Thapsigargin, a tumor promoter, discharges intracellular Ca2+ stores by specific inhibition of the endoplasmic reticulum Ca2(+)-ATPase

Volume-sensitive Ca influx and release from intracellular pools in gastric parietal cells

The effects of osmotically induced changes in cell volume on cytoplasmic free Ca (Cai) were studied in parietal cells from intact rabbit gastric glands using digital image processing of fura-2 fluorescence. In resting unstimulated cells, Cai was unaffected by either cell swelling or shrinking when osmolarity was varied between 200 and 400 mosM (isotonicity 290 mosM). However, when cells were swelled in a 165 mosM solution (55% tonicity), a biphasic Ca increased was observed. On average, Cai increased transiently from 80 to 218 nM before stabilizing at approximately 140 nM. The peak was due to release from intracellular pools because it was present in Ca-free solutions while the sustained elevation was dependent on external Ca. In carbachol-stimulated cells, Ca influx was most sensitive to cell shrinkage. For example, addition of 25 mM sucrose (108% tonicity) caused a 30% decrease in the sustained carbachol-stimulated Cai increase (plateau). In contrast, carbachol-stimulated cells were relatively insensitive to cell swelling, with a 30% decrease in tonicity causing only a 15% increase in the plateau. However, as in the unstimulated cells, extreme (55% tonicity) swelling caused additional increases in Cai levels. The carbachol-dependent effects of changes in cell volume on Cai could be mimicked by treating cells with thapsigargin, an inhibitor of Ca pumps of intracellular membranes that also has been shown to stimulate Ca entry. Thus, although extreme swelling conditions (55% tonicity) could elicit Cai increases in either the presence or absence of agonist, agonist was required to observe Cai decreases due to cell shrinkage.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium Requirement for Ethylene-Dependent Responses

Ethylene, a gaseous plant hormone, plays a role in plant development, defense, and climacteric fruit ripening. Both genetic and biochemical evidence suggest that the response of plants to ethylene is mediated by a specific ethylene receptor. The signal emanating from the receptor-effector complex is then presumably transduced via an unknown cascade pathway. We have used the plant pathogenesis response, exemplified by the induction of the pathogenesis-related gene chitinase, as a paradigm to investigate ethylene-dependent signal transduction in the plant cell. We showed that calcium is necessarily involved in the ethylene-mediated pathogenesis response. Blocking calcium fluxes with chelators inhibited ethylene-dependent induction of chitinase accumulation, but not ethylene independent induction. Artificially increasing cytosolic calcium levels by treatments with the calcium ionophore ionomycin or the calcium pump blocker thapsigargin stimulated chitinase accumulation. Plants grown in calcium-poor soil showed a 10-fold reduction in leaf extractable calcium. Their leaves exhibited a reduced pathogenesis reaction to ethylene and were impaired in another hormone response mediated by calcium, i.e., abscisic acid-controlled closure of guard cells. The addition of calcium to leaves excised from calcium-deficient plants restored their sensitivity to ethylene. Ethylene participates in the control of seedling growth, promoting the so-called "triple response" that results in distinct morphological development, such as hypocotyl hook formation. This effect, similar to the ethylene-promoted pathogenesis response, was found to be calcium dependent. The results indicate that calcium is required for a variety of ethylene-dependent processes.

Calcium Requirement for Ethylene-Dependent Responses

Ethylene, a gaseous plant hormone, plays a role in plant development, defense, and climacteric fruit ripening. Both genetic and biochemical evidence suggest that the response of plants to ethylene is mediated by a specific ethylene receptor. The signal emanating from the receptor-effector complex is then presumably transduced via an unknown cascade pathway. We have used the plant pathogenesis response, exemplified by the induction of the pathogenesis-related gene chitinase, as a paradigm to investigate ethylene-dependent signal transduction in the plant cell. We showed that calcium is necessarily involved in the ethylene-mediated pathogenesis response. Blocking calcium fluxes with chelators inhibited ethylene-dependent induction of chitinase accumulation, but not ethylene independent induction. Artificially increasing cytosolic calcium levels by treatments with the calcium ionophore ionomycin or the calcium pump blocker thapsigargin stimulated chitinase accumulation. Plants grown in calcium-poor soil showed a 10-fold reduction in leaf extractable calcium. Their leaves exhibited a reduced pathogenesis reaction to ethylene and were impaired in another hormone response mediated by calcium, i.e., abscisic acid-controlled closure of guard cells. The addition of calcium to leaves excised from calcium-deficient plants restored their sensitivity to ethylene. Ethylene participates in the control of seedling growth, promoting the so-called "triple response" that results in distinct morphological development, such as hypocotyl hook formation. This effect, similar to the ethylene-promoted pathogenesis response, was found to be calcium dependent. The results indicate that calcium is required for a variety of ethylene-dependent processes.

Brefeldin A, thapsigargin, and AIF4- stimulate the accumulation of GRP78 mRNA in a cycloheximide dependent manner, whilst induction by hypoxia is independent of protein synthesis

The glucose regulated proteins (GRPs) are major structural components of the endoplasmic reticulum (ER) and are involved in the import, folding, and processing of ER proteins. Expression of the glucose regulated proteins (GRP78 and GRP94) is greatly increased after cells are exposed to stress agents (including A23187 and tunicamycin) which inhibit ER function. Here, we demonstrate that three novel inhibitors of ER function, thapsigargin (which inhibits the ER Ca(2+)-ATPase), brefeldin A (an inhibitor of vesicle transport between the ER and Golgi) and AIF4-, (which inhibits trimeric G-proteins), can increase the expression of both GRP78 and 94. The common characteristic shared by activators of GRP expression is that they disrupt some function of the ER. The increased levels of GRPs may be a response to the accumulation of aberrant proteins in the ER or they may be increased in response to structural/functional damage to the ER. The increased accumulation of GRP78 mRNA after exposure of cells to either thapsigargin, brefeldin A, AIF4-, A23187, or tunicamycin can be blocked by pre-incubation in cycloheximide. In contrast, accumulation of GRPs after exposure to hypoxia was independent of cycloheximide. In addition, the protein kinase inhibitor genistein blocked the thapsigargin induced accumulation of GRP78 mRNA, whereas the protein phosphatase inhibitor okadaic acid caused increased accumulation of GRP78 mRNA. The data indicates that there are at least 2 mechanisms for induced expression of GRPs, one of which involves a phosphorylation step and requires new protein synthesis (e.g., thapsigargin, A23187) and one which is independent of both these steps (hypoxia).

