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.     

Inhibition of phosphatidylcholine synthesis by vasopressin and angiotensin in rat hepatocytes.

The addition of 1 microM-vasopressin or -angiotensin to isolated rat hepatocytes induced a fast transient inhibition of the rate of incorporation of [Me-3H]choline into phosphatidylcholine. The cationophore A23187 induced a similar inhibition of phosphatidylcholine synthesis. The addition of micromolar Ca2+ to rat liver microsomes inhibited the activity of CDP-choline: 1,2-diacylglycerol cholinephosphotransferase. This inhibition is due a decrease in the Vmax. of the enzyme without affecting the Km for CDP-choline. It is concluded that Ca2+ regulates phosphatidylcholine synthesis in rat liver.     

Mechanism of hepatic glycogen synthase inactivation induced by Ca2+-mobilizing hormones. Studies using phospholipase C and phorbol myristate acetate.

Incubation of hepatocytes with the protein kinase C activator and tumour promoter 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) produced a time- and concentration-dependent inactivation of glycogen synthase, but no change in phosphorylase. The same rate and extent of inactivation occurred in hepatocytes depleted of Ca2+ by treatment with the Ca2+ chelator EGTA. When hepatocytes were treated with the Ca2+-mobilizing hormone vasopressin (10 nM), the rate of glycogen synthase inactivation was similar to that observed with PMA (1 microM). Depletion of intracellular Ca2+ stores with EGTA abolished the ability of vasopressin to mobilize Ca2+ and activate phosphorylase without abolishing its ability to inactivate glycogen synthase and increase 1,2-diacylglycerol (DAG), the endogenous activator of protein kinase C. Protein kinase C, either in membranes or after partial purification, was shown to be activated in vitro by PMA in the presence of very low concentrations of Ca2+. Exogenous phospholipase C from Clostridium perfringens, at low concentrations, inactivated glycogen synthase and increased DAG without affecting cell Ca2+ or phosphorylase. It is proposed that the inactivation of glycogen synthase elicited by the Ca2+-mobilizing hormones is due, at least in part, to generation of DAG and activation of protein kinase C.     

On the source of the vasopressin-induced increases in diacylglycerol in hepatocytes

In hepatocytes pre-labelled with [3H]glycerol, vasopressin increased by 20% the amount of radioactivity present in diacylglycerols. The effect of vasopressin was partially dependent on Ca2+. The magnitude of the increase in [3H]diacylglycerol was 5-times the sum of the radioactivity present in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. No stimulation by vasopressin of the initial rate of incorporation of radioactivity into diacylglycerols was observed in cells incubated in the presence of 10 mM [3H]glycerol. Treatment of hepatocytes labelled with either [3H]ethanolamine or [3H]choline with vasopressin, ionophore A23187 or phospholipase C increased the amount of radioactivity present in trichloroacetic acid extracts of the cells. The effect of vasopressin was dependent on extracellular Ca2+. It is concluded that in hepatocytes vasopressin increases diacylglycerols by a process which does not principally involve the conversion of phosphoinositides to diacylglycerol or the de novo synthesis of diacylglycerol from glycerol 3-phosphate, but does involve the Ca2+-dependent conversion of phosphatidylethanolamine and phosphatidylcholine to diacylglycerol.     

Phosphatidate accumulation in hormone-treated hepatocytes via a phospholipase D mechanism

