Calmodulin activates inositol 1,4,5-trisphosphate 3-kinase activity in pig aortic smooth muscle

Inositol 1,4,5-trisphosphate 3-kinases: functions and regulations

Inositol 1,4,5-trisphosphate 3-kinase (IP3 3-kinase/IP(3)K) plays an important role in signal transduction in animal cells by phosphorylating inositol 1,4,5-trisphosphate (IP3) to inositol 1,3,4,5-tetrakisphosphate (IP(4)). Both IP(3) and IP(4) are critical second messengers which regulate calcium (Ca(2+)) homeostasis. Mammalian IP3Ks are involved in many biological processes, including brain development, memory, learning and so on. It is widely reported that Ca(2+) is a canonical second messenger in higher plants. Therefore, plant IP3K should also play a crucial role in plant development. Recently, we reported the identification of plant IP3K gene (AtIpk2beta/AtIP3K) from Arabidopsis thaliana and its characterization. Here, we summarize the molecular cloning, biochemical properties and biological functions of IP3Ks from animal, yeast and plant. This review also discusses potential functions of IP3Ks in signaling crosstalk, inositol phosphate metabolism, gene transcriptional control and so on.

A wave of IP3 production accompanies the fertilization Ca2+ wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free Ca2+ that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca2+ wave involves an increase in inositol-1,4,5-trisphosphate (IP3) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP3-dependent Ca2+ mobilizing machinery. We propose a detailed model of the fertilization Ca2+ wave in Xenopus eggs that requires an accompanying wave of IP3 production. While the Ca2+ wave is initiated by a localized increase of IP3 near the site of sperm-egg fusion, the Ca2+ wave propagates via IP3 production correlated with the Ca2+ wave-possibly via Ca(2+)-mediated PLC activation. Such a Ca(2+)-mediated IP(3) production wave has not been required previously to explain the fertilization Ca2+ wave in eggs; we argue this is necessary to explain the observed IP3 dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP3 levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP3 levels in unfertilized eggs, but after fertilization, [IP3] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degrees away once the Ca2+ wave passed that region about 2 min after fertilization. Prior to the Ca2+ wave reaching that region the IP3 levels were undetectable. Since significant IP3 could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP3 production.

Role of PRIP-1, a novel Ins(1,4,5)P3 binding protein, in Ins(1,4,5)P3-mediated Ca2+ signaling

PRIP-1 was isolated as a novel inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] binding protein with a domain organization similar to phospholipase C-delta1 (PLC-delta1) but lacking the enzymatic activity. Further studies revealed that the pleckstrin homology (PH) domain of PRIP-1 is the region responsible for binding Ins(1,4,5)P3. In this study we aimed to clarify the role of PRIP-1 at the physiological concentration in Ins(1,4,5)P3-mediated Ca2+ signaling, as we had previously used COS-1 cells overexpressing PRIP-1 (Takeuchi et al., 2000, Biochem J 349:357-368). For this purpose we employed PRIP-1 knock out (PRIP-1-/-) mice generated previously (Kanematsu et al., 2002, EMBO J 21:1004-1011). The increase in free Ca2+ concentration in response to purinergic receptor stimulation was lower in primary cultured cortical neurons prepared from PRIP-1-/- mice than in those from wild type mice. The relative amounts of [3H]Ins(1,4,5)P3 measured in neurons labeled with [3H]inositol was also lower in cells from PRIP-1-/- mice. In contrast, PLC activities in brain cortex samples from PRIP-1-/- mice were not different from those in the wild type mice, indicating that the hydrolysis of Ins(1,4,5)P3 is enhanced in cells from PRIP-1-/- mice. In vitro analyses revealed that type1 inositol polyphosphate 5-phosphatase physically interacted with a PH domain of PRIP-1 (PRIP-1PH) and its enzyme activity was inhibited by PRIP-1PH. However, physical interaction with these two proteins did not appear to be the reason for the inhibition of enzyme activity, indicating that binding of Ins(1,4,5)P3 to the PH domain prevented its hydrolyzation. Together, these results indicate that PRIP-1 plays an important role in regulating the Ins(1,4,5)P3-mediated Ca2+ signaling by modulating type1 inositol polyphosphate 5-phosphatase activity through binding to Ins(1,4,5)P3.

Differential expression of Ca(2+)-binding proteins on follicular dendritic cells in non-neoplastic and neoplastic lymphoid follicles

We studied the Ca(2+)-capture ability of follicular dendritic cells (FDCs) in tonsillar secondary lymphoid follicles (LFs) and the expression of six Ca(2+)-binding proteins (CBPs), caldesmon, S-100 protein, calcineurin, calbindin-D, calmodulin, and annexin VI in LFs of various lymphoid tissues and caldesmon and S-100 protein in neoplastic follicles of follicular lymphomas. First, Ca(2+)-capture cytochemistry revealed extensive Ca(2+) capture in the nuclei and cytoplasm of FDCs, but little or none in follicular lymphocytes. All six CBPs were localized immunohistochemically in the LFs and were always present in the basal light zone. Immunoelectron microscopic staining of FDCs was classified into two patterns: caldesmon was distributed in the peripheral cytoplasm like a belt; S-100 protein, calcineurin, calbindin-D, and calmodulin were distributed diffusely in the cytosol. Annexin VI was, however, negative on FDCs. Immunocytochemistry also demonstrated CBP-positive FDCs within FDC-associated clusters isolated from germinal centers. In situ hybridization revealed diffuse calmodulin mRNA expression throughout the secondary LFs. These data indicate that the CBPs examined may regulate Ca(2+) in the different subcellular sites of FDCs, and the roles of CBPs may be heterogeneous. We also investigated the distribution of caldesmon and S-100 protein in follicular lymphomas on paraffin-embedded tissue sections. FDCs within grades I and II neoplastic follicles clearly expressed caldesmon, but not S-100 protein, except a part of grade II neoplastic follicles. FDCs within grade III follicles showed no caldesmon, but frequently expressed S-100 protein. These results demonstrate that the caldesmon and S-100 protein staining patterns of grade I follicular lymphomas are different from those of grade III follicular lymphomas and suggest that FDC networks in grade I neoplastic follicles may be similar to those in the light zone within non-neoplastic follicles, FDC networks in grade III neoplastic follicles may be similar to those in dark and basal light zones within non-neoplastic follicles, and grade II follicles may be intermediate between grade I and grade III follicles.

