Effects Of Thapsigargin Research Articles

Control of pulmonary vascular smooth muscle tone by sarcoplasmic reticulum Ca2+ pump blockers: thapsigargin and cyclopiazonic acid.

The effect of thapsigargin (TG) and cyclopiazonic acid (CPA) on the mechanical activity of the rat pulmonary artery were investigated. In chemically (beta-escin)-skinned arterial strips, application of TG (0.1-1 microM) or CPA (0.5-10 microM) prior and throughout the loading procedure of the internal Ca2+ stores (0.3 microM free Ca2+ ions for 8-10 min) concentration dependently inhibited the subsequent contractile response induced by noradrenaline (NA, 10 microM) or caffeine (25 mM). In intact strips repeatedly incubated in a Ca(2+)-containing solution (2.5 mM for 10 min), followed by incubation in a Ca(2+)-free solution 12 min before NA-stimulation, TG and CPA not only inhibited the NA-induced contraction but also increased the tension which appeared during the exposure time to Ca2+. The two phenomena developed with similar time courses. The increase in tension during the readmission of Ca2+ ions was not antagonized by verapamil (10 microM) or nifedipine (1 microM) but was blocked by La3+ (50 microM) and Co2+ (1 mM) ions. The amplitude of the verapamil-insensitive TG (or CPA)-induced contraction was dependent on the external [Ca2+] [0.1-10 mM, concentration for half maximal effect (EC50) = 0.85 mM], not modified by the reduction of the external [Na+] (from 130 to 10 mM) and decreased by depolarization of the strip using K(+)-rich (30-120 mM) solutions. Under the latter condition, 38 +/- 9 and 83 +/- 4% reduction (n = 5) was observed in the presence of 60 and 120 mM K+ respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracrine role of progesterone in fibronectin production and deposition by chicken ovarian granulosa cells in vitro: effect of extracellular calcium.

The role of extracellular Ca2+ in progesterone-induced fibronectin production and deposition by hen granulosa cells was studied. Granulosa cells isolated from preovulatory follicles of the chicken ovary were incubated in Ca(2+)-deficient or Ca(2+)-containing medium 199, and the amount of total fibronectin produced (fibronectin deposited in the matrix, secreted into the medium, or associated with cells) was measured by ELISA. The quantity of fibronectin deposited as well as the total amount of fibronectin produced in the presence or absence of exogenous progesterone was suppressed in Ca(2+)-deficient medium. It required greater concentrations of exogenous progesterone to significantly increase fibronectin production in Ca(2+)-deficient medium than in Ca(2+)-replete medium. Cyanoketone, an inhibitor of progesterone synthesis, suppressed total fibronectin produced by unstimulated cells both in the absence and presence of Ca2+. However, the inhibitory effect of cyanoketone was significantly less in Ca(2+)-replete medium than in Ca(2+)-deficient medium. Exogenous progesterone reversed completely the inhibitory effects of cyanoketone. In the presence of cyanoketone, progesterone caused a greater (3-fold) increase in fibronectin production in Ca(2+)-replete medium than in the absence of Ca2+. Thapsigargin, an agent that mobilizes Ca2+ from internal stores, suppressed basal and progesterone-induced fibronectin production in the absence and presence of cyanoketone. However, the inhibitory effect of thapsigargin was significantly reduced in Ca(2+)-replete medium.(ABSTRACT TRUNCATED AT 250 WORDS)

P1-182 - The effects of thapsigargin (TG), cyclopiazonic acid (CPA) and 2,5-di(tert-butyl)-1,4-benzohydroquinon (tBuBHQ) on rat myocardium

P1-182 - The effects of thapsigargin (TG), cyclopiazonic acid (CPA) and 2,5-di(tert-butyl)-1,4-benzohydroquinon (tBuBHQ) on rat myocardium

Modulation of cytosolic Ca 2+ concentration by thapsigargin and cyclopiazonic acid in human aortic endothelial cells