The rubella virus E1 glycoprotein is arrested in a novel post-ER, pre-Golgi compartment

Evidence is accumulating that a distinct compartment(s) exists in the secretory pathway interposed between the rough ER (RER) and the Golgi stack. In this study we have defined a novel post-RER, pre-Golgi compartment where unassembled subunits of rubella virus (RV) E1 glycoprotein accumulate. When RV E1 is expressed in CHO cells in the absence of E2 glycoprotein, transport of E1 to the Golgi complex is arrested. The compartment in which E1 accumulates consists of a tubular network of smooth membranes which is in continuity with the RER but has distinctive properties from either the RER, Golgi, or previously characterized intermediate compartments. It lacks RER and Golgi membrane proteins and is not disrupted by agents which disrupt either the RER (thapsigargin, ionomycin) or Golgi (nocodazole and brefeldin A). However, luminal ER proteins bearing the KDEL signal have access to this compartment. Kinetically the site of E1 arrest lies distal to or at the site where palmitylation occurs and proximal to the low temperature 15 degrees C block. Taken together the findings suggest that the site of E1 arrest corresponds to, or is located close to the exit site from the ER. This compartment could be identified morphologically because it is highly amplified in cells overexpressing unassembled E1 subunits, but it may have its counterpart among the transitional elements of non-transfected cells. We conclude that the site of E1 arrest may represent a new compartment or a differentiated proximal moiety of the intermediate compartment.

Stored calcium modulates inositol phosphate synthesis in cultured smooth muscle cells

Cytosolic Ca2+ concentrations ([Ca2+]cyt) and [3H]inositol phosphates ([3H]InsP) were correlated while varying the Ca2+ content of the sarcoplasmic reticulum (SR) in cultured A7r5 cells at rest and during activation with [Arg8]-vasopressin (AVP). Thapsigargin (TG) raised and superfusion with 0 Ca2+ lowered [Ca2+]cyt, but both treatments decreased SR Ca2+ and AVP-evoked Ca2+ transients. Neither TG nor 0 Ca2+ affected basal [3H]InsP, but both treatments increased AVP-evoked synthesis of [3H]InsP. Exposure for several minutes to 40 mM K+ solution, BAY K 8644, or low-Na+ solution all elevated [Ca2+]cyt and, thereby, increased SR Ca2+, as manifested by augmented AVP-evoked Ca2+ transients. In all three cases, AVP-evoked, but not basal, [3H]InsP were reduced. The inhibitory effect of 40 mM K+ on AVP-evoked [3H]InsP synthesis was blocked when SR Ca2+ uptake was prevented by TG. Brief (30-s) exposures to 40 mM K+, which elevated [Ca2+]cyt but not SR Ca2+ loading, did not modify AVP-evoked [3H]InsP synthesis or Ca2+ transients. These results demonstrate an inverse relationship between SR Ca2+ content and evoked [3H]-InsP synthesis. Moreover, they suggest that SR Ca2+ may serve as a signal that modulates sarcolemmal [3H]InsP formation.

Deduced amino acid sequence and E1-E2 equilibrium of the sarcoplasmic reticulum Ca(2+)-ATPase of frog skeletal muscle. Comparison with the Ca(2+)-ATPase of rabbit fast twitch muscle

The cDNA encoding a Ca(2+)-transport ATPase of frog (Rana esculenta) skeletal muscle was isolated and characterized. The deduced amino acid sequence, consisting of 994 residues, showed 89% identity to the fast twitch muscle sarcoplasmic reticulum Ca(2+)-ATPases of chicken and rabbit. Northern blot analysis using a fragment of this cDNA as probe detected a 5.0 kb message in frog skeletal muscle but did not detect any mRNA encoding sarcoplasmic reticulum Ca(2+)-ATPase in frog cardiac muscle. The enzymatic properties of the amphibian skeletal muscle Ca(2+)-ATPase were compared with those of the rabbit fast twitch muscle Ca(2+)-ATPase by functional expression of the cDNAs in COS-1 cells. The amphibian Ca(2+)-ATPase displayed a reduced apparent affinity for Ca2+ and an increased apparent affinity for the inhibitors, vanadate and thapsigargin, relative to the mammalian enzyme. This may be explained by a mechanism in which relatively more of the E2 conformation accumulated in the frog Ca(2+)-ATPase than in the mammalian enzyme.

Cyclic nucleotides and intracellular-calcium homeostasis in human platelets

The relationship between agonist-sensitive calcium compartments and those discharged by the Ca(2+)-ATPase inhibitor thapsigargin were studied in human platelets. In this context, calcium mobilization from intracellular pools and manganese influx was investigated in relation to the effect of altered cyclic-nucleotide levels. For maximal calcium release from intracellular stores, thapsigargin, compared to a receptor agonist like thrombin, requires the platelet's self-amplification mechanism, known to generate thromboxane A2. With this lipid mediator formed, thapsigargin released calcium and stimulated manganese influx in a manner similar to thrombin. Blocking the thromboxane receptor by addition of sulotroban (BM13.177) or, alternatively, increasing platelet cAMP or cGMP using prostacyclin or sodium nitroprusside, dramatically reduced the ability of thapsigargin to release calcium from intracellular compartments. The same experimental conditions significantly reduced the rate of manganese influx initiated by thapsigargin compared to thrombin. The experiments indicate that thapsigargin-sensitive compartments play only a minor role in inducing manganese influx compared to the receptor-sensitive compartment. Cyclic nucleotides accelerate the redistribution of an agonist-elevated platelet calcium into the thapsigargin-sensitive compartment, from which calcium can be released by inhibition of the Ca(2+)-ATPase. In human platelets, thapsigargin-induced calcium increase and influx were responsible for only part the calcium release resulting from inhibition of the corresponding ATPase; another part results from the indirect effect of thapsigargin acting via thromboxane-A2-receptor activation. Cyclic nucleotides are therefore an interesting regulatory device which can modify the thapsigargin response by not allowing the self-amplification mechanism of platelets to operate.

Intercellular calcium signaling via gap junctions in glioma cells

Calcium signaling in C6 glioma cells in culture was examined with digital fluorescence video microscopy. C6 cells express low levels of the gap junction protein connexin43 and have correspondingly weak gap junctional communication as evidenced by dye coupling (Naus, C. C. G., J. F. Bechberger, S. Caveney, and J. X. Wilson. 1991. Neurosci. Lett. 126:33-36). Transfection of C6 cells with the cDNA encoding connexin43 resulted in clones with increased expression of connexin43 mRNA and protein and increased dye coupling, as well as markedly reduced rates of proliferation (Zhu, D., S. Caveney, G. M. Kidder, and C. C. Naus. 1991. Proc. Natl. Acad. Sci. USA. 88:1883-1887; Naus, C. C. G., D. Zhu, S. Todd, and G. M. Kidder. 1992. Cell Mol. Neurobiol. 12:163-175). Mechanical stimulation of a single cell in a culture of non-transfected C6 cells induced a wave of increased intracellular calcium concentration ([Ca2+]i) that showed little or no communication to adjacent cells. By contrast, mechanical stimulation of a single cell in cultures of C6 clones expressing transfected connexin43 cDNA induced a Ca2+ wave that was communicated to multiple surrounding cells, and the extent of communication was proportional to the level of expression of the connexin43 cDNA. These results provide direct evidence that intercellular Ca2+ signaling occurs via gap junctions. Ca2+ signaling through gap junctions may provide a means for the coordinated regulation of cellular function, including cell growth and differentiation.

