADP content or cell oxygen consumption. When basal Caf was examined over a wide range of cell dye content (from 0.04 to 180 nmol dye/mg protein) an inverse relationship was observed. At low dye content, Caf was 300-380 nM and, as dye content was increased, Caf progressively fell to 60 nM. Using low dye content cells, in which minimal alteration in Ca steady state would be expected, the role for plasma membrane Na-Ca exchange was examined using either medium sodium substitution or ouabain. While medium sodium substitution increased Caf, prolonged treatment with ouabain had no effect on Caf despite a clear increase in cell sodium content. The lack of effect of ouabain suggests that Na-Ca exchange-mediated Ca efflux plays a minimal role in the regulation of basal Caf. However, exchange-mediated Ca efflux may play a role in Caf regulation when cytosolic calcium is elevated.

The Ca2+-dependent actions of the alpha-adrenergic agonist phenylephrine on hepatic glycogenolysis differ from those of vasopressin and angiotensin

The stimulation of hepatic glycogenolysis by the Ca2+-dependent hormones phenylephrine, vasopressin and angiotensin II was studied as a function of intracellular and extracellular Ca2+. In the isolated perfused rat liver the decline in glucose formation was monophasic ('half-life' approximately equal to 3 min) with vasopressin (1 nM) or angiotensin II (0.05 microM), but biphasic (half-life of 4.8 min and 17.6 min) in the presence of the alpha-agonist phenylephrine (0.01 mM), indicating either a different mode of mobilization or the mobilization of additional intracellular calcium stores. Under comparable conditions an elevated [Ca2+] level was maintained in the cytosol of hepatocytes for at least 10 min in the presence of phenylephrine, but not vasopressin. Titration experiments performed in the isolated perfused liver to restore cellular calcium revealed differences in the hormone-mediated uptake of Ca2+. The onset in glucose formation above that seen in the absence of exogenous calcium occurred at approximately 30 microM or 70-80 microM Ca2+ in the presence of phenylephrine or vasopressin respectively. The shape of the response curve was sigmoidal for vasopressin and angiotensin II, but showed a distinct plateau between 0.09 mM and 0.18 mM in the presence of phenylephrine. The plateau was also observed at phenylephrine concentrations as low as 0.5 microM. The formation of plateaus observed after treatment of the liver with A 23187, but not after EGTA, is taken as an indication that intracellular calcium stores are replenished. A participation of the mitochondrial compartment could be excluded by pretreatment of the liver with the uncoupler 2,4-dinitrophenol. Differences in the Ca2+ dependence of the glycogenolytic effects of these hormones were also revealed by kinetic analysis. It is concluded that phenylephrine differs from vasopressin and angiotensin II in that, in addition to a more common, non-mitochondrial pool, which is also responsive to the vasoactive peptides, the agonist mobilizes Ca2+ from a second, non-mitochondrial pool. The results are consistent with the proposal that Ca2+ transport across subcellular membranes may be subject to different hormonal control.

Ca2+ ions, an allosteric activator of calcium uptake in rat liver mitochondria

In a previous investigation, I have shown that the kinetics of the Ca uniporter change fundamentally when mitochondria have transitorily lost their membrane potential. The sigmoidal kinetics, usually observed in liver mitochondria, became almost hyperbolic. This means an increase in the affinity for calcium, and hence a considerable acceleration of Ca uptake in the range of low, e.g., physiological calcium concentration. In this investigation I show that extramitochondrial calcium released from the deenergized mitochondria causes the allosteric activation of the Ca uniporter. The dependence of the allosterical activation on the extramitochondrial Ca2+ concentration and on time is described. It is also reported that it is possible to activate allosterically the Ca uniporter of energized mitochondria by a short-term elevation of the extramitochondrial Ca2+ concentration. The process of activation is reversible. It is quickly reversed by the addition of chelators for Ca2+, and it is slowly reversed when the activating Ca2+ has to be removed by the mitochondrial Ca uniporter, though the bulk of extramitochondrial calcium is taken up by it very quickly. Several kinetics of the Ca uniporter are described. The implications of continually changing kinetics of the Ca uniporter are considered for carbon tetrachloride intoxication and the action of alpha 1-adrenergic agonists in liver cells.

Studies with verapamil and nifedipine provide evidence for the presence in the liver cell plasma membrane of two types of Ca2+ inflow transporter which are dissimilar to potential-operated Ca2+ channels

The addition of 500 microM verapamil or nifedipine to isolated hepatocytes incubated in the presence of 1.3 mM Ca2+ caused 20% inhibition of Ca2+ inflow as measured by the initial rate of 45Ca2+ exchange. No stimulation of 45Ca2+ exchange was observed in the presence of the Ca2+ agonist CGP 28392. An increase in the concentration of extracellular K+ from 6 to 60 mM (to depolarize the plasma membrane) increased the initial rate of 45Ca2+ exchange by 30%. In the presence of 60 mM K+, 400 microM verapamil inhibited the initiate rate of 45Ca2+ exchange by 50%. Verapamil and nifedipine completely inhibited vasopressin-induced Ca2+ inflow as determined by measurement of the initial rate of 45Ca2+ exchange and of glycogen phosphorylase a activity. This effect of verapamil was completely reversed by increasing the extracellular concentration of Ca2+. The concentrations of Ca2+ antagonist which gave 50% inhibition of vasopressin- or K+-stimulated Ca2+ inflow were in the range 50-100 microM, about 50-fold greater than the concentration which gave 50% inhibition of the beating of electrically-stimulated myocardial muscle cells. In the absence of vasopressin, verapamil caused a transient increase in glycogen phosphorylase a activity by a process which is largely independent of Ca2+. It is concluded that verapamil and nifedipine inhibit the transport of Ca2+ across the hepatocyte plasma membrane through a putative Ca2+ transporter which is activated by vasopressin and which differs in nature from potential-operated Ca2+ channels in excitable cells and from the Ca2+ transporter present in hepatocytes in the absence of hormone.

A model for receptor-regulated calcium entry

A model is proposed for the mechanism by which activation of surface membrane receptors causes sustained Ca2+ entry into cells from the extracellular space. Reassessment of previously published findings on the behavior of receptor-regulated intracellular Ca2+ pools leads to the conclusion that when such pools are empty, a pathway from the extracellular space to the pool is opened; conversely when the pool is filled, the pathway is closed and it becomes relatively stable to depletion by low Ca2+ media or chelating agents. The biphasic nature of agonist-activated Ca2+-mobilization is thus seen as an initial emptying of the intracellular Ca2+ pool by inositol (1,4,5) trisphosphate, followed by rapid entry of Ca2+ into the pool and, in the continued presence of inositol (1,4,5) trisphosphate, into the cytosol. On withdrawal of agonist, inositol (1,4,5) trisphosphate is then rapidly degraded, the pathway from the pool to the cytosol is closed, and rapid entry from the outside continues until the Ca2+ content of the pool reaches a level that inactivates Ca2+ entry. This capacitative model allows for Ca2+ release and Ca2+ entry to be controlled by a single messenger, inositol (1,4,5) trisphosphate.