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To study the role of the inositol 1,3,4,5-trisphosphate–binding protein GAP1IP4BP in store-operated Ca2+ entry, we established a human erythroleukemia (HEL) cell line in which the expression of GAP1IP4BP was substantially reduced by transfection with a vector containing antisense DNA under control of a Rous Sarcoma virus promoter and the Escherichia coli LacI repressor (AS-HEL cells). Control cells were transfected with vector lacking antisense DNA (V-HEL cells). GAP1IP4BP protein, which is a member of the GTPase-activating protein (GAP1) family, was reduced by 85% in AS-HEL cells and was further reduced by 96% by treatment with isopropylthio-β-d- galactoside to relieve LacI repression. The loss of GAP1IP4BP was associated with both a membrane hyperpolarization and a substantially increased Ca2+ entry induced by thrombin or thapsigargin. The activation of intermediate conductance Ca2+-activated K+ channels in AS-HEL cells (not seen in V-HEL cells) was responsible for the membrane hyperpolarization and the enhanced Ca2+ entry, and both were blocked by charybdotoxin. Stimulated V-HEL cells did not hyperpolarize and basal Ca2+ influx was unaffected by charybdotoxin. In V-HEL cells hyperpolarized by removal of extracellular K+, the thapsigargin-stimulated Ca2+ influx was increased. Expression of mRNA for the human Ca2+-activated intermediate conductance channel KCa4 was equivalent in both AS-HEL and V-HEL cells, suggesting that the specific appearance of calcium-activated potassium current (IK(Ca)) in AS-HEL cells was possibly due to modulation of preexisting channels. Our results demonstrate that GAP1IP4BP, likely working through a signaling pathway dependent on a small GTP-binding protein, can regulate the function of K(Ca) channels that produce a hyperpolarizing current that substantially enhances the magnitude and time course of Ca2+ entry subsequent to the release of internal Ca2+ stores.

Receptor-activated formation of inositol phosphates results in mobilization of intracellular stored Ca2+ in a variety of cells, including vas deferens derived DDT1 MF-2 cells. Stimulation of the histamine H-1 receptor on these cells caused a pronounced formation of inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P-4) with respect to that of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3). In this study, the role of inositol phosphates, in particular Ins(1,3,4,5)P-4 on the internal Ca2+-releasing process was investigated in permeabilized and histamine-stimulated intact DDT1 MF-2 cells. In permeabilized cells, Ins(1,4,5)P-3 induced a concentration-dependent release of intracellular stored Ca2+. Addition of Ins(1,3,4,5)P-4 did not cause Ca2+ mobilization, but its presence enhanced the amount of Ca2+ released by Ins(1,4,5)P-3, thereby increasing the total Ca2+-releasing capacity. The effect of both inositol phosphates was inhibited by heparin, known to block Ins(1,4,5)P-3-sensitive receptors. Thus, the additional amount of Ca2+ released by Ins(1,3,4,5)P-4 is mediated, either via Ins(1,4,5)P-3-sensitive Ca2+ channels, or via different heparin-sensitive Ca2+ channels activated by both Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4. Histamine H-1 receptor stimulation in intact cells induced a Ca2+-dependent K+ current, representing Ca2+ release from internal stores if receptor-activated Ca2+ entry from the extracellular space was prevented under Ca2+-free conditions or in the presence of La3+. This transmembrane current was abolished in the presence of intracellularly applied heparin. Depletion of Ins(1,4,5)P-3-sensitive Ca2+ stores by internal application of Ins(1,4,5)P-3 reduced the histamine evoked K+ current to some extent if the contribution of external Ca2+ was excluded. However, depletion of both Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4-sensitive Ca2+ compartments in advance caused abolition of the histamine-activated Ca2+ regulated K+ current. These results show that Ins(1,3,4,5)P-4 plays an important role in the Ca2+-releasing process in DDT1 MF-2 cells. It contributes to the development of the intracellular Ca2+ signal following histamine H-1 receptor stimulation by enhancing the total Ins(1,4,5)P-3-sensitive Ca2+-releasing capacity via a discrete Ca2+ compartment.

Proteins which bind with high affinity Ins 1,3,4,5-P4 or Ins 1,4,5-P3 were solubilized from porcine cerebellar membranes. Both binding activities were separated by heparin-agarose chromatography. The Ins 1,3,4,5-P4 receptor was partially purified with an approximately 1000-fold enrichment as compared to the membrane preparation. In the receptor-enriched preparation the Ins 1,3,4,5-P413 binding protein had an affinity (Kd) for Ins 1,3,4,5-P4 of 4.6 nM. Ins 1,3,4,5,6-P5 displaced [3H]Ins 1,3,4,5-P4 binding with a comparable affinity. The Ins 1,3,4,5-P4 binding protein displayed high selectivity for Ins 1,3,4,5-P4 over other inositol-phosphates (IC50 for Ins 1,4,5,6-P4 150 nM, for Ins-P6 1 microM and for Ins 1,3,4-P3 5 microM). Most importantly, Ins 1,4,5-P3 did not displace [3H]Ins 1,3,4,5-P4 binding at concentrations up to 10 microM. Binding of Ins 1,3,4,5-P4 was maximal in the pH range between 4.5 and 6, was stable with Ca2+ concentration varied from 1 nM to 1 mM, and was suppressed by heparin (IC50 about 2 nM). The high affinity receptor for Ins 1,3,4,5-P4 reported here, which is distinct from the Ins 1,4,5-P3 receptor might allow to evaluate the possible functional role of Ins 1,3,4,5-P4 in the cellular signal transduction.