Inositol 1,4,5-trisphosphate receptor immunoreactivity in SH-SY5Y human neuroblastoma cells is reduced by chronic muscarinic receptor activation

Inositol 1,4,5-trisphosphate (InsP3) receptor immunoreactivity in SH-SY5Y human neuroblastoma cells was monitored with a monoclonal antibody raised against the mouse cerebellar InsP3 receptor. Recognition of a protein corresponding to the InsP3 receptor (molecular mass, approximately 275 kDa) was inhibited markedly following exposure of cells to 0.1 mM carbachol. This effect was half-maximal and maximal at approximately 2 and approximately 6 h, respectively; was blocked by atropine; but was not mimicked by thapsigargin, K+, or phorbol 12-myristate 13-acetate. However, the decrease in immunoreactivity following exposure of cells to carbachol for 5 h was blocked if the extracellular Ca2+ concentration was reduced from 1.3 mM to 200 nM. This manipulation also reduced markedly carbachol-induced increases in InsP3 concentration at 5 h. These data indicate that chronic muscarinic stimulation of phosphoinositide hydrolysis reduces InsP3 receptor concentration in SH-SY5Y cells, perhaps via a mechanism that involves prolonged elevation of InsP3 levels.

Identification of a novel enhancer element mediating calcium-dependent induction of gene expression in response to either epidermal growth factor or activation of protein kinase C

The VL30 family of defective murine retroviruses consists of 100 to 200 members, of which fewer than 5% appear to be transcriptionally active. A genomic clone of the transcriptionally active VL30 element RVL-3 was identified and sequenced. Genetic analysis indicated that a triple-repeat sequence within the RVL-3 long terminal repeat is capable of functioning as an inducible enhancer element responding to a variety of agonists. In Rat-1 fibroblasts, the ability of the RVL-3 enhancer to mediate induction of gene expression from a heterologous promoter in response to either epidermal growth factor (EGF) or phorbol ester treatment required coelevation of intracellular calcium. Two CArG boxes present in the triple-repeat sequence appeared to exert a negative effect on gene expression, as mutation of these sequences elevated the basal level of expression observed without altering the fold induction in response to either EGF or protein kinase C activation. In the presence of these CArG elements, mutation of AP-1-like sites adjacent to the CArG elements significantly inhibited the ability of either EGF or phorbol esters to induce gene expression. The effect of mutating these AP-1-like sites was relieved by simultaneous mutation of the CArG sites, indicating that interactions among these sites modulate RVL-3 expression. Mutational analysis and gel mobility shift experiments have identified a third sequence within the VL30 triple-repeat element that is required for the induction of gene expression and serves as a binding site for nuclear proteins. Sequence comparisons indicate that this enhancer element has not been described previously.

Depletion of the inositol 1,4,5-trisphosphate-sensitive intracellular Ca2+ store in vascular endothelial cells activates the agonist-sensitive Ca(2+)-influx pathway

Previous studies in non-excitable cells have suggested that depletion of internal Ca2+ stores activates Ca2+ influx from the extracellular space via a mechanism that does not require stimulation of phosphoinositide hydrolysis. To test this hypothesis in vascular endothelial cells, the effect of the Ca(2+)-ATPase/pump inhibitor 2,5-di-t-butylhydroquinone (BHQ) on cytosolic free Ca2+ concentration ([Ca2+]i) was examined. BHQ produced a dose-dependent increase in [Ca2+]i, which remained elevated over basal values for several minutes and was substantially inhibited in the absence of extracellular Ca2+. Application of bradykinin after BHQ demonstrated that the BHQ-sensitive compartment partially overlapped the bradykinin-sensitive store. Similar results were obtained with thapsigargin and cyclopiazonic acid, two other Ca(2+)-ATPase inhibitors. Although BHQ had no effect on phosphoinositide hydrolysis, both 45Ca2+ influx and efflux were stimulated by this agent. These results suggest that depletion of the agonist-sensitive Ca2+ store is sufficient for activation of Ca2+ influx. Several characteristics of the Ca(2+)-influx pathway activated by internal store depletion were compared with those of the agonist-activated pathway. Bradykinin-stimulated Ca2+ influx was increased at alkaline extracellular pH (pHo), and was inhibited by extracellular La3+, by depolarization of the membrane, and by the novel Ca(2+)-influx blocker 1-(beta-[3-(4-methoxyphenyl)propoxy]-4- methoxyphenethyl)-1H-imidazole hydrochloride (SKF 96365). Additionally, bradykinin stimulated influx of both 45Ca2+ and 133Ba2+, consistent with the hypothesis that the agonist-activated influx pathway is permeable to both of these bivalent cations. Likewise, activation of Ca2+ influx by BHQ, thapsigargin and cyclopiazonic acid was blocked by La3+, membrane depolarization and SKF 96365, but was unaffected by nitrendipine or BAY K 8644. Furthermore, Ca2+ influx stimulated by BHQ was increased at alkaline pHo and BHQ stimulated the influx of both 45Ca2+ and 133Ba2+ to the same extent. These results demonstrate that the agonist-activated Ca(2+)-influx pathway and the pathway activated by depletion of the agonist-sensitive internal Ca2+ store are indistinguishable.

The Ca(2+)-transport ATPases from the plasma membrane

The initial studies on the plasma membrane (PM) Ca(2+)-transport ATPases were made in the erythrocyte, a structure that can not be taken as representing a typical eukaryotic cell. In other cell types however, the study of the PM Ca(2+)-transport ATPase is complicated by the simultaneous expression of related Ca(2+)-pumps in intracellular stores. Whereas there are as yet no known specific inhibitors for the PM Ca(2+)-transport ATPase, a number of selective inhibitors for the endo(sarco)plasmic reticulum Ca2+ pumps have been described: thapsigargin, cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone. With the recent introduction of the molecular biological approach, it became quickly obvious that a family of at least 5 different PM Ca(2+)-transport ATPase genes govern the tissue-dependent expression of PM Ca2+ pumps. Moreover alternative splicing of the primary gene transcripts was found to further enhance the number of pump variants. The PM Ca(2+)-transport ATPase are subject to modulatory control by calmodulin, by acidic phospholipids, and by the known families of protein kinases. Each of the ensuing effects are mutually related and interdependent. The wide variety PM Ca2+ pump isoforms and their regulation by such an intricate modulatory network allows the distinct tissues to adapt most adequately to the prevailing tissue and stimulus specific requirements.

