Mechanism underlying histamine-induced intracellular Ca2+ movement in PC3 human prostate cancer cells

A2B Adenosine Receptor Antagonists with Picomolar Potency

The A_2B adenosine receptor (A_2BAR) was proposed as a novel target for the (immuno)therapy of cancer since A_2BAR blockade results in antiproliferative, antiangiogenic, antimetastatic, and immunostimulatory effects. In this study, we explored the structure-activity relationships of xanthin-8-yl-benzenesulfonamides mainly by introducing a variety of linkers and substituents attached to the sulfonamide residue. A new, convergent strategy was established, which facilitated the synthesis of the target compounds. Many of the new compounds exhibited subnanomolar affinity for the A_2BAR combined with high selectivity. Functional groups were introduced, which will allow the attachment of dyes and other reporter groups. 8-(4-((4-(4-Bromophenyl)piperazin-1-yl)sulfonyl)phenyl)-1-propylxanthine (34, PSB-1901) was the most potent A_2B-antagonist ( K_i 0.0835 nM, K_B 0.0598 nM, human A_2BAR) with >10 000-fold selectivity versus all other AR subtypes. It was similarly potent and selective at the mouse A_2BAR, making it a promising tool for preclinical studies. Computational studies predicted halogen bonding to contribute to the outstanding potency of 34.

Calpain2 mediates Rab5-driven focal adhesion disassembly and cell migration

The early endosome protein Rab5 was recently shown to promote cell migration by enhancing focal adhesion disassembly through mechanisms that remain elusive. Focal adhesion disassembly is associated to proteolysis of talin, in a process that requires calpain2. Since calpain2 has been found at vesicles and endosomal compartments, we hypothesized that Rab5 stimulates calpain2 activity, leading to enhanced focal adhesion disassembly in migrating cells. We observed that calpain2 co-localizes with EEA1-positive early endosomes and co-immunoprecipitates with EEA1 and Rab5 in A549 lung carcinoma cells undergoing spreading, whereas Rab5 knock-down decreased the accumulation of calpain2 at early endosomal-enriched fractions. In addition, Rab5 silencing decreased calpain2 activity, as shown by cleavage of the fluorogenic substrate tBOC-LM-CMAC and the endogenous substrate talin. Accordingly, Rab5 promoted focal adhesion disassembly in a calpain2-dependent manner, as expression of GFP-Rab5 accelerated focal adhesion disassembly in nocodazole-synchronized cells, whereas pharmacological inhibition of calpain2 with N-acetyl-Leu-Leu-Met prevented both focal adhesion disassembly and cell migration induced by Rab5. In summary, these data uncover Rab5 as a novel regulator of calpain2 activity and focal adhesion proteolysis leading to cell migration.

(S)-lacosamide inhibition of CRMP2 phosphorylation reduces postoperative and neuropathic pain behaviors through distinct classes of sensory neurons identified by constellation pharmacology

Chronic pain affects the life of millions of people. Current treatments have deleterious side effects. We have advanced a strategy for targeting protein interactions which regulate the N-type voltage-gated calcium (CaV2.2) channel as an alternative to direct channel block. Peptides uncoupling CaV2.2 interactions with the axonal collapsin response mediator protein 2 (CRMP2) were antinociceptive without effects on memory, depression, and reward/addiction. A search for small molecules that could recapitulate uncoupling of the CaV2.2-CRMP2 interaction identified (S)-lacosamide [(S)-LCM], the inactive enantiomer of the Food and Drug Administration-approved antiepileptic drug (R)-lacosamide [(R)-LCM, Vimpat]. We show that (S)-LCM, but not (R)-LCM, inhibits CRMP2 phosphorylation by cyclin dependent kinase 5, a step necessary for driving CaV2.2 activity, in sensory neurons. (S)-lacosamide inhibited depolarization-induced Ca influx with a low micromolar IC50. Voltage-clamp electrophysiology experiments demonstrated a commensurate reduction in Ca currents in sensory neurons after an acute application of (S)-LCM. Using constellation pharmacology, a recently described high content phenotypic screening platform for functional fingerprinting of neurons that uses subtype-selective pharmacological agents to elucidate cell-specific combinations (constellations) of key signaling proteins that define specific cell types, we investigated if (S)-LCM preferentially acts on certain types of neurons. (S)-lacosamide decreased the dorsal root ganglion neurons responding to mustard oil, and increased the number of cells responding to menthol. Finally, (S)-LCM reversed thermal hypersensitivity and mechanical allodynia in a model of postoperative pain, and 2 models of neuropathic pain. Thus, using (S)-LCM to inhibit CRMP2 phosphorylation is a novel and efficient strategy to treat pain, which works by targeting specific sensory neuron populations.

