Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate in rat parotid glands

Aberrant 3H labelling of ATP during in vivo labelling of Ehrlich mouse ascites tumour cells with [2-3H]inositol is significant in the study of isomers of InsP3 and InsP4

Neutralized perchloric acid extracts of intra-abdominally proliferating Ehrlich mouse ascites-tumour cells harvested after 24 h exposure to [2-3H]inositol were analysed by Mono Q HR5/5 anion-exchange h.p.l.c. using an ammonium formate/phosphoric acid gradient or by ambient-pressure small-column anion-exchange chromatography (Bio-Rad AG 1-X8, 200-400 mesh). The results showed that cellular ATP contained aberrant 3H label in excess of 3H in the isomers of InsP3 and InsP4. The putative ATP 3H radioactivity showed: (i) h.p.l.c. run time as the material causing the largest A254 peak traced, (ii) precise spiking with ATP and [14C]ATP and (iii) specific absorption to charcoal. Moreover, enzymic conversion of ATP into ADP changed putative ATP 3H into putative ADP 3H. In addition, aberrant 3H labelling of cellular ADP and GTP was detected, although at a lower level.

Effects of okadaic acid on the human isolated bronchus

The effects of okadaic acid, a polyether derivative of a 38-carbon monocarboxylic fatty acid obtained from a culture of the marine dinoflagellate, Prorocentrum lima, were studied on the human isolated bronchus. In low concentrations (0.01 and 0.03 microM), okadaic acid had no significant effect of its own on the human isolated bronchus, but in higher concentrations (0.1-10 microM) it induced a series of contractions and relaxations. The first contraction was of low intensity (5% of maximum response to acetylcholine 3 mM) and occurred early. The second contraction had a higher amplitude (30% of maximum response to acetylcholine 3 mM) and reached its peak with okadaic acid 0.3 microM. At higher concentrations (1-10 microM), following a relaxation phase, a later rebound contraction occurred between 70 and 120 min and corresponded to 40% of the maximum response to acetylcholine 3 mM. In addition, okadaic acid inhibited or abolished the contractile response evoked by either KCl 60 mM or acetylcholine 3 mM with IC50 of 0.04 and 0.12 microM, respectively. The second contraction evoked by 0.3 microM okadaic acid was partially inhibited in the presence of the Ca2+ channel blocker, nicardipine 1 microM, or after incubation of the human bronchus in a Ca(2+)-free solution and it was completely abolished in the presence of CdSO4 0.1 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphoinositide 3-phosphatase segregates from phosphatidylinositol 3-kinase in EGF-stimulated A431 cells and fails to in vitro hydrolyse phosphatidylinositol(3,4,5)trisphosphate

Beside 4- and 5-phosphatases playing a role in the interconversion between the D-3 phosphorylated polyphosphoinositides, the only enzyme described so far to be responsible for a phosphomonesterasic activity on the D-3 position of inositol lipids is a specific 3-phosphatase that hydrolyzes PtdIns(3)P in NIH 3T3 cells. We report here the presence of a potent 3-phosphatase activity in different cell types. This activity is detected both in cytosol and membranes of A431 cells and is inhibited by VO4(-3) and Zn2+. Interestingly, the cytosolic activity from A431 cells selectively hydrolyzes in vitro PtdIns(3)P and PtdIns(3,4)P2, whereas PtdIns(3,4,5)P3 remains a very poor substrate under the same conditions. Finally, assays of phosphatidylinositol 3-kinase and 3-phosphatase activities in the pool of phosphotyrosine-containing proteins isolated from EGF-stimulated A431 cells suggest a compartmentation of these two antagonistic activities during cell activation.

Translocation of the alpha-isozyme of protein kinase C during stimulation of rat parotid acinar cells by phorbol ester and carbachol

Protein kinase C activity was detected in the cytosolic fraction of quiescent parotid acinar cells; the particulate fraction contained a much smaller proportion of the enzyme. Protein kinase C activity was increased in the membrane fraction and decreased in the cytosol after exposure of intact cells to phorbol 12-myristate 13-acetate (PMA) or the muscarinic-receptor agonist carbachol. The effect of PMA was potentiated by a subthreshold concentration of ionomycin. Immunoblot analysis with anti-protein kinase C antibodies revealed that the protein kinase C-alpha isoform is expressed in rat parotid cells. Other Ca(2+)-dependent isoforms were not detected. Further, agonist stimulation caused the redistribution of protein kinase C-alpha from cytosol to a membrane fraction. Agonists may promote parotid acinar cell activity, including amylase secretion, by increasing the affinity of protein kinase C-alpha for the membrane fraction, presumably via a rise in Ca2+ and diacylglycerol derived from polyphosphoinositide hydrolysis.

Ca2+ signalling in exocrine acinar cells: the diffusional properties of cellular inositol 1,4,5-trisphosphate and its role in the release of Ca2+

The correlation between acetylcholine induced changes in the intracellular free, Ca2+ concentration ([Ca2+]i), and the inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) content in isolated acini from the rat parotid and lacrimal glands was investigated. Applying digital image processing on Fura-2 loaded acini, we observed that Ca2+ increases either simultaneously throughout the acinar configurations or that occasionally, the rise near the lumen can precede the rise near the basal part by 50-100 ms. Measurements on cell suspensions revealed a correlation between changes in [Ca2+]i and changes in the cellular Ins(1,4,5)P3 content, and it is concluded that in the individual cells Ins(1,4,5)P3 is released to the cytosol within the first second after stimulation. Applying a diffusion coefficient for cytoplasmic Ins(1,4,5)P3 of 2.83 x 10(-6) cm2/s (Allbritton et al., 1992, Science, 258, 1812-1815), we have calculated the concentration profile for this messenger in a sphere with a radius of 10 microns where Ins(1,4,5)P3 is released in the center following a monoexponential function with a rate constant of 4 s-1. Assuming that Ins(1,4,5)P3 concentrations of 1 or 5% of the maximum value is able to release Ca2+, we calculated that Ca2+ waves can appear at a rate of 100 or 40 microns/s. The present data are consistent with Ins(1,4,5)P3 being a cellular messenger, that by diffusion, initiates the Ca2+ release from the cellular pools within the first fraction of a second.

Nerve growth factor stimulates the production of inositol 1,3,4- and 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate in PC12 cells

In PC12 cells, preincubated with [3H]inositol, nerve growth factor (NGF) stimulated an approximately 100% increase in the levels of [3H]inositol 1,3,4-trisphosphate ([3H]-Ins(1,3,4)P3], [3H]inositol 1,4,5-trisphosphate ([3H]Ins(1,4,5)P3], and [3H]inositol 1,3,4,5-tetrakisphosphate ([3H]Ins(1,3,4,5)P4] as early as 5-15 s after addition of NGF. This NGF-mediated response was apparent only when the cells had been cultured in the absence of fetal bovine serum (FBS). PC12 cells cultured in FBS-containing medium did not display NGF-mediated increases in [3H]Ins(1,3,4)P3, [3H]Ins(1,4,5)P3, and [3H]Ins(1,3,4,5)P4 levels. Using cells cultured in the absence of FBS, epidermal growth factor (EGF) and fibroblast growth factor also stimulated production of [3H]Ins(1,3,4)P3, [3H]Ins(1,4,5)P3, and [3H]Ins(1,3,4,5)P4. Lavendustin A, a tyrosine kinase inhibitor, inhibited both the EGF- and NGF-stimulated increases in the levels of these tritiated inositol phosphates. These results suggest that NGF stimulates the production of Ins(1,3,4)P3, Ins(1,4,5)P3, and Ins(1,3,4,5)P4 and that this response is dependent on tyrosine kinase activity. Furthermore, although the production of Ins(1,3,4)P3, Ins(1,4,5)P3, and Ins(1,3,4,5)P4 may be a common response to factors stimulating neuronal differentiation, it is not sufficient for stimulation of neuronal differentiation.

