Rapid breakdown of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in rat hepatocytes stimulated by vasopressin and other Ca2+-mobilizing hormones

Role of inositol lipid breakdown in the generation of intracellular signals. State of the art lecture

Many hormones, neurotransmitters, and secretagogues act by increasing the intracellular free Ca2+ concentration in target cells. The initial event following binding of agonists to specific receptors in the plasma membrane involves a receptor-mediated activation of a guanosine nucleotide-binding protein (G protein), which induces a Ca2+-independent activation of phospholipase C. This novel, presently uncharacterized G protein is inactivated by pertussis toxin-catalyzed adenosine 5'-diphosphate ribosylation in some but not all cell