Evidence for an Extracellular Reception Site for Abscisic Acid in Commelina Guard Cells

The phytohormone abscisic acid (ABA) triggers stomatal closing as a physiological response to drought stress. Several basic questions limit an understanding of the mechanism of ABA reception in guard cells. Whether primary ABA receptors are located on the extracellular side of the plasma membrane, within the intracellular space of guard cells, or both remains unknown. Furthermore, it is not clear whether ABA must be transported into guard cells to exert control over stomatal movements. In the present study, a combination of microinjection into guard cells and physiological assays of stomatal movements have been performed to determine primary sites of ABA reception in guard cells. Microinjection of ABA into guard cells of Commelina communis L. resulted in injected cytosolic concentrations of 50 to 200 [mu]M ABA and in additional experiments in lower concentrations of approximately 1 [mu]M ABA. Stomata with ABA-loaded guard cells (n > 180) showed opening similar to stomata with uninjected guard cells. The viability of guard cells following ABA injection was demonstrated by neutral red staining as well as monitoring of stomatal opening. Extracellular application of 10 [mu]M ABA inhibited stomatal opening by 98% at pH 6.15 and by 57% at pH 8.0. The pH dependence of extracellular ABA action may suggest a contribution of an intracellular ABA receptor to stomatal regulation. The findings presented here show that intracellular ABA alone does not suffice to inhibit stomatal opening under the imposed conditions. Furthermore, these data provide evidence that a reception site for ABA-mediated inhibition of stomatal opening is on the extracellular side of the plasma membrane of guard cells.     

The effects of fatty acids on phosphoinositide synthesis and myo-inositol accumulation in exocrine pancreas

The effects of arachidonic acid (20:4) on phosphoinositide turnover were examined in rat pancreatic acinar cells prelabeled with myo-[3H]inositol. Arachidonic acid (50 microM) increased the accumulation of myo-[3H]inositol, but not that of [3H]inositol monophosphate, [3H]inositol bisphosphate, or [3H]inositol trisphosphate. By contrast, 10 microM carbamoylcholine increased the accumulation of all four compounds. A combination of arachidonic acid plus carbamoylcholine caused a selective and marked accumulation of myo-[3H]inositol, which was abolished by 10 mM LiCl. Arachidonic acid (10-100 microM) produced a concentration-dependent inhibition of myo-[3H]inositol incorporation into phosphoinositides and markedly depressed carbamoylcholine-induced increases in myo-[3H]inositol incorporation into inositol phospholipids. Several other unsaturated and saturated fatty acids failed to elicit a synergistic response with carbamoylcholine in stimulating myo-[3H]inositol accumulation and did not retard the incorporation of myo-[3H]inositol into phosphoinositides. The fact that eicosapentaenoic acid (20:5), but not arachidic acid (20:0), mimicked the depressant effect of arachidonate on phosphoinositide labeling suggests that the degree of unsaturation of the fatty acid, rather than chain length, is important for inhibition of phosphoinositide synthesis. The arachidonate-induced decrease in myo-[3H]inositol incorporation was accompanied by a reduction in the steady state level of [32P]phosphatidylinositol 4,5-bisphosphate. The mass of arachidonic acid liberated in response to carbamoylcholine was measured by gas chromatography-mass spectrometry, and the time course of stimulated arachidonate accumulation paralleled that of inositol phosphate accumulation and amylase release. These observations suggest that in exocrine pancreas, endogenous arachidonic acid serves as a negative feedback regulator of phosphoinositide turnover.     