Light evoked inositol trisphosphate release in the rat retina in vitro

Light exposure not only elicits a photic response but may also alter the metabolism and functional properties of the retina. This may be evoked by the stimulation of phospholipid derived second messenger systems. In this study, we investigated the light-evoked release of inositol 1,4,5-triphosphate in the isolated rat retina in vitro by means of high performance liquid chromatography (HPLC) detection. After prelabelling of isolated retinae with tritiated myo-inositol in darkness, they were exposed to no light or to white fluorescent light of 10,000 lux illuminance for 3,5 and 10 sec, respectively. We observed a 200% increase in the release of inositol 1,4,5-trisphosphate compared to basal values in darkness after 3 sec of light exposure with a decline after 5 sec and a return to basal values after 10 sec indicating a rapid breakdown of inositol 1,4,5-trisphosphate. Our data confirm previous studies in the amphibian retina and photoreceptors and demonstrate for the first time a light evoked inositol 1,4,5-trisphosphate release in the mammalian retina.

Reoxygenation following hypoxia stimulates lipid peroxidation and phosphatidylinositol breakdown in kidney cortical slices

Reoxygenation of hypoxic (120 min at 37 degrees) rabbit kidney cortical slices in vitro resulted in a rapid increase in lipid peroxidation and phosphatidylinositol hydrolysis. No changes in phosphatidylinositol breakdown occurred during hypoxia or upon reoxygenation in the absence of calcium. Incubation of renal slices with carbon tetrachloride resulted in increased lipid peroxidation but had no effect on phosphatidylinositol breakdown. It is concluded that altered intracellular calcium homeostasis during reoxygenation is involved in mediating increased phosphatidylinositol hydrolysis through activation of a specific phospholipase C, but that oxidative stress per se does not have a significant effect on the inositol phosphate secondary messenger response in this model system.

Inositol Trisphosphate Metabolism in Subcellular Fractions of Barley (Hordeum vulgare L.) Mesophyll Cells

Phosphatases in cytosolic fractions, vacuoles, and vacuolar membranes from barley (Hordeum vulgare L.) leaves were found to dephosphorylate inositol 1,4,5-trisphosphate (IP3). 1,4-inositol bisphosphate (1,4-IP2) is the main product of IP3 dephosphorylation by the cytosolic fraction. The activity was strictly Mg2+ dependent. In contrast, IP3 dephosphorylation activity of both the soluble vacuolar and the tonoplast fractions was inhibited up to 50% by Mg2+. When vacuolar membranes were incubated with IP3, 1,4-IP2 was produced only under neutral and slightly alkaline conditions. Under acidic conditions, however, dephosphorylation yielded putative 4,5-inositol bisphosphate. Li+ (20 mM) and Ca2+ (100 [mu]M) strongly inhibited activity in the soluble vacuolar fraction but had only a slight effect on the activities of the cytosolic and tonoplast fractions.

Quantification of inositol phospholipid breakdown in isolated rat hepatocytes

The hydrolysis of inositol phospholipids induced by vasopressin in hepatocytes during 60 min was quantified chemically. There was a large release of myo-inositol which was abolished by Li+, indicating that it was derived from inositol phosphates and not from phospholipase D action on PtdIns. There was also a large release of inositol phosphates which was increased approx. 2-fold by Li+ at 30 min, but then remained constant, suggesting that inositol phospholipid breakdown declined substantially beyond this time. In cells prelabelled with myo-[3H]inositol and treated with Li+, [3H]PtdIns(4,5)P2 decreased maximally (50%) at 15 s and then recovered to a level at 5 min that was maintained at 25% below control for 40 min. [3H]PtdIns4P and [3H]PtdIns showed slower decreases to approx. 30% below control at 15 min, but with no further changes. Labelled Ins(1,4,5)P3 and Ins(1,3,4)P3 showed 2-4-fold increases within 30 s and then declined to values that were maintained at a constant level above the control, except for [3H]Ins(1,3,4)P3, which showed a second increase. [3H]Ins(1,4)P2 showed a very large increase over 10 min, whereas [3H]Ins4P and [3H]Ins1P showed little change before 6 and 15 min respectively. The total [3H]inositol phosphates showed little further increase after 20 min. These data are consistent with a rapid, but not sustained, hydrolysis of PtdIns-(4,5)P2, but not of PtdIns, by phospholipase C, but do not exclude PtdIns4P as a substrate. Phosphatidate was rapidly increased by vasopressin, whereas diacylglycerol was increased after a 1-2 min lag. Both were maintained at levels 2-3-fold above control for 60 min. The vasopressin-induced increase in inositol phosphates plus myo-inositol (approx. 120 nmol/100 mg) was greater than the increase in diacylglycerol plus phosphatidate (approx. 60 nmol/100 mg) between 10 and 40 min. This indicates that there was substantial further metabolism of these lipids. Addition of 75 mM ethanol resulted in rapid production of phosphatidylethanol in response to vasopressin and a 35% reduction in phosphatidate, but no decrease in diacylglycerol. In summary, the results indicate that inositol phospholipid hydrolysis by phospholipase C can account for most of the diacylglycerol and phosphatidate that accumulate during 60 min of vasopressin action, but that these phospholipids are probably not the major source of the phosphatidate that is formed during the first 2 min by phospholipase D, or of the diacylglycerol and phosphatidate that are formed beyond 30 min.

Li(+)-induced structural changes of synaptic ribbons are related to the phosphoinositide metabolism in photoreceptor synapses

Synaptic ribbons are specialized cytoskeletal components of the presynaptic exocytotic machinery in photoreceptors. In cone photoreceptors, these structures are highly dynamic disappearing during darkness and reforming in the light phase. In this study we wanted to characterize the second messengers involved in the regulation of these cyclic changes. We show that synaptic ribbons in cone photoreceptors are very sensitive to both Li+ and inositol 1,4,5-trisphosphate suggesting that inositol polyphosphates might play a physiological role in the disassembly of synaptic ribbons. The presence of the phosphoinositide pathway was demonstrated in isolated photoreceptor synapses. The phosphoinositide metabolism in photoreceptor synapses was dark-activated and correlated with the disappearance of synaptic ribbons during dark-adaptation. Since Li+ and inositol 1,4,5-trisphosphate only influenced synaptic ribbons in cones but not in rods the dark-activated phosphoinositide metabolism should be largely ascribed to cone synapses.

