The high affinity state of inositol 1,4,5-trisphosphate receptor is a functional state

Redox regulation of store-operated Ca2+ entry

SIGNIFICANCE

Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ signaling mechanism triggered by Ca2+ depletion of the endoplasmic reticulum (ER) and by a variety of cellular stresses. Reactive oxygen species (ROS) are often concomitantly produced in response to these stresses, however, the relationship between redox signaling and SOCE is not completely understood. Various cardiovascular, neurological, and immune diseases are associated with alterations in both Ca2+ signaling and ROS production, and thus understanding this relationship has therapeutic implications.

RECENT ADVANCES

Several reactive cysteine modifications in stromal interaction molecule (STIM) and Orai proteins comprising the core SOCE machinery were recently shown to modulate SOCE in a redox-dependent manner. Moreover, STIM1 and Orai1 expression levels may reciprocally regulate and be affected by responses to oxidative stress. ER proteins involved in oxidative protein folding have gained increased recognition as important sources of ROS, and the recent discovery of their accumulation in contact sites between the ER and mitochondria provides a further link between ROS production and intracellular Ca2+ handling.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Future research should aim to establish the complete set of SOCE controlling molecules, to determine their redox-sensitive residues, and to understand how intracellular Ca2+ stores dynamically respond to different types of stress. Mapping the precise nature and functional consequence of key redox-sensitive components of the pre- and post-translational control of SOCE machinery and of proteins regulating ER calcium content will be pivotal in advancing our understanding of the complex cross-talk between redox and Ca2+ signaling.

ROS and SOCE: recent advances and controversies in the regulation of STIM and Orai

Store-operated Ca(2+) entry (SOCE) is a widespread mechanism in cells to raise cytosolic Ca(2+) and to refill Ca(2+) stores. T cells critically rely on SOCE mediated by stromal interaction molecules (STIM) and Orai molecules for their activation and regulation of gene transcription; cells such as muscle cells, neurons or melanocytes probably utilize SOCE for the transmission of inducible receptor-mediated function as well as for generalized Ca(2+) homeostasis mechanisms. Exposure to environmental or cell-intrinisic reactive oxygen species (ROS) can affect several components involved in Ca(2+) homeostasis and thus alter multiple pathways. While all cells have a capacity to produce intracellular ROS, exposure of immune and skin cells to extracellular oxidative stress is particularly high during inflammation and/or with UV exposure. This review briefly summarizes cell-intrinsic sources of ROS and focuses on current findings and controversies regarding the regulation of STIM and Orai by oxidative modifications. We also introduce melanocytes as a new model system to study the function of STIM and Orai isoforms under physiological conditions that include exposure to UV light as an activating stimulus.

Expression of a truncated form of inositol 1,4,5-trisphosphate receptor type III in the cytosol of DT40 triple inositol 1,4,5-trisphosphate receptor-knockout cells

In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ channel playing a major role in Ca2+ signaling. Three isoforms of IP3R have been identified and most cell types express different proportions of each isoform. The DT40 B lymphocyte cell line lacking all three IP3R isoforms (DT40IP3R-KO cells) represents an excellent model to re-express any recombinant IP3R and analyze its specific properties. In the study presented here, we confirmed that DT40IP3R-KO cells do not express any IP3-sensitive Ca2+ release channel. However, with an immunoblot approach and a [3H]IP3 binding approach we demonstrated the presence of a C-terminally truncated form of IP3R type III in the cytosolic fraction of DT40IP3R-KO cells. We further showed that this truncated IP3R retained the ability to couple to the Ca2+ entry channel TRPC6. Therefore, a word of caution is offered about the interpretation of results obtained in using DT40IP3R-KO cells to study the cellular mechanisms of Ca2+ entry.