To clarify the agonist-induced Ca 2+ entry mechanism, effects of thapsigargin and cyclopiazonic acid, selective inhibitors of endoplasmic reticulum Ca 2+-ATPase, on intracellular free Ca 2+ concentration ([Ca 2+] i) were studied in cultured human aortic endothelial cells loaded with the fluorescent Ca 2+ indicator fura-2. Thapsigargin (1–1000 nM) and cyclopiazonic acid (0.1–100 μM) produced a biphasic change in [Ca 2+] i, which consisted of a transient peak elevation followed by a long-lasting decline of [Ca 2+] i in a concentration-dependent manner. In the presence of thapsigargin or cyclopiazonic acid, the rapid transient elevation of [Ca 2+] i elicited by histamine was attenuated in a time-dependent manner. The slow declining phase of the response to thapsigargin and cyclopiazonic acid was completely eliminated by removal of extracellular Ca 2+, and it was also prevented by reduction of the extracellular Cl − concentration to 40 mM or by the Cl − channel blocker N-phenylanthranilic acid. These findings suggest that the initial transient rising phase and the slow declining phase of [Ca 2+] i in response to thapsigargin and cyclopiazonic acid reflect a blockade of Ca 2+ uptake into the endoplasmic reticulum and the Cl −-sensitive Ca 2+ entry activated by the depletion of agonist-sensitive intracellular Ca 2+ stores, respectively, in human aortic endothelial cells.

Self-association accompanies inhibition of Ca-ATPase by thapsigargin

Recent studies have demonstrated a relationship between the activity of the Ca-ATPase of sarcoplasmic reticulum and its state of self-association. In the present study, the effects of thapsigargin (TG), a toxin that specifically inhibits the Ca-ATPase of rabbit skeletal muscle sarcoplasmic reticulum membrane, were studied by detecting the time-resolved phosphorescence anisotropy (TPA) decay of the Ca-ATPase that had been labeled with the phosphorescent probe erythrosin-isothiocyanate (ErITC). Anisotropy decays were fit to a function that consisted of three exponential decays plus a constant background, as well as to a function describing explicitly the uniaxial rotation of proteins in a membrane. In the absence of TG, the anisotropy was best-fit by a model representing the rotation of three populations, corresponding to different-sized oligomeric species in the membrane. The addition of stoichiometric amounts of TG to the Ca-ATPase promptly decreased the overall apparent rate of decay, indicating decreased rotational mobility. A detailed analysis showed that the principal change was not in the rates of rotation but rather in the population distribution of the Ca-ATPase molecules among the different-sized oligomers. TG decreased the proportion of small oligomers and increased the proportion of large ones. Preincubation of the ErITC-SR in 1 mM Ca2+, which stabilizes the E1 conformation relative to E2, was found to protect partially against the changes in the TPA associated with the presence of the inhibitor. These results are consistent with the hypothesis that TG inhibits the Ca-ATPase by stabilizing it in an E2-like conformation, which promotes the formation of larger aggregates of the enzyme. When combined with the effects of other inhibitors on the Ca-ATPase, these results support a general model for the coupling of enzyme conformation and self-association in this system.

Thapsigargin protects human erythrocyte Ca 2+-ATPase from proteolysis

The effect of thapsigargin on the activation by partial proteolysis of the plasma membrane Ca 2+-ATPase was studied in intact human erythrocyte membranes and in the purified enzyme. The enzyme was maximally activated in the absence of thapsigargin within 1 min of exposure to trypsin. However, in the presence of thapsigargin maximal activation was achieved only after 5 min trypsin digestion. Thapsigargin did not alter the pattern of proteolysis as revealed by SDS-PAGE of the tryptic fragments, although it slowed down the rate of appearance of the fragments. Thapsigargin also enhanced the activation of the enzyme by calmodulin. These findings suggest that, although thapsigargin at low concentrations has no effect on the catalytic activity of the Ca 2+-ATPase in vitro in the absence of calmodulin, it could interfere with its regulation in vivo.

Evidence for a direct interaction of thapsigargin with voltage-dependent Ca2+ channel