Dietary omega-3 polyunsaturated fatty acids inhibit phosphoinositide formation and chemotaxis in neutrophils

Earlier studies demonstrated that dietary omega-3 polyunsaturated fatty acid (PUFA) supplementation attenuates the chemotactic response of neutrophils and the generation of leukotriene (LT) B4 by neutrophils stimulated with calcium ionophore; however, the mechanisms and relationship of these effects were not examined. Neutrophils and monocytes from eight healthy individuals were examined before and after 3 and 10 wk of dietary supplementation with 20 g SuperEPA daily, which provides 9.4 g eicosapentaenoic acid (EPA) and 5 g docosahexaenoic acid. The maximal neutrophil chemotactic response to LTB4, assessed in Boyden microchambers, decreased by 69% after 3 wk and by 93% after 10 wk from prediet values. The formation of [3H]inositol tris-phosphate (IP3) by [3H]inositol-labeled neutrophils stimulated by LTB4 decreased by 71% after 3 wk (0.033 +/- 0.013% [3H] release, mean +/- SEM) and by 90% after 10 wk (0.011 +/- 0.011%) from predict values (0.114 +/- 0.030%) as quantitated by beta-scintillation counting after resolution on HPLC. LTB4-stimulated neutrophil chemotaxis and IP3 formation correlated significantly (P < 0.0001); each response correlated closely and negatively with the EPA content of the neutrophil phosphatidylinositol (PI) pool (P = 0.0003 and P = 0.0005, respectively). Neither the affinities and densities of the high and low affinity LTB4 receptors on neutrophils nor LTB4-mediated diglyceride formation changed appreciably during the study. Similar results were observed in neutrophils activated with platelet-activating factor (PAF). The summed formation of LTB4 plus LTB5 was selectively inhibited in calcium ionophore-stimulated neutrophils and was also inhibited in zymosan-stimulated neutrophils. The inhibition of the summed formation of LTB4 plus LTB5 in calcium ionophore-stimulated neutrophils and in zymosan-stimulated neutrophils did not correlate significantly with the EPA content of the PI pool. The data indicate that dietary omega-3 PUFA supplementation inhibits the autoamplification of the neutrophil inflammatory response by decreasing LTB4 formation through the inactivation of the LTA epoxide hydrolase and independently by inhibiting LTB4- (and PAF) stimulated chemotaxis by attenuating the formation of IP3 by the PI-selective phospholipase C. This is the initial demonstration that dietary omega-3 PUFA supplementation can suppress signal transduction at the level of the PI-specific phospholipase C in humans.

Quantitative analysis of inositol lipids and inositol phosphates in synaptosomes and microvessels by column chromatography: comparison of the mass analysis and the radiolabelling methods

Chromatographic methods that measure both the mass and the radiolabelling of various inositol lipids and inositol phosphates in tissues have been developed. The mass of phosphatidylinositol (PtdIns), phosphatidylinositol-4-monophosphate [PtdIns(4)P] and phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] was quantitated by measuring the inorganic phosphate, whereas inositol monophosphate (IP), inositol bisphosphate (IP2), inositol trisphosphate (IP3) and inositol tetrakisphosphate (IP4) were quantitated by using an enzymic method. The radiolabelling of various inositol lipids and inositol phosphates was determined by incubating the tissue samples with [3H]myo-inositol, separating individual inositol lipids and inositol phosphates, and measuring the radioactivity in each compound. Although the mass analysis method was sensitive enough to measure low levels of inositol lipids or inositol phosphates, the method was laborious and time-consuming. Compared with the enzymic method, the radiolabelling method was simple and fast, but it gave variable results. This study demonstrated differences in inositol lipid and inositol phosphate levels by radiolabelling and mass measurements, and agonist-stimulated phosphatidylinositol turnover of synaptosomes versus the blood-brain barrier as represented by microvessels. Although the mass of PtdIns, PtdIns(4)P and PtdIns(4,5)P2 was comparable in synaptosomes and microvessels, the incorporation of [3H]myo-inositol into phosphorylated PtdIns in microvessels was less than that in synaptosomes.

Losartan inhibits the angiotensin II-induced stimulation of the phosphoinositide signalling system in vascular smooth muscle cells

2-n-Butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)bip hen yl-4-yl)methyl]imidazole, potassium salt (Losartan) (previous name, DuP 753 or MK 954) is a nonpeptide angiotensin II receptor antagonist. This study was performed to investigate the ability of Losartan to inhibit the angiotensin II-induced stimulation of the phospoinositide signalling system and the angiotensin II-induced hypertrophy in aortic vascular smooth muscle cells of normotensive Wistar-Kyoto rats. 10(-7) M Losartan abolished the angiotensin II-induced formation of inositol 1,4,5-trisphosphate in vascular smooth muscle cells. 10(-6) M Losartan completely abolished the angiotensin II-induced elevation of the intracellular free Ca2+ concentration ([Ca2+]i). 10(-6) M Losartan lacked effects on the [Arg8]vasopressin-induced elevation of [Ca2+]i. In addition, 10(-6) M completely inhibited the angiotensin II-induced stimulation of Na+/H+ exchange in the vascular smooth muscle cells. 10(-10) to 10(-6) M Losartan inhibited the angiotensin II-induced cell protein synthesis in a concentration-dependent manner, yielding to an effective concentration (ED50) of 6.2 +/- 1.8 x 10(-8) M (n = 4). Losartan did not affect the platelet-derived growth factor-BB-induced increase in cell protein. These results show that Losartan is a highly specific angiotensin II receptor antagonist which inhibits angiotensin II-induced cell growth and thus may have beneficial effects on the development and regression of vascular hypertrophy.

Elevation of cytosolic [Ca2+] due to intracellular Ca2+ release retards carbachol stimulation of divalent cation entry in rat parotid gland acinar cells

This study examines the activation of divalent cation entry into rat parotid gland acinar cells by using Mn2+ as a Ca2+ surrogate cation. Following muscarinic-cholinergic stimulation of dispersed parotid acini with carbachol (10 microM), the onset of internal Ca2+ release (cytosolic [Ca2+], [Ca2+]i, increase) and the stimulation of Mn2+ entry (increase in fura2 quenching) are not simultaneously detected. [Ca2+]i elevation, due to intracellular release, is detected almost immediately following carbachol addition and peak [Ca2+]i increase occurs at 6.0 +/- 0.8 sec. However, there is an interval (apparent lag) between carbachol addition and the detection of stimulated Mn2+ entry. This apparent lag is decreased from 26 +/- 3.1 sec to 9.2 +/- 1.5 sec when external Mn2+ ([Mn2+]0) is increased from 12.5 to 500 microM. It is not decreased further with increase in [Mn2+]0 from 500 microM to 1 mM (9.8 +/- 2.1 sec), although both intracellular free Mn2+ and [Mn2+-fura2]/[fura2] increase. Thus, at [Mn2+]0 < 500 microM, the observed lag time is partially due to a limitation in the magnitude of Mn2+ entry. Furthermore, neither peak [Ca2+]i nor the time required to reach peak [Ca2+]i is significantly altered by [Mn2+]0 (12.5 microM to 1 mM). At every [Mn2+]0 tested (i.e., 12.5 microM-1 mM), the apparent lag is significantly greater than the time required to reach peak [Ca2+]i. However, when carbachol stimulation of the [Ca2+]i increase is attenuated by loading the acini with the Ca2+ chelator, 2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA), there is no detectable lag in carbachol stimulation of Mn2+ entry (with 1 mM [Mn2+]0). Importantly, in BAPTA-loaded acini, carbachol stimulates Mn2+ entry via depletion of the internal Ca2+ pool and not via direct activation of other divalent cation entry mechanisms. Based on these results, we suggest that the apparent lag in the detection of carbachol stimulation of Mn2+ entry into parotid acinar cells is due to a retardation of Mn2+ entry by the initial increase in [Ca2+]i, due to internal release, which most likely occurs proximate to the site of divalent cation entry.