Visualizing Changes in Cytosolic-Free Ca2+ during the Response of Stomatal Guard Cells to Abscisic Acid

In this paper, we report the results of a detailed investigation into abscisic acid (ABA)[mdash]stimulated elevations of guard cell cytosolic-free Ca2+ ([Ca2+]cyt). Fluorescence ratio photometry and ratio imaging techniques were used to investigate this phenomenon. Guard cells of open and closed (opened to 10 to 12 [mu]m before treatment with ABA) stomata were microinjected with the fluorescent Ca2+ indicator Indo-1. Resting [Ca2+]cyt ranged from 50 to 350 nM. ABA (100 nM) stimulated an increase in [Ca2+]cyt in 68 and 81% of guard cells microinjected in the open and closed configuration, respectively. All stomata were observed to close in response to ABA. Increases ranged from 100 to 750 nM above the resting concentration and were arbitrarily grouped into five "classes." ABA-stimulated increases in [Ca2+]cyt were not uniformly distributed across the cytosol of guard cells. Rapid transient increases in [Ca2+]cyt were also observed in the guard cells of stomata microinjected in the closed configuration. We concluded that the ABA-induced turgor loss in guard cells is a Ca2+-dependent process.     

Metabolism of inositol phosphates in parotid cells: implications for the pathway of the phosphoinositide effect and for the possible messenger role of inositol trisphosphate

Rat parotid acinar cells were used to investigate the time course of formation and breakdown of inositol phosphates in response to receptor-active agents. In cells preincubated with [3H]inositol and in the presence of 10 mM LiCl (which blocks hydrolysis of inositol phosphate), methacholine (10(-4)M) caused a substantial increase in cellular content of [3H]inositol phosphate, [3H]inositol bisphosphate and [3H]inositol trisphosphate. Subsequent addition of atropine (10(-4) M) caused breakdown of [3H]inositol trisphosphate and [3H]inositol bisphosphate and little change in accumulated [3H]inositol phosphate. The data could be fit to a model whereby inositol trisphosphate and inositol bisphosphate are formed from phosphodiesteratic breakdown of phosphatidylinositol bisphosphate and phosphatidylinositol phosphate respectively, and inositol phosphate is formed from hydrolysis of inositol bisphosphate rather than from phosphatidyl-inositol. Consistent with this model was the finding that [3H]inositol trisphosphate and [3H]inositol bisphosphate levels were substantially increased in 5 sec while an increase in [3H]inositol phosphate was barely detectable at 60 sec. These results indicate that in the parotid gland the phosphoinositide cycle is activated primarily by phosphodiesteratic breakdown of the polyphosphoinositides rather than phosphatidyl-inositol. Also, the results show that formation of inositol trisphosphate is probably sufficiently rapid for it to act as a second messenger signalling internal Ca2+ release in this tissue.     

Insulin-stimulated phosphoinositide metabolism in isolated fat cells

Treatment of isolated fat cells with insulin produced increases of up to 4.8-fold in the incorporation of [3H]inositol into phosphatidylinositol. This effect of insulin was both time- and dose-dependent with half-maximal stimulation at 30 microunits/ml of insulin. Insulin increased the labeling of phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate but not phosphatidylinositol 4-monophosphate in cells which had been preincubated with [3H]inositol for 90 min. Incubation of the cells in a Ca2+-free buffer increased the basal level of phosphatidylinositol labeling and enhanced the effect of insulin. Glucagon and isoprenaline, both of which stimulate lipolysis, had no effect on phosphatidylinositol labeling but did potentiate insulin-stimulated incorporation of [3H]inositol into phosphatidylinositol. Phosphoinositide breakdown was measured by the accumulation of inositol phosphates. Insulin did not increase the level of the inositol phosphates at all concentrations of the hormone tested. By comparison, phenylephrine and vasopressin were able to stimulate phosphoinositide breakdown. Pretreatment of the cells with insulin enhanced the effect of phenylephrine on inositol phosphates' accumulation, suggesting that insulin may potentiate phenylephrine-mediated phosphoinositide turnover. From these data we conclude that insulin stimulates the de novo synthesis of phosphatidylinositol and phosphatidylinositol 4,5-biphosphate, but has no effect on phosphoinositide breakdown.     

Pertussis toxin does not inhibit muscarinic-receptor-mediated phosphoinositide hydrolysis or calcium mobilization.