The thiol reagent, thimerosal, irreversibly inhibits meiosis reinitiation in mouse oocyte when applied during a very early and narrow temporal window: a pharmacological analysis

The effect of the sulfhydryl reagent, thimerosal (TMS) on meiosis resumption in germinal vesicle (GV)-stage denuded mouse oocytes was studied. It irreversibly inhibits both GV breakdown (GVBD) and the first polar body (pb1) extrusion in concentration- and time-dependent manners, the most striking result being the very early and narrow temporal window during which denuded primary oocytes released from their follicle are susceptible to a pulse of the drug. This inhibition is bypassed by dithiothreitol (DTT) with an efficiency declining with time, while thiosalicylic acid (TA), an analog of TMS devoid of the mercury atom, has no effect on meiosis reinitiation. These results strongly suggest that the inhibitory effect of TMS is a consequence of its sulfhydryl group oxidising activity. The molecular target(s) of this inhibitory oxidation should however be identified. In contrast to DTT, okadaic acid (OA), known to bypass the inhibitory effect of drugs interfering with protein kinase activities, only induces chromatin condensation and GVBD in TMS-pulsed oocytes with a delay of about 8 hr as compared to the control situation. This confirms that a very early thiol oxidation induced by TMS exerts a much more dramatic effect on resumption on meiosis than any pharmacological manipulation of protein kinase activities leading to activation of MPF.

FK506 blocks intracellular Ca2+ oscillations in bovine adrenal glomerulosa cells

The inositol 1,4,5-trisphosphate (InsP(3)) receptor is a ligand-gated Ca(2+) channel playing an important role in the control of intracellular Ca(2+). In the study presented here, we demonstrate that angiotensin (AngII), phorbol ester (PMA), and FK506 significantly increase the level of InsP(3) receptor phosphorylation in intact bovine adrenal glomerulosa cells. With a back-phosphorylation approach, we showed that the InsP(3) receptor is a good substrate for protein kinase C (PKC) and that FK506 increases the level of PKC-mediated InsP(3) receptor phosphorylation. With a microsomal preparation from bovine adrenal cortex, we showed that PKC enhances the release of Ca(2+) induced by a submaximal dose of InsP(3). We also showed that FK506 blocks intracellular Ca(2+) oscillations in isolated adrenal glomerulosa cells by progressively increasing the intracellular Ca(2+) concentration to a high plateau level. This effect is consistent with an inhibitory role of FK506 on calcineurin dephosphorylation of the InsP(3) receptor, thus keeping the receptor in a phosphorylated, high-conductance state. Our results provide further evidence for the crucial role of the InsP(3) receptor in the regulation of intracellular Ca(2+) oscillations and show that FK506, by maintaining the phosphorylated state of the InsP(3) receptor, causes important changes in the Ca(2+) oscillatory process.

Thiol-reactive agents biphasically regulate inositol 1,4,5-trisphosphate binding and Ca(2+) release activities in bovine adrenal cortex microsomes

Within all endocrine cells, the inositol 1,4,5-trisphosphate (InsP(3)) receptor plays an important role in regulation of the intracellular Ca(2+) concentration. In the present study we showed that a single short-term treatment with either N-ethylmaleimide (known to decrease InsP(3) receptor activity) or thimerosal (known to increase InsP(3) receptor activity) caused time-dependent biphasic effects on the InsP(3) binding activity of bovine adrenal cortex microsomes. The early potentiating effect of thiol-reactive agents translated into a 2-fold increase in binding affinity and Ca(2+) release efficiency. The late dampening effect of thiol-reactive agents translated into a continuous reduction of the maximal binding capacity of the microsomes with a concomitant decrease in Ca(2+) release efficiency. Under these conditions, Western blot analyses demonstrated that the level of InsP(3) receptor protein was not modified. Sequential treatments with thimerosal and the reducing agent dithiothreitol showed that the InsP(3) receptor can readily oscillate between high and low affinity states that are related to its alkylation state. Our results suggest a common mode of action of thiol-reactive agents on the InsP(3) receptor. These results also support the contention that cellular mechanisms of thiol group modification could play important roles in regulation of the intracellular Ca(2+) concentration.