Use of phosphorofluoridate analogues of D-myo-inositol 1,4,5-trisphosphate to assess the involvement of ionic interactions in its recognition by the receptor and metabolising enzymes

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] analogues fluoridated at 4- or 5-phosphate or both were analysed to assess the involvement of ionic interactions between the phosphates of Ins(1,4,5)P3 and the proteins that recognize it, such as metabolic enzymes and the InsP3 receptor. These analogues were effective in inhibiting type I Ins(1,4,5)P3 5-phosphatase activity with much the same potency as Ins(1,4,5)P3, although the enzyme showed a lower Km value as pH values increased. In contrast, the analogues were less potent ligands than Ins(1,4,5)P3 in both the assay of [3H]Ins(1,4,5)P3 binding to the receptors and the phosphorylation of [3H]Ins(1,4,5)P3 catalysed by Ins(1,4,5)P3 3-kinase. These results suggest that ionic interactions with the dianionic 4- and 5-phosphates of Ins(1,4,5)P3 are involved in recognition by the receptor and the kinase, but not by the phosphatase.

Cutting Edge: Role of the Inositol Phosphatase SHIP in B Cell Receptor-Induced Ca2+ Oscillatory Response

Src homology-2 domain-containing inositol polyphosphate 5′-phosphatase (SHIP) is a recently identified protein that has been implicated as an important signaling molecule. Although SHIP has been shown to participate in the FcγRIIB-mediated inhibitory signal, the functional role of SHIP in activation responses by immunoreceptor tyrosine-based activation motif-bearing receptors such as B cell receptor (BCR) remains unclear. Indeed, it has been proposed that SHIP serves as a linking molecule for the regulation of the extracellular signal-regulated kinase pathway in BCR signaling, because SHIP associates with Shc. We now report that SHIP-deficient DT40 B cells display enhanced Ca2+ mobilization in response to BCR ligation, whereas extracellular signal-regulated kinase activation is unaffected. This Ca2+ enhancement is due to a sustained intracellular Ca2+ increase or to long-lasting Ca2+ oscillations by loss of SHIP, as revealed by single-cell Ca2+ imaging analysis. These results demonstrate the importance of SHIP in B cell activation by the modulation of Ca2+ mobilization.

Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the oocytes of Xenopus laevis

The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured in Xenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3] and the intracellular free Ca2+ concentration ([Ca2+]). At low [IP3] (100 nM) and high [Ca2+] (>/=1 microM), IP3 was metabolized predominantly by inositol 1,4, 5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 microM or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both </=400 nM), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was approximately 300 micron suggesting that IP3 acts as a global messenger in oocytes. In contrast to IP3, inositol 1,3,4, 5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was an order of magnitude or greater than that in small mammalian cells.

D-myo-inositol 1,4,5-trisphosphate 3-kinase A is activated by receptor activation through a calcium:calmodulin-dependent protein kinase II phosphorylation mechanism

D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] 3-kinase, the enzyme responsible for production of D-myo-inositol 1,3,4,5-tetrakisphosphate, was activated 3- to 5-fold in homogenates of rat brain cortical slices after incubation with carbachol. The effect was reproduced in response to UTP in Chinese hamster ovary (CHO) cells overexpressing Ins(1,4,5)P3 3-kinase A, the major isoform present in rat and human neuronal cells. In ortho-32P-labelled cells, the phosphorylated 53 kDa enzyme could be identified after receptor activation by immunoprecipitation. The time course of phosphorylation was very similar to that observed for carbachol (or UTP)-induced enzyme activation. Enzyme phosphorylation was prevented in the presence of okadaic acid. Calmodulin (CaM) kinase II inhibitors (i.e. KN-93 and KN-62) prevented phosphorylation of Ins(1,4,5)P3 3-kinase. Identification of the phosphorylation site in transfected CHO cells indicated that the phosphorylated residue was Thr311. This residue of the human brain sequence lies in an active site peptide segment corresponding to a CaM kinase II-mediated phosphorylation consensus site, i.e. Arg-Ala-Val-Thr. The same residue in Ins(1,4,5)P3 3-kinase A was also phosphorylated in vitro by CaM kinase II. Phosphorylation resulted in 8- to 10-fold enzyme activation and a 25-fold increase in sensitivity to the Ca2+:CaM complex. In this study, direct evidence is provided for a novel regulation mechanism for Ins(1,4,5)P3 3-kinase (isoform A) in vitro and in intact cells.