Pertussis toxin was used to examine the role of the inhibitory guanine nucleotide regulatory protein, Ni, in muscarinic-receptor-mediated stimulation of phosphoinositide turnover and calcium mobilization. In cultured chick heart cells, pertussis-toxin treatment inhibited muscarinic-receptor-mediated attenuation of isoprenaline-stimulated cyclic AMP accumulation. This finding is consistent with the proposal that pertussis toxin blocks the capacity of Ni to couple muscarinic receptors to adenylate cyclase. In contrast, treatment of chick heart cells or 1321N1 human astrocytoma cells with pertussis toxin did not block muscarinic-receptor-mediated stimulation of phosphoinositide hydrolysis, as measured by [3H]inositol phosphate accumulation in the presence of Li+. Pertussis-toxin treatment also had little effect on basal and muscarinic-receptor-stimulated phosphatidylinositol synthesis, as measured by the incorporation of [3H]inositol into phosphatidylinositol. Activation of muscarinic receptors also enhances the rate of unidirectional 45Ca2+ efflux in 1321N1 cells; this response, like phosphoinositide hydrolysis, was not prevented by pertussis-toxin treatment. Our data suggest that muscarinic receptors are not coupled to phosphoinositide hydrolysis or calcium mobilization through Ni.     

Phospholipase C is required for the control of stomatal aperture by ABA

The calcium-releasing second messenger inositol 1,4,5-trisphosphate is involved in the regulation of stomatal aperture by ABA. In other signalling pathways, inositol 1,4,5-trisphosphate is generated by the action of phospholipase C. We have studied the importance of phospholipase C in guard cell ABA-signalling pathways. Immunolocalisation of a calcium-activated phospholipase C confirmed the presence of phospholipase C in tobacco guard cells. Transgenic tobacco plants with considerably reduced levels of phospholipase C in their guard cells were only partially able to regulate their stomatal apertures in response to ABA. These results suggest that phospholipase C is involved in the amplification of the calcium signal responsible for reductions in stomatal aperture in response to ABA. As full ABA-induced inhibition of stomatal opening was not observed, our results support a role for the action of other calcium-releasing second messengers in the guard cell ABA-signalling pathway. It is not known whether these different calcium-releasing second messengers act in the same or parallel ABA-signalling pathways.     

The phosphoinositide pathway of lymphoid cells: labeling after permeabilization by alveolysin, a bacterial sulfhydryl-activated cytolysin.

An improved method allowing incorporation of [3H]myo-inositol into the phosphoinositide pool of human lymphoid cells is described. The procedure devised involves cell permeabilization with a thiol-activated membranolytic toxin, alveolysin, and optimization of the phosphoinositide labeling and extraction. In these conditions 4 to 10% of the added [3H]myo-inositol is found intracellularly and half of this amount (2-5%) is incorporated into the phosphoinositide pool in only 1 h as compared to the classical 0.2 to 0.3% incorporation obtained after 10 to 20 h. The integrity of coupling between receptors and phospholipase C was assessed by the inositol phosphate production after cell stimulation by various agonists.     

Enhanced arterial contractility to noradrenaline in diabetic rats is associated with increased phosphoinositide metabolism.

The purpose of this study was to investigate whether the increased contractile responsiveness of aortae from male rats with 12-14 week streptozotocin-induced diabetes to noradrenaline is associated with alterations in phosphoinositide metabolism. The contractile response to noradrenaline (10 microM) in both the presence and absence of extracellular calcium was significantly enhanced in aortae from diabetic rats. No significant differences were found between control and diabetic arteries in the basal incorporation of 32P and [3H]myo-inositol into phosphoinositides, or in the basal accumulation of [32P]phosphatidic acid and [3H]inositol phosphates. However, noradrenaline (10 microM) caused significantly greater breakdown of [32P]phosphatidylinositol 4,5-bisphosphate and formation of [32P]phosphatidic acid and [3H]inositol phosphates in diabetic aortae than in control preparations. The production of [3H]inositol phosphates induced by noradrenaline was selectively reduced by the alpha 1-adrenoceptor antagonist, prazosin, in both control and diabetic tissues. These results indicate that phosphoinositide metabolism in response to noradrenaline via stimulation of alpha 1-adrenoceptors is enhanced in aortae from chronic streptozotocin-diabetic rats. The increase in inositol 1,4,5-trisphosphate and 1,2-diacylglycerol production that presumably results could be responsible, at least in part, for the enhanced contractile response of aortae from diabetic rats to noradrenaline.     