Isolated rat hepatocytes responded to a variety of Ca2+-mobilizing agents (vasopressin, angiotensin II, epinephrine, epidermal growth factor, ATP, and ADP) with a rapid increase in phosphatidate mass, as measured by a sensitive new method. When hepatocytes were incubated with vasopressin (10(-8) M), phosphatidate levels increased 2-3-fold in 2 min, but there was no significant increase in diacylglycerol at this time. Changes in the fatty acid composition of phosphatidate also preceded those in diacylglycerol. De novo synthesis of phosphatidate from [3H]glycerol was unaffected by vasopressin in short-term incubation. Incubation of washed rat liver plasma membranes with GTP gamma S caused a time-dependent increase in phosphatidate. When membranes were incubated with GTP gamma S and [gamma-32P]ATP, no incorporation of 32P into phosphatidate was observed. This excludes the phospholipase C-diacylglycerol kinase pathway and suggests that a phospholipase D activity produced the phosphatidate. At submaximal concentrations of GTP gamma S, ATP and ADP stimulated membrane phosphatidate formation, presumably by acting through P2-purinergic receptors. Only phosphatidylcholine, among the major phospholipids, decreased in the membranes in response to GTP gamma S. The fatty acid composition of the phosphatidate produced in response to vasopressin in hepatocytes also suggests that phosphatidylcholine may be the source of hormonally elicited phosphatidate. We conclude that Ca2+-mobilizing hormones mainly increase phosphatidate levels in hepatocytes by a mechanism that does not involve phosphorylation of diacylglycerol or de novo synthesis but involves a guanine nucleotide-binding protein coupled to phospholipase D.     

Carbachol induces a rapid and sustained hydrolysis of polyphosphoinositide in bovine tracheal smooth muscle measurements of the mass of polyphosphoinositides, 1,2-diacylglycerol, and phosphatidic acid

The effects of carbachol on polyphosphoinositides and 1,2-diacylglycerol metabolism were investigated in bovine tracheal smooth muscle by measuring both lipid mass and the turnover of [3H]inositol-labeled phosphoinositides. Carbachol induces a rapid reduction in the mass of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-monophosphate and a rapid increase in the mass of 1,2-diacylglycerol and phosphatidic acid. These changes in lipid mass are sustained for at least 60 min. The level of phosphatidylinositol shows a delayed and progressive decrease during a 60-min period of carbachol stimulation. The addition of atropine reverses these responses completely. Carbachol stimulates a rapid loss in [3H]inositol radioactivity from phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-monophosphate associated with production of [3H]inositol trisphosphate. The carbachol-induced change in the mass of phosphoinositides and phosphatidic acid is not affected by removal of extracellular Ca2+ and does not appear to be secondary to an increase in intracellular Ca2+. These results indicate that carbachol causes phospholipase C-mediated polyphosphoinositide breakdown, resulting in the production of inositol trisphosphate and a sustained increase in the actual content of 1,2-diacylglycerol. These results strongly suggest that carbachol-induced contraction is mediated by the hydrolysis of polyphosphoinositides with the resulting generation of two messengers: inositol 1,4,5-trisphosphate and 1,2-diacylglycerol.     

Antigen-induced biphasic diacylglycerol formation in RBL-2H3 cells: the late sustained phase due to phosphatidylcholine hydrolysis is dependent on protein kinase C

Exposure to antigen (Ag) caused a biphasic 1,2-diacylglycerol (DG) production in [3H]myristic acid-labeled RBL-2H3 cells; the early, small transient phase and the second large sustained phase. The accumulation of phosphatidic acid (PA) or phosphatidylethanol (PEt) in the presence of ethanol was paralleled by the second-phase DG generation. Ag-induced formation of phosphocholine and choline in [3H]choline-labeled cells suggested the hydrolysis of phosphatidylcholine (PC) by phospholipases C and D. Treatment with phorbol myristate (PMA) or A23187 caused increases in [3H]DG and water-soluble [3H]choline metabolites. In protein kinase C (PKC) down-regulated cells, PEt formation was markedly reduced. In these cells DG production induced by Ag and A23187 was largely suppressed, thus indicating that PKC would play an important regulatory role for PC hydrolysis. However, because the A23187 treatment showed significant accumulation of water-soluble choline metabolites in PKC down-regulated cells, an increase in intracellular Ca2+ is another factor regulating PC hydrolysis. Taken together, these results may indicate that PC hydrolysis in response to Ag is dependent on PKC and Ca2+.     