Changes in polyphosphoinositide levels in rat liver nuclei in response to prolactin, a known hepatic mitogen

The effect of prolactin action on nuclear polyphosphoinositide synthesis was investigated in isolated rat liver nuclei. An increased uptake of phosphate from [gamma 32P] adenosinetriphosphate was observed in both phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate with a maximum response at 10(-12) M concentration of hormone. Pulse-chase experiments in isolated nuclei following prolactin treatment indicate that the observed increase in accumulation of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate is mainly due to a decrease in their rate of turnover possibly induced by a change in activity of polyphosphoinositide-specific monoesterases. In vitro prolactin also reduces the activity of nuclear phospholipase C specific for phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Moreover, this feature is strongly supported by the concomitant decrease in nuclear diacylglycerol mass. Thus these data suggest that once prolactin reaches the nucleus an intranuclear signalling is evoked through inositol lipid metabolism.

Effects of chronic anethole trithione and amitriptyline treatment on rat parotid gland signalling

The present study examines the mechanism(s) of action of anethole trithione (Sulfarlem S25) compared to the sialogogue pilocarpine. The chronic effects (7 days of treatment) of anethole trithione, pilocarpine and/or amitriptyline on autonomic receptor binding (homogenates) were measured together with parallel tests of stimulation-induced rises in delta [Ca2+]i in collagenase-isolated rat parotid acini. The results revealed that chronic treatment with amitriptyline resulted in significantly increased rises in delta [Ca2+]i after stimulation with 20 microM of carbachol or adrenaline, and a significant increase in muscarinic acetylcholine receptor density. In addition, anethole trithione also increased cholinergic and adrenergic responsiveness. The double treatment of amitriptyline and anethole trithione or amitriptyline and pilocarpine did, however, prevent the rise in delta [Ca2+]i observed under conditions when these drugs were administered alone. Furthermore, anethole trithione, but not pilocarpine, was able to prevent the amitriptyline-induced upregulation in muscarinic acetylcholine receptor density. In conclusion, the experimental data presented in this study are compatible with the hypothesis that anethole trithione might stimulate some post-receptor effect in the coupling to the secretory response. In addition, this study supports the beneficial effects of anethole trithione in treating drug-induced xerostomia.

Growth factors downregulate vascular smooth muscle thromboxane receptors independent of cell growth

Growth factors, in addition to being mitogenic, may modulate vascular smooth muscle differentiation. We tested whether serum or defined growth factors could regulate thromboxane A2 (TxA2) receptors in cultured rabbit aorta smooth muscle cells. Fetal bovine serum (10%) stimulated cell proliferation and DNA synthesis in subconfluent cell cultures. Binding of the thromboxane A2 agonist [1S-(1 alpha 2 beta(5Z),3 alpha(1E,3S),4 alpha)]-7-[3-(3-hydroxy-4-p- iodophenoxy-1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-hep tenoic acid showed a 41% decrease in TxA2 receptors in cells treated with 10% serum compared with serum-deprived (0.1%) controls. Receptor downregulation by serum was gradually reversible upon serum withdrawal. Compared with serum-deprived cells, those exposed to 10% serum also had diminished TxA2-stimulated phosphatidylinositol hydrolysis. Regulatory actions of serum on TxA2 receptors were distinguished from mitogenic effects with heparin, which prevented cell growth but did not inhibit serum-induced downregulation of TxA2 receptors. Furthermore, low concentrations of platelet-derived growth factor and basic fibroblast growth factor decreased TxA2 receptors without stimulating cell proliferation or DNA synthesis. These observations describe a previously unrecognized regulatory action of growth factors on a vascular smooth muscle vasoconstrictor receptor, an action which is independent of effects on cell proliferation or DNA synthesis.

The metabolism of microinjected inositol trisphosphate in Xenopus oocytes

Microinjection of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) into Xenopus oocytes evokes a complex physiological response composed of a transient and a slow depolarizing chloride current. We investigated the relationship between intracellular levels of Ins(1,4,5)P3 and the kinetics of the physiological response. Microinjected Ins(1,4,5)P3 was slowly degraded following first order kinetics of disappearance (t1/2 = 10 min). The degradation products were inositol bisphosphate (InsP2), inositol monophosphate (InsP) and inositol, as well as inositol tetrakisphosphate (InsP4). The rate of degradation of injected 3[H]-Ins(1,4)P2 was much greater (t1/2 = 3 min), indicating that the conversion of InsP3 to InsP2 may be the rate-limiting step in the degradation process. The slow degradation of 3[H]-Ins(1,4,5)P3 was not a result of its conversion to Ins(1,3,4)P3 since no accumulation of InsP3 was observed within 10 min of microinjection of 3[H]-Ins(1,3,4,5)P4. Activation of protein kinase C (PK-C) with a phorbol ester transiently increased the rate of conversion of 3[H]-Ins(1,4,5)P3 to InsP2. This, however, did not significantly affect the overall kinetics of 3[H]-Ins(1,4,5)P3 disappearance. Our results indicate that the kinetics of Ins(1,4,5)P3 degradation do not correlate well with the termination of both the rapid and the slow components of the physiological response. The termination of the slow component of the response, however, may be related to the decay of Ins(1,4,5)P3-induced 45Ca efflux, which lasted about 10 min.

Light does not induce an increase in cyclic-GMP content of squid or Limulus photoreceptors

Both slices of squid (Loligo) retinas and Limulus ventral photoreceptors, with or without prior stimulation by flashes of bright light, were rapidly frozen and analysed for cyclic-GMP content by radioimmunoassay. Measurement of the freezing process indicated that freezing occurred at about 100 msec after the light flash. No significant changes in cyclic-GMP content were induced by light in either tissue. These data do not confirm previous reports that cyclic-GMP content of invertebrate photoreceptors is markedly increased by illumination.

Modified kinetics of platelet-derived growth factor-induced Ca2+ increases in NIH-3T3 cells overexpressing phospholipase C gamma 1

The effects of platelet-derived growth factor (PDGF) on cytosolic free Ca2+ concentration ([Ca2+]i) and inositol phosphates were studied in NIH-3T3 fibroblasts transfected with cDNA for phospholipase C gamma 1 (PLC gamma 1) to yield a 7-fold overexpression of this enzyme, compared with cells containing normal levels of PLC gamma 1. In a study published recently [Margolis, Zilberstein, Franks, Felder, Kremer, Ullrich, Rhee, Skorecki & Schlessinger (1990) Science 248, 607-610] it was reported that this overexpression of PLC gamma 1 caused a specific potentiation of the inositol phosphate response to PDGF, but this was not associated with an enhancement of the [Ca2+]i response. In the present study, measurements of the time course and isomeric profile of PDGF-induced inositol phosphate formation demonstrated that the initial rate of Ins(1,4,5)P3 formation was also enhanced in the PLC gamma 1-overexpressing cells, yielding a 10-fold greater increase at 1 min compared with the parental NIH-3T3 cells. By contrast, bradykinin-induced phosphoinositide metabolism was unchanged in PLC gamma 1-transfected cells. Measurements of [Ca2+]i in cell populations and single cells showed a significant latent period following PDGF addition prior to the [Ca2+]i increases in both cell lines, which decreased in a dose-dependent manner with increasing PDGF concentration. The duration of the latent period was decreased and the maximal rate of [Ca2+]i rise was increased in the PLC gamma 1-overexpressing cells at all doses of PDGF examined. In single-cell measurements these cells also responded to PDGF with a greater peak amplitude of [Ca2+]i. Both intracellular Ca2+ mobilization and Ca2+ influx across the plasma membrane were enhanced in the PLC gamma 1-overexpressing cells. There was no difference between the two cell lines in either the latency or the magnitude of the [Ca2+]i increases induced by bradykinin. These data provide further evidence that PLC gamma 1 is responsible for the PDGF-induced stimulation of Ins(1,4,5)P3 formation. Moreover, in contrast to earlier conclusions, the modified kinetics of the [Ca2+]i changes in PLC gamma 1-overexpressing cells suggest that Ins(1,4,5)P3 does play a predominant second messenger role in the PDGF-induced [Ca2+]i increases. The data also indicate that the latent period may be a function of the time required to reach a threshold level of Ins(1,4,5)P3, rather than an intrinsic property of the PDGF receptor.