Purification and properties of D-myo-inositol 1,4,5-trisphosphate 3-kinase from bovine iris sphincter smooth muscle: effects of protein phosphorylation in vitro and in intact muscle

Stimulation of bovine iris sphincter muscle with carbachol (10 microM) increased accumulation of Ins(1,4,5)P3 (InsP3) and Ins(1,3,4,5)P4 (InsP4) by 86 and 32% respectively. Addition of isoproterenol (5 microM) to muscle pretreated with carbachol reduced the 3H-radioactivity in InsP3 by 30% and increased that of InsP4 by 41%. InsP3 3-kinase was predominantly localized in the soluble fraction (110,000 g supernatant) of the iris sphincter. The enzyme was purified from this fraction by sequential chromatography on DEAE-cellulose, calmodulin (CAM)-agarose affinity, and Mono-Q anion-exchange columns. The specific activity of the purified enzyme was 1.94 mumol/min per mg protein with a purification of 114-fold, compared with the cytosolic fraction of the muscle. SDS/PAGE showed the enzyme to be associated with a protein band corresponding to 50 kDa. In the presence of 10 microM Ca2+, CaM dose-dependently stimulated the enzyme. InsP3 3-kinase specifically phosphorylated InsP3 with an apparent K(m) of 0.56 microM and a Vmax. of 2.5 mumol/min per mg protein. The stimulatory effect of CaM was due to a change in Vmax. and not in its K(m). The enzyme was maximally active at pH 7.0-7.5. Phosphorylation of the purified InsP3 3-kinase with protein kinase A increased its activity; in contrast, phosphorylation with protein kinase C inhibited the enzyme activity. Treatment of the intact iris sphincter with isoproterenol or phorbol 12,13-dibutyrate resulted in stimulation of InsP3 3-kinase activity in the soluble fraction and this activation was preserved on SDS/PAGE and renaturation. These results indicate that the bovine iris sphincter contains a Ca-CaM-dependent InsP3 3-kinase which can be differentially regulated, both in vitro and in intact muscle, by protein kinases A and C.

D-myo-inositol 1,4,5-trisphosphate analogues substituted at the 3-hydroxyl group

D-myo-Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) analogues derived at 3-OH with a bulky substituent were chemically synthesized and structural features of vicinity surrounding the 3-OH of Ins(1,4,5)P3, recognized by metabolic enzymes and by the receptor were explored. 3-Benzoyl-, 3-methylbenzoyl- and 3-para-aminobenzoyl-Ins(1,4,5)P3 inhibited the dephosphorylation of [3H]Ins(1,4,5)P3 by the 5-phosphatase present in erythrocyte ghosts, but the potency varied. The inhibitory potency for the former two compounds was slightly lower than that for Ins(1,4,5)P3, while that for the latter compound was higher. Transfer of the amino group to the meta-position of the benzoyl group led to a less potent analogue. In an assay of [3H]Ins(1,4,5)P3 3-kinase at a low Ca2+ concentration, catalyzed by rat brain cytosol, 3-meta-aminobenzoyl-Ins(1,4,5)P3 was the most potent among compounds examined, including Ins(1,4,5)P3 in inhibiting the phosphorylation, whereas both 3-benzoyl- and 3-methylbenzoyl-Ins(1,4,5)P3 at concentrations up to 30 microM, were without effect. All analogues examined were effective in inhibiting [3H]Ins(1,4,5)P3 binding to purified Ins(1,4,5)P3 receptor, but all 3-derived analogues were less potent and 3-benzoyl-Ins(1,4,5)P3 was the least potent. It would thus appear that the space in the vicinity surrounding the 3-hydroxyl group of Ins(1,4,5)P3 is sterically restrictive with regard to recognition by metabolic enzymes and the receptor, whereas the amino group providing arms for either the electrostatic interaction or the hydrogen bond, makes the analogues more potent.

Calmodulin: effects of cell stimuli and drugs on cellular activation

The activity, localization and cellular content of CaM can be regulated by drugs, hormones and neurotransmitters. Regulation of physiological responses of CaM can depend upon local Ca(2+)-entry domains in the cells and phosphorylation of CaM target proteins, which would either decrease responsiveness of CaM target enzymes or increase CaM availability for binding to other target proteins. Despite the abundance of CaM in many cells, persistent cellular activation by a variety of substances can lead to an increase in CaM, reflected both in the nucleus and other cellular compartments. Increases in CaM-binding proteins can accompany stimuli-induced increases in CaM. A role for CaM in vesicular or protein transport, cell morphology, secretion and other cytoskeletal processes is emerging through its binding to cytoskeletal proteins and myosins in addition to the more often investigated activation of target enzymes. More complete knowledge of the physiological regulation of CaM can lead to a greater understanding of its role in physiological processes and ways to alter its actions through pharmacology.

Cellular actions of inositol phosphates and other natural calcium and magnesium chelators

Naturally occurring chelators of Ca2+ and Mg2+ have largely been unrecognized due to their low binding affinities. They include carbohydrate and cyclitol phosphates, nucleotides and nucleic acids. The calciotrophic inositol phosphates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 form chelates within the range of Ca2+ concentrations found in biological systems. As well as being a likely source of experimental artifact where these compounds have been investigated at unphysiological cation concentrations, chelation may have important physiological roles. The autoregulation of Ca2+ entry into the cell cytosol is one, whereas the coupling of chelation with enzyme or receptor interactions offers a general mechanism for divalent cation control of diverse biological processes. Inositol monophosphate 1-phosphatase and inositol polyphosphate 1-phosphatase are two related enzymes which may conform to this mechanism. If so, it would provide a possible explanation for their sensitivity to divalent cations and for their non-competitive inhibition by lithium ion.