The effect of thapsigargin, an inhibitor of the sarco-endoplasmic reticulum Ca(2+)-ATPase, on voltage-dependent Ca2+ channels has been investigated in the A7r5 cell line and in membrane preparations from rat aorta, heart and brain. Patch-clamp technique showed that, at micromolar concentrations, thapsigargin inhibited the L-type Ca2+ channel current in A7r5 cells. It depressed the current at all voltages without change in the steady state inactivation curve. The rates of inactivation of the Ca2+ current were highly variable among the cells suggesting that more than one component of L-type Ca2+ current coexist in A7r5 cells, differing in the kinetics of inactivation. Thapsigargin appeared to be more potent on the slower-inactivating Ca2+ current than on the faster-inactivating one. In the same range of concentrations, thapsigargin inhibited the specific binding of 3H(+)-isradipine in intact cells while 45Ca2+ uptake in intracellular stores of skinned cells was inhibited at nanomolar concentrations. The equilibrium dissociation constant of 3H(+)-isradipine was increased in the presence of thapsigargin as a result of an increase of the dissociation rate constant indicating that the inhibitory effect of the antagonist cannot be attributed to a simple competitive interaction with the dihydropyridine binding site. Maximum binding capacity was unaffected. A similar pattern of inhibition of 3H(+)-isradipine binding was observed in membrane preparations from rat aorta, heart and brain. Those results indicate that, at micromolar concentrations, thapsigargin inhibits the voltage-dependent Ca2+ current by a direct interaction with the L-type Ca2+ channels.

Activation of recombinant trp by thapsigargin in Sf9 insect cells

The mammalian protein responsible for Ca2+ release-activated current (Icrac) may be homologous to the Drosophila protein designated trp. Thus the activity of trp, and another Drosophila protein designated trp-like or trpl, may be linked to depletion of the internal Ca2+ store via the so-called capacitative Ca2+ entry mechanism. To test this hypothesis, the effect of thapsigargin, a selective inhibitor of the endoplasmic reticulum Ca2+ pump, on trp- and trpl-induced whole cell membrane current was determined using the baculovirus Sf9 insect cell expression system. The results demonstrate that trp and trpl form Ca(2+)-permeable cation channels. The trpl encodes a nonselective cation channel that is constitutively active under basal nonstimulated conditions and is unaffected by thapsigargin, whereas trp is more selective for Ca2+ than Na+ and is activated by depletion of the internal Ca2+ store. Although evaluation of cation selectivity suggests that trp is not identical to the channel responsible for Icrac, these channels must share some structural feature(s) since both are activated by thapsigargin. A unique proline-rich region in the COOH-terminal tail of trp, which is absent in trpl, may be necessary for capacitative Ca2+ entry.

Na+/Ca2+ exchange-mediated calcium entry in human lymphocytes.

Regulation of cytosolic Ca2+ and cytosolic Na+ is critical for lymphocyte cation homeostasis and function. To examine the influence of cytosolic Na+ on Ca2+ regulation in human peripheral blood lymphocytes, Ca2+ entry and cytosolic Ca2+ (measured with fura-2) were monitored in cells in which cytosolic Na+ was increased and/or the Na+ gradient was decreased by reduction of external Na+ concentration. Ouabain-treated cells (0.1 mM for 30 min at 37 degrees C), suspended in Na(+)-free medium, showed a 30-65% increase in Ca2+ uptake compared to cells in 140 mM Na+ medium. Enhanced Ca2+ influx was entirely dependent on ouabain pretreatment and reversal of the Na+ gradient. Na pump inhibition or Na ionophore addition and subsequent exposure to Na(+)-free medium resulted in a sustained elevation of cytosolic Ca2+. As preincubation of cells in Ca(2+)-free medium further enhanced the ouabain-dependent increase in cytosolic Ca2+, the effects of the microsomal Ca(2+)-ATPase inhibitor thapsigargin on Ca2+ influx and cytosolic Ca2+ were studied. Thapsigargin stimulated Ca2+ entry following ouabain pretreatment and reversal of the Na+ gradient; the effects of thapsigargin were retained in the presence of LaCl3, a potent inhibitor of store-dependent calcium influx pathways. These results show lymphocytes demonstrate Na+/Ca2+ exchange activity and suggest the Na+/Ca2+ exchanger modulates cytosolic Ca2+ following intracellular Ca2+ store depletion.