Production of inositol phosphates and reactive oxygen metabolites in quartz-dust-stimulated human polymorphonuclear leukocytes

The present paper explores phosphoinositide turnover in quartz-stimulated human polymorphonuclear leukocytes. Separation of inositol phosphates was carried out with a new ion-pair, reverse-phase high performance liquid chromatographic method applying a gentle tetrabutyl ammonium phosphate buffer gradient. The method separates inositol monophosphates, inositol 1,4-bisphosphate, inositol trisphosphates and inositol 1,3,4,5-tetrakisphosphate. Reactive oxygen metabolites, indices for leukocyte activation, were measured with a luminometric assay. Quartz increased the production of reactive oxygen metabolites, preceded by facilitated inositol phosphate turnover. This finding provides evidence that inositol phosphate second messengers may be involved in quartz-induced leukocyte activation and subsequent production of reactive oxygen metabolites.

Role of endogenous fatty acids in the control of hepatic gluconeogenesis

Inhibition of endogenous long chain fatty acids oxidation by tetradecylglycidate (TDGA) impeded gluconeogenesis from lactate or from low concentrations of pyruvate (less than 0.5 mM). The inhibitory effect of TDGA was overcome by medium and short chain fatty acid or by concentrations of pyruvate about 0.5 mM, but not by 10-fold higher concentrations of lactate. Despite decreased energy demand when gluconeogenesis was inhibited by TDGA, the pyruvate-induced increase in hepatic oxygen consumption was similar to the control, indicating that pyruvate transport across the mitochondrial membrane and/or its decarboxylation was not altered, and therefore can not be responsible for the inhibition of gluconeogenesis. Neither does a deficiency of acetyl-CoA explain the decrease in the gluconeogenic flux since high pyruvate loads (greater than 0.5 mM), beta-hydroxybutyrate or even ethanol was capable of overcoming the inhibitory effect of TDGA in the absence of significant changes in the hepatic content of acetyl-CoA. At low (less than 0.3 mM), presumably physiological, pyruvate concentrations, its rate of mitochondrial utilization is limited by the activity of the monocarboxylate transporter. Agents that reduced the mitochondrial NAD system, and therefore reduced flux through pyruvate dehydrogenase, like beta-hydroxybutyrate or ethanol, stimulated gluconeogenesis when fatty acid oxidation was inhibited. The latter observations indicate that the primary role of endogenous fatty acid, when substrate availability is limiting, is to spare mitochondrial pyruvate by decreasing its oxidation, and therefore shifting the partitioning between the carboxylation and decarboxylation reactions toward the former.

Regulation of the formation of inositol phosphates by calcium, guanine nucleotides and ATP in digitonin-permeabilized bovine adrenal chromaffin cells

Both micromolar Ca2+ and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulated the formation of inositol phosphates (InsPs) in digitonin-permeabilized chromaffin cells prelabelled with [3H]inositol. The production of InsPs was potentiated by ATP. Guanosine 5'-[beta-thio]diphosphate (GDP[S]) caused a GTP-reversible shift to higher concentrations in the Ca(2+)-concentration-response curve for the release of InsPs without changing the maximal response. GTP[S] caused a shift to lower concentrations of Ca2+ and also increased the maximal response. The effects of GTP[S] and Ca2+ were synergistic. Although as much as 80% of the InsPs were derived from phosphatidylinositol 4-phosphate (PtdInsP) or 4,5-bisphosphate (PtdInsP2), the amount of InsPs produced could be several times the total amount of PtdInsP and PtdInsP2 in the cells and was largely accounted for by a decrease in PtdIns. The levels of labelled PtdInsP and PtdInsP2 increased on stimulation with Ca2+, but decreased on stimulation with GTP[S] or the combination of Ca2+ and GTP[S]. Preincubation with Ca2+ and ATP amplified the subsequent GTP[S]-induced production of InsPs. ATP and its gamma-thio and beta gamma-imido analogues stimulated the formation of InsPs in intact cells. However, only ATP potentiated the responses to Ca2+ and GTP[S] in permeable cells. Our main conclusions are: (1) a GTP-binding protein participates in the Ca(2+)-induced production of InsPs by phospholipase C, and (2) ATP markedly potentiates the stimulated formation of InsPs, an effect with arises from its role in polyphosphoinositide synthesis and does not involve purinergic receptor activation in permeabilized cells. The data also suggest that the different effects of Ca2+ and GTP[S] on polyphosphoinositide synthesis probably contribute to the synergistic action of Ca2+ and GTP[S] on the generation of InsPs.

Potentiation of vascular smooth muscle cell activity by cyclosporin A

Pretreatment of rat vascular smooth muscle cells with cyclosporin A caused concentration- and time-dependent enhancement of both angiotensin II- and platelet-derived growth factor-stimulated cellular functions, which may be related to a rise in vascular tone. In particular, cyclosporin A increased cytosolic free calcium, and augmented the agonist-induced formation of inositol polyphosphate. In addition, it markedly increased the sensitivity of the cells to platelet-derived growth factor-mediated stimulation of thymidine incorporation. The potentiation of vascular smooth muscle cell activity by cyclosporin A may be a novel mechanism by which it exerts an adverse vasoconstrictor effect.

The effect of ischaemia and reperfusion on sarcolemmal inositol phospholipid and cytosolic inositol phosphate metabolism in the isolated perfused rat heart

In this study the mass of polyphosphoinositides as well as the turnover of [3H]inositol phospholipids and [3H]inositol phosphates during ischaemia and short periods of reperfusion were studied in the isolated perfused rat heart. Since the phosphoinositides located within the sarcolemma are precursors for release of inositoltrisphosphate (InsP3) and diacylglycerol, sarcolemmal membranes (rather than whole tissue) isolated at the end of the experimental procedure, were used. Hearts were prelabelled with [3H]inositol and subsequently perfused with 10 mM LiCl to block the phosphatidylinositol (PI) pathway. The results showed that 20 min of global ischaemia depressed the amount of [3H]inositol present in both sarcolemmal phosphatidylinositol-4-phosphate (PI-4-P) and phsophatidylinositol-4,5-bisphosphate (PI-4,5-P2), as well as in the cytosolic [3H]inositol phosphates, [3H]InsP2 and [3H]InsP3. The mass of the sarcolemmal inositol phospholipids remained unchanged during ischaemia. Reperfusion caused an immediate (within 30 sec) increase in the amount of [3H]inositol in sarcolemmal PI, PI-4-P and PI-4,5-P2. PI-4-P levels showed a transient increase after 30 seconds postischaemic reperfusion, while the mass of the other sarcolemmal inositol phospholipids, PI and PI-4,5-P2, remained unchanged. [3H]InsP, [3H]InsP2 and [3H]InsP3 also increased significantly in comparison to ischaemic hearts after only 30 sec postischaemic reperfusion. In summary, the results obtained indicate inhibition of the PI pathway during ischaemia with an immediate significant stimulation upon reperfusion. In view of the capacity of InsP3 to mobilize Ca2+, the possibility exists that stimulation of this pathway during reperfusion may play a role in the intracellular Ca2+ overload, characteristic of postischaemic reperfusion.