Ca2+ clearance mechanisms in cancer cell lines and stromal cells of the prostate

BACKGROUND

Three prostatic cell lines, PC3, LNCaP, and DU 145, are used as established models to study cell signaling in prostate cancer. Recently, stromal cell lines of the prostate, such as P21, were also introduced. Here we investigate a basic and important mechanism of living cells: Ca(2+) homeostasis in PC3, DU 145, and P21.

METHODS

We examined Ca(2+) clearance mechanisms by monitoring the kinetics of recovery from histamine stimulation under conditions which inhibit prospect mechanisms for storing or extrusion of Ca(2+) from the cytosol by photometry.

RESULTS

Despite the fact that in all three cell lines the Ca(2+) ATPase of the plasma membrane and the SERCA are most important for Ca(2+) homeostasis, inhibition of PMCA in epithelial cells has a greater effect than in stromal cells. Furthermore, the proportion of PMCA and SERCA differs in PC3 and DU145 cells. PMCA is most effective at reaching resting [Ca(2+) ]i in the final recovery stage. In contrast to DU 145 and P21 cells, PC3 are the only cells substantially affected by the inhibition of the mitochondrial uniporter. In all cell lines the role of the sodium calcium exchanger is marginal.

CONCLUSION

These results demonstrate that not only cancer and stromal cell lines show significant differences in the modes and extent of their use of Ca(2+) clearance mechanisms, but also the cancer cell lines themselves.

Terfenadine induces anti-proliferative and apoptotic activities in human hormone-refractory prostate cancer through histamine receptor-independent Mcl-1 cleavage and Bak up-regulation

Although the results of several studies have underscored the regulatory effect of H1-histamine receptors in cell proliferation of some cancer cell types, its effect in prostate cancers remains unclear. We have therefore studied the effect of terfenadine (an H1-histamine receptor antagonist) in prostate cancer cell lines. Our data demonstrate that terfenadine was effective against PC-3 and DU-145 cells (two prostate cancer cell lines). In contrast, based on the sulforhodamine B assay, loratadine had less potency while fexofenadine and diphenhydramine had little effect. Terfenadine induced the cleavage of Mcl-1 cleavage into a pro-apoptotic 28-kDa fragment and up-regulation of Bak, resulting in the loss of mitochondrial membrane potential (ΔΨm) and the release of cytochrome c and apoptosis-inducing factor into the cytosol. The activation of caspase cascades was detected to be linked to terfenadine action. Bak up-regulation was also examined at both the transcriptional and translational levels, and Bak activation was validated based on conformational change to expose the N terminus. Terfenadine also induced an indirect-but not direct-DNA damage response through the cleavage and activation of caspase-2, phosphorylation and activation of Chk1 and Chk2 kinases, phosphorylation of RPA32 and acetylation of Histone H3; these processes were highly correlated to severe mitochondrial dysfunction and the activation of caspase cascades. In conclusion, terfenadine induced apoptotic signaling cascades against HRPCs in a sequential manner. The exposure of cells to terfenadine caused the up-regulation and activation of Bak and the cleavage of Mcl-1, leading to the loss of ΔΨm and activation of caspase cascades which further resulted in DNA damage response and cell apoptosis.

Effect of celecoxib on Ca(2+) handling and viability in human prostate cancer cells (PC3)