The yeast Ca(2+)-ATPase homologue, PMR1, is required for normal Golgi function and localizes in a novel Golgi-like distribution

PMR1, a Ca(2+)-adenosine triphosphatase (ATPase) homologue in the yeast Saccharomyces cerevisiae localizes to a novel Golgi-like organelle. Consistent with a Golgi localization, the bulk of PMR1 comigrates with Golgi markers in subcellular fractionation experiments, and staining of PMR1 by indirect immunofluorescence reveals a punctate pattern resembling Golgi staining in yeast. However, PMR1 shows only partial colocalization with known Golgi markers, KEX2 and SEC7, in double-label immunofluorescence experiments. The effect of PMR1 on Golgi function is indicated by pleiotropic defects in various Golgi processes in pmr1 mutants, including impaired proteolytic processing of pro-alpha factor and incomplete outer chain glycosylation of invertase. Consistent with the proposed role of PMR1 as a Ca2+ pump, these defects are reversed by the addition of millimolar levels of extracellular Ca2+, suggesting that Ca2+ disposition is essential to normal Golgi function. Absence of PMR1 function partially suppresses the temperature-sensitive growth defects of several sec mutants, and overexpression of PMR1 restricts the growth of others. Some of these interactions are modulated by changes in external Ca2+ concentrations. These results imply a global role for Ca2+ in the proper function of components governing transit and processing through the secretory pathway.

Identification of a novel enhancer element mediating calcium-dependent induction of gene expression in response to either epidermal growth factor or activation of protein kinase C

The VL30 family of defective murine retroviruses consists of 100 to 200 members, of which fewer than 5% appear to be transcriptionally active. A genomic clone of the transcriptionally active VL30 element RVL-3 was identified and sequenced. Genetic analysis indicated that a triple-repeat sequence within the RVL-3 long terminal repeat is capable of functioning as an inducible enhancer element responding to a variety of agonists. In Rat-1 fibroblasts, the ability of the RVL-3 enhancer to mediate induction of gene expression from a heterologous promoter in response to either epidermal growth factor (EGF) or phorbol ester treatment required coelevation of intracellular calcium. Two CArG boxes present in the triple-repeat sequence appeared to exert a negative effect on gene expression, as mutation of these sequences elevated the basal level of expression observed without altering the fold induction in response to either EGF or protein kinase C activation. In the presence of these CArG elements, mutation of AP-1-like sites adjacent to the CArG elements significantly inhibited the ability of either EGF or phorbol esters to induce gene expression. The effect of mutating these AP-1-like sites was relieved by simultaneous mutation of the CArG sites, indicating that interactions among these sites modulate RVL-3 expression. Mutational analysis and gel mobility shift experiments have identified a third sequence within the VL30 triple-repeat element that is required for the induction of gene expression and serves as a binding site for nuclear proteins. Sequence comparisons indicate that this enhancer element has not been described previously.

Regulation of transin/stromelysin and VL30 gene expression by intracellular calcium

Changes in intracellular Ca++ levels are observed as a second messenger in response to a number of cellular agonists, including epidermal growth factor, transforming growth factor beta 1, and endothelin-1. The role of elevated intracellular Ca++ in transducing the effects of these three agonists on gene expression has been studied using two target genes: transin/stromelysin-1 and the endogenous murine retrovirus VL30. Although the effects of EGF and TGF beta 1 on transin/stromelysin-1 mRNA expression appear to be independent of these agonists' effects on intracellular Ca++ levels, elevated Ca++ interacted synergistically with activators of pkC to induce transin expression, even though neither agent alone could induce transin/stromelysin-1 expression. In contrast, the integrated VL30 retrovirus could be induced by Ca++ ionophores alone, and induction of VL30 mRNA by other agonists was blocked if intracellular Ca++ levels were held below a threshold value of 165 nM with Ca++ chelators. Genetic analysis of the VL30 upstream regulatory region indicated that a triple-repeat element present in the VL30 long-terminal repeat could function as an inducible enhancer, but responsiveness to either EGF or pkC activation required the concomitant elevation of intracellular Ca++. Because EGF was capable of inducing expression even in pkC-depleted cells, providing Ca++ levels were elevated, these results indicate that elevated intracellular Ca++ is capable of interacting synergistically with multiple signaling pathways to stimulate increased gene expression.

Oxidized glutathione causes sensitization of calcium release to inositol 1,4,5-trisphosphate in permeabilized hepatocytes

The effects of GSSG on Ca2+ mobilization by Ins(1,4,5)P3 were studied in permeabilized rat hepatocytes. Incubation with GSSG (2 mM) increased the sensitivity to Ins(1,4,5)P3 for Ca2+ release, with no effect on the size of the Ca2+ pool that could be released with maximal concentrations of Ins(1,4,5)P3. GSSG decreased the EC50 for Ins(1,4,5)P3 from a control value of 578 +/- 23 nM to 137 +/- 21 nM. GSSG had no effect on the metabolism of Ins(1,4,5)P3 in permeabilized cells, and sensitization of Ca2+ release was still observed when the poorly metabolizable analogue inositol 1,4,5-trisphosphorothioate was used. GSSG did not affect the ATP-dependent Ca2+ pump or the extent of loading of intracellular Ca2+ pools. In addition, the enhancement of Ins(1,4,5)P3-sensitivity by GSSG occurred under conditions where the Ca2+ pumps were blocked with thapsigargin or by chelation of medium Ca2+ just before Ins(1,4,5)P3 addition. The effect of GSSG was time- and dose-dependent, maximal effects being observed after 5 min incubation with 2 mM-GSSG. Cystine mimicked the GSSG-induced increase in Ins(1,4,5)P3-sensitivity, and the effects could be reversed by dithiothreitol (DTT). DTT, GSH glutathione and cysteine had no effect when added alone. Other agents known to react with protein thiols, including N-ethylmaleimide, p-chloromercuribenzoic acid and Ag+, did not affect the sensitivity to Ins(1,4,5)P3, but were inhibitors of ATP-dependent Ca2+ uptake. The data suggest that the sensitivity of the intracellular Ca2+ pools to release by Ins(1,4,5)P3 can be modulated by the formation of mixed disulphides with GSSG or other oxidized thiols.