InsP3 and Ins(1,3,4,5)P4 act in synergy to stimulate influx of extracellular Ca2+ in Xenopus oocytes

To investigate the role of D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] in the regulation of Ca2+ influx, we injected inositol phosphates into Xenopus oocytes and measured Ca(2+)-gated Cl- current to assay intracellular free Ca2+ concentration ([Ca2+]i). To assess Ca2+ influx, we removed extracellular Ca2+ or added the inorganic Ca2+ channel blocker Mn2+ to the extracellular bath and measured the resulting change in Cl- current. Ins(1,3,4,5)P4 did not cause Ca2+ influx when injected alone or when preceded by an injection of Ca2+. In contrast, Ins(1,3,4,5)P4 stimulated Ca2+ influx when injected after the poorly metabolized inositol trisphosphate (InsP3) analogues D-myo-inositol 1,4,5-trisphosphorothioate [Ins(1,4,5)P3S3] or D-myo-inositol 2,4,5-trisphosphate [Ins(2,4,5)P3]. These results indicate that Ins(1,3,4,5)P4 is not sufficient to stimulate Ca2+ influx but acts in synergy with InsP3s to cause Ca2+ influx. We also studied the effect of Ca2+ influx on the immediate metabolism of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in single oocytes. Ca2+ influx shunted the metabolism of Ins(1,4,5)P3 toward the formation of Ins(1,3,4,5)P4 and away from D-myo-inositol 1,4-bisphosphate [Ins(1,4)P2]. These results suggest that there is a positive feedback regulatory mechanism in which Ca2+ influx stimulates Ins(1,3,4,5)P4 production and Ins(1,3,4,5)P4 stimulates further Ca2+ influx.

Maturational regulation of inositol 1,4,5-trisphosphate metabolism in rabbit airway smooth muscle

Airway reactivity has been shown to vary with age; however, the mechanism(s) underlying this process remain unidentified. To elucidate the role of ontogenetic changes in phosphoinositide-linked signal transduction, we examined whether age-related differences in tracheal smooth muscle (TSM) contractility to carbachol (CCh) are associated with developmental changes in the production and metabolism of the second messenger, inositol 1,4,5-trisphosphate (Ins (1,4,5)P3). In TSM segments isolated from 2-wk-old and adult rabbits, both the maximal isometric contractile force and sensitivity (i.e., -logED50) to CCh (10(-10)-10(-4) M) were significantly greater in the immature vs. adult tissues (P less than 0.001). Similarly, Ins(1,4,5)P3 accumulation elicited by either receptor-coupled stimulation with CCh (10(-10)-10(-4) M) or post-receptor-mediated guanine nucleotide binding protein activation of permeabilized TSM with GTP gamma S (100 microM) was also significantly enhanced in 2-wk-old vs. adult TSM. Measurement of the activities of the degradative enzymes for Ins(1,4,5)P3 demonstrated that: (a) mean +/- SE maximal Ins(1,4,5)P3 3'-kinase activity was significantly reduced in the immature vs. adult TSM (i.e., approximately 71.7 +/- 6.0 vs. 137.8 +/- 10.0 pmol/min per mg protein, respectively; P less than 0.005); (b) by contrast, maximal Ins(1,4,5)P3 5'-phosphatase activity was significantly increased in the immature vs. adult TSM (i.e., 27.9 +/- 1.2 vs. 15.6 +/- 1.5 nmol/min per mg protein, respectively; P less than 0.001); and (c) the Km values for Ins(1,4,5)P3 5'-phosphatase were 14- and 19-fold greater than those for Ins(1,4,5)P3 3'-kinase in the 2-wk-old and adult TSM, respectively. Collectively, the findings suggest that the age-related decrease in agonist-induced rabbit TSM contractility is associated with a diminution in Ins(1,4,5)P3 accumulation which is attributed, at least in part, to ontogenetic changes in the relative activities of the degradative enzymes for Ins(1,4,5)P3.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.

Inositol trisphosphate kinase activity in hypertrophied rat heart

In the present experiment, we demonstrated that IP3 kinase activity was increased in SHRSP heart compared to WKY heart. IP3 kinase activity in the heart was highest in the cytosolic fraction in both SHRSP and WKY. Its activity progressively increased with age in 5- to 20-week SHRSP. The activity reached about three times the level of 5-week-old SHRSP in 40-week-old SHRSP. On the other hand, in WKY it was 1.3-fold at 40 weeks compared with that at 5 weeks. We determined the effect of divalent cations on IP3 kinase activity. Ca2+ stimulated its activity in a dose-dependent manner at 10(-9) to 10(-6) M. In SHRSP it was enhanced about 2.1-fold at 10(-6) M of Ca2+, but in WKY it was 1.5-fold at 1.0(-6) M of Ca2+. Mn2+ also stimulated IP3 kinase activity in both groups of animals, while, Fe2+, Zn2+, and Cu2+ inhibited IP3 kinase activity. In our experiment IP3 kinase activity was increased in SHRSP and its activity was markedly affected by divalent cations. These data suggest that the accumulations of IP3 and IP4 after hormonal stimulation play a physiologic role, possibly by alteration of Ca2+ levels in cardiac tissue.

Sphingosine increases inositol trisphosphate in rat parotid acinar cells by a mechanism that is independent of protein kinase C but dependent on extracellular calcium

In rat parotid acinar cells prelabelled with [3H]-inositol, sphingosine stimulated the accumulation of [3H]-inositol polyphosphates. When the cells were exposed to sphingosine, [3H]-inositol trisphosphate (InsP3) was accumulated in a time- and dose-dependent manner. When the extracellular Ca2+ was chelated by 1 mM EGTA, the effect of sphingosine on InsP3 accumulation was completely inhibited. Ionophores, A23187 and ionomycin, had no significant effect on InsP3 accumulation. An inhibitor of protein kinase C, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), failed to stimulate InsP3 accumulation. In the homogenate of parotid acinar cells, InsP3 3-kinase and 5-phosphomonoesterase activities were not affected by sphingosine. These results suggest that sphingosine activates phosphoinositide turnover by a mechanism dependent upon extracellular Ca2+, but different from that of an ionophore, and independent of protein kinase C.