Sugar and Organic Acid Accumulation in Guard Cells of Vicia faba in Response to Red and Blue Light

Changes in neutral sugar and organic acid content of guard cells were quantitated by high-performance liquid chromatography during stomatal opening in different light qualities. Sonicated Vicia faba epidermal peels were irradiated with 10 [mu]mol m-2 s-1 of blue light, a fluence rate insufficient for the activation of guard cell photosynthesis, or 125 [mu]mol m-2 s-1 of red light, in the presence of 1 mM KCl, 0.1 mM CaCl2. The low-fluence-rate blue light stimulated an average net stomatal opening of 4.7 [mu]m in 2 h, whereas the saturating fluence rate of red light stimulated an average net opening of 3.8 [mu]m in 2 h. Under blue light, the malate content of guard cells increased to 173% of the initial level during the first 30 min of opening and declined as opening continued. Sucrose levels continuously rose throughout the blue light-stimulated opening, reaching 215% of the initial level after 2 h. The starch hydrolysis products maltose and maltotriose remained elevated at all times. Under red light, guard cells showed very little increase in organic acid or maltose levels, whereas sucrose levels increased to 208% of the initial level after 2 h. Total measured organic metabolite concentrations were correlated with stomatal apertures in all cases except where substantial malate increases occurred. These results support the hypothesis that light quality modulates alternative mechanisms of osmotic accumulation in guard cells, including potassium uptake, photosynthetic sugar production, and starch breakdown.     

Lithium-induced accumulation of inositol 1-phosphate during cholecystokinin octapeptide- and acetylcholine-stimulated phosphatidylinositol breakdown in dispersed mouse pancreas acinar cells

Dispersed mouse pancreas acinar cells were prepared in which phosphatidylinositol had been labeled with myo[2-3H]inositol. During incubation with 0.3 microM cholecystokinin octapeptide (CCK-8) for 15 min, there was a loss of [3H]phosphatidylinositol radioactivity (23%) and a 3-fold gain in trichloroacetic acid-soluble radioactivity. Replacement of NaCl by up to 58 mM LiCl did not significantly affect the amount of CCK-8-stimulated [3H]phosphatidylinositol breakdown or the gain in acid-soluble radioactivity. However, in normal medium, the product of phosphatidylinositol breakdown was almost all inositol, whereas in Li+-containing medium, the product was almost all inositol 1-phosphate. Similar results were obtained with acetylcholine which, in the presence of Li+, gave a dose-responsive increase in inositol 1-phosphate over the concentration range of 0.1 to 10 microM. No increased accumulation of [3H]inositol diphosphate or [3H]inositol triphosphate was detected in stimulated cells. Time courses in the presence of Li+ indicated that the formation of inositol 1-phosphate preceded the formation of inositol. Addition of up to 50 mM myoinositol to the incubation medium showed no diluting effect on the amount of [3H]inositol 1-phosphate found. The accumulation of inositol 1-phosphate is presumably due to the known ability of Li+ to inhibit myoinositol 1-phosphatase. The results provide clear evidence that stimulated phosphatidylinositol breakdown involves a phospholipase C type of phosphodiesterase activity. 1.25 mM Li+ gave half-maximal inositol 1-phosphate accumulation. This is close to the range of plasma Li+ levels which is used therapeutically in psychiatric disorders. In unstimulated cells, [3H]inositol 1-phosphate accumulation in the presence of Li+ corresponded to a breakdown rate for [3H]phosphatidylinositol of 2 to 3%/h.     