Modulation of inositol 1,4,5-trisphosphate binding to the various inositol 1,4,5-trisphosphate receptor isoforms by thimerosal and cyclic ADP-ribose

Three different genes encode the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R), an intracellular Ca2+ channel involved in cellular Ca2+ signaling. The IP3-binding characteristics of the various IP3R isoforms differ, but until now no specific activators or inhibitors of IP3 binding have been described. We compared the effects of oxidizing reagents, in particular thimerosal, and of cyclic ADP-ribose (cADPR) on IP3 binding to the various IP3R isoforms. We therefore expressed the N-terminal 581 amino acids of the three IP(3)R isoforms as recombinant proteins in the soluble fraction of Escherichia coli (ligand-binding sites [lbs] 1, 2, and 3) as well as the full-length IP3R1 and IP3R3 in Spodoptera frugiperda (Sf9) insect cells. Thimerosal (100 microM) stimulated IP3 binding to lbs-1 (1.4-fold) and lbs-3 (2.5-fold), but had no effect on lbs-2. Thimerosal acted on lbs-1 and lbs-3 by decreasing the Kd for IP3 binding (from 46 +/- 4 nM to 20 +/- 2 nM and from 54 +/- 21 nM to 19 +/- 7 nM for lbs-1 and -3, respectively) without modifying the Bmax. Similarly, IP3 binding to microsomes of Sf9 insect cells overexpressing the full-length IP3R1 was 1.2-fold stimulated by thimerosal. Thimerosal, however, did not affect IP3 binding to Sf9-IP3R3 microsomes, suggesting that in situ thimerosal will only directly affect ligand binding to the type 1 isoform. cADPR (50 microM) stimulated IP3 binding to Sf9-IP3R1 microsomes (1.5-fold), but not to Sf9-IP3R3 microsomes. In addition, cADPR inhibited IP3 binding to lbs-1 and lbs-2 by decreasing the affinity for IP3 1.8- and 2.8-fold, respectively, while IP3 binding to lbs-3 was not affected. These results suggest that a regulatory site for cADPR is present in the ligand-binding domain of IP3R1 and 2, but not of IP3R3.

Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation

Fertilization in mammalian eggs is characterized by the presence of intracellular calcium ([Ca(2+)]i) oscillations. In mouse eggs, these oscillations cease after a variable period of time and this is accompanied by a decrease in inositol 1,4,5-trisphosphate receptor (IP3R) responsiveness and down-regulation of the IP3R type 1 (IP3R-1). To investigate the signaling pathway responsible for inducing IP3R-1 down-regulation during fertilization, mouse eggs were exposed to or injected with several Ca(2+)-releasing agonists and the amounts of IP3R-1 immunoreactivity evaluated by Western blotting. Exposure to ethanol or ionomycin, which induce a single [Ca(2+)]i rise, failed to signal down-regulation of IP3R-1. However, [Ca(2+)]i oscillations induced by injection of boar sperm fractions (SF), which presumably stimulate production of IP3, or adenophostin A, an IP3R agonist, both induced down-regulation of IP3R-1 of a magnitude similar to or greater than that observed after fertilization. Exposure to thimerosal, an oxidizing agent that modifies the IP3R without stimulating production of IP3, also initiated down-regulation of IP3R-1, although oscillations initiated by SrCl(2) failed to evoke down-regulation of IP3R-1. The degradation of IP3R-1 in mouse eggs appears to be mediated by the proteasome pathway because it was inhibited by preincubation with lactacystin, a very specific proteasome inhibitor. We therefore suggest that persistent stimulation of the phosphoinositide pathway in mouse eggs by the sperm during fertilization or by injection of SF leads to down-regulation of the IP3R-1.