1. The effects of the specific protein kinase C (PKC) inhibitor, GF109203X, were measured on the cytoplasmic Ca2+ concentration ([Ca2+]i), and on histamine H1 receptor- and thapsigargin-mediated increases in [Ca2+]i in DDT1 MF-2 smooth muscle cells. 2. After pretreatment of cells with GF109203X (5 microM, 45 min), the histamine (100 microM)-induced initial rise in [Ca2+]i, representing Ca2+ mobilization from internal stores, was inhibited (by 59 +/- 7%). The slowly declining phase of the histamine induced Ca2+ response, reflecting Ca2+ entry, was enhanced (83 +/- 26%) in the presence of the PKC inhibitor. 3. The histamine induced release of Ca2+ from internal stores, measured after blocking Ca2+ entry with LaCl3 was inhibited by GF109203X in a concentration-dependent manner (IC50: 3.1 +/- 1.1 microM). 4. Histamine-induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) was not changed in the presence of GF109203X. 5. The PKC activating phorbol ester, phorbol 12-myristate 13-acetate (PMA, 1 microM), strongly reduced histamine-induced Ins(1,4,5)P3 formation (58 +/- 16%). This effect was reversed by GF109203X (5 microM). Furthermore, PMA diminished histamine evoked Ca2+ release (50 +/- 6%) and blocked Ca2+ entry completely. 6. The rise in [Ca2+]i caused by blocking endoplasmic reticulum Ca2(+)-ATPase with thapsigargin (1 microM), was strongly reduced (57 +/- 3%) after pretreatment of cells with GF109203X. Downregulation of PKC by long-term pretreatment of cells with PMA (1 microM, 48 h) did not abolish this effect of GF109203X (48 +/- 3% inhibition). 7. In permeabilized DDT, MF-2 cells preloaded with 45Ca2+ in the presence of GF109203X, the amount of 45Ca2+ released by Ins(1,4,5)P3 (10 microM) was markedly reduced (42 +/- 9%). GF109203X did not release Ca2+ itself and did not impair Ins(1,4,5)P3 receptor function. 8. Uptake of 45Ca2+ by intact cells, representing Ca2+ entry, was enhanced by GF109203X (65 +/- 11%), by histamine (24 +/- 6%) and also by thapsigargin (121 +/- 10%). The GF109203X- and the thapsigargin-induced uptake of 45Ca2+ were not additive. 9. These data suggest that GF109203X reduces the filling-state of intracellular Ins(1,4,5)P3 sensitive Ca2+ stores by inhibiting the Ca2+ uptake into these stores, thereby promoting store-dependent (capacitive) Ca2+ entry.

The histamine H-1 receptor mediated increase in cytoplasmic Ca2+ ([Ca2+](i)) was measured in the presence of the known phospholipase C (PLC) inhibitor, neomycin. Neomycin (1 mM) inhibited the histamine (100 mu M) induced rise in [Ca2+](i) to the same extent as observed after blocking Ca2+ entry with LaCl3. Likewise, the increase in [Ca2+](i) after re-addition of CaCl2 (2 mM) to extracellular Ca2+ deprived and histamine pretreated cells was strongly reduced by neomycin. However, neomycin did not inhibit the histamine induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) or the release of Ca2+ from internal stores. These results show that neomycin blocks histamine induced Ca2+ entry independent of phospholipase C activation. Inhibition of intracellular store Ca2+-ATPase by thapsigargin (1 mu M), elicited an increase in [Ca2+](i) due to a leakage from the stores, subsequently followed by store-dependent Ca2+ entry. Thapsigargin induced Ca2+ entry was also completely blocked by neomycin. These results indicate that neomycin inhibits histamine and thapsigargin induced Ca2+ entry. This inhibition is most likely exerted at the level of plasma membrane Ca2+ channels.