Transport of Envelope Proteins of Sendai Virus, HN and F0, Is Blocked at Different Steps by Thapsigargin and Other Perturbants to Intracellular Ca2+11

The effects of thapsigargin (Tg), a specific inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum (ER), on replication of Sendai virus (HVJ) in BALB3T3 cells were examined. In the presence of Tg, the cells infected with HVJ did not release viral particles to the culture medium. Tg inhibited almost completely the expression of viral envelope proteins, HN and F0, on the cell surface, although it did not affect the synthesis of viral proteins. Two other inhibitors of Ca(2+)-ATPase of the ER, 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ) and cyclopiazonic acid (CPA), as well as Ca(2+)-ionophores such as A23187 and ionomycin, also inhibited the expression of HN protein on the cell surface. Tg seemed to inhibit the intracellular transport or maturation of the viral membrane proteins by perturbing intracellular distribution of Ca2+ ions. In the presence of Tg, HN protein remained sensitive to endoglycosidaseH (endoH) for 3 h after its synthesis. On the other hand, F0 protein became resistant to endoH and sensitive to neuraminidase even in the presence of Tg. These results indicate that the transport of HN protein is blocked at the ER or the cis-Golgi region, while that of F0 protein is blocked at the post-Golgi stage in the presence of Tg.

Different calcium storage pools in vascular smooth muscle cells from spontaneously hypertensive and normotensive Wistar???Kyoto rats

To evaluate whether the distribution of intracellular free calcium may be impaired in primary hypertension. Cytosolic free calcium and stored calcium were investigated in cultured vascular smooth muscle cells from spontaneously hypertensive rats (SHR). The concentrations of intracellular and stored calcium were investigated in cultured vascular smooth muscle cells from spontaneously hypertensive rats aged 6 months from the Münster strain (SHR) and from age-matched normotensive Wistar-Kyoto (WKY) rats. Vascular smooth muscle cells were grown on coverslips, and fluorescence measurements of the intracellular calcium concentration were performed using fura-2. The different effects of thapsigargin, a selective Ca-ATPase inhibitor, and of angiotensin II (Ang II) on the calcium storage pools were investigated. In the absence of external calcium thapsigargin produced a dose-dependent transient increase in the concentration of intracellular calcium in vascular smooth muscle cells. The thapsigargin-induced maximum peak increase in the concentration of intracellular calcium was not significantly different in SHR and WKY rats. After depletion of the thapsigargin-sensitive calcium pools the addition of 100 nmol/l Ang II produced a rise in the concentration of intracellular calcium in vascular smooth muscle cells from SHR and WKY rats. Using vascular smooth muscle cells from the SHR the Ang II-induced increase in the concentration of intracellular calcium was not significantly different in the presence and absence of thapsigargin, indicating that the calcium pools depleted by thapsigargin and Ang II do not overlap significantly in vascular smooth muscle cells from SHR. In contrast, in the WKY rats the response to Ang II was significantly diminished after depletion of the thapsigargin-sensitive pool. When Ang II and thapsigargin were administered in the reverse order, i.e. Ang II before thapsigargin, the thapsigargin response was diminished in the WKY rats but not in the SHR. SHR differ from WKY rats in having vascular smooth muscle cells that contain thapsigargin-sensitive calcium storage pools that are distinct from the Ang II-sensitive calcium pools.

Effect of tri- n-butyltin on intracellular Ca 2+ concentration of mouse thymocytes under Ca 2+-free condition

Effect of tri- n-butyltin at concentrations ranging from 100 nM to 1 μM on the intracellular Ca 2+ concentration of mouse thymocytes was examined under Ca 2+-free conditions in comparison with those of 50 nM A23187, 100 nM thapsigargin and 10 μM cyclopiazonic acid, using the fluorescent dye for intracellular Ca 2+, fluo-3. Tri- n-butylin persistently increased the intensity of fluo-3 fluorescence while A23187, thapsigargin and cyclopiazonic acid produced a transient augmentation of the fluorescence. Pretreatment with A23187 greatly decreased the fluorescence responses induced by 1 μM tri- n-butyltin. However, the effect of thapsigargin and cyclopiazonic acid on the tri- n-butyltin-induced response was much weaker than that of A23187. In the presence of tri- n-butyltin, the transient response produced by A23187 was greatly prolonged. Results may suggest that tri- nbutyltin increases the membrane Ca 2+ permeability of the intracellular organelles (cellular calcium stores) and decreases the Ca 2+ pump activity of thymocyte membrane, resulting in a sustained increase in the intracellular Ca 2+ concentration under Ca 2+-free concentration.

Androgens increase intracellular calcium concentration and inositol 1,4,5-trisphosphate and diacylglycerol formation via a pertussis toxin-sensitive G-protein.