Bradykinin Activates a Phospholipase D that Hydrolyzes Phosphatidylcholine in PC12 Cells

In PC12 pheochromocytoma cells whose phospholipids had been prelabelled with [3H]palmitic acid, bradykinin increased the production of [3H]phosphatidic acid. The increase in [3H]phosphatidic acid occurred within 1-2 min. before the majority of the increase in [3H]diacylglycerol. When the phospholipids were prelabeled with [3H]choline, bradykinin increased the intracellular release of [3H]choline. The production of phosphatidic acid and choline suggests that bradykinin was increasing the activity of phospholipase D. Transphosphatidylation is a unique property of phospholipase D. In cells labeled with [3H]palmitic acid, bradykinin stimulated the transfer of phosphatidyl groups to both ethanol and propanol to form [3H]phosphatidylethanol and [3H]phosphatidylpropanol, respectively. The effect of bradykinin on [3H]phosphatidic acid and [3H]phosphatidylethanol formation was partially dependent on extracellular Ca2+. In cells treated with nerve growth factor, carbachol also increased [3H]phosphatidylethanol formation. To investigate the substrate specificity of phospholipase D, cells were labeled with [14C]stearic acid and [3H]palmitic acid, and then incubated with ethanol in the absence or presence of bradykinin. The 14C/3H ratio of the phosphatidylethanol that accumulated in response to bradykinin was almost identical to the 14C/3H ratio of phosphatidylcholine. The 14C/3H ratio in phosphatidic acid and diacylglycerol was higher than the ratio in phosphatidylcholine. These data provide additional support for the idea that bradykinin activates a phospholipase D that is active against phosphatidylcholine. The hydrolysis of phosphatidylcholine by phospholipase D accounts for only a portion of the phosphatidic acid and diacylglycerol that accumulates in bradykinin-stimulated cells: bradykinin evidently stimulates several pathways of phospholipid metabolism in PC12 cells.     

Epidermal growth factor-induced hydrolysis of phosphatidylcholine by phospholipase D and phospholipase C in human dermal fibroblasts

The enzymatic pathways for formation of 1,2-diradylglyceride in response to epidermal growth factor in human dermal fibroblasts have been investigated. 1,2-Diradylglyceride mass was elevated 2-fold within one minute of addition of EGF. Maximal accumulation (4-fold) occurred at 5 minutes. Since both diacyl and ether-linked diglyceride species occur naturally and may accumulate following agonist activation, we developed a novel method to determine separately the alterations in diacyl and ether-linked diglycerides following stimulation of fibroblasts with EGF. Utilizing this method, it was found that approximately 80% of the total cellular 1,2-diradylglyceride was diacyl, the remaining 20% being ether-linked. Addition of EGF caused accumulation of 1,2-diacylglyceride without alteration in the level of ether-linked diglyceride. Thus, the observed induction of 1,2-diradylglyceride by EGF was due exclusively to increased formation of 1,2-diacylglyceride. In cells labelled with [3H]choline, the water soluble phosphatidylcholine hydrolysis products, phosphorylcholine and choline, were increased 2-fold within 5 minutes of addition of EGF. No hydrolysis of phosphatidylethanolamine, phosphatidylserine, or phosphatidylinositol was observed. Quantitation by radiolabel and mass revealed equivalent elevations in phosphorylcholine and choline, suggesting stimulation of both phospholipase C and phospholipase D activities. To identify the presence of EGF-induced phospholipase D activity, cells were labelled with exogenous [3H]1-0-hexadecyl, 2-acyl phosphatidylcholine and its conversion to phosphatidic acid in response to EGF determined. Radiolabelled phosphatidic acid was detectable in 15 seconds after addition of EGF and was maximal (3-fold) at 30 seconds. Consistent with the presence of EGF-induced phospholipase D activity, treatment of cells with EGF, in the presence of [14C]ethanol, resulted in the rapid formation of [14C]phosphatidylethanol, the product of phospholipase D-catalyzed transphosphatidylation. The formation of phosphatidylethanol, which competes for the formation of phosphatidic acid by phospholipase D, did not diminish the induction of 1,2-diglyceride by EGF. These data suggest that the phosphatidic acid formed by phospholipase D-catalyzed hydrolysis of phosphatidylcholine is not a major precursor of the observed increased 1,2-diglyceride. Thus, the induction of 1,2-diacylglycerol by EGF may occur primarily via phospholipase C-catalyzed hydrolysis of phosphatidylcholine.     