Celecoxib has been shown to have an antitumor effect in previous studies, but the mechanisms are unclear. Ca(2+) is a key second messenger in most cells. The effect of celecoxib on cytosolic free Ca(2+) concentrations ([Ca(2+)](i)) in human suspended PC3 prostate cancer cells was explored by using fura-2 as a fluorescent dye. Celecoxib at concentrations between 5 and 30 μM increased [Ca(2+)](i) in a concentration-dependent manner. The Ca(2+) signal was reduced partly by removing extracellular Ca(2+). Celecoxib-induced Ca(2+) influx was not blocked by L-type Ca(2+) entry inhibitors or protein kinase C/A modulators [phorbol 12-myristate 13-acetate (PMA), GF109203X, H-89], but was inhibited by the phospholipase A(2) inhibitor, aristolochic acid. In Ca(2+)-free medium, 30 μM of celecoxib failed to induce a [Ca(2+)](i) rise after pretreatment with thapsigargin (an endoplasmic reticulum [ER] Ca(2+) pump inhibitor). Conversely, pretreatment with celecoxib inhibited thapsigargin-induced Ca(2+) release. Inhibition of phospholipase C with U73122 did not change celecoxib-induced [Ca(2+)](i) rises. Celecoxib induced slight cell death in a concentration-dependent manner, which was enhanced by chelating cytosolic Ca(2+) with BAPTA. Collectively, in PC3 cells, celecoxib induced [Ca(2+)](i) rises by causing phospholipase C-independent Ca(2+) release from the ER and Ca(2+) influx via non-L-type, phospholipase A(2)-regulated Ca(2+) channels. These data may contribute to the understanding of the effect of celecoxib on prostate cancer cells.

Safrole-Induced Ca2+Mobilization and Cytotoxicity in Human PC3 Prostate Cancer Cells

The effect of the carcinogen safrole on intracellular Ca2+ mobilization and on viability of human PC3 prostate cancer cells was examined. Cytosolic free Ca2+ levels ([Ca2+]i) were measured by using fura-2 as a probe. Safrole at concentrations above 10 microM increased [Ca2+]i in a concentration-dependent manner with an EC50 value of 350 microM. The Ca2+ signal was reduced by more than half after removing extracellular Ca2+ but was unaffected by nifedipine, nicardipine, nimodipine, diltiazem, or verapamil. In Ca2+-free medium, after treatment with 650 microM safrole, 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor) failed to release Ca2+. Neither inhibition of phospholipase C with U73122 nor modulation of protein kinase C activity affected safrole-induced Ca2+ release. Overnight incubation with 0.65-65 microM safrole did not affect cell viability, but incubation with 325-625 microM safrole decreased viability. Collectively, the data suggest that in PC3 cells, safrole induced a [Ca2+]i increase by causing Ca2+ release from the endoplasmic reticulum in a phospholipase C- and protein kinase C-independent fashion, and by inducing Ca2+ influx. Safrole can decrease cell viability in a concentration-dependent manner.

Desipramine-induced Ca2+ movement and cytotoxicity in PC3 human prostate cancer cells

The effect of the antidepressant desipramine on intracellular Ca(2+) movement and viability in prostate cancer cells has not been explored previously. The present study examined whether desipramine could alter Ca(2+) handling and viability in human prostate PC3 cancer cells. Cytosolic free Ca(2+) levels ([Ca(2+)](i)) in populations of cells were measured using fura-2 as a probe. Desipramine at concentrations above 10 microM increased [Ca(2+)](i) in a concentration-dependent manner. The responses saturated at 300 microM desipramine. The Ca(2+) signal was reduced by half by removing extracellular Ca(2+), but was unaffected by nifedipine, nicardipine, nimodipine, diltiazem or verapamil. In Ca(2+)-free medium, after treatment with 300 microM desipramine, 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor) failed to release Ca(2+) from endoplasmic reticulum. Conversely, desipramine failed to release more Ca(2+) after thapsigargin treatment. Inhibition of phospholipase C with U73122 did not affect desipramine-induced Ca(2+) release. Overnight incubation with 10-800 microM desipramine decreased viability in a concentration-dependent manner. Chelation of cytosolic Ca(2+) with BAPTA did not reverse the decreased cell viability. Collectively, the data suggest that in PC3 cells, desipramine induced a [Ca(2+)](i) increase by causing Ca(2+) release from endoplasmic reticulum in a phospholipase C-independent fashion and by inducing Ca(2+) influx. Desipramine decreased cell viability in a concentration-dependent, Ca(2+)-independent manner.