Abnormal response to a human B cell growth factor in patients with common variable immunodeficiency (CVI)

Patients with common variable immunodeficiency (CVI) generally fail to produce antigen-specific IgG. We have identified a lymphokine called high molecular weight B cell growth factor (HMW BCGF) that expands an IgG-producing subpopulation of B cells. The B cells from 15 of 16 patients with CVI evaluated in this study failed to proliferate to HMW BCGF, although they proliferated normally to another BCGF, low molecular weight BCGF (LMW BCGF). Nevertheless, 11 patients had more than normal numbers of B cells expressing HMW BCGF receptors. The HMW BCGF receptors on the B cells of three patients with CVI studied were the same molecular weight as the normal HMW BCGF receptor. Examination of B cells from four patients with CVI for intracellular signals produced in normal B cells after stimulation with HMW BCGF revealed that B cells from patients with CVI failed to developed significant increases in cyclic adenosine monophosphate or phosphoinositides after HMW BCGF stimulation. However, cytoplasmic phosphoinositides in the B cells from all four patients with CVI were already increased above what is observed in normal B cells before stimulation with HMW BCGF (either freshly isolated or Staphylococcus aureus Cowan I-activated B cell). Thus, the failure of B cells from patients with CVI to respond to HMW BCGF may be related to their abnormal activation in vivo. Since HMW BCGF expands a subpopulation of memory B cells, the inability of CVI B cells to respond to HMW BCGF may contribute to their abnormal secondary responses to antigens.

Receptor-coupled phosphoinositide hydrolysis in human retinal pigment epithelium

Carbachol and histamine stimulated phosphoinositide (PPI) hydrolysis in cultured human retinal pigment epithelium (RPE), as reflected by an accumulation of 3H-inositol phosphates in the presence of 10 mM Li+. Carbachol increased PPI hydrolysis to greater than 600% of basal with an EC50 of 60 microM; stimulation was linear up to 60 min. This activation likely occurred via the M3 muscarinic cholinergic receptor based on the IC50 values for 4-diphenylacetoxy-N-methylpiperidine methiodide (0.47 nM), pirenzepine (280 nM), and 11-[[2-[(diethylamino)methyl]-1-piperidinyl]-acetyl]-5,11- dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (1.4 microM). Carbachol-mediated PPI hydrolysis was decreased by 80% in the absence of extracellular Ca2+. Histamine stimulated PPI turnover in a linear manner by 180% with an EC50 of 20 microM by the H1 histaminergic receptor. Serotonin, glutamate, norepinephrine, and dopamine were inactive. In human RPE, the resting cytoplasmic Ca2+ concentration, as determined by fura-2 fluorescence, was 138 +/- 24 nM. On the addition of carbachol, there was a 180% increase in peak intracellular Ca2+; addition of histamine increased intracellular Ca2+ by 187%. These results suggest receptor-mediated, inositol lipid hydrolysis is coupled to intracellular Ca2+ flux in human RPE.

Thrombin stimulates the production of phosphatidylinositol 3,4-bisphosphate in human erythroleukemia cells

The human erythroleukemic cell line, HEL, which has numerous platelet markers, shows enhanced inositol phosphate production in response to thrombin. We investigated the production of phosphoinositides in HEL cells and showed that thrombin stimulates the turnover of several phosphoinositides including the synthesis of phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). Phosphatidylinositol 3-monophosphate is also produced in HEL cells and its synthesis is not stimulated by thrombin. Pretreatment of HEL cells with the stable prostacyclin analog iloprost inhibits the thrombin-induced increase in the production of PtdIns(3,4)P2. 3-Phosphorylated phosphoinositides have been implicated in signal transduction and regulation of cell proliferation in other cells and may be involved in signal transduction in HEL cells.

Induction of proliferation by high molecular weight B cell growth factor or low molecular weight B cell growth factor is associated with increases in intracellular calcium in different subpopulations of human B lymphocytes

The activation of resting B cells with anti-surface Ig is associated with transient increases in intracellular calcium. In the present study, we demonstrate that stimulation of B cells which have already been activated by Staphylococcus aureus Cowan I (Sac), with high molecular weight B cell growth factor (HMW-BCGF) or low molecular weight B cell growth factor (LMW-BCGF), but not IL-2, IL-4, or interferon-gamma, is associated with an increase in intracellular calcium, which is modest compared to that seen with anti-Ig (approximately 100 nM vs approximately 400 nM). The increases in intracellular calcium induced by HMW-BCGF or LMW-BCGF occur in distinct but overlapping subpopulations of B cells. Thus, increases in intracellular calcium in human B cells occur not only upon activation but also upon the induction of proliferation by certain (but not all) B cell growth factors. Presumably, the effect of increasing intracellular calcium during the induction of proliferation is to modify a different group of intracellular molecules than those induced during activation.

Determination of mass changes in phosphatidylinositol 4,5-bisphosphate and evidence for agonist-stimulated metabolism of inositol 1,4,5-trisphosphate in airway smooth muscle

Stimulation of muscarinic receptors in bovine tracheal smooth muscle (BTSM) causes a sustained increase in muscle tone, but a transient increase in the second messenger Ins(1,4,5)P3. To examine whether this brief increase in Ins(1,4,5)P3 mass results from transient formation or is due to agonist-stimulation of Ins(1,4,5)P3 metabolism, we have studied the relationship between mass changes in PtdIns(4,5)P2 and Ins(1,4,5)P3 accumulation, and changes in [3H]InsP3, [3H]PtdIns, [3H]PtdInsP1 and [3H]PtdInsP2 in carbachol-stimulated myo-[3H]inositol-prelabelled BTSM slices. Carbachol (0.1 mM) caused a rapid transient increase in Ins(1,4,5)P3 concentration (basal, 12.9 +/- 0.8 pmol/mg of protein; 5 s carbachol treatment, 27.1 +/- 1.5 pmol/mg of protein), with values returning to basal levels by 30 s, but a sustained accumulation of total [3H]InsP3s, with [3H]Ins(1,3,4)P3 being the predominant isomer present at later time points. In contrast, PtdIns(4,5)P2 mass, determined by radioreceptor assay of Ins(1,4,5)P3 in desalted alkaline hydrolysates of acidified chloroform/methanol tissue extracts, declined rapidly (basal, 941 +/- 22 pmol/mg of protein; 120 s carbachol, 365 +/- 22 pmol/mg of protein; t1/2 14 s) and remained at this new steady-state level for at least 20 min in the continued presence of carbachol. Addition of 10 microM-atropine 2 min after carbachol caused a prompt return of PtdIns(4,5)P2 concentration to prestimulated values (t1/2 210 s). Ongoing resynthesis of PtdIns(4,5)P2 after carbachol stimulation was demonstrated in [3H]inositol-labelled tissue by observing a persistent increase in the specific radioactivity of [3H]PtdInsP2, shown to be exclusively [3H]PtdIns(4,5)P2, over a 10 min period. These findings strongly suggest the occurrence of persistent receptor-mediated increases in PtdIns(4,5)P2 hydrolysis and Ins(1,4,5)P3 formation which, in conjunction with the transient accumulation of Ins(1,4,5)P3 observed, provide evidence that regulation of the metabolism of Ins(1,4,5)P3 is a major determinant of Ins(1,4,5)P3 concentration in this tissue under agonist-stimulated conditions.