Activation of phospholipase D by the fucose-sulfate glycoconjugate that induces an acrosome reaction in spermatozoa

We report for the first time that phospholipase D activity in sea urchin spermatozoa can be regulated by a component of egg jelly known to induce an acrosome reaction. The fucose-sulfate glycoconjugate (FSG) of egg jelly that induces an acrosome reaction in spermatozoa caused Ca2+-dependent increases in 1,2-diacylglycerol and phosphatidic acid. Diacylglycerol concentrations were increased 2-fold, and phosphatidic acid concentrations were elevated up to 10-fold 2 min after the addition of FSG to spermatozoa. FSG also caused increases in choline, but not in choline phosphate concentrations. Neither phosphorylation of diacylglycerol nor de novo synthesis from glycerol were significant routes of synthesis of phosphatidic acid during the acrosome reaction. When spermatozoa were incubated with FSG in the presence of ethanol, phosphatidylethanol was produced. As ethanol concentrations in the extracellular medium were increased from 0.1 to 2.5%, the amount of phosphatidylethanol increased, whereas phosphatidic acid concentrations decreased, suggesting a competitive transphosphatidylation reaction catalyzed by phospholipase D. Furthermore, when a phosphatidylcholine pool in spermatozoa was radiolabeled using [3H]1-O-alkyl-2-lyso-glycerol-3-phosphorylcholine, the subsequent addition of FSG caused a 4-fold accumulation of [3H]phosphatidic acid. FSG-induced elevations in [3H]phosphatidic acid were positively correlated with the percent of cells that had undergone an acrosome reaction.     

Effects of hyperthyroidism on stimulation of [1-14C]oleate oxidation to14CO2 in isolated hepatocytes from fed rats by the catecholamines,vasopressin, and angiotensin II

Possible effects of adrenaline, noradrenaline, vasopressin, and angiotensin II to increase 14CO2 production from [1-14C]oleate were examined in hepatocytes from fed L-triiodothyronine (T3)-treated or control rats. Rates of 14CO2 production were decreased and rates of ketogenesis increased in hepatocytes from T3-treated rats. These changes were accompanied by a marked shift of the 3-hydroxybutyrate:acetoacetate concentration ratio towards acetoacetate. Rates of glucose and lactate release were decreased. Whereas the Ca2+-mobilizing hormones increased 14CO2 production from [1-14C]oleate by 64-84% with hepatocytes from control rats, they increased 14CO2 production from [1-14C]oleate by on 24-32% with hepatocytes from T3-treated rats. The magnitude of the response to the Ca2+-mobilizing hormones in hepatocytes from T3-treated rats was increased by the addition of 3-mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase, to the incubation medium (increases of 52-88%). In the presence of 3-mercaptopicolinate, the 3-hydroxybutyrate:acetoacetate concentration ratio in hepatocytes from fed, T3-treated rats was similar to that in hepatocytes from control rats in the absence of 3-mercaptopicolinate. The results demonstrate that hyperthyroidism per se does not lead to a loss of sensitivity, in terms of oleate oxidation, either to the catecholamines or to vasopressin and angiotensin II. The impaired ability of hepatocytes from T3-treated rats to respond to these hormones is a consequence of decreased net glycolytic flux or a more oxidized mitochondrial redox state.     