Bone is a target tissue of androgens, but the mechanisms by which they act on bone are still unclear. This study examines the early (5-60 s) effects of 1 pM to 1 microM testosterone on cytosolic free Ca2+ concentration ([Ca2+]i) and inositol 1,4,5-trisphosphate (InsP3) and diacyglycerol (DAG) formation in confluent male rat osteoblasts. 10 pM to 10 nM testosterone increased [Ca2+]i within 5 s via Ca2+ influx as shown by the effects of EGTA and the Ca2+ channel blockers nifedipine and verapamil and via Ca2+ mobilization from the endoplasmic reticulum as shown by the effects of thapsigargin and neomycin. 10 pM to 10 nM testosterone increased InsP3 and DAG formation within 10 s. Testosterone immobilized on bovine serum albumin (testosterone (O-carboxymethyl)oxime/bovine serum albumin) and its derivative, (O-carboxymethyl)oxime, rapidly increased [Ca2+]i and InsP3 and DAG formation and were full agonists, although they were less potent than the free steroid. Cyproterone acetate, a nuclear antagonist, did not block the increase in [Ca2+]i and InsP3 and DAG formation induced by testosterone. Finally, neomycin and pertussis toxin totally abolished the effects of testosterone on InsP3 and DAG. These results suggest that male rat osteoblasts bear nongenomic unconventional cell-surface receptors for testosterone that belong to the class of the membrane receptors coupled to a phospholipase C via a pertussis toxin-sensitive G-protein.

ＰＡＦ精製用イムノアフィニティーミニカラムを活用したＰＡＦの測定 ｔｈａｐｓｉｇａｒｇｉｎによるＰＡＦ産生こう進とｄｅｘａｍｅｔｈａｓｏｎｅの効果

Thapsigargin, a hexaoxygenated tetraacylated sesquiterpene lactone isolated from the roots of Thapsia garganica L., is one of non-TPA type tumor promoters that stimulate prostaglandin E2 production at very low concentrations and induce inflammation when applied topically. Effects of thapsigar-gin on platelet activating factor (PAF) production by rat peritoneal macrophages were examined. Levels of PAF in the conditioned medium and in the cells were determined by radioimmunoassay after extraction of PAF using immunoaffinity mini-columns. It was demonstrated that thapsigargin at concentrations of 3 to 100 ng/ml stimulated production of cell-associated PAF when measured 10 min after incubation. Levels of PAF in the conditioned medium were less than the detectable amount.The stimulative effect by thapsigargin reached a plateau at a concentration of 30 ng/ml. Time course experiments revealed that the levels of cell-associated PAF showed a peak 10 to 15 min after incubation with thapsigargin at a concentration of 30 ng/ml. The levels were then declined until 40 min after incubation. When macrophages were preincubated for 3 h with dexamethasone at concentra-tions of 0.01, 0.1 and l μg/ml, thapsigargin (30 ng/ ml) -stimulated PAF production in the cells at 10 min were decreased. However, the maximum inhibition was about 50% even at 1μg/ml of dexamethasone. These date suggest that by treatment with thapsigargin, PAF is partly produced through formation of lyso-PAF by activated phospholipase A2 since dexamethasone shows partial inhibition of thapsigargin-induced PAF production, and thap-sigargin-stimulated PAF production may partly account for proinflammatory activity of thapsigar-gin.

Effects of thapsigargin and cyclopiazonic acid on twitch force and sarcoplasmic reticulum Ca2+ content of rabbit ventricular muscle.

Thapsigargin (TG) and cyclopiazonic acid (CPA) are reported to be specific high-affinity inhibitors of the sarcoplasmic reticulum (SR) Ca2+ pump in isolated membranes and cells, with TG causing complete pump inhibition at nanomolar concentrations. To evaluate the effectiveness of TG and CPA in small multicellular cardiac preparations, we used rapid cooling contractures (RCCs) to assess the SR Ca2+ load. In contrast to observations in single myocytes, TG caused remarkably slow and incomplete SR Ca2+ depletion in multicellular preparations. A 45-minute exposure to 500 microM TG at 30 degrees C and 0.5-Hz stimulation only decreased RCCs by 76 +/- 5% (and 100 microM CPA reduced RCCs by 59 +/- 10% [mean +/- SEM]). In contrast, 10 minutes with 20 mM caffeine completely abolished RCCs. This confirms that there was still a caffeine-sensitive pool of Ca2+ in the TG-treated muscle. The time constant of rest decay of RCCs was accelerated by both TG (from 83 +/- 18 to 26 +/- 6 seconds) and CPA (from 68 +/- 11 to 10 +/- 5 seconds). This might be expected since Ca2+ leaking from the SR during rest cannot be taken back up as efficiently, favoring Ca2+ extrusion by the sarcolemmal Na(+)-Ca2+ exchanger. TG and CPA decreased twitch force (by 44 +/- 7% and 40 +/- 11%, respectively) and increased twitch duration, presumably because of the SR effects. We conclude that complete blockade of SR Ca2+ uptake by TG or CPA in multicellular preparations cannot be assumed, even at high [TG] or [CPA], unless evaluated (eg, by RCC).