Evidence for two pathways of receptor-mediated Ca2+ entry in hepatocytes

Receptor-mediated Ca2+ entry was studied in fura-2-loaded isolated hepatocytes. Emptying of internal Ca2+ stores by treatment with either the Ca(2+)-mobilizing hormone vasopressin or the inhibitors of the microsomal Ca2+ pump, 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ) or thapsigargin, stimulated Ca2+ entry, as indicated by a rise in the cytosolic free Ca2+ concentration after Ca2+ was added to cells suspended in nominally Ca(2+)-free medium. The enhancement of Ca2+ entry was proportional to the degree of depletion of the intracellular Ca2+ pool and occurred also after removal of vasopressin from its receptor. In contrast, the stimulation of Mn2+ entry by vasopressin required the continuous presence of the agonist, since it was prevented by the addition of vasopressin receptor antagonist. This effect was observed under conditions where refilling of the agonist-sensitive pool was prevented by using nominally Ca(2+)-free medium. Unlike vasopressin, tBuBHQ or thapsigargin did not stimulate Mn2+ entry. These results suggest the existence of two pathways for receptor-mediated Ca2+ entry in hepatocytes, a 'capacitative' pathway that is sensitive to the Ca2+ content in the Ins(1,4,5)P3-sensitive Ca2+ pool and does not allow Mn2+ entry, and a second pathway that depends on receptor occupation, seems to require a second messenger for activation, and permits influx of Mn2+.

Modulation of evoked contractions in rat arteries by ryanodine, thapsigargin, and cyclopiazonic acid

The contribution of sarcoplasmic reticulum (SR) Ca2+ release to evoked tension in rat arterial rings was studied by comparing the effects of ryanodine (an SR Ca2+ channel opener) and thapsigargin and cyclopiazonic acid (CPA) (two Ca(2+)-ATPase inhibitors). Isometric tension was evoked by serotonin (5-HT), 30-50 mM external K+, and 10 mM caffeine in rings of aorta and a small (second-order) branch of the superior mesenteric artery (SMA). Resting tension was unaffected by 10 microM ryanodine or 1-5 microM thapsigargin, but 20 microM CPA raised resting tension in aortic rings and evoked spontaneous contractions in some SMA rings. Ryanodine (10 microM) or 1-5 microM thapsigargin partially depleted the SR Ca2+ stores (indicated by reduced caffeine-evoked contractions) and attenuated 5-HT- and high K(+)-evoked contractions in aortic rings but augmented 5-HT- and high K(+)-evoked contractions in SMA. Caffeine completely emptied the SR Ca2+ stores in the presence of ryanodine but not thapsigargin in both the aorta and SMA; thus, thapsigargin may selectively affect one component of a heterogeneous SR. When the aortic Ca2+ stores were empty (i.e., caffeine contractions were abolished), the 5-HT- and high K(+)-evoked contractions in the aorta were also augmented. CPA rapidly emptied the SR Ca2+ stores in both the aorta and SMA. CPA augmented the 5-HT-evoked contractions in the SMA and in five of nine aortic rings but attenuated evoked contractions in the remaining aortic rings. The attenuation or abolition of the caffeine contractions implies that ryanodine, thapsigargin, and CPA all deplete the SR Ca2+ stores. The attenuated responses to 5-HT and high K+ observed when the aortic SR Ca2+ stores were only partially depleted are consistent with the idea that evoked SR Ca2+ release is a large component of the Ca2+ transient in the aorta. The augmentation of 5-HT- and high K(+P)-evoked responses after partial (SMA) or complete (aorta) depletion of the SR Ca2+ stores suggests that evoked release of SR Ca2+ normally regulates Ca2+ entry by negative feedback and/or that the SR normally buffers the evoked rise in cytosolic Ca2+.

Thapsigargin stimulates Ca2+ entry in vascular smooth muscle cells: nicardipine-sensitive and -insensitive pathways

We have investigated the role of the sarcoplasmic reticulum Ca2+ pool in regulating Ca2+ entry in vascular smooth muscle cells using a receptor-independent means of mobilizing the intracellular Ca2+ pool. Thapsigargin (TG) has been shown to inhibit the endoplasmic reticulum Ca(2+)-ATPase, mobilize intracellular Ca2+, and activate Ca2+ entry in nonmuscle tissues. When smooth muscle cells were treated with 0.2 microM TG, cytosolic Ca2+ concentrations rose gradually over 8 min to a peak value of 365 +/- 18 nM. Cytosolic Ca2+ remained elevated for at least 20 min and was supported by continued entry of extracellular Ca2+. TG also stimulated entry of Mn2+ and 45Ca2+ from outside the cell. Importantly, TG-induced Ca2+ entry and Mn2+ entry were found to occur through mechanisms that were independent of L-type Ca2+ channel activation because influx was not inhibited by concentrations of nicardipine that were found to block either endothelin- or 100 mM extracellular K(+)-induced cation influx. The mechanism through which TG activates cation entry appears to involve mobilization of the inositol 1,4,5-trisphosphate-responsive intracellular Ca2+ pool. In permeabilized cells, TG prevented ATP-stimulated Ca2+ uptake into the sarcoplasmic reticulum and slowly released sequestered Ca2+. The Ca2+ pool involved was responsive to inositol 1,4,5-trisphosphate. However, TG did not initiate the formation of inositol polyphosphates. Thus TG mobilizes the sarcoplasmic reticulum Ca2+ pool and activates Ca2+ entry through a nicardipine-insensitive Ca2+ channel in vascular smooth muscle. The mechanism is independent of inositol polyphosphate formation.

Toxicodynamics of tumour promoters of mouse skin. III. Specific binding of the tumour promoter thapsigargin as measured by the cold-acetone filter assay

A method is described for measuring rapid, specific, and saturable binding of the skin irritant and tumour-promoting secretagogue thapsigargin (sesquiterpene lactone) to the microsomal fraction from mouse brain. Employing the tritium-labelled compound its apparent dissociation constant, Kd, and the maximal amount of binding Bmax are shown to be 9.8 nM and 1.9 pmol/mg protein respectively. Such a Kd for thapsigargin is similar to (a) its IC50 value for inhibiting Ca2+ uptake in the microsomal fraction from rat brain and (b) its EC50 values for inducing a rise in the cytoplasmic Ca2+ concentration of human platelets and histamine release from rat peritoneal mast cells. A positive correlation is found between the binding affinities of thapsigargin, thapsitranstagin, and trilobolide, their potencies as secretagogues and their lipophilicities. This correlation does not extend to the skin-irritant activities of the compounds thus emphasizing that their mechanism of action is unlike that of 12-O-tetradecanoylphorbol 13-acetate.