A dual antagonist for chemokine CCR3 receptor and histamine H1 receptor

Eosinophilic chemokines and histamine play distinct but important roles in allergic diseases. Inhibition of both eosinophilic chemokines and histamine, therefore, is an ideal strategy for the treatment of allergic inflammation, such as asthma, allergic rhinitis, and atopic dermatitis. YM-344484 was found to potently inhibit both the CCL11-induced Ca2+ influx in human CCR3-expressing cells (Kb=1.8 nM) and histamine-induced Ca2+ influx in histamine H1 receptor-expressing PC3 cells (Kb=47 nM). YM-344484 also inhibited the CCL11-induced chemotaxis of human CCR3-expressing cells (IC50=6.2 nM) and CCL11-induced eosinophil-derived neurotoxin release from human eosinophils (IC50=19 nM). Orally administered YM-344484 inhibited the increase in histamine-induced vascular permeability in mice (82% inhibition at a dose of 10 mg/kg) and the accumulation of eosinophils in a mouse asthma model (74% at a dose of 300 mg/kg). These results indicate that YM-344484, a novel and functional dual antagonist for chemokine CCR3 receptor and histamine H1 receptor, is an attractive candidate for development as a novel anti-allergic inflammation drug.

Effect of capsazepine on cytosolic Ca2+ levels and proliferation of human prostate cancer cells

Capsazepine has been widely used as a selective antagonist of vanilloid type 1 receptors; however, its other in vitro effect on most cell types is unknown. In human PC3 prostate cancer cells, the effect of capsazepine on intracellular Ca(2+) concentrations ([Ca(2+)](i)) and cytotoxicity was investigated by using fura-2 and tetrazolium, respectively. Capsazepine caused a rapid rise in [Ca(2+)](i) in a concentration-dependent manner with an EC(50) value of 75 microM. Capsazepine-induced [Ca(2+)](i) rise was reduced by 60% by removal of extracellular Ca(2+), suggesting that the capsazepine-induced [Ca(2+)](i) rise was contributed by extracellular Ca(2+) influx and intracellular Ca(2+). Consistently, the capsazepine (200 microM)-induced [Ca(2+)](i) rise was decreased by La(3+) by half. In Ca(2+)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a monophasic [Ca(2+)](i) rise, after which the effect of capsazepine on [Ca(2+)](i) was inhibited by 80%. Conversely, pretreatment with capsazepine partly reduced thapsigargin-induced [Ca(2+)](i) rise. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca(2+) mobilizer)-induced, but not capsazepine-induced, [Ca(2+)](i) rise. These findings suggest that in human PC3 prostate cancer cells, capsazepine increases [Ca(2+)](i) by evoking Ca(2+) influx and releasing Ca(2+) from the endoplasmic reticulum via a phospholiase C-independent manner. Overnight incubation with capsazepine (200 microM) killed 37% of cells, which could not be prevented by chelating intracellular Ca(2+) with BAPTA.

The effect of histamine on field-stimulated contractions of the guinea-pig prostate

The aim of this study was to investigate the effect of histamine on field-stimulated contractions of the guinea-pig prostate using isolated preparations in organ bath experiments. The histamine receptor subtype involved in potentiating the twitch responses was characterised, and the presence of any post-synaptic effects was determined. In addition, the effects of histamine on nerve-stimulated contractions of the ventral and dorsal prostate as well as the coagulating gland were compared in preliminary experiments. Histamine (300 microM) approximately doubled the magnitude of the twitch contractions in the ventral and dorsal lobes of the prostate (nerve stimulation parameters were 10 Hz for 2 s every 50 s, 0.5 ms duration at supramaximal voltage). In the coagulating glands, histamine increased the contractions more than fourfold. Further experiments were performed only on the ventral glands. The H(1) receptor antagonist, mepyramine, exhibited an apparent competitive antagonism against the histamine-induced potentiations of the twitch responses (apparent pK(B) value = 9.21+/-0.17 (n=5). The H(2) receptor antagonist, ranitidine, produced a small, significant shift to the right, as did the time control. The H(3) receptor antagonist, thioperamide, had no significant effect on the concentration-response curve. The effects of histamine (10 microM) on exogenously applied acetylcholine (Ach), noradrenaline (NA) and adenosine 5'-triphosphate (ATP) were determined. In each case, the contractile responses were significantly potentiated: ATP (5.8-fold P<0.0001, n=8), NA (1.6-fold P<0.05, n=10) and Ach (2.1-fold P<0.0007, n=10). This is the first study that has shown the effects of histamine on field-stimulated contractions in the prostate of any species. It is concluded that histamine, acting on H(1) receptors, elicits a considerable potentiation of the nerve-stimulated twitch contractions in the guinea-pig prostate. One mechanism whereby histamine exerts its potentiating effect is a post-synaptic enhancement of the response to ATP in particular, but also to NA and Ach. It is suggested that H(1)-receptor and purinoceptor antagonists may have a role to play in the treatment of benign prostatic hyperplasia (BPH).