Depolarizing stimuli and neurotransmitters utilize separate pathways to activate protein kinase C in sympathetic neurons.

Several types of extracellular signals affect the function of peripheral neurons. Depolarizing stimuli cause sudden increases in permeability to various ions leading to propagation of nerve impulses and release of transmitter substances. Neurons also receive external signals via neurotransmitter receptors located on the membrane. Different types of receptors present on sympathetic neurons are believed to modulate stimulation-evoked release of norepinephrine. We have investigated the effects of depolarizing stimuli and neurotransmitters on different signaling pathways in homogeneous cultures of chick sympathetic neurons. Depolarizing stimuli (35 mM KCl; electrical stimulation, 1 Hz for 5 min) and neurotransmitters (acetylcholine and 5-hydroxytrypatmine) enhanced membrane binding of protein kinase C by 2-5-fold. 35 mM KCl increased formation of 1,2-diacylglycerol and hydrolysis of [3H]phosphatidycholine without affecting [3H] phosphoinositide hydrolysis. Neurotransmitters increased [3H]inositol phosphates and 1,2-diacylglycerol without affecting the hydrolysis of [3H]phosphatidylcholine. 5-Hydroxytryptamine and acetylcholine (muscarinic component) did not increase Ca2+ concentration in the Indo-1-loaded neuronal cell body or the growth cone, but 35 mM KCl and electrical stimulation caused a marked increase in Ca2+ concentration in both regions of sympathetic neurons. We believe this to be the first demonstration of these two types of signalling mechanisms co-existing in sympathetic neurons; depolarization activate the phosphatidylcholine pathway and neurotransmitters activate the phosphatidylinositol pathway. The importance of two pathways in controlling neuronal Ca2+ concentration and the release of transmitter is discussed.     

Evidence from studies employing radioactively labelled fatty acids that the stimulation of flux through the diacylglycerol pool is an early action of vasopressin on hepatocytes.

1. In isolated hepatocytes prelabelled with [14C]-arachidonic, -stearic, -linoleic, -oleic or -palmitic acids, vasopressin increased the amount of radioactivity present in diacylglycerols. The largest increase was observed in cells labelled with arachidonic or stearic acids. 2. In cells prelabelled with [14C]- or [3H]-arachidonic acid, the onset of the increase in radioactivity in diacylglycerols induced by vasopressin was slow, the increase was partly dependent on the presence of extracellular Ca2+, and was associated with an increase in radioactivity present in phosphatidic acid which was more rapid in onset. Vasopressin decreased the amount of [3H]arachidonyl-phosphatidylinositol 4,5-bisphosphate, but the magnitude of this decrease was less than 10% of the observed increase in radioactivity in [3H]arachidonyl-diacylglycerol. 3. The concentration of vasopressin which gave half-maximal increase in [14C]arachidonyl-diacylglycerol at low extracellular Ca2+ was 10-fold higher than that which gave half-maximal stimulation of 45Ca2+ efflux. Phenylephrine, but not glucagon, also increased the amount of [14C]arachidonyl-diacylglycerol. 4. It is concluded that an early action of vasopressin on the liver cell is to increase the flux of carbon from phospholipids, including the phosphoinositides, to diacylglycerols.     