Stimulation of leukotriene production and membrane translocation of 5-lipoxygenase by cross-linking of the IgE receptors in RBL-2H3 cells

Recent studies in rat basophilic leukemia cells (RBL-2H3) have shown that two pharmacological agents, ionomycin and thapsigargin, induce leukotriene C4 production and translocation of 5-lipoxygenase from cytosol to membrane, primarily by causing an influx of extracellular calcium. In the present study, we investigate the induction of these events by receptor activation. Cross-linking of high-affinity IgE receptors (Fc epsilon RI) by antigen in RBL-2H3 cells leads to leukotriene C4 production and membrane translocation of 5-lipoxygenase. As in the ionomycin-stimulated cells, leukotriene C4 production in antigen-stimulated cells is calcium-dependent since the amount of leukotriene C4 produced correlates quantitatively with the increase in intracellular free calcium concentration ([Ca2+]i). However, the increase in [Ca2+]i required for equivalent leukotriene C4 production by antigen is not as high as it is using ionomycin. In addition, no threshold [Ca2+]i level is required for leukotriene production by antigen, which is in contrast to the ionomycin stimulation that a [Ca2+]i level of 300-400 nM is required. Furthermore, antigen causes an additive increase in leukotriene C4 production in cells stimulated by the ionomycin. These results suggest that another as yet unidentified intracellular pathway acts in conjunction with Ca2+ for leukotriene synthesis in antigen-stimulated cells. Antigen stimulation causes 20-30% of the total cell 5-lipoxygenase to associate with membranes (compared with 10% in unstimulated cells) as demonstrated by enzyme activity assay and by Western Blot using antibodies to 5-lipoxygenase.(ABSTRACT TRUNCATED AT 250 WORDS)

ERK1 and ERK2, two microtubule-associated protein 2 kinases, mediate the phosphorylation of tyrosine hydroxylase at serine-31 in situ

Tyrosine hydroxylase (TH) is phosphorylated at four sites in situ and in vivo, and the protein kinases that phosphorylate three of these sites (Ser8,Ser19,Ser40) have been identified. In intact cells, the phosphorylation of the fourth site (Ser31) is increased in response to phorbol esters or nerve growth factor (NGF). Here, we show that Ser31 is phosphorylated by ERK1 and ERK2, two myelin basic protein and microtubule-associated protein kinases. Extracts of NGF- or bradykinin-treated PC12 rat pheochromocytoma cells were fractionated on Mono Q columns. Protein kinase activity toward Ser31 in TH was present in two peaks corresponding to myelin basic protein kinase activities previously identified as ERK1 and ERK2. Phosphorylation of purified TH in vitro by both kinases was selective for Ser31 up to at least 0.6 mol of phosphate per mol of TH subunit. Treatment of intact PC12 cells with bradykinin or NGF increased both the phosphorylation of TH-Ser31 in situ and the catalytic activity of ERKs (measured subsequently in vitro with myelin basic protein as substrate). Pretreatment of the cells with genistein (a protein-tyrosine kinase inhibitor) decreased the bradykinin- but not the NGF-induced changes in both TH-Ser31 phosphorylation and ERK activity. Genistein also inhibited the increases in Ser31 phosphorylation produced by phorbol dibutyrate, muscarine, and Ba2+. The data indicate that ERK activity is responsible for phosphorylating TH at Ser31 in intact cells and suggest that TH-Ser31 phosphorylation may be regulated by multiple signaling pathways that converge at or prior to the activation of the ERKs.

NPY and carbachol raise Ca2+ in SK-N-MC cells by three different mechanisms. Evidence for inositol phosphate-independent Ca2+ mobilization by NPY

We have compared the mechanism of NPY- and carbachol-stimulated Ca2+ increases in SK-N-MC cells. NPY stimulated Ca2+ mobilization via a pertussis toxin-sensitive mechanism. Carbachol stimulated Ca2+ mobilization and influx via pertussis toxin-insensitive and -sensitive mechanisms, respectively. Carbachol but not NPY stimulated inositol phosphate accumulation by a pertussis toxin-insensitive mechanism. We conclude that carbachol promotes Ca2+ influx via a pertussis toxin-sensitive G protein and Ca2+ mobilization via a pertussis toxin-insensitive G-protein coupling to inositol phosphate generation; NPY stimulates Ca2+ mobilization via a pertussis toxin-sensitive G protein without apparent involvement of inositol phosphates.

Ins(1,3,4,5)P4 promotes sustained activation of the Ca(2+(-dependent Cl- current in isolated mouse lacrimal cells

Infusion of 50 microM-Ins(1,3,4,5)P4 in addition to 500 microM-Ins(1,4,5)P3 into mouse lacrimal cells via a patch-clamp pipette promoted sustained activation of the Ca(2+)-dependent Cl- current, which could not be achieved with 500 microM-Ins(1,4,5)P3 alone. It has been proposed that Ins(1,3,4,5)P4 facilitates Ca2+ influx in the presence of Ins(1,4,5)P3 [Morris, Gallacher, Irvine & Petersen (1987) Nature (London) 330, 653-655], but a subsequent study in mouse lacrimal cells [Bird, Rossier, Hughes, Shears, Armstrong & Putney (1991) Nature (London) 352, 162-165] showed that a high concentration of Ins(1,4,5)P3 could mobilize both intra- and extra-cellular Ca2+ in the absence of Ins(1,3,4,5)P4. My data confirm these findings, but also show that Ins(1,3,4,5)P4 can stimulate additional Ca2+ influx even when the Ins(1,4,5)P3-dependent intracellular Ca2+ pools have been depleted.

Two roles of Ca2+ in agonist stimulated Ca2+ oscillations

We propose a mechanism for agonist-stimulated Ca2+ oscillations that involves two roles for cytosolic Ca2+: (a) inhibition of inositol-1,4,5-trisphosphate (IP3) stimulated Ca2+ release from the endoplasmic reticulum (ER) and (b) stimulation of the production of IP3 through its action on phospholipase C (PLC), via a Gq protein related mechanism. Relying on quantitative experiments by Parker, I., and I. Ivorra (1990. Proc. Natl. Acad. Sci. USA. 87:260-264) on the inhibition of Ca2+ release from the ER using caged-IP3, we develop a kinetic model of inhibition that allows us to simulate closely their experiments. The model assumes that the ER IP3 receptor is a tetramer of independent subunits that can bind both Ca2+ and IP3. Upon incorporation of the action of Ca2+ on PLC that leads to production of IP3, we observe in-phase-oscillations of Ca2+ and IP3 at intermediate values of agonist stimulation. The oscillations occur on a time scale of 10-20 s, which is comparable to the time scale for inhibition in Xenopus oocytes. Analysis of the mechanism shows that Ca(2+)-inhibition of IP3-stimulated Ca2+ release from the ER is an essential step in the mechanism. We also find that the effect of Ca2+ on PLC can lead to an indirect increase of cytosolic Ca2+, superficially resembling "Ca(2+)-induced Ca(2+)-release." The mechanism that we propose appears to be consistent with recent experiments on REF52 cells by Harootunian, A. T., J. P. Y. Kao, S. Paranjape, and R. Y. Tsien. (1991. Science [Wash. DC]. 251:75-78.) and we propose additional experiments to help test its underlying assumptions.