Cellular mechanisms of bradykinin-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have demonstrated that bradykinin hyperpolarizes the cell membrane of subconfluent MDCK cells by increase of the potassium conductance. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of bradykinin on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate diester (TPA). In untreated cells, bradykinin leads to a transient increase of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate, increase of Cai, activation of potassium channels and hyperpolarization of the cell membrane. The effects of bradykinin on PD and Cai are still present in the absence of extracellular calcium. In cells pretreated with pertussis toxin the effect of bradykinin on inositol trisphosphate formation is almost abolished but bradykinin still leads to a transient increase of Cai and PD in the presence and absence of extracellular calcium. In cells pretreated with TPA the bradykinin-induced increase of inositol trisphosphate formation is blunted, the bradykinin-induced increase of Cai abolished, but the bradykinin-induced hyperpolarization still present. The observations indicate that bradykinin increases Cai in part by phorbol ester and pertussis toxin sensitive activation of phospholipase C. In addition, bradykinin is capable of enhancing Cai by utilizing pertussis toxin insensitive mechanisms. Furthermore, bradykinin is able to transiently enhance the potassium conductance without a general increase of intracellular calcium.

Potential mechanisms involved in the negative coupling between serotonin 5-HT1A receptors and carbachol-stimulated phosphoinositide turnover in the rat hippocampus

Serotonin 5-HT1A receptors have been reported to be negatively coupled to muscarinic receptor-stimulated phosphoinositide turnover in the rat hippocampus. In the present study, we have investigated further the pharmacological specificity of this negative control and attempted to elucidate the mechanism whereby 5-HT1A receptor activation inhibits the carbachol-stimulated phosphoinositide response in immature or adult rat hippocampal slices. Various 5-HT1A receptor agonists were found to inhibit carbachol (10 microM)-stimulated formation of total inositol phosphates in immature rat hippocampal slices with the following rank order of potency (IC50 values in nM): 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (11) greater than ipsapirone (20) greater than gepirone (120) greater than RU 24969 (140) greater than buspirone (560) greater than 1-(m-trifluoromethylphenyl)piperazine (1,500) greater than methysergide (5,644); selective 5-HT1B, 5-HT2, and 5-HT3 receptor agonists were inactive. The potency of the 5-HT1A receptor agonists investigated as inhibitors of the carbachol response was well correlated (r = 0.92) with their potency as inhibitors of the forskolin-stimulated adenylate cyclase in guinea pig hippocampal membranes. 8-OH-DPAT (10 microM) fully inhibited the carbachol-stimulated formation of inositol di-, tris-, and tetrakisphosphate but only partially antagonized (-40%) inositol monophosphate production. The effect of 8-OH-DPAT on carbachol-stimulated phosphoinositide turnover was not prevented by addition of tetrodotoxin (1 microM), by prior destruction of serotonergic afferents, by experimental manipulations causing an increase in cyclic AMP levels (addition of 10 microM forskolin), or by changes in membrane potential (increase in K+ concentration or addition of tetraethylammonium). Prior intrahippocampal injection of pertussis toxin also failed to alter the ability of 8-OH-DPAT to inhibit the carbachol response. Carbachol-stimulated phosphoinositide turnover in immature rat hippocampal slices was inhibited by the protein kinase C activators phorbol 12-myristate 13-acetate (10 microM) and arachidonic acid (100 microM). Moreover, the inhibitory effect of 8-OH-DPAT on the carbachol response was blocked by 10 microM quinacrine (a phospholipase A2 inhibitor) but not by BW 755C (100 microM), a cyclooxygenase and lipoxygenase inhibitor. These results collectively suggest that 5-HT1A receptor activation inhibits carbachol-stimulated phosphoinositide turnover by stimulating a phospholipase A2 coupled to 5-HT1A receptors, leading to arachidonic acid release. Arachidonic acid could in turn activate a gamma-protein kinase C with as a consequence an inhibition of carbachol-stimulated phosphoinositide turnover. This inhibition may be the consequence of a phospholipase C phosphorylation and/or a direct effect on the muscarinic receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular mechanisms of ATP-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have shown that ATP enhances intracellular calcium concentration and activates potassium channels in Madin Darby canine kidney (MDCK)-cells, thus leading to hyperpolarization of the cell membrane. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of ATP on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoyl phorbol 13-acetate diester (TPA). In untreated cells, ATP leads to a sustained hyperpolarization and an increase of inositol 1,4,5-trisphosphate (IP3), inositol 1,3,4,5-tetrakisphosphate (IP4), and Cai. In the absence of extracellular calcium, the effect of ATP on PD and Cai is only transient. In cells pretreated with pertussis toxin, the effect of ATP on inositol trisphosphate is almost abolished, but ATP still leads to an increase of PD and Cai, which is sustained in the presence, and transient in the absence, of extracellular calcium. In cells pretreated with TPA, the effect of ATP on inositol trisphosphate is reduced and the effect on Cai blunted; but ATP still leads to a hyperpolarization of the cell membrane, which is sustained in the presence, and transient in the absence, of extracellular calcium. The observations indicate that ATP activates phospholipase C by a phorbol ester and pertussis toxin sensitive mechanism. In addition, ATP enhances Cai by pertussis toxin insensitive mechanisms allowing recruitment of calcium from both, extracellular fluid and intracellular stores. Calcium then activates the potassium channels and thus leads to the hyperpolarization of the cell membrane.

Measurement of picomole amounts of any inositol phosphate isomer separable by h.p.l.c. by means of a bioluminescence assay

An assay is described which allows the determination of the mass of any individual inositol phosphate (InsP) isomer by combining a popular h.p.l.c. separation method with simple desalting, dephosphorylation and final measurement of Ins liberated using an inositol dehydrogenase-NADH-linked bioluminescence reaction. The limit of sensitivity of this assay is about 1 pmol of Ins. routinely 5 pmol. About 40 mg wet wt. of guinea pig small intestine longitudinal smooth muscle contains 5 pmol of Ins(1,4,5)P3. For Inst(1,3,4)P3 or Ins(1,3,4,5)P4 slightly more smooth muscle is needed, and for major isomers of InsP1 or InsP2 10 mg wet wt. or less of tissue can be used. A 35 mm tissue culture plate with a confluent layer of rat fibroblasts contains about 30 pmol of Ins(1,4,5)P3. The method was applied to the measurement of the masses of Ins1P1. Ins4P1, Ins(1,4)P2, Ins(1,3,4)P3, Ins(1,4,5)P3, Ins(1,3,4,5)P4 and InsP5. The h.p.l.c. elution profiles of radiolabelled InsPS generated from [32P]Pi-labelled human Erythrocytes, [3H]Ins-labelled cultured rat fibroblasts and [3H]Ins-labelled smooth muscle fragments from guinea pig small intestine were compared with the h.p.l.c. elution profiles of their masses.