NPC-14686 (Fmoc-l-homophenylalanine)-induced CaCa2+ increases and death in human prostate cancer cells

The effect of NPC-14686, a potential anti-inflammatory drug, on cytosolic free Ca2+ levels ([Ca2+]i) and growth in PC3 human prostate cancer cells was examined by using fura-2 as a fluorescent Ca2+ indicator and WST-1 as a fluorescent growth dye. NPC-14686 at concentrations above 10 microM increased [Ca2+]i in a concentration-dependent manner with an EC50 value of 100 microM. NPC-14686-induced Ca2+ influx was confirmed by Mn2+ quench of fura-2 fluorescence. The Ca2+ signal was also reduced by removing extracellular Ca2+. Pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor) to deplete the endoplasmic reticulum Ca2+ nearly abolished 200 microM NPC-14686-induced Ca2+ release; and conversely pretreatment with NPC-14686 completely inhibited thapsigargin-induced Ca2+ release. The Ca2+ release induced by 200 microM NPC-14686 was not affected by inhibiting phospholipase C with 2 microM U73122. Overnight treatment with 1-500 microM NPC-14686 decreased cell viability in a concentration-dependent manner. These findings suggest that in human PC3 prostate cancer cells, NPC-14686 increases [Ca2+]i by evoking extracellular Ca2+ influx and releasing intracellular Ca2+ from the endoplasmic reticulum via a phospholiase C-independent manner. NPC-14686 may be cytotoxic to prostate cancer cells.

EFFECTS OF ECONAZOLE ON Ca2+ LEVELS IN AND THE GROWTH OF HUMAN PROSTATE CANCER PC3 CELLS

1. Econazole is used clinically as an antifungal drug with many different in vitro effects. However, the effects of econazole on prostate cancer cells are unknown. The effects of econazole on intracellular Ca2+ concentrations ([Ca2+]i) in and the proliferation of human PC3 prostate cancer cells was explored in the present study using fura-2 and tetrazolium as fluorescent dyes. 2. At a concentration of 0.1 micromol/L, econazole started to increase [Ca2+]i in a concentration-dependent manner. The econazole-induced increase in [Ca2+]i was reduced by 48% by removal of extracellular Ca2+, suggesting that the econazole-induced increase in [Ca2+]i was composed of extracellular Ca2+ influx and intracellular Ca2+. 3. This econazole-induced Ca2+ influx was via an L-type Ca2+ channel-like pathway. In Ca2+-free medium, 1 micromol/L thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic increase in [Ca2+]i, after which the effect of econazole to increase [Ca2+]i was substantially inhibited. Conversely, pretreatment with 5 micromol/L econazole to deplete intracellular Ca2+ stores totally prevented thapsigargin from releasing more Ca2+. 4. The phospholipase C (PLC) inhibitor U73122 (2 micromol/L) abolished the increase in [Ca2+]i induced by 10 micromol/L ATP (a Ca2+ mobilizer that needs inositol 1,4,5-trisphosphate). 5. Overnight incubation with 1-30 micromol/L econazole inhibited proliferation of PC3 cells in a concentration-dependent manner. 6. These findings suggest that, in PC3 cells, econazole increases [Ca2+]i by stimulating Ca2+ influx into cells and Ca2+ release from the endoplasmic reticulum via a PLC-independent mechanism. Econazole is cytotoxic at submicromolar concentrations.

Novel effect of Y-24180, a presumed specific platelet activation factor receptor antagonist, on Ca2+ levels and growth of human prostate cancer cells