Cell signaling and estrogens in female rat osteoblasts: a possible involvement of unconventional nonnuclear receptors.

Estrogen deficiency is associated with bone loss, and estrogen replacement is an effective treatment of this osteoporotic process. This study examines the early (5-120 s) effects of 17 beta-estradiol on the intracellular calcium and phospholipid metabolism in confluent female rat osteoblasts. The cytosolic free Ca2+ concentration ([Ca2+]i) was determined using fura-2/AM as Ca2+ probe. Cells were labeled with myo-[2-3H]inositol or [14C]arachidonic acid for inositol or lipid determination. Inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) production were determined by either mass measurement or anion-exchange chromatography or by thin-layer chromatography, respectively. 17 beta-Estradiol (1 pM to 1 nM) increased [Ca2+]i in a biphasic manner within 10 s via Ca2+ influx from the extracellular milieu, as shown by the effects of the calcium chelator EGTA and the Ca2+ channel blockers nifedipine and verapamil, and via Ca2+ mobilization from the endoplasmic reticulum (ER), as shown by the effects of thapsigargin. 17 beta-Estradiol (1 pM to 1 nM) induced a biphasic and concomitant increase in IP3 and DAG formation. Estradiol immobilized on bovine serum albumin (BSA) [E-(O-carboxymethyl)oxime BSA] and its derivative (O-carboxymethyl)oxime rapidly increased ([Ca2+]i, IP3, and DAG and were full agonists, although they were less potent than the free estradiol. They had the same action time course and acted via Ca2+ influx and Ca2+ mobilization from ER. Tamoxifen, a potent inhibitor of genomic steroid responses, did not block the rapid increase in Ca2+, IP3, and DAG induced by estradiol. Finally, inhibitor of phospholipase C (neomycin) and pertussis toxin abolished the effects of 17 beta-estradiol on IP3 and DAG formation. These results suggest that female rat osteoblasts bear non-genomic unconventional cell surface receptors for estradiol, belonging to the class of the membrane receptors coupled to a phospholipase C via a pertussis toxin-sensitive G protein.

Turnover of inositol pentakisphosphates, inositol hexakisphosphate and diphosphoinositol polyphosphates in primary cultured hepatocytes.

We have used a non-transformed cell model, the primary cultured hepatocyte, to explore the turnover of inositol hexakisphosphate, multiple isomers of inositol pentakisphosphate and two novel diphosphoinositol polyphosphates. All of these compounds gradually accumulated radioactivity throughout a 70 h period of labelling with [3H]inositol. However, a rapid metabolic rate was revealed upon inhibition of diphosphoinositol polyphosphate biphosphatase(s) with 1 mM fluoride for 40 min: this treatment elevated levels of [3H]diphosphoinositol polyphosphates up to 10-fold, indicating that their cellular pools were normally turning over at least 10 times every 40 min. This was accompanied by a turnover of about 10% of the pool of inositol hexakisphosphate. Control experiments established that 200 nM vasopressin brought about a typical activation of phospholipase C in hepatocytes after 62 h of primary culture. This agonist treatment did not affect steady-state levels of [3H]inositol pentakisphosphates, [3H]inositol hexakisphosphate or [3H]diphosphoinositol polyphosphates. However, prolonged treatment of hepatocytes with 2 microM thapsigargin reduced steady-state levels of [3H]diphosphoinositol polyphosphates by 50-70%. This effect of thapsigargin was also observed in the presence of fluoride, indicating that thapsigargin inhibited the rate of synthesis of diphosphoinositol polyphosphates.