Cellular mechanisms of adrenaline-induced hyperpolarization in renal epitheloid MDCK cells

The effects of adrenaline on the potential difference across the cell membrane, on formation of inositol phosphates and on intracellular Ca2+ ([Ca2+]i) were analysed in cells without or with pretreatment with pertussis toxin or phorbol 12-myristate 13-acetate (PMA). In untreated cells, adrenaline leads to a sustained hyperpolarization, a stimulation of Ins(1,4,5)P3 and Ins(1,3,4,5,)P4 formation and a transient increase in [Ca2+]i from 78 +/- 7 to 555 +/- 43 nM, followed by a plateau of 260 +/- 23 microM. In the absence of extracellular Ca2+ the effect of adrenaline on both potential difference and [Ca2+]i is transient. In cells pretreated with pertussis toxin, the effects of adrenaline on InsP3 and [Ca2+]i are still preserved, but the effect on potential difference is transient. In cells pretreated with PMA, the effect of adrenaline on InsP3 formation is severely decreased and that on [Ca2+]i abolished, whereas a transient hyperpolarizing effect is still present. This transient hyperpolarization is abolished by additional pretreatment with pertussis toxin. The observations suggest that adrenaline hyperpolarizes the cell membrane of MDCK cells by several distinct mechanisms. First, adrenaline stimulates the formation of InsP3 and InsP4, which at least in part accounts for the release of intracellular Ca2+ and the entry of Ca2+ from the extracellular fluid. Stimulation of phospholipase C is not mediated by pertussis-toxin-sensitive G-proteins, but apparently is inhibited by activation of protein kinase C. Second, adrenaline hyperpolarizes the cell membrane by a mechanism independent from increase in [Ca2+]i which is sensitive to pertussis toxin but is, at least in part, insensitive to PMA.

Inositol 1,4,5-triphosphate phosphatase activity in membranes isolated from amphibian skeletal muscle [corrected]

The hydrolysis of [3H]inositol 1,4,5-trisphosphate by a soluble fraction and by isolated transverse tubule and sarcoplasmic reticulum membranes from frog skeletal muscle was studied. Transverse tubule membranes displayed rates of hydrolysis several-fold higher than those of sacroplasmic reticulum and soluble fraction; Km and Vmax were 25.2 microM and 44.1 nmol/mg/min, respectively. Transverse tubule membranes sequentially hydrolyzed inositol trisphosphate to inositol bisphosphate, inositol 1-phosphate and inositol, indicating that these membranes have inositol bis- and monophosphatases in addition to inositol trisphosphatase.

Stimulation by epidermal growth factor of inositol phosphate production in plasma membranes from A431 cells

Plasma membranes were isolated from A431 cells previously labelled with myo-[3H]inositol during exponential growth, using a rapid procedure on Percoll gradients. They displayed a significant phospholipase (PLC) activity against phosphoinositides, which was stimulated by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), epidermal growth factor (EGF) and fetal calf serum (FCS) (24%, 11% and 97% over controls, respectively). The effect of EGF was not significantly increased by GTP gamma S. Upon addition of cytosol, EGF promoted an almost 100% stimulation of inositol 1,4,5-trisphosphate and inositol bisphosphate generation, which displayed an absolute requirement for GTP gamma S. This dose-dependent effect of cytosol was linear until 60 micrograms/ml of cytosolic protein and decreased afterwards; it was abolished by heat treatment and trypsin hydrolysis, and it was not reproduced by an identical amount of bovine serum albumin. The same biphasic stimulation was observed with phosphotyrosyl proteins immunopurified from cytosol of A431 cells previously stimulated by EGF. Since phosphotyrosyl proteins displayed PLC activity, our data suggest that soluble protein substrates of EGF receptor tyrosine kinase, including PLC, could be involved in the regulation of phosphoinositide hydrolysis in response to EGF. Using phosphatidyl[3H]inositol 4,5-bisphosphate (PIP2) dispersed with unlabelled phosphatidylethanolamine and phosphatidylserine as an exogenous substrate, no stimulation of PLC activity by EGF could be detected, either with membranes or with membranes plus cytosol. It is concluded that EGF might stimulate hydrolysis of phosphoinositides by PLC through complex interactions between plasma membrane and cytosolic factors which still remain to be identified.

Sialagogue-stimulated protein phosphorylation related to ornithine decarboxylase induction in cultured rat parotid explants

Both beta-adrenergic (isoproterenol) and cholinergic (carbachol) sialagogues increase amylase secretion, ornithine decarboxylase activity and DNA synthesis in murine parotid gland in vivo and in vitro. These agonists enhanced the incorporation of labelled inorganic orthophosphate into parotid proteins in rat parotid explants cultured on siliconized lens paper floating on serum-free 199 medium. Analysis of the labelled proteins by SDS-PAGE and autoradiography revealed that isoproterenol enhanced the phosphorylation of four proteins with apparent molecular weights of 17, 20, 31 and 32 kDa and carbachol stimulated the phosphorylation of 31 and 32 K proteins. Isoproterenol-dependent ornithine decarboxylase induction and phosphorylation of the proteins were selectively suppressed by monensin but not by polymyxin B, whereas carbachol-dependent ornithine decarboxylase induction and protein phosphorylation were inhibited by polymyxin B but not by monensin. Neither monensin nor polymyxin B suppressed isoproterenol- or carbachol-stimulated amylase secretion. Time course experiments showed that sialagogue-stimulated protein phosphorylation preceded the increase of ornithine decarboxylase activity and had almost disappeared when it was maximal. Propranolol and atropine, antagonists of isoproterenol and carbachol, respectively, completely inhibited not only amylase secretion and ornithine decarboxylase induction but also protein phosphorylation stimulated by the corresponding agonists. These findings suggest that increased phosphorylation of specific proteins is associated with sialagogue-stimulated ornithine decarboxylase induction but not amylase secretion.

Properties of muscarinic receptors mediating second messenger responses in the rabbit aorta

The second messenger response of cultured smooth muscle and endothelial cells of the rabbit aorta to acetylcholine (ACh) was investigated. ACh induced a concentration-dependent accumulation of inositol-monophosphate (InsP1) in the smooth muscle cells and a concentration-dependent reduction in basal production of adenosine 3',5'-cyclic monophosphate (cAMP) in the endothelial cells. Atropine, scopolamine and nitric oxide (NO) inhibited the ACh-induced accumulation of InsP1. The IC50 values were 55 +/- 4.3 nM, 81.2 +/- 6.5 microM and 13.3 +/- 3.5 microM, respectively. On the other hand, the inhibition of reduction in basal production of cAMP was inhibited by scopolamine (IC50 = 55.3 +/- 4.3 nM) and atropine (IC50 = 63.2 +/- 5.2 microM). Pirenzepine inhibited both the ACh-induced accumulation of InsP1 (IC50 = 1.5 +/- 0.01 microM) and reduction of basal production of cAMP (IC50 = 812 +/- 33.5 nM). However, unlike scopolamine or atropine, the M1-selective ligand was not selective to either the endothelial or smooth muscle receptors. These results demonstrate for the first time the second messenger response of the action of ACh on the endothelial muscarinic receptors and the inhibition of InsP1 formation by NO. In addition, the results also support our earlier findings that the muscarinic receptors in the endothelium and smooth muscle of the rabbit aorta can be differentiated by atropine and scopolamine, namely, the endothelial receptors have high affinity for scopolamine but extremely low affinity for atropine whilst the reverse holds true for the smooth muscle receptors.

High-performance reversed-phase ion-pair chromatographic study of myo-inositol phosphates. Separation of myo-inositol phosphates, some common nucleotides and sugar phosphates

A detailed study of all the major chromatographic variables affecting the retention behaviour and separation of myo-inositol phosphates in reversed-phase ion-pair chromatographic systems was carried out. The parameters studied included the eluent concentration of the pairing ion, the eluent concentration of the organic modifier and the buffer salt, the pH of the eluent, the minimum column plate count necessary for the separation of the inositol trisphosphate isomers and isocratic and gradient modes of separation. The retention behaviour of some common nucleotides and sugar phosphates was also investigated as these phosphates present chromatographic interference problems in biochemical studies based on the cellular incorporation of [32P]Pi. The separation methods developed appear to be superior to established anion-exchange separation techniques in terms of separation speed and "mildness" of the chromatographic conditions.