In human prostate cancer PC3 cells, the effect of Y-24180, a presumed specific platelet activation factor (PAF) receptor antagonist, on intracellular Ca2+ concentration ([Ca2+]i) was measured by using fura-2 as a Ca2+-sensitive fluorescent probe. Y-24180 (1-10 microM) caused a rapid and sustained [Ca2+]i rise in a concentration-dependent manner. The [Ca2+]i rise was prevented by 40% by removal of extracellular Ca2+, but was not changed by dihydropyridines, verapamil and diltiazem. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which the increasing effect of 10 microM Y-24180 on [Ca2+]i was reduced by 67%; conversely, depletion of Ca2+ stores with 10 microM Y-24180 abolished thapsigargin-induced [Ca2+]i rise. U73122, an inhibitor of phospholipase C, inhibited ATP-, but not Y-24180-induced [Ca2+]i rise. Activation of protein kinase C with phorbol-12-myristate-13-acetate (PMA) enhanced Y-24180-induced [Ca2+]i rise by 70%. Overnight treatment with 0.1-10 microM Y-24180 inhibited cell proliferation in a concentration-dependent manner. Collectively, these results suggest that Y-24180 acts as a potent and cytotoxic Ca2+ mobilizer in prostate cancer cells, by stimulating both extracellular Ca2+ influx and intracellular Ca2+ release. Since alterations in Ca2+ movement may interfere with many cellular signalling processes unrelated to modulation of PAF receptors, caution must be applied in using this reagent as a selective PAF receptor antagonist.

Nordihydroguaiaretic acid-induced Ca2+ handling and cytotoxicity in human prostate cancer cells

The effect of nordihydroguaiaretic acid (NDGA), a compound commonly used as a lipoxygenases inhibitor, on intracellular free Ca2+ levels ([Ca2+]i) in PC3 human prostate cancer cells was investigated. [Ca2+]i was measured by using the Ca2+ -sensitive dye fura-2. NDGA increased [Ca2+]i in a concentration-dependent manner with an EC50 of 30 microM. The Ca2+ signal comprised a gradual and sustained increase. Removal of extracellular Ca2+ partly decreased the NDGA-induced [Ca2+]i increase, suggesting that the Ca2+ signal was due to both extracellular Ca2+ influx and intracellular Ca2+ release. NDGA-induced Ca2+ influx was independently confirmed by measuring NDGA-induced Mn2+ -coupled quench of fura-2 fluorescence. The NDGA-induced Ca2+ influx was not affected by L-type Ca2+ channel blockers. In Ca2+ -free medium, the NDGA-induced [Ca2+]i increase was abolished by pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor), and conversely, pretreatment with NDGA abolished thapsigargin-induced [Ca2+]i increase. NDGA-induced intracellular Ca2+ release was not altered by inhibition of phospholipase C. Overnight treatment with 20-50 microM NDGA inhibited cell proliferation rate in a concentration-dependent manner. Several other lipoxygenases inhibitors did not alter [Ca2+]i. Collectively, this study shows that in prostate cells, NDGA induced a [Ca2+]i increase via releasing stored Ca2+ from the endoplasmic reticulum in a manner independent of phospholipase C activity, and by causing Ca2+ influx. NDGA also caused cytotoxicity at higher concentrations.

EFFECT OF THE ANTIDEPRESSANT MAPROTILINE ON CA2+ MOVEMENT AND PROLIFERATION IN HUMAN PROSTATE CANCER CELLS

1. The effect of maprotiline, an antidepressant, on human prostate cells is unclear. In the present study, the effect of maprotiline on [Ca2+]i and growth in PC3 human prostate cancer cells was measured using the fluorescent dyes fura-2 and tetrazolium, respectively. 2. Maprotiline caused a rapid, concentration-dependent increase in [Ca2+]i (EC50 = 200 micromol/L). The maprotiline-induced [Ca2+]i increase was reduced by removal of extracellular Ca2+ or pretreatment with nicardipine. 3. The maprotiline-induced Mn2+ influx-associated fura-2 fluorescence quench directly suggests that maprotiline caused Ca2+ influx. 4. In Ca(2+)-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, caused a monophasic [Ca2+]i increase, after which the effects of maprotiline of increasing [Ca2+]i were abolished. In addition, pretreatment with maprotiline reduced a major portion of the thapsigargin-induced increase in [Ca2+]i. 5. U73122, an inhibitor of phospholipase C, abolished the ATP (but not maprotiline)-induced increase in [Ca2+]i. 6. Overnight incubation with 1-10 micromol/L maprotiline did not alter cell proliferation, although incubation with 30-50 micromol/L maprotiline decreased cell proliferation. 7, These findings suggest that maprotiline rapidly increases [Ca2+]i in human prostate cancer cells by stimulating both extracellular Ca2+ influx and intracellular Ca2+ release and that it may modulate cell proliferation in a concentration-dependent manner.