Phosphoinositide metabolism and the calcium response to concanavalin A in S49 T-lymphoma cells. A comparison with thymocytes.

Comparisons were made between transformed S49 T-lymphoma cells and normal murine thymocytes in their polyphosphoinositides, inositol polyphosphates and cytosolic free calcium concentrations ([Ca2+]i), and the effects of the T-cell mitogen concanavalin A (Con A) on these properties. 1. The ratios of the polyphosphoinositides to phosphatidylinositol in both exponential-phase S49 cells and mitogen-stimulated thymocytes (G1 phase) were greater than in quiescent (G0-phase) thymocytes. 2. In response to Con A, the amount of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in S49 cells decreased slightly (17% in 30 min), and this was sufficient to account for the small amounts of inositol phosphates that accumulated. In contrast, it has been shown previously that Con A stimulates a rapid resynthesis of PtdInsP2 in thymocytes and the amounts of inositol phosphates released rapidly exceed the steady-state amount of the PtdInsP2 precursor [Taylor, Metcalfe, Hesketh, Smith & Moore (1984) Nature (London) 312, 462-465]. 3. The [Ca2+]i did not differ significantly in S49 cells and thymocytes before the addition of Con A, and the increases in [Ca2+]i in response to Con A were similar in both types of cell. 4. The [Ca2+]i increase in response to Con A was inhibited by similar concentrations of intracellular cyclic AMP (2-10 microM) in S49 cells and thymocytes, suggesting that similar regulatory mechanisms act on this response in both types of cell. The data demonstrate that the basal [Ca2+]i and phosphoinositide metabolism is similar in both the normal cells and their transformed counterparts. In addition, they suggest that the activated Con A receptors generate very similar signals in the two cell types, and that any perturbations of primary signal transduction to the secondary phosphoinositide and [Ca2+]i responses in the S49 phenotype are quantitative rather than qualitative.     

Altered levels of phosphoinositide metabolites and activation of guanine-nucleotide dependent phospholipase C in rat hepatic tumors.

The metabolism of phosphatidylinositol was studied in normal quiescent hepatocytes, hepatocellular carcinomas induced by single dose of diethylnitrosamine, followed by 2-acetylaminofluorene and partial hepatectomy (Solt-Farber model), and in an established hepatoma cell line, JB1. The JB1 hepatoma cell line and hepatocellular carcinomas demonstrated a 4- to 5-fold higher rate of turnover of [3H]-inositol and [3H]-glycerol than the control hepatocytes. Significantly, elevated levels of second messengers inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol were noted in hepatic tumor cells within 4 hr of labeling with precursor molecules, whereas no detectable level of 3H-labeled inositol trisphosphate was noted in quiescent hepatocytes, even after incubation with 10 mM LiCl for 30 min. Approximately 2.5-fold higher specific activities of a guanine nucleotide and Ca+2 dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C were detected in the hepatocellular carcinoma cells. The cellular location of the phospholipase C activity was also different, being membrane bound in hepatocytes and equally distributed between cytosolic and membrane factions in the hepatomas. These data are consistent with the hypothesis that the enhanced production of diacylglycerol and inositol 1,4,5-trisphosphate in hepatocellular carcinomas may be due to the activation of a guanine nucleotide dependent phosphatidylinositol 4,5-bisphosphate specific phospholipase C. These data are the first to compare phosphoinositide turnover in normal liver and hepatic tumor cells and suggest that the sustained levels of second messengers is closely associated with the transformation and enhanced growth rate in hepatic tumor cells.     