Ca2(+)-mobilizing hormones stimulate Ca2+ efflux from hepatocytes

Treatment of hepatocytes with 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ), a novel mobilizer of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool, produces a sustained elevation of [Ca2+]i (Kass, G. E. N., Duddy, S. K., and Orrenius, S. (1989) J. Biol. Chem. 264, 15192-15198). Exposure of hepatocytes to the Ca2(+)-mobilizing hormones, vasopressin, angiotensin II, or ATP following [Ca2+]i elevation by tBuBHQ produced a rapid return of [Ca2+]i to basal or near basal levels. Release of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool by tBuBHQ following pretreatment with vasopressin or angiotensin II resulted in a [Ca2+]i transient and not the sustained [Ca2+]i elevation observed in the absence of the Ca2(+)-mobilizing hormones. The G-protein activator, NaF plus AlCl3, mimicked both effects of the Ca2(+)-mobilizing hormones on [Ca2+]i. The mechanism for Ca2+ removal from the cytosol by Ca2(+)-mobilizing hormones did not involve cyclic nucleotides nor did it require protein kinase C activation or cyclo- and lipoxygenase-dependent metabolites of arachidonic acid. Furthermore, the hormone-mediated decrease in [Ca2+]i did not involve the pertussis toxin-sensitive Gi-protein. Removal of the tBuBHQ-mobilized Ca2+ from the cytosol of hepatocytes by Ca2(+)-mobilizing hormones was mediated by stimulation of a Ca2+ efflux pathway. Thus, in addition to initiating [Ca2+]i transients by releasing Ca2+ from the inositol 1,4,5-trisphosphate-sensitive Ca2+ store and stimulating Ca2+ influx, Ca2(+)-mobilizing hormones also regulate the termination of the [Ca2+]i transient by stimulating a Ca2+ efflux pathway.     

The role of protein kinase C in the inactivation of hepatic glycogen synthase by calcium-mobilizing agonists.

The regulation of glycogen synthase by Ca2+-mobilizing hormones was studied by using rat liver parenchymal cells in primary culture. Long-term exposure of hepatocytes to 4 beta-phorbol 12-myristate 13-acetate (TPA) resulted in a decrease in vasopressin or ATP inhibition of glycogen synthesis and glycogen synthase activity, without any change in the activation of glycogen phosphorylase. In contrast, treatment with TPA did not diminish the effects of glucagon, isoprenaline or A23187 on glycogen synthase or phosphorylase. TPA treatment for 18 h did not change specific [3H]vasopressin binding, but abolished protein kinase C activity in a concentration-dependent manner. The effects of TPA to decrease protein kinase C activity and to reverse the inactivation of glycogen synthase by vasopressin were well correlated and were mimicked by mezerein, but not by 4 alpha-phorbol. However, 1 microM-TPA totally inhibited protein kinase C activity, but reversed only 60% of the vasopressin effect on glycogen synthase. It is therefore concluded that Ca2+-mobilizing hormones inhibit glycogen synthase partly, but not wholly, through a mechanism involving protein kinase C.     

Activation of phospholipase D in a rat mast (RBL 2H3) cell line. A possible unifying mechanism for IgE-dependent degranulation and arachidonic acid metabolite release.

RBL 2H3 cells (a model of mast cell function) were sensitized with anti-TNP IgE (0.5 micrograms/ml) and triggered to secrete both histamine and arachidonic acid (AA) metabolites by the addition of TNP-OVA (0 to 100 ng/ml). After a 3-min delay, the release of both groups of mediators proceeded in a parallel manner. In cells labeled with [14C]-AA, TNP-OVA produced a rapid increase in phosphatidic acid (PA), and subsequently, 1,2-diacylglycerol (DAG) and intracellular AA levels. Concurrently, there was a decrease in [14C]-AA labeled phosphatidylcholine. The release of labeled AA from phosphatidylcholine in response to TNP-OVA was paralleled by a liberation of free choline but no evidence of liberation of phosphorylcholine. When ethanol (0.05 to 2% v/v) was included in the culture medium, phosphatidylethanol was synthesized at the expense of PA and DAG, with a resulting inhibition of secretion. D,1 propranolol, an inhibitor of PA phosphohydrolase, inhibited the IgE-dependent production of [14C]-DAG, and [14C]-free fatty acid but not [14C]-PA. The IgE-dependent release of both histamine and AA metabolites was completely inhibited by pretreatment with propranolol. Taken together, the above results suggest that phospholipase D is activated upon cross-bridging of IgE receptors on the surface of RBL 2H3 cells and that this may be a pivotal step in the signal transduction cascade leading to the release of both presynthesized and de novo synthesized mediators.     