Hormonal stimulation of Mg2+ uptake in hepatocytes. Regulation by plasma membrane and intracellular organelles

Collagenase dispersed rat liver hepatocytes release Mg2+ when stimulated with norepinephrine or accumulate Mg2+ when stimulated with vasopressin, respectively. Mg2+ fluxes in either direction account for a net loss or gain of approximately 10% of total cell magnesium and are rapidly reversible. Both stimulated Mg2+ efflux and Mg2+ influx require physiological concentration of extracellular NaCl and Ca2+. In the absence of extracellular Na+, Mg2+ efflux, but not influx, can be observed in the presence of extracellular Cl-. Under these conditions, the efflux is inhibited by the Cl-/HCO3- exchanger inhibitor 4,4'-dinitrostilbene-2,2'-disulfonic acid. In hepatocytes, Mg2+ influx, but not efflux, is completely inhibited by thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ ATPase. Several lines of evidence, such as measurements of cytosolic Ca2+ or of cytosolic Ca2+ buffering, indicate that the effect of thapsigargin in inhibiting Mg2+ influx could not be explained by an increase in cytosolic Ca2+. Instead, the inhibition of hepatocyte Mg2+ influx was found to be the result of the depletion of the Ca2+ stored within the endoplasmic reticulum.

Activation by calcium alone of chloride secretion in T84 epithelial cells

1. The goal of this study was to determine if an increase in cytoplasmic calcium concentration ([Ca2+]i), in the absence of additional second messengers derived from membrane phospholipid turnover, is a sufficient signal to induce chloride secretion across monolayers of the human colonic epithelial line, T84. 2. Thapsigargin was used to increase [Ca2+]i by inhibiting the endomembrane Ca(2+)-ATPase. [Ca2+]i was monitored in monolayers by fura-2 fluorescence spectroscopy, chloride secretion by measuring changes in short circuit current (Isc) in modified Ussing chambers, and inositol phosphates were measured by radio-h.p.l.c. of extracts of cells prelabelled with [3H]-inositol. 3. Thapsigargin increased [Ca2+]i and Isc in parallel, without increasing any inositol phosphates. The effect of thapsigargin on Isc was abolished by the intracellular calcium chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N"-tetraacetic acid (BAPTA). 4. Increasing [Ca2+]i with thapsigargin did not prevent a subsequent calcium response to carbachol or histamine if extracellular calcium was available. In the absence of extracellular calcium, only one such release of calcium to hormonal stimulation occurred when cells were pretreated with thapsigargin, and a second response to either carbachol histamine was essentially abolished. 5. Addition of carbachol or histamine to thapsigargin-treated cells mounted in Ussing chambers caused a transient further increase in Isc followed by termination of the response, even though [Ca2+]i continued to rise. 6. We conclude that an elevation in [Ca2+]i is a sufficient signal to induce chloride secretion in T84 cells. Rather than being required to stimulate secretory responses, additional second messengers induced by hormonal secretagogues (such as inositol phosphates) may in fact serve to limit the secretory response.

Evidence that the Ca 2+ inflow pathway in hepatocytes stimulated by thapsigargin is similar to that activated by vasopressin

Experiments were conducted to characterize the thapsigargin-stimulated plasma membrane Ca 2+ inflow pathway in hepatocytes. Ca 2+ inflow was estimated by measurement of the initial rate of activation of glycogen phosphorylase a following the addition of Ca 2+ to cells previously incubated in the absence of added Ca 2+. Pretreatment of hepatocytes with thapsigargin caused a substantial stimulation of the rate of Ca 2+ activation of glycogen phosphorylase a. This was interpreted to reflect a stimulation of plasma membrane Ca 2+ inflow. The effect of thapsigargin on plasma membrane Ca 2+ inflow was approximately 65% of the magnitude of the effect caused by vasopressin. When thapsigargin and vasopressin were combined as a stimulus, the degree of stimulation was similar to that caused by vasopressin alone. The thapsigargin-induced stimulation of the rate of Ca 2+ activation of glycogen phosphorylase a was inhibited in a concentration-dependent manner by both Zn 2+ and 1-{β-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl-1 H-imidazole hydrochloride (SK&F 96365). The concentration of each agent required for half-maximal inhibition was approximately 20 μM. It is concluded from: (i) the apparent lack of additivity in the responses of thapsigargin and vasopressin, and (ii) the sensitivity to inhibitors, that the Ca 2+ inflow pathway in hepatocytes stimulated by thapsigargin is likely to be similar to that which is activated by vasopressin.