Antigen receptor-mediated regulation of sustained polyphosphoinositide turnover in a human T cell line. Evidence for a receptor-regulated pathway for production of phosphatidylinositol 4,5-bisphosphate

Stimulation of the human T cell line, Jurkat, by the addition of monoclonal antibodies reactive with the T cell antigen receptor complex (CD3/Ti) leads to sustained increases in levels of inositol 1,4,5-trisphosphate. To investigate the possibility that the production of polyphosphoinositides is regulated during CD3/Ti stimulation, we studied Jurkat cells whose inositol phospholipids had been labeled to steady state with [3H]inositol, as well as Jurkat cells during nonequilibrium labeling with [32P]orthophosphate. The addition of CD3 monoclonal antibodies led to a 4-5-fold increase in [3H]inositol trisphosphate that was sustained for greater than 20 min. Within 60 s of CD3/Ti stimulation, [3H] phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and [3H]phosphatidylinositol 4-phosphate (PtdIns4P) decreased by 65 and 35%, respectively. This change in [3H]PtdIns(4,5)P2 persisted for greater than 20 min. The decrease in [3H]PtdIns4P, however, was transient, and, after 5 min, the levels of [3H]PtdIns4P were comparable in stimulated and unstimulated cells. To examine the rate of flux through inositol phospholipids, we measured the CD3/Ti-stimulated changes in the ratio, 32P cpm/3H cpm, in each inositol phospholipid. CD3/Ti stimulation led to accelerated fluxes through PtdIns(4,5)P2 and phosphatidylinositol (PtdIns) that were maintained for greater than 20 min. After the initial 30 s, however, there was no detectable effect of anti-CD3 on flux through Ptsins4p. This observation suggested that, during CD3/Ti stimulation, production of PtdIns(4,5)P2 from PtdIns might occur via a small pool of PtdIns4P with a very high turnover. The existence of such a pool was established by determining that, in stimulated cells, the 32P-specific activity of the 1-position phosphate of PtdIns(4,5)P2 was 8-10-fold that of PtdIns4P. We conclude that, during the initial 60 s of CD3/Ti stimulation, there is a substantial depletion of cellular PtdIns(4,5)P2 and PtdIns4P. Thereafter, a CD3/Ti-regulated pathway generates PtdIns(4,5)P2 from PtdIns through a small, but highly labile, pool of PtdIns4P.     

Characterization of an actin-myosin head interface in the 40-113 region of actin using specific antibodies as probes.

A method of membrane permeabilization of T lymphocytes with the bacterial cytotoxin streptolysin O has allowed the effect of guanine nucleotide analogues on phosphatidylinositol metabolism and protein kinase C (PKC) activation to be investigated. The data demonstrate that, in permeabilized cells, phosphorylation of the gamma subunit of the CD3 antigen can be induced in response to the PKC activator phorbol 12,13-dibutyrate, the polyclonal mitogen phytohaemagglutinin (PHA) and the stimulatory guanine nucleotide analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]). Application of a pseudo-substrate inhibitor of PKC indicated that CD3gamma-chain phosphorylation induced in response to all three agonists was mediated by PKC. PHA and GTP[S] also stimulated inositol phospholipid turnover and inositol phosphate accumulation. The kinetics and concentration-dependence of PHA-induced inositol phospholipid hydrolysis correlated with PHA-induced CD3gamma phosphorylation, suggesting that PHA may regulate CD3gamma phosphorylation via diacylglycerol produced as a consequence of inositol phospholipid hydrolysis. However, there was an inconsistency in that PHA induced greater (greater than 200%) levels of inositol phospholipid turnover than did GTP[S], but much weaker (less than 50%) levels of CD3-antigen phosphorylation. There was also a discrepancy between GTP[S] effects on phosphatidylinositol turnover and PKC activation, in that the half-maximal GTP[S] concentration for inositol phosphate production and CD3gamma-chain phosphorylation was 0.75 microM and 75 microM respectively. Moreover, 10 microM-GTP[S] induced maximal inositol phosphate production, but only 10% of maximal CD3gamma-chain phosphorylation. The data are consistent with the idea that other signal-transduction pathways, in addition to those involving inositol phosphate production, exist for the regulation of PKC in T lymphocytes.     