Inositol 1,4,5-trisphosphate affinity chromatography

Inositol 1,4,5-trisphosphate (IP3) affinity columns were made by coupling IP3 analogs to a supporting matrix. Sepharose 4B. IP3 5-phosphatase activity. IP3 3-kinase activity and IP3 binding activity from rat brain were absorbed on the IP3 columns. and were eluted by increasing KC1 concentration. This purification procedure increased the specific activities of these parameters 5-200-fold. Thus Sepharose 4B immobilized IP3 analogs can specifically interact with IP3-binding proteins, demonstrating that IP3 affinity columns are a good method for purifying such proteins. Furthermore, our results suggest that IP3 analogs can be linked to other molecules to make useful derivatives without loss of their biological activities.

Inhibitory action of alpha-human atrial natriuretic peptide on noradrenaline-induced synthesis of myo-inositol 1,4,5-trisphosphate in the smooth muscle cells of rabbit aorta

1. Interactions between the synthesis of myo-inositol 1,4,5-trisphosphate (IP3) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) in the smooth muscle cells of the rabbit aorta were investigated. 2. In the presence or absence of vascular endothelium, noradrenaline (NA; 5 microM) consistently reduced the amount of phosphatidylinositol 4,5-bisphosphate (PI-P2) and increased both phosphatidic acid (PA) and IP3. 3. In the presence or absence of endothelium, acetylcholine (ACh; 100 microM but not 5 microM) slightly increased the amount of IP3, but exposure to ACh (100 microM) 4 min after application of NA did not modify NA-induced synthesis of IP3. 4. ACh (100 microM) markedly enhanced the synthesis of cyclic GMP in the presence of endothelium but not in the endothelium-denuded tissues. 5. Prazosin (5 microM) but not dibutyryl cyclic GMP (db-cyclic GMP; 100 microM) blocked the hydrolysis of PI-P2 induced by 5 microM NA. Synthesis of IP3 induced by NA, as estimated with [3H]-inositol was not modified by application of 100 microM db-cyclic AMP or db-cyclic GMP. 6. alpha-Human atrial natriuretic peptide (alpha-hANP; 0.1 microM) increased cyclic GMP in the presence or absence of endothelium. alpha-hANP (0.1 microM) consistently inhibited the hydrolysis of PI-P2 induced by 5 microM NA. 7. The results indicate that synthesis of IP3 is inhibited neither by the synthesis of cyclic GMP in the cytosol nor by cyclic GMP itself. However, synthesis of IP3 through hydrolysis of PI-P2 may be inhibited by an interaction between some steps in the IP3 synthetic process and by the activation of the alpha-hANP-guanylate cyclase process at the sarcolemma.

Ca2+/calmodulin independent inositol 1,4,5-trisphosphate 3-kinase activity in guinea pig peritoneal macrophages

1. The Ca2+/calmodulin (CaM) independent activity of inositol 1,4,5-trisphosphate (InsP3) 3-kinase in macrophages could be separated from the dependent activity by serial column chromatography, gel filtration, Orange A and DEAE-5PW. 2. An InsP3 analog which has an aminobenzoyl group on the 2nd carbon of the inositol ring inhibited the conversion of [3H]InsP3 to [3H]InsP4 (inositol 1,3,4,5-tetrakisphosphate) in a dose-dependent manner. The concentration required for half-maximal inhibition (IC50) with the Ca2+/CaM independent enzyme activity was also dependent on the free Ca2+ concentration, as with the dependent activity. 3. These results suggest that a conformational change in the enzyme occurs in response to a change in free Ca2+ concentration, and thus the potency to recognize the InsP3 analog would change, even when the Ca2+/CaM independent enzyme activity was used.

Ca2+ and calmodulin-sensitive inositol trisphosphate kinase from bovine parathyroid

A Ca2+ and calmodulin-activated inositol 1,4,5 trisphosphate kinase activity was detected in both soluble and membrane fractions from bovine parathyroid glands. Ca2+ activated the soluble enzyme in the concentration range 100 nM to 1 microM, which corresponds to the Ca2+ concentration range observed in the intact cell following maximal variation in extracellular Ca2+, the principal regulator of parathyroid hormone release. The Ca2+ sensitivity of the enzyme was absolutely dependent upon calmodulin. A similar activity was detected in the membranes but could be progressively removed by repeated washing at low ionic strength. This, together with data demonstrating binding of the enzyme to the hydrophobic matrix, Phenyl-Sepharose, suggests that the association of the enzyme with the membrane is likely to involve a significant hydrophobic component. The organic base, amiloride was identified as an inhibitor of the activity, the degree of inhibition being most marked in the presence of Ca2+ and calmodulin (K0.5 approx. 0.1 mM). The Ca2+ concentration dependence of the IP3 kinase suggests that inositol 1,3,4,5 tetrakisphosphate may be a messenger in the signal transduction pathway for the feedback inhibition of PTH secretion by extracellular Ca2+.

Mode of activation of bovine brain inositol 1,4,5-trisphosphate 3-kinase by calmodulin and calcium

The effect of Ca2+ and calmodulin (CaM) on the activation of purified bovine brain Ins(1,4,5)P3 kinase was quantified and interpreted according to the model of sequential equilibria generally used for other calmodulin-stimulated systems. Two main conclusions can be drawn. (i) CaM.Ca3 and CaM.Ca4 together are the biologically active species in vitro, as is the case for the great majority of other calmodulin targets. (ii) These species bind in a non-co-operative way to the enzyme with an affinity constant of 8.23 x 10(9) M-1, i.e. approx 10-fold higher than for most calmodulin-activated target enzymes. The dose-response curve of the activation of Ins(1,4,5)P3 kinase by calmodulin is not significantly impaired by melittin and trifluoperazine, whereas under very similar assay conditions the half-maximal activation of bovine brain cyclic AMP phosphodiesterase requires over 30-50-fold higher concentrations of CaM when 1 microM melittin or 20 microM-trifluoperazine is present in the assay medium. Similarly, 1 microM of the anti-calmodulin peptides seminalplasmin and gramicidin S, as well as 20 microM of N-(6-aminohexyl)-5-chloro-1-naphthalene-sulphonamide (W7), do not inhibit the activation process. These data suggest that binding and activation of Ins(1,4,5)P3 kinase require surface sites of calmodulin which are different from those involved in the binding of most other target enzymes or of model peptides.