Comparison of the effects of thapsigargin and BAY K 8644 on spontaneous mechanical activity in rat portal vein and contractile responses of rat cardiac muscle

The effect of thapsigargin, 10(-9)-10(-6) M, and Bay K 8644, 10(-9)-10(-7) M, was studied on isolated portal veins and cardiac muscles from rats. In rat portal veins thapsigargin induced a concentration dependent increase in the amplitude of the spontaneous mechanical activity without increasing the frequency of spontaneous activity. Thapsigargin was less effective than Bay K 8644 in increasing the amplitude of the mechanical activity. In contrast to thapsigargin Bay K 8644, 10(-6) M increases the frequency of the mechanical activity. Atropine, 10(-6) M, and phentolamine, 10(-6) M, had no effect on the thapsigargin and Bay K 8644 induced increase in mechanical activity. Nitrendipine, 10(-6) M, totally abolished the mechanical response in preparations stimulated by thapsigargin and Bay K 8644. In rat atrial and papillary muscles Bay K 8644 increases the frequency in right atrium and tension in both atrial and papillary muscles. Thapsigargin was without effect on the frequency and tension in the cardial preparations. In conclusion, thapsigargin increases the amplitude of spontaneous activity in rat portal veins. In contrast to Bay K 8644 thapsigargin was less effective in increasing the amplitude and had no effect on the frequency of spontaneous activity; furthermore, thapsigargin was without effect on cardiac muscles. The results support the view that an endoplasmatic Ca2(+)-pump sensitive to thapsigargin is of importance for spontaneous activity in portal veins while such pump is of minor importance for contractile activity in cardiac muscles.

Ca(2+)-induced Ca2+ release in insulin-secreting cells

The sulphydryl reagent thimerosal (50 microM) released Ca2+ from a non-mitochondrial intracellular Ca2+ pool in a dose-dependent manner in permeabilized insulin-secreting RINm5F cells. This release was reversed after addition of the reducing agent dithiothreitol. Ca2+ was released from an Ins(1,4,5)P3-insensitive pool, since release was observed even after depletion of the Ins(1,4,5)P3-sensitive pool by a supramaximal dose of Ins(2,4,5)P3 or thapsigargin. The Ins(1,4,5)P3-sensitive pool remained essentially unaltered by thimerosal. Thimerosal-induced Ca2+ release was potentiated by caffeine. These findings suggest the existence of Ca(2+)-induced Ca2+ release also in insulin-secreting cells.

Depletion of intracellular calcium stores activates a calcium current in mast cells

In many cell types, receptor-mediated Ca2+ release from internal stores is followed by Ca2+ influx across the plasma membrane. The sustained entry of Ca2+ is thought to result partly from the depletion of intracellular Ca2+ pools. Most investigations have characterized Ca2+ influx indirectly by measuring Ca(2+)-activated currents or using Fura-2 quenching by Mn2+, which in some cells enters the cells by the same influx pathway. But only a few studies have investigated this Ca2+ entry pathway more directly. We have combined patch-clamp and Fura-2 measurements to monitor membrane currents in mast cells under conditions where intracellular Ca2+ stores were emptied by either inositol 1,4,5-trisphosphate, ionomycin, or excess of the Ca2+ chelator EGTA. The depletion of Ca2+ pools by these independent mechanisms commonly induced activation of a sustained calcium inward current that was highly selective for Ca2+ ions over Ba2+, Sr2+ and Mn2+. This Ca2+ current, which we term ICRAC (calcium release-activated calcium), is not voltage-activated and shows a characteristic inward rectification. It may be the mechanism by which electrically nonexcitable cells maintain raised intracellular Ca2+ concentrations and replenish their empty Ca2+ stores after receptor stimulation.

Okadaic acid inhibits angiotensin II stimulation of Ins(1,4,5)P3 and calcium signalling in rat hepatocytes

OKA2 and CL-A significantly inhibit the ability of angiotensin II, ATP and vasopressin to raise [Ca2+]i in rat hepatocytes, with a partial inhibition of the initial spike, and a complete inhibition of the following plateau. In contrast, the [Ca2+]i response to thapsigargin, which releases intracellular calcium stores through a mechanism independent of inositol phosphates, is much less affected. The ability of angiotensin II to stimulate Ins(1,4,5)P3 production is also reduced by OKA, with kinetics consistent with the inhibited [Ca2+]i response. Since OKA and CL-A are potent and selective inhibitors of phosphoprotein phosphatases, these results provide further evidence that agonist-stimulated Ins(1,4,5)P3 signalling can be inhibited by protein phosphorylation.

Quantal calcium release and calcium entry in the pancreatic acinar cell

In the past decade, there have been remarkable advances in our understanding of the calcium messenger system that mediates the effects of various agonists. The purpose of the present article is to describe two areas of current interest in the calcium signaling field--quantal calcium release and calcium entry into the cell--using the pancreatic acinar cell as a model. Proposed mechanisms describing these phenomena and the role they play in the kinetics of calcium movements in the cell are discussed.

Transient membrane hyperpolarizations due to spontaneous fluctuations of the cytosolic Ca2+ in osteoblast-like cells

The recently developed nystatin modification of the patch clamp technique allows stable whole-cell recordings without affecting the intracellular Ca2+ buffering capacity and thereby may provide a means to indirectly monitor spontaneous changes in the intracellular Ca2+ concentrations. To test this hypothesis, we applied the nystatin method to the well-characterized ROS 17/2.8 osteoblast-like cell system, where rises of the intracellular Ca2+ are known to cause transient hyperpolarizations via activation of Ca2+ -dependent K+ channels. Additionally to minor fluctuations (10-20 mV) around a mean potential of -42.1 +/- 4.2 mV, we observed spontaneously occurring, transient hyperpolarizations to membrane potentials as negative as -80 mV. These transient hyperpolarizations were not eliminated by Ca2+ entry blockers but abolished by intracellular infusion of 10 mM EGTA. Thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, hyperpolarized the cells close to the K+ reversal potential. Moreover, voltage-clamp studies revealed an intermittendly activating Ca2+-dependent K+ conductance. These results strongly suggest that the nystatin method is particularly suitable to study Ca(2+)-dependent channels and thereby spontaneous changes in the intracellular Ca2+.