Modulation of Phosphoinositide Hydrolysis by NaF and Aluminum in Rat Cortical Slices

NaF stimulated phosphoinositide hydrolysis in rat cortical slices. The production of [3H]inositol monophosphate was rapid for the first 15 min of incubation with NaF, followed by a plateau. The major product detected was [3H]inositol monophosphate, although significant amounts of [3H]inositol bisphosphate and [3H]inositol trisphosphate were also produced. The stimulation of [3H]inositol monophosphate production by NaF was concentration dependent between 2 and 20 mM NaF. Addition of 10 or 100 microM AlCl3 or aluminum maltol did not alter the effect of NaF, whereas at 500 microM, these aluminum preparations resulted in significant inhibition. Increasing the concentration of K+ from 5 to 20 mM potentiated [3H]inositol monophosphate production induced by carbachol but not by NaF. Incubation with 1 microM phorbol 12-myristate 13-acetate, a phorbol ester, inhibited carbachol-induced, but not NaF-induced, [3H]inositol monophosphate production. These results further support the hypothesis that a guanine nucleotide binding protein that can be activated by NaF is involved in phosphoinositide hydrolysis in brain. The use of NaF provides a means to bypass receptors to study intracellular regulatory sites of phosphoinositide metabolism without disrupting cells.     

Thyrotropin-releasing hormone rapidly activates the phosphodiester hydrolysis of polyphosphoinositides in GH3 pituitary cells. Evidence for the role of a polyphosphoinositide-specific phospholipase C in hormone action

The early actions of thyrotropin-releasing hormone (TRH) have been studied in hormone-responsive clonal GH3 rat pituitary cells. Previous studies had demonstrated that TRH promotes a "phosphatidylinositol response" in which increased incorporation of [32P]orthophosphate into phosphatidylinositol and phosphatidic acid was observed within minutes of hormone addition. The studies described here were designed to establish whether increased labeling of phosphatidylinositol and phosphatidic acid resulted from prior hormone-induced breakdown of an inositol phosphatide. GH3 cells were prelabeled with [32P]orthophosphate or myo-[3H]inositol. Addition of TRH resulted in the rapid disappearance of labeled polyphosphoinositides, whereas levels of phosphatidylinositol and other phospholipids remained unchanged. TRH-promoted polyphosphoinositide breakdown was evident by 5 S and maximal by 15 s of hormone treatment. Concomitant appearance of inositol polyphosphates in [3H]inositol-labeled cells was observed. In addition, TRH rapidly stimulated diacylglycerol accumulation in either [3H]arachidonic- or [3H]oleic acid-labeled cultures. These results indicate that TRH rapidly causes activation of a polyphosphoinositide-hydrolyzing phospholipase C-type enzyme. The short latency of this hormone effect suggests a proximal role for polyphosphoinositide breakdown in the sequence of events by which TRH alters pituitary cell function.     

Nitric oxide is a signaling intermediate during bicarbonate-induced stomatal closure in Pisum sativum

The role of nitric oxide (NO) during bicarbonate-induced stomatal closure was studied in the abaxial epidermis of Pisum sativum . A few experiments were done with 10 μ M ABA, for comparison. The presence of 2 m M sodium bicarbonate or 10 μ M ABA induced an increase of NO in guard cells. Elevation of NO by sodium nitroprusside induced stomatal closure and enhanced further the closure by bicarbonate. The bicarbonate induced increase in NO of guard cells, or stomatal closure was prevented partially by 2-phenyl-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide, an NO scavenger and N -nitro- l -Arg-methyl ester, an inhibitor of NO synthase (NOS). These results suggested that guard cells generated NO on exposure to bicarbonate and that NOS was involved at least partially in such NO production. Time course experiments revealed that on exposure to bicarbonate or ABA, the rise in guard cell NO production peaked within 10 min. Experiments using pharmacological compounds like wortmannin/LY294002 (phosphatidylinositol 3 kinase inhibitors), 1 H -(1,2,4)-oxadiazole-[4,3 a ]quinoxalin-1-one (guanylyl cyclase inhibitor), nicotinamide (cyclic adenosine diphosphate ribose antagonist), guanosine 5'-O-(2-thiodiphosphate) (G-protein antagonist) suggested a role of phosphatidylinositol 3-phosphate or G-proteins during bicarbonate-induced stomatal closure.     