Synthesis, acid-base behavior, and binding properties of 6-modified myo-inositol 1,4,5-tris(phosphate)s

myo-Inositol 1,4,5-tris(phosphate) was modified at position 6. The analogues synthesized are reported in this publication are 6-deoxy-myo-inositol 1,4,5-tris(phosphate), 6-fluoro-6-deoxy-myo-inositol 1,4,5-tris(phosphate), epi-inositol 1, 4,5-tris(phosphate), and 6-amino-6-deoxy-myo-inositol 1,4, 5-tris(phosphate). These derivatives showed poor affinity for the Ins(1,4,5)P(3) receptors. The inframolecular acid-base behavior and the cooperative effects between the phosphate groups could help explain the loss of affinity of these 6-modified analogues.

Cd2+ and Co2+ at micromolar concentrations mobilize intracellular Ca2+ via the generation of inositol 1,4,5-triphosphate in bovine chromaffin cells

To understand the mechanisms underlying the Cd2+- and Co2+-induced intracellular Ca2+ mobilization, we measured the levels of inositol phosphates using bovine chromaffin cells. Studies using HPLC indicated that Cd2+, Co2+ and methacholine significantly increased the generation of 1,4,5-IP3. The results suggest that Cd2+ and Co2+ mobilize Ca2+ from IP3-sensitive Ca2+ stores, possibly through the presumptive Cd2+ receptor.

The sulphydryl oxidizing reagent diamide affects phosphoinositide-mediated signal transduction: implications for the pathogenesis of Alzheimer's disease

In fura-2-labelled human platelets, the thiol oxidising agent diamide decreases the intracellular calcium response to thrombin and serotonin without affecting the basal calcium levels. The effect of diamide on the thrombin response could be prevented by pre-treatment with dithiothreitol (DTT) and reduced when DTT was added 60 s after diamide. The effects of diamide and hydrogen peroxide on the thrombin response were additive. Hydrogen peroxide also produced a calcium response per se, but this response was not affected by diamide. Hydrogen peroxide increased rat brain phosphoinositide hydrolysis and reduced the response to carbachol and noradrenaline, whereas diamide was without effect. The binding of [3H]inositol-1,4,5-trisphosphate to human platelet membranes was inhibited by diamide but not by hydrogen peroxide. Thus diamide affects the phosphoinositide signal transduction pathway in a qualitatively different manner from that found with hydrogen peroxide. It is suggested that oxidative stress may contribute to the disturbances in the phosphoinositide transduction pathway that are found in Alzheimer's disease.

Redox modulation of calcium entry and release of intracellular calcium by thimerosal in GH4C1 pituitary cells

In the present work we have investigated the actions of the oxidizing sulfhydryl reagent thimerosal on different mechanisms which regulate intracellular free Ca2+ concentration ([Ca2+]i) in GH4C1 pituitary cells. In intact Fura-2 loaded cells, low concentrations of thimerosal potentiated the spike phase of the TRH-induced (thyrotropin-releasing hormone) rise in [Ca2+]i, whereas high thimerosal concentrations inhibited it. The effect of thimerosal on the plateau phase was always inhibitory. The effect of thimerosal on the IP3-induced calcium release (IICR) was studied in permeabilized cells using the Ca2+ indicator Fluo-3. A low concentration of thimerosal (10 microM) stimulated IICR: the Ca2+ release induced by 300 nM inositol-1,4,5-trisphosphate (IP3) was enhanced in cells treated with thimerosal for 1 or 6 min (67 +/- 11 nM and 34 +/- 5 nM, respectively) as compared to control cells (17 +/- 2 nM). On the other hand, a high concentration of thimerosal (100 microM) inhibited IICR: when IP3 (10 microM) was added after a 5 min preincubation with thimerosal, the IP3-induced rise in [Ca2+]i (46 +/- 14 nM) was 57% smaller as compared with that seen in control cells (106 +/- 10 nM). The effect of thimerosal on the voltage-operated Ca2+ channels (VOCCs) was studied by depolarizing intact Fura-2 loaded cells by addition of 20 mM K+ to the cuvette. The depolarization-evoked increase in [Ca2+]i was inhibited in a dose-dependent manner by thimerosal. Direct evidence for an inhibitory effect of thimerosal on VOCCs was obtained by using the whole-cell configuration of the patch-clamp technique: thimerosal (100 microM) potently inhibited the Ba2+ currents through VOCCs. In addition, our results indicated that thimerosal inhibited the caffeine-induced increase in [Ca2+]i, and activated a capacitative Ca2+ entry pathway. The actions of thimerosal were apparently due to its oxidizing activity because the effects were mostly reversed by the thiol-reducing agent dithiothreitol (DTT). We conclude that, in GH4C1 pituitary cells, the mobilization of intracellular calcium and the different Ca2+ entry pathways are sensitive to redox modulation.