Hydrolysis of chylomicron arachidonate and linoleate ester bonds by lipoprotein lipase and hepatic lipase

Chylomicrons labeled with [3H]arachidonic and [14C]linoleic acid were incubated with bovine milk lipoprotein lipase or rat postheparin plasma, containing both lipoprotein lipase and hepatic lipase. During incubation with bovine lipoprotein lipase, [3H]arachidonic acid was released from chylomicron triacylglycerols at a slower rate than [14C]linoleic acid. Only small amounts of [14C]linoleic acid were found as 1,2(2,3)-diacylglycerols, whereas a transient accumulation as [14C]monoacylglycerols was observed. In contrast, significantly more [3H]arachidonic acid was found as 1,2(2,3)-diacylglycerols than as monoacylglycerols at all time intervals investigated. The initial pattern of triacylglycerol hydrolysis by postheparin plasma was similar to that of bovine lipoprotein lipase. However, in contrast to the results obtained with bovine lipoprotein lipase, little [3H]1,2(2,3)-diacylglycerol accumulated. The addition of antiserum to hepatic lipase increased the amount of 3H found in 1,2(2,3)-diacylglycerols and inhibited the formation of free [3H]arachidonic acid. The antiserum also caused a significant inhibition of the hydrolysis of [3H]-but not of [14C]triacylglycerol. With regard to chylomicron phospholipids, the rate of hydrolysis of [14C]linoleoyl phosphatidylcholine with milk lipoprotein lipase was twofold higher than that of the [3H]arachidonyl phosphatidylcholine. However, the hepatic lipase of postheparin plasma had similar activity towards the two phosphatidylcholine species. Postheparin plasma rapidly hydrolyzed chylomicron 3H-labeled and 14C-labeled phosphatidylethanolamine to the same degree, and lipoprotein lipase similarly hydrolyzed 3H-labeled and 14C-labeled phosphatidylethanolamine at approximately equal rates. Antiserum to hepatic lipase inhibited the postheparin plasma hydrolysis of phosphatidylethanolamine and 3H-labeled phosphatidylcholine by about 60%, but the 14C-labeled phosphatidylcholine by only 27%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age features of glycerolipid metabolism in rat liver under short term exposure to thyroxine

The age specificity of the regulation by thyroid hormones of 1,2-diacylglycerol production in rat liver has been studied. It was found that L-thyroxine-stimulation of the 3-month old rats liver cells resulted in a rapid rise in 1,2-diacylglycerol concentration in hepatocytes and simultaneous degradation of phospholipids. The endogenous phosphatidylcholine and phosphatidylethanolamine are the sources of 1,2-diacylglycerol in a liver. Under the action of hormone liver cells of young rats may product 1,2-diacylglycerol from exogenous 1-acyl, 2-[14C]arachidonyl-phosphatidylethanolamine. Thyroxine had no effect on de novo 1,2-diacylglycerols formation and their release from triacylglycerol. In liver cells of elder rats, 1,2-diacylglycerol and individual phospholipids content are unaffected by hormones.     

The fucose-sulfate glycoconjugate that induces an acrosome reaction in spermatozoa stimulates inositol 1,4,5-trisphosphate accumulation