Partial purification of inositol polyphosphate 1-phosphomonoesterase with characterization of its substrates and products by nuclear magnetic resonance spectroscopy

A study of the enzyme activities that degrade Ins(1,3,4)P3 in rat brain showed that it was dephosphorylated primarily by a Mg2+-dependent inositol polyphosphate 1-phosphomonoesterase to Ins(3,4)P2 and then to Ins(3)P by a 4-phosphomonoesterase. A less active enzyme activity with the properties of a 4-phosphomonoesterase that converted Ins(1,3,4)P3 to Ins(1,3)P2 was also detected. The inositol polyphosphate 1-phosphomonoesterase was separated from the 4-phosphomonoesterase and the inositol monophosphate phosphomonoesterase by chromatography on phosphocellulose, DE-52 anion exchange and hydroxylapatite columns. Kinetic characterization of the partially purified inositol polyphosphate 1-phosphomonoesterase indicated that both Ins(1,3,4)P3 and Ins(1,4)P2 were substrates with apparent Km values of 0.9 microM and 0.7 microM, respectively. Either substrate was a competitive inhibitor of the other substrate and dephosphorylation of both substrates was directly inhibited by Li+ in an uncompetitive manner. These data strongly suggest that a single enzyme dephosphorylates both Ins(1,3,4)P3 and Ins(1,4)P2. The 4-phosphomonoesterase that dephosphorylated Ins(3,4)P2 to Ins(3)P was insensitive to Mg2+ and Li+ and was probably the same enzyme that degraded Ins(1,3,4)P3 to Ins(1,3)P2. The isomeric configurations of the major inositol polyphosphates formed from the degradation of Ins(1,3,4,5)P4 were determined using 1H- and 31P-NMR spectroscopy, and confirmation of the structures assigned to Ins(1,3,4,5)P4, Ins(1,3,4)P3 and Ins(3,4)P2 was obtained.

Stimulation of hepatic inositol 1,4,5-trisphosphate kinase activity by Ca2+-dependent and -independent mechanisms

A cytosolic fraction derived from rat hepatocytes was used to investigate the regulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase, the enzyme which converts Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The activity was doubled by raising the free Ca2+ concentration of the assay medium from 0.1 microM to 1.0 microM. A 5 min preincubation of the hepatocytes with 100 microM-dibutyryl cyclic AMP (db.cAMP) plus 100 nM-tetradecanoylphorbol acetate (TPA) resulted in a 40% increase in Ins(1,4,5)P3 kinase activity when subsequently assayed at 0.1 microM-Ca2+. This effect was smaller at [Ca2+] greater than 0.5 microM, and absent at 1.0 microM-Ca2+. Similar results were obtained after preincubation with 100 microM-db.cAMP plus 300 nM-vasopressin (20% increase at 0.1 microM-Ca2+; no effect at 1.0 microM-Ca2+). Preincubation with vasopressin, db.cAMP or TPA alone did not alter Ins(1,4,5)P3 kinase activity. It is proposed that these results, together with recent evidence implicating Ins(1,3,4,5)P4 in the control of Ca2+ influx, could be relevant to earlier findings that hepatic Ca2+ uptake is synergistically stimulated by cyclic AMP analogues and vasopressin.

Cyclosporin A augments vasoconstrictor-induced rise in intracellular free calcium in rat renal mesangial cells

Pretreatment of rat renal mesangial cells with the immunosuppressive drug cyclosporin A caused a dose-dependent increase in the angiotensin II, [Arg8]vasopressin and noradrenalin-stimulated rise in intracellular free calcium as measured with quin 2. Cyclosporin A had no significant effect on basal cytosolic free calcium. However, cyclosporin A increased the basal 45Ca2+ influx. This stimulated 45Ca2+ influx was not blocked by nifedipine (10(-6) M). Cyclosporin A also augmented the angiotensin II, [Arg8]vasopressin and noradrenalin-stimulated efflux of 45Ca2+ from mesangial cells. These results suggest that cyclosporin A stimulates transmembrane Ca2+ influx in mesangial cells and also augments the vasoconstrictor-induced increases in cytosolic free calcium.

Characterization of inositol 1,3,4-trisphosphate phosphorylation in rat liver

Liver homogenates phosphorylated Ins 1,3,4-P3 to an InsP4 isomer that was distinct from Ins 1,3,4,5-P4. This InsP4 isomer accumulated in vasopressin stimulated hepatocytes prelabeled with myo-[3H]inositol with a time course that lagged behind Ins 1,3,4-P3 formation. The Ins 1,3,4-P3 kinase responsible for its formation was partially purified from rat liver. The enzyme had a Km for Ins 1,3,4-P3 of 0.29 microM, a Km for ATP of 141 microM and was not affected by changes in free Ca2+ in the physiological range. The relationship of this new InsP4 isomer to the inositol phosphate signaling pathway is discussed.