This study was carried out to identify the cellular component activating the histamine-stimulated Ca2+ entry in vas-deferens-derived DDT1 MF-2 cells. H-1-histaminoceptor stimulation resulted in a rise in intracellular Ca2+ concentration, caused by Ca2+ release from inositol phosphate-sensitive Ca2+ stores and Ca2+ entry from the extracellular space, accompanied by a transient Ca2+-activated outward K+ current. The histamine-evoked K+ current was still observed after preventing inositol phosphate-induced Ca2+ mobilization by intracellularly applied heparin. This current was activated by Ca2+ entry from the extracellular space, because it was abolished in the presence of the Ca2+-channel blocker La3+ or under Ca2+-free conditions. H-1 histaminoceptor-activated Ca2+ entry was also observed in the presence of intracellularly applied Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4, depleting their respective Ca2+ stores and pre-activating the inositol phosphate-regulated Ca2+ entry. Thus the ability of histamine to activate Ca2+ entry independently of Ca2+ mobilization and the formation of inositol phosphates suggests that another component is involved to initiate the Ca2+-entry process. It was observed that H-1-histaminoceptor stimulation resulted in a pronounced release of arachidonic acid (AA) in DDT1 MF-2 cells. Exogenously applied AA induced a concentration-dependent increase in internal Ca2+ due to activation of Ca2+ entry from the extracellular space. Slow inactivation of the AA-sensitive Ca2+ channels is suggested by the slow decline in Ca2+ entry. In accord, the histamine-induced Ca2+ entry was not observed with AA-pre-activated Ca2+ channels. Inhibition of the lipoxygenase and cyclo-oxygenase pathway did not affect the AA-induced Ca2+ entry. The histamine-induced change in internal Ca2+ and the concomitant Kf current were decreased in the presence of AA and caused by Ca2+ mobilization from internal stores. Blocking this internal Ca2+ release by heparin, in the presence of AA, resulted in abolition of the histamine-induced Ca2+-regulated K+ current. These observations show that AA, released on H-1-histaminoceptor stimulation in DDT1 MF-2 cells, is functioning as a second messenger to activate plasmamembrane Ca2+ channels promoting Ca2+ entry from the extracellular space.

Different effects of injected IP 4 and IP 3 , on membrane Cl − currents were observed in Xenopus oocytes previously microinjected with GH 3 , cell poly(A) + RNA. IP 3 induced a biphasic current response similar to that induced by TRH, whereas IP 4 regularly evoked a monophasic current response consisting of a prolonged inward current with superimposed oscillations. Membrane currents elicited by TRH were enhanced by prior injection of IP 4 , but not of IP 3 . In contrast to IP 3 -induced membrane currents, those evoked by IP 4 were independent of injection depth. Phorbol ester led to enhancement of the initial current after injection of IP 3 , whereas the current oscillations were greater following injection of IP 4 . The results indicate that, in Xenopus oocytes, IP 4 plays a role different from that of IP 3 within the signal transduction pathway initiated by ligand stimulation of a neuropeptide receptor.

The Arg82 gene of Saccharomyces cerevisiae encodes a transcriptional regulator that phosphorylates inositol 1,4,5-trisphosphate [Saiardi et al. (1999) Curr. Biol. 9, 1323–1326]. However, some controversy has surrounded the nature of the reaction products. We now show that Arg82 phosphorylates inositol 1,3,4,5-tetrakisphosphate to inositol pentakisphosphate, which is itself converted to two isomers of diphosphoinositol tetrakisphosphate, one of which has never previously been identified. One of the diphosphoinositol phosphates was further phosphorylated by a yeast cell lysate. We propose that Arg82 is an ancestral precursor of two distinct and specific enzyme families: inositol 1,4,5-trisphosphate kinases and diphosphoinositol polyphosphate synthases.

Heparin was found to inhibit the Ca2+ release induced by inositol 1,4,5-trisphosphate (IP3) in permeabilized pancreatic β-cells obtained from obese hyperglycemic mice. The effect of heparin was dose-dependent and not due to inhibition of Ca2+ uptake into the IP3-sensitive pool. The effect appeared specific for heparin and was not reproduced by other polysaccharides such as chondroitin sulfates. Heparin might consequently be a useful tool when investigating the molecular mechanism whereby IP3 mobilizes Ca2+.

A membrane preparation from porcine cerebellum displays high-affinity binding sites for [3H]inositol 1,3,4,5-tetrakisphosphate ([3H]InsP4) with a dissociation constant (Kd) of 1.0 nM and a density of 220 fmol/mg protein. Specific binding was maximal in the presence of 25 mM phosphate and at pH 5.0. The receptor site was specific for InsP4, since Ins(1,3,4,5,6)P5 and Ins(1,4,5,6)P4 displaced binding of InsP4 with EC50 values of 0.2 and 0.3 μM, respectively. Ins(1,4,5)P3 and other inositol phosphates were less effective. Using this InsP4 receptor, an assay for measuring tissue content of InsP4 was developed. The detection limit of the assay was 0.1 pmol. In the same tissue samples the amount of Ins(1,4,5)P3 was determined in parallel with a similar assay using a binding protein preparation from beef liver.