The effects of thapsigargin on [Ca2+]i in isolated rat mesenteric artery vascular smooth muscle cells

The effects of thapsigargin were studied on single cells isolated from side branches of the rat mesenteric artery. Thapsigargin (150 nM) produced a transient increase of [Ca2+]i. This transient rise of [Ca2+]i was unaffected by removing external Ca2+ ions. This suggests that thapsigargin is releasing Ca2+ ions from an intracellular store. In the absence of thapsigargin both noradrenaline and caffeine also produced a transient increase of [Ca2+]i. These increases were abolished by prior exposure to thapsigargin. Correspondingly, the effects of thapsigargin were abolished by prior exposure to caffeine. These results show that thapsigargin releases Ca2+ from the noradrenaline and caffeine-sensitive stores.

Regulation of extracellular calcium entry in endothelial cells: role of intracellular calcium pool

We have investigated the role of the intracellular Ca2+ pool in regulating Ca2+ entry into vascular endothelial cells. The intracellular Ca2+ pool was mobilized using either thapsigargin (TG) or 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), inhibitors of the endoplasmic reticulum Ca(2+)-adenosinetriphosphatase (ATPase). Mobilization of intracellular Ca2+ stores with either inhibitor depleted intracellular Ca2+ and greatly reduced subsequent mobilization of the inositol 1,4,5-trisphosphate (IP3)-sensitive intracellular Ca2+ pool by bradykinin. However, bradykinin-induced mobilization of the IP3-sensitive intracellular Ca2+ pool only partially reduced the subsequent response of cells to TG and BHQ. Mobilization of the intracellular Ca2+ pool by either TG or BHQ led to a concentration-dependent elevation of cytosolic Ca2+ concentrations ([Ca2+]i) without initiating inositol polyphosphate formation. In contrast to the rapidly developing, transient rise in Ca2+ concentration initiated by bradykinin, maximal concentrations of TG and BHQ stimulated a slowly developing, prolonged elevation of [Ca2+]i that required extracellular Ca2+ and could be blocked by extracellular Ni2+. Extracellular Ca2+ entered the cell through an activated cation entry pathway, since bradykinin, TG, and BHQ stimulated Mn2+ and 45Ca2+ entry. Bradykinin-stimulated 45Ca2+ uptake reached a peak within 2 min, whereas 45Ca2+ influx initiated by TG or BHQ continued for at least 8 min. Importantly, the [Ca2+]i response after low concentrations of BHQ was more transient than that seen after TG. The return of [Ca2+]i to basal values after low concentrations of BHQ was associated with reversal of Ca(2+)-ATPase inhibition and refilling of the IP3-sensitive Ca2+ pool. The continued elevation of [Ca2+]i and prolonged Ca2+ entry seen with TG was associated with continued Ca(2+)-ATPase inhibition and an empty IP3-sensitive Ca2+ pool. We conclude that mobilization of intracellular Ca2+ stores induces Ca2+ entry in endothelial cells which continues until the intracellular Ca2+ pool is refilled.

Intracellular calcium-mediated activation of hepatic Na+/H+ exchange by arginine vasopressin and phenylephrine

The effect of Ca++ mobilizing agonists arginine vasopressin and phenylephrine on Na+/H+ exchange was studied in freshly isolated hepatocytes and isolated perfused rat livers. The activity of Na+/H+ exchange was determined from the rate of H+ efflux, 22Na uptake and pHi recovery. Arginine vasopressin and phenylephrine stimulated H+ efflux and 22Na uptake in isolated rat hepatocytes and increased the rate of pHi recovery from acid-loaded hepatocytes. These effects were inhibited by amiloride. Arginine vasopressin- and phenylephrine-induced increases in H+ efflux were also dependent on extracellular Na+. Arginine vasopressin- and phenylephrine-induced increases in intracellular Ca++ concentration, H+ efflux, 22Na uptake and intracellular pH recovery were decreased in hepatocytes preloaded with the Ca(++)-buffering agent [bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] (MAPTA). Na+/H+ exchange-dependent intracellular pH recovery from cytosolic acidification was stimulated by thapsigargin, which increases intracellular calcium concentration by inhibiting endoplasmic reticulum Ca++ ATPase. Arginine vasopressin- and phenylephrine-induced increases in intracellular pH recovery were not dependent on extracellular Ca++ and were inhibited by calmidazolium, a calmodulin inhibitor. Arginine vasopressin and phenylephrine also increased H+ efflux in the absence but not in the presence of amiloride in perfused rat livers without affecting biliary HCO3- excretion. These results indicate that arginine vasopressin and phenylephrine activate Na+/H+ exchange in rat hepatocytes, an effect mediated in part by intracellular Ca++ and calmodulin kinase. Furthermore, sinusoidal Na+/H+ exchange does not appear to be involved in biliary HCO3- excretion.

Calcium signaling mechanisms in the gastric parietal cell

Gastric hydrochloric acid (HCl) secretion is stimulated in vivo by histamine, acetylcholine, and gastrin. In vitro studies have shown that histamine acts mainly via a cAMP-dependent pathway, and acetylcholine acts via a calcium-dependent pathway. Histamine also elevates intracellular calcium ([Ca2+]i) in parietal cells. Both gastrin and acetylcholine release histamine from histamine-containing cells. In humans, rats, and rabbits, there is considerable controversy as to whether or not gastrin receptors are also present on the parietal cell. We utilized digitized video image analysis techniques in this study to demonstrate gastrin-induced changes in intracellular calcium in single parietal cells from rabbit in primary culture. Gastrin also stimulated a small increase in [14C]-aminopyrine (AP) accumulation, an index of acid secretory responsiveness in cultured parietal cells. In contrast to histamine and the cholinergic agonist, carbachol, stimulation of parietal cells with gastrin led to rapid loss of the calcium signaling response, an event that is presumed to be closely related to gastrin receptor activation. Moreover, different calcium signaling patterns were observed for histamine, carbachol, and gastrin, Previous observations coupled with present studies using manganese, caffeine, and ryanodine suggest that agonist-stimulated increases in calcium influx into parietal cells do not occur via voltage-sensitive calcium channels or nonspecific divalent cation channels. It also appears to be unlikely that release of intracellular calcium is mediated by a muscle or neuronal-type ryanodine receptor. We hypothesize that calcium influx may be mediated by either a calcium exchange mechanism or by an unidentified calcium channel subtype that possesses different molecular characteristics as compared to muscle, nerve, and certain secretory cell types such as, for example, the adrenal chromaffin cell. Release of intracellular calcium may be mediated via both InsP3-sensitive and -insensitive mechanisms. The InsP3-insensitive calcium pools, if present, do not appear, however, to possess ryanodine receptors capable of modulating calcium efflux from these storage sites.