The hydrolysis of phosphatidylinositol may generate multiple second messengers, including inositol phosphates, 1,2-diacylglycerol, arachidonic acid, and phosphatidic acid. Here, we describe for the first time in spermatozoa that accumulation of one of these potential second messengers, inositol 1,4,5-trisphosphate (1,4,5-IP3), can be stimulated by the fucose-sulfate glycoconjugate (FSG) that induces an acrosome reaction. Sea urchin spermatozoa were labeled with myo-[3H]inositol and incubated with FSG. The amount of [3H]1,4,5-IP3 obtained from FSG-treated cells was up to 10 times that from untreated cells. Increases in the amount of [3H]1,4,5-IP3 were detected within 30 s after addition of FSG (2.5-fold) and were highest at 2 min after addition. Previously, it was shown that FSG induces Ca2+-dependent increases in cyclic AMP concentrations (Kopf, G. S., and Garbers, D. L. (1980) Biol. Reprod. 22, 1118-1126). Increases in [3H]1,4,5-IP3 accumulation caused by FSG were also dependent on extracellular Ca2+. The Ca2+ channel blockers, verapamil and nifedipine, inhibited increases in both [3H]1,4,5-IP3 and cyclic AMP, and the addition of concentrations of extracellular Ca2+ higher than 9.6 mM could reduce the inhibition. When spermatozoa were incubated in Ca2+-free seawater, FSG-induced increases in [3H]1,4,5-IP3 and cyclic AMP concentrations were blocked; addition of extracellular Ca2+ restored the responses. Other treatments that result in the induction of an acrosome reaction, including the addition of monovalent cation H+ exchangers, nigericin and gramicidin S, and incubation in seawater at alkaline pH (pH 8.8), also stimulated accumulation of [3H]1,4,5-IP3 and cyclic AMP.     

How far does phospholipase C activity depend on the cell calcium concentration? A study in intact cells.

The dependence of phospholipase C activity on the cytosolic Ca2+ concentration ([Ca2+]i) was studied in intact liver cells treated with the Ca2+-mobilizing hormone vasopressin, or not so treated. Phospholipase C (PLC) activity was estimated from the formation of [3H]inositol trisphosphate (InsP3) and the degradation of [3H]phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The [Ca2+]i of the cells was clamped from 29 to 1130 nM by quin2 loading. This wide concentration range was obtained by loading the hepatocytes with a high concentration of the Ca2+ indicator in low-Ca2+ medium or by using the Ca2+ ionophore ionomycin in medium containing Ca2+. In resting cells, in which [Ca2+]i was 193 nM, treatment with 0.1 microM-vasopressin which stimulates liver PLC maximally, tripled InsP3 content and raised [Ca2+]i to 2 microM within 15 s. Lowering [Ca2+]i partially decreased cell InsP3 content as well as the ability of vasopressin to stimulate InsP3 formation maximally. At 29 nM, the lowest Ca2+ concentration obtained in isolated liver cells, basal InsP3 content was 64% of that measured in control cells. Addition of vasopressin no longer affected [Ca2+]i, but significantly increased InsP3 by 200%, although less than in the controls (300%). The maintenance of the greater part of the PLC response at constant [Ca2+]i indicated that, in the liver, InsP3 formation does not result from an increase in [Ca2+]i. The effects of lowering [Ca2+]i were reversible. When low cell [Ca2+]i was restored to a normal value, resting InsP3 content and the ability of vasopressin to stimulate InsP3 formation maximally by 300% were also restored. Raising [Ca2+]i from 193 to 1130 nM had little effect on the InsP3 content or the vasopressin-mediated increase in InsP3. In agreement with the stimulation of PLC activity by vasopressin, cell [3H]PtdInsP2 and total PtdInsP2 were degraded by application of this hormone for 15 s. In contrast, when [Ca2+]i was lowered to 29 nM, basal [3H]PtdInsP2 and total PtdInsP2 were increased by about 30%, [3H]PtdInsP2 was further increased by vasopressin, but total PtdInsP2 was not changed. These results show that, in intact hepatocytes, PLC is little affected by [Ca2+]i concentrations above 193 nM, but is partially dependent on Ca2+ below that value. They suggest that, in addition to activating PLC activity, vasopressin might stimulate PtdInsP2 synthesis, presumably via phosphatidylinositol-phosphate kinase, and that this pathway might predominate in cells with low [Ca2+]i.