Insulin and oxytocin effects on phosphoinositide metabolism in adipocytes

The effects of hormones on phosphoinositide metabolism were examined in rat adipocytes prelabeled with 32Pi or [3H]inositol. Oxytocin and vasopressin produced large decreases in labeled polyphosphoinositides and increases in phosphatidic acid and inositol phosphates, whereas insulin was without effect, although it stimulated lipogenesis from glucose. Likewise, insulin did not elevate 1,2-diacylglycerol measured chemically by high pressure liquid or thin-layer chromatography in fat cells or pads. It also did not increase the radioactivity in 1,2-diacylglycerol in ghosts prepared from fat cells previously labeled with [3H]arachidonic acid, although oxytocin and vasopressin increased this. It is therefore concluded that insulin does not stimulate the breakdown of polyphosphoinositides to yield 1,2-diacylglycerol and inositol phosphates in adipocytes and that the insulin-like actions of oxytocin must be due to other changes. Insulin induced small, but significant and equal increases (40% at 30 min) in the incorporation of [3H] inositol into phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in adipocytes. The effects were not dependent upon glucose and were not evident before 15 min. Oxytocin also produced large increases in the labeling of the three phosphoinositides. Insulin stimulated the incorporation of [3H]glycerol into the three phosphoinositides and also phosphatidic acid, phosphatidylserine, and phosphatidylethanolamine by 50-100% in cells incubated without glucose. No changes in the labeling of glycerol 3-phosphate, lysophosphatidic acid, phosphatidylcholine, and triacylglycerol were detected, and there was a small increase (30%) in 1,2-diacylglycerol labeling. It is concluded that insulin increases the synthesis of phosphatidylinositol, phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, phosphatidylethanolamine, and phosphatidylserine in fat cells partly by stimulating a reaction(s) located between glycerol 3-phosphate and phosphatidic acid in the biosynthetic pathway.     

Breakdown of polyphosphoinositides and not phosphatidylinositol accounts for muscarinic agonist-stimulated inositol phospholipid metabolism in rat parotid glands.

The molecular mechanisms underlying the ability of muscarinic agonists to enhance the metabolism of inositol phospholipids were studied using rat parotid gland slices prelabelled with tracer quantities of [3H]inositol and then washed with 10 mM unlabelled inositol. Carbachol treatment caused rapid and marked increases in the levels of radioactive inositol 1-phosphate, inositol 1,4-bisphosphate, inositol 1,4,5-trisphosphate and an accumulation of label in the free inositol pool. There were much less marked changes in the levels of [3H]phosphatidylinositol, [3H]phosphatidylinositol 4-phosphate and [3H]phosphatidylinositol 4,5-bisphosphate. At 5 s after stimulation with carbachol there were large increases in [3H]inositol 1,4-bisphosphate and [3H]inositol 1,4,5-trisphosphate, but not in [3H]inositol 1-phosphate. After stimulation with carbachol for 10 min the levels of radioactive inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate greatly exceeded the starting level of radioactivity in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate respectively. When carbachol treatment was followed by addition of sufficient atropine to block all the muscarinic receptors the radioactive inositol phosphates rapidly returned towards control levels. The carbachol-evoked changes in radioactive inositol phosphate and phospholipid levels were blocked in the presence of 2,4-dinitrophenol (an uncoupler of oxidative phosphorylation). The results suggest that muscarinic agonists stimulate a polyphosphoinositide-specific phospholipase C and that these lipids are continuously replenished from the labelled phosphatidylinositol pool. [3H]Inositol 1-phosphate in the stimulated glands probably arises via hydrolysis of inositol 1,4-bisphosphate and not directly from phosphatidylinositol.