Threshold for inositol 1,4,5-trisphosphate action

We developed a unidirectional 45Ca2+ efflux technique in which 60 cumulative doses of inositol 1,4,5-trisphosphate (InsP3), each lasting 6 s, were subsequently added to permeabilized A7r5 cells. This technique allowed an accurate determination of the threshold for InsP3 action, which was around 32 nM InsP3 under control conditions. The InsP3-induced Ca2+ release was characterized by an initial rapid phase, after which the normalized rate progressively decreased. The slowing of the release was associated with a shift of the threshold to higher InsP3 concentrations. Stimulatory concentrations of thimerosal (10 microM) shifted the threshold to 4.5 nM InsP3 and increased both the cooperativity and the maximal normalized rate of Ca2+ release. This low threshold was maintained when the thimerosal concentration was increased to inhibitory levels (100 microM) but then the effects on the cooperativity and on the normalized rate of Ca2+ release disappeared. Oxidized glutathione (5 mM) was much less effective in stimulating the release and did not have an effect on the threshold or on the cooperativity. ATP (5 mM) stimulated the release despite a shift in threshold toward higher InsP3 concentrations. Luminal Ca2+ did not affect the threshold for InsP3 action but stimulated the normalized release at each InsP3 concentration. The inhibitory effect of 10 microM free cytosolic Ca2+ was associated with a shift in threshold to higher InsP3 concentrations and a decreased cooperativity of the release process. We conclude that this novel technique of accurately measuring the threshold for InsP3 action under various experimental conditions has allowed us to refine the analysis of the kinetic parameters involved in the regulation of the InsP3 receptor.

Effect of the aminosteroid, U73122, on Ca2+ uptake and release properties of rat liver microsomes

The putative phospholipase C inhibitor, U73122, transiently increases the cytosolic free Ca2+ concentration in rabbit pancreatic acinar cells by stimulating the release of Ca2+ from intracellular stores [Willems, Van de Put, Engbersen, Bosch, Van Hoof & De Pont (1994) Pflügers Arch. 427, 233-243]. In order to elucidate the exact mechanism of action of U73122 we studied its effects on both Ca(2+)-stimulated Mg(2+)-dependent ATPase activity and Ca(2+)-stimulated ATP-dependent Ca2+ uptake in rat liver microsomes. In addition, we studied its effects on Ca2+ release from steady-state loaded microsomes. The effects of U73122 were compared with those of thimerosal, described in the literature as inhibiting Ca(2+)-ATPases and sensitizing inositol 1,4,5-trisphosphate-operated Ca2+ release channels, and thapsigargin, a specific inhibitor of sarcoplasmic and endoplasmic reticulum Ca(2+)-ATPases. Both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 11 microM) dose-dependently inhibited Ca(2+)-stimulated Mg(2+)-dependent ATPase activity, without significantly affecting Mg(2+)-stimulated ATPase activity. Similarly, both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 14 microM) dose-dependently inhibited ATP-dependent Ca2+ uptake. At concentrations beyond 20 microM, U73122 stimulated Ca2+ release from steady-state loaded microsomes at a rate considerably higher than obtained with a maximally inhibitory concentration of thapsigargin (1 microM). This observation, which was not reached with equally inhibitory concentrations of thimerosal, demonstrates that higher U73122 concentrations cause an additional increase of passive Ca2+ leak. The data presented demonstrate that U73122 stimulates the release of actively stored Ca2+ primarily through inhibition of the internal Ca2+ pump.