Calcium modulates the generation of inositol 1,3,4-trisphosphate in human platelets by the activation of inositol 1,4,5-trisphosphate 3-kinase

We observed that more total inositol trisphosphate (InsP3) was formed when human platelets were stimulated with agonists (15-hydroxy-9,11-azo-prosta-5,13-dienoic acid or thrombin) in the presence of extracellular Ca2+ than in its absence. Analysis of the InsP3 by h.p.l.c. indicated that the increased InsP3 formed in the presence of extracellular Ca2+ was primarily the 1,3,4-trisphosphate [Ins(1,3,4)P3]. In addition, more inositol 1,3,4,5-tetrakisphosphate (InsP4) was formed in the presence of extracellular Ca2+. Experiments conducted with electrically permeabilized platelets demonstrated that conversion of [3H]Ins(1,4,5)P3 to [3H]InsP4 in platelets was Ca2+-dependent, with half-maximal conversion observed at approx. 2.5 microM-Ca2+. By contrast, dephosphorylation of [3H]InsP4 to [3H]Ins(1,3,4)P3 was not activated by Ca2+. A partially purified preparation of Ins(1,4,5)P3 3-kinase from human platelets was found to be insensitive to Ca2+, but addition of calmodulin restored Ca2+-sensitivity to the kinase, increasing its activity about 5-fold. These results show that in human platelets the metabolism of Ins(1,4,5)P3 is regulated by Ca2+-calmodulin, and suggest that the metabolites of Ins(1,4,5)P3 may also have important second-messenger functions in platelets, and are consistent with the hypothesis that the activation of phospholipase C is not dependent on extracellular Ca2+.

Inositol phosphates: proliferation, metabolism and function

After the initial discovery of receptor-linked generation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) it was generally assumed that Ins(1,4,5)P3 and its proposed breakdown products inositol(1,4)bisphosphate (Ins(1,4)P2) and Ins1P, along with cyclic inositol monophosphate, were the only inositol phosphates found in significant amounts in animal cells. Since then, three levels of complexity have been introduced. Firstly, Ins(1,4,5)P3 can be phosphorylated to Ins(1,3,4,5)P4, and the subsequent metabolism of these two compounds has been found to be intricate and probably different between various tissues. The functions of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are almost certainly to regulate cytosolic Ca2+ concentrations, but the reasons for the labyrinth of the metabolic pathways after their deactivation by a specific 5-phosphatase remain obscure. Secondly, inositol pentakis- and hexakisphosphates have been found in many animal cells other than avian erythrocytes. It has been shown that their synthesis pathway is entirely separate from the inositol phosphates discussed above, both in terms of many of the isomers involved and probably in the subcellular localization; some possible functions of InsP5 and InsP6 are discussed here. Thirdly, cyclic inositol polyphosphates have been reported in stimulated tissues; the evidence for their occurrence in vivo and their possible physiological significance are also discussed.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Protein phosphorylation and hormone action

Many key regulatory proteins exist in cells as either a phosphorylated or a dephosphorylated form, their steady-state levels of phosphorylation reflecting the relative activities of the protein kinases and protein phosphatases that catalyse the interconversion process. Phosphorylation of seryl or threonyl (and occasionally tyrosyl) residues triggers small conformational changes in these proteins that alter their biological properties. Hormones and other extracellular signals transmit information to the interior of the cell by activating transmembrane signalling systems that control the production of a relatively small number of chemical mediators, termed 'second messengers'. These substances regulate the activities of protein kinases and phosphatases, and so alter the phosphorylation states of many intracellular proteins, accounting for the diversity of action of hormones. In this lecture I review recent work which demonstrates that a wide variety of cellular processes are controlled by relatively few protein kinases and protein phosphatases with pleiotropic actions. These enzymes provide the basis of an interlocking network that allows extracellular signals to coordinate biochemical functions.

Ca2+/calmodulin-sensitive inositol 1,4,5-trisphosphate 3-kinase in rat and bovine brain tissues

Inositol 1,4,5-trisphosphate (Ins P3) 3-kinase catalyzes the ATP-dependent phosphorylation of Ins P3 to Inositol 1,3,4,5-tetrakisphosphate (Ins P4). Ca2+/calmodulin (CaM)-sensitivity of Ins P3 3-kinase was measured in the crude soluble fraction from rat brain and different anatomic regions of bovine brain. Kinase activity was inhibited in the presence of EGTA (free Ca2+ below 1 nM) as compared to Ca2+ (10 microM free Ca2+) or Ca2+ (10 microM free Ca2+) and CaM (1 microM). Ca2+-sensitivity was also seen for the cAMP phosphodiesterase measured under the same assay conditions, but was not for the Ins P3 5-phosphatase. DEAE-cellulose chromatography of the soluble fraction of rat brain or bovine cerebellum resolved a Ca2+/CaM-sensitive Ins P3 3-kinase (maximal stimulation at 1 microM Ins P3 substrate level was 2.0-3.0 fold).

Metabolism of inositol 1,3,4,5-tetrakisphosphate by human erythrocyte membranes. A new mechanism for the formation of inositol 1,4,5-trisphosphate

Human erythrocyte membranes metabolize inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] to inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] in the presence of Mg2+. In the absence of Mg2+ a less rapid conversion of Ins(1,3,4,5)P4 into Ins(1,4,5)P3 was revealed. Such an enzyme activity, if present in hormonally sensitive cells, could provide a mechanism for maintaining constant concentrations of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, important for stimulation of Ca2+ entry after Ca2+ mobilization.

Regulation of inositol 1,4,5-trisphosphate metabolism in insulin-secreting RINm5F cells

Factors underlying the transience of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] accumulation following muscarinic stimulation of RINm5F cells were examined. Transience was not due to a protein kinase C-mediated stimulation of Ins(1,4,5)P3 dephosphorylation, since pretreatment of cells with tetradecanoyl-phorbol acetate (TPA) did not alter the rate of this conversion. However, preincubation with TPA did inhibit carbamoylcholine-stimulated Ins(1,4,5)P3 formation. In permeabilized cells, the conversion of Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] was slightly enhanced in the presence of TPA or cyclic AMP, but much more markedly by raising the Ca2+ concentration from 10(-7) M to 10(-6) or 10(-5) M. In intact cells the most rapid rate of accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 occurred in the first 2 s following stimulation, whereas the levels of inositol 1,4-bisphosphate were not increased until after 5 s. This suggests that Ins(1,4,5)P3 kinase is chiefly responsible for the early disposal of Ins(1,4,5)P3 following cellular stimulation. The results are consistent with the proposal that the transient accumulation of Ins(1,4,5)P3 is due both to its enhanced metabolism via the Ca2+-calmodulin-sensitive Ins(1,4,5)P3 kinase, as well as a down-regulation of phosphatidylinositol 4,5-bisphosphate hydrolysis.

Purification and characterization of inositol 1,4,5-trisphosphate 3-kinase from pig aortic smooth muscle

Inositol 1,4,5-trisphosphate (InsP3) 3-kinase, which phosphorylates InsP3 to form inositol 1,3,4,5-tetrakisphosphate, was purified to apparent homogeneity by (NH4)2SO4 fractionation and sequential chromatographic steps on DEAE-sepharose, calmodulin-Affi-Gel and DEAE-5PW h.p.l.c. The purified enzyme had a specific activity of 24.4 nmol of inositol tetrakisphosphate formed/min per mg of protein, which represented a purification of approx. 195-fold with a 0.29% recovery, compared with the cytosol fraction of the muscle. SDS/polyacrylamide-gel electrophoresis showed a single protein-staining band of Mr 93,000. Moreover, the major protein peak, of Mr 84,000, was detected by TSK gel G3000SW gel-permeation chromatography of the purified sample. As this value was approximately consistent with the Mr determined by SDS/polyacrylamide-gel-electrophoretic analysis, the InsP3 3-kinase might be a monomeric enzyme. The purified enzyme had a Km for InsP3 of 0.4 microM, with an optimum pH range of 5.8-7.7. The enzyme was maximally activated by calmodulin, with a stoichiometry of 1:1.

Partial purification and some properties of rat brain inositol 1,4,5-trisphosphate 3-kinase

An enzyme which catalyses the ATP-dependent phosphorylation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was purified approx. 180-fold from rat brain cytosol by (NH4)2SO4 precipitation, chromatography through hydroxyapatite, anion-exchange fast protein liquid chromatography and gel-filtration chromatography. Gel filtration on Sepharose 4B CL gives an Mr of 200 x 10(3) for the native enzyme. The inositol tetrakisphosphate (InsP4) produced by the enzyme has the chromatographic, chemical and metabolic properties of Ins(1,3,4,5)P4. Ins(1,4,5)P3 3-kinase displays simple Michaelis-Menten kinetics for both its substrates, having Km values of 460 microM and 0.44 microM for ATP and Ins(1,4,5)P3 respectively. When many of the inositol phosphates known to occur in cells were tested, only Ins(1,4,5)P3 was a substrate for the enzyme; the 2,4,5-trisphosphate was not phosphorylated. Inositol 4,5-bisphosphate and glycerophosphoinositol 4,5-bisphosphate were phosphorylated much more slowly than Ins(1,4,5)P3. CTP, GTP and adenosine 5'-[gamma-thio]triphosphate were unable to substitute for ATP. When assayed under conditions of first-order kinetics, Ins(1,4,5)P3 kinase activity decreased by about 40% as the [Ca2+] was increased over the physiologically relevant range. This effect was insensitive to the presence of calmodulin and appeared to be the result of an increase in the Km of the enzyme for Ins(1,4,5)P3. Preincubation with ATP and the purified catalytic subunit of cyclic AMP-dependent protein kinase did not affect the rate of phosphorylation of Ins(1,4,5)P3 when the enzyme was assayed at saturating concentrations of Ins(1,4,5)P3 or at concentrations close to its Km for this substrate.

Cyclosporin A augments angiotensin II-stimulated rise in intracellular free calcium in vascular smooth muscle cells

Pretreatment of rat vascular smooth muscle cells with the immunosuppressive drug cyclosporin A caused concentration- and time-dependent increases in both the amplitude and duration of the angiotensin II-induced rise in cytosolic free calcium, as measured with quin 2. Cyclosporin A had no significant effect on basal quin 2 fluorescence. However, cyclosporin A increased the basal 45Ca2+ influx. This stimulation of 45Ca2+ influx was not blocked by nifedipine (10(-6) M). Cyclosporin A also augmented the angiotensin II-stimulated influx and efflux of 45Ca2+. These results demonstrate that cyclosporin A increases the permeability of the plasma membrane for Ca2+ and also augments the angiotensin II-induced increases in cytosolic free calcium.

Calmodulin inhibitors, W-7 and TFP, block the calmodulin-independent activation of NADPH-oxidase by arachidonate in a cell-free system

The calmodulin inhibitor, N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7), or trifluoperazine inhibited not only Fc gamma-receptor mediated cytosolic free Ca2+ increase and O2- generation in macrophages, but also an arachidonate-induced activation of NADPH-oxidase in a cell-free system. Although these results suggested the involvement of Ca2+-calmodulin system, the cell-free activation of NADPH-oxidase occurred in the presence of EGTA and addition of calmodulin had no effect. Furthermore W-7 shifted the optimal concentration of arachidonate required for the activation to a higher level, suggesting that W-7 may block the interaction between arachidonate and NADPH-oxidase system rather than inhibiting a Ca2+-calmodulin system.