Regulation of phosphatidylinositide transduction system in the rat spinal cord during aging

Age-related functional alterations in a variety of neurotransmitter systems result in modulation of interneuronal communications which has some relevance in neurological deficits observed in the aging process. The synergistic interactions between protein kinase and inositol 1,4,5-trisphosphate (insP3)/Ca2+ pathways underlie a variety of cellular responses to external stimuli. To determine whether age-dependent changes occur in the regulation of protein kinase C and inositol 1,4,5-trisphosphate/Ca2+ pathways, insP3 contents as a marker for the release of intracellular calcium, saturation binding analysis of Ins P3 receptor using [3H]inositol 1,4,5-trisphosphate, slot/northern blot analysis of Ins P3 receptor-encoding mRNA transcripts, and the activities of Ca2+/phospholipid-dependent protein kinase C isozymes were investigated in the rat spinal cord. Inositol 1,4,5-trisphosphate content and [3H]inositol 1,4,5-trisphosphate binding site density (Bmax) were quantified in the spinal cords of young (three months old), adult (12 months old) and senescent (25 months old) male Fischer 344 rats. Spinal cord content of inositol 1,4,5-trisphosphate was increased (P < 0.01) in the 25-month old compared to the three- and 12-month old animals. The density of Ins P3 receptor in particulate membranes derived from the 25-month old rats was reduced (P < or = 0.01), but the binding affinity (Kd) was increased (P < or = 0.04) by a factor of 2.2 and 3.2 at 25 months of age when compared with three- and 12-month old animals, respectively. Young and middle-aged animals showed no differences in both inositol 1,4,5-trisphosphate contents and [3H]inositol 1,4,5-trisphosphate binding site density. The quantity of Ins P3 receptor mRNA was significantly increased with age in the order 25 >> 12 > 3 months of age. Total functional cytosolic and membrane-associated PKC activities were decreased (P < or = 0.05) in the 25-month compared to the three- and 12-month old rats in which activity remained unchanged. Total membrane/cytosolic activity ratios were unchanged by the aging process. In all cases, the activities of membrane-associated conventional protein kinase C isozymes (alpha, beta and gamma), determined by immunoprecipitation followed by in situ quantification of protein kinase C activities in the immunoprecipitates, showed age-dependent decline. The activities of protein kinase C-alpha and beta were significantly decreased in age-related manner. However, the activity of the gamma-isozyme was not significantly changed at 12- and 25-months of age, although it was higher (P < or = 0.03) in young rats. Western blot analyses using affinity purified polyclonal antibodies specific for each isozyme indicated a single protein with an apparent molecular mass of approximately 80 x 10(3) molec. weight for all isozymes except for the beta isozyme that also had an appreciable immunoreactive band at approximately 36 x 10(3) molec. weight. Overall, the aging process did not affect the electropheretic mobility of each isozyme. With decreased protein kinase C activity, the present data suggest that the aging process would decrease protein kinase C-induced phosphorylation of membrane proteins including Ins P3 receptor. A significant change in Ins P3 receptor affinity combined with increased levels of Ins P3 receptor mRNA-encoding transcripts in senescent rats suggests not only a modification (possibly by phosphorylation) of Ins P3 receptor protein but also the existence of multiple (spliced) variants of Ins P3 receptor in spinal neurons with increasing age. The present data indicate that the spinal contents of inositol 1,4,5-trisphosphate increased with age, but with decreased efficacy and number of inositol 1,4,5-trisphosphate-activatable Ca2+ channels in the spinal cord of senescent rats. These age-related changes may contribute to the attenuated responsiveness of spinal cord neurons by phosphoinositide-coupled receptors during the aging process.

Pharmacological modulators of the inositol 1,4,5-trisphosphate receptor

Elevation of cytosolic calcium concentrations, induced by many neurotransmitters, plays a crucial role in neuronal function. Some neurotransmitters produce the second messenger InsP3 which activates an intracellular calcium channel (InsP3 receptor) usually located in the endoplasmic reticulum. This article undertakes a comprehensive survey of most pharmacological modulators of the InsP3 receptor so far reported. This review discusses in detail competitive antagonists, non-competitive antagonists and thiol reactive reagents, highlighting their modes of action and in some cases indicating drawbacks in their use.

Bidirectional activity of the endoplasmic reticulum Ca(2+)-ATPase of bovine adrenal cortex

It is generally accepted that the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor play major roles in the complex mechanisms by which agonists increase intracellular Ca2+ concentration. In these mechanisms, the endoplasmic reticulum Ca(2+)-ATPase has been attributed an accessory role of refilling the intracellular Ca2+ store. In the present study, the activity of the microsomal Ca(2+)-ATPase of bovine adrenal cortex was investigated. We show that the Ca(2+)-pumping activity of the Ca(2+)-ATPase is related to the ADP/ATP ratio. Our results also show that a brisk increase of the ADP/ATP ratio upon addition of exogenous ADP triggered a rapid release of Ca2+ from preloaded microsomes. ADP released Ca2+ in a dose-dependent manner with an EC50 of 2.98 +/- 0.78 mM. ADP-induced Ca2+ release was not prevented by heparin, ruling out the participation of the inositol 1,4,5-trisphosphate receptor. ADP-induced Ca2+ release could not be attributed to the mere inhibition of the Ca(2+)-ATPase, since the rate of ADP-induced Ca2+ release was 20 times faster than the rate of Ca2+ release induced by a maximal concentration of thapsigargin (2 microM). ADP-induced Ca2+ release experiments performed in the presence of [32P]PO4 revealed a concomitant production of [32P]ATP. ADP-induced [32P]ATP production was dose-dependent, with an EC50 of 5.50 +/- 0.70 mM. ADP-induced [32P]ATP production was prevented by ionomycin (10 microM) and by high concentrations of extramicrosomal Ca2+. These results demonstrate that the microsomal Ca(2+)-ATPase of adrenal cortex possesses a bidirectional activity that depends on ADP concentrations, the Ca2+ gradient across the microsomal membrane, and probably also ATP concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of mersalyl on inositol trisphosphate receptor binding and ion channel function

A number of thiol-reactive agents induce repetitive Ca2+ spiking in cells by a mechanism thought to involve sensitization of the inositol 1,4,5-trisphosphate receptor (IP3R). To further define the basis of this interaction, we have studied the effect of several thiol-reactive agents on [3H]IP3 binding, IP3-gated channel activity, and conformation of the IP3R in membranes from hepatocytes, cultured WB rat liver epithelial cells, and cerebellum microsomes. At 4 degrees C, the organomercurial thiol-reactive agent mersalyl markedly stimulates (3-4fold) [3H]IP3 binding to permeabilized hepatocytes. The closely related molecule, thimerosal, has only a small stimulatory effect under these conditions, and GSSG or N-ethylmaleimide are without effect. The stimulatory effect of mersalyl was associated with a decrease in Kd of the IP3R with no change in Bmax. Mersalyl was without effect on detergent-solubilized hepatocyte binding sites or on the [3H]IP3 binding activity of cerebellum microsomes. In contrast to thimerosal, which potentiates IP3-mediated Ca2+ release, mersalyl blocked IP3-gated Ca2+ channels. Mersalyl pretreatment of WB membranes altered the pattern of immunoreactive receptor fragments generated upon subsequent cleavage of the receptor with proteinase K. This effect was not reproduced by thimerosal and was also not observed in experiments on cerebellum microsomes. We conclude that the WB cell and brain IP3 receptors are differently regulated by modification of thiol groups. Reaction of the WB cell IP3 receptor with mersalyl alters its conformation and modifies the accessibility of sites on the protein that are cleaved by proteinase K. In the presence of mersalyl, the receptor has high affinity for IP3 but is inactive as a Ca2+ channel. This contrasts with the high affinity receptor/active Ca2+ channel induced by thimerosal, suggesting that even closely related thiol agents may interact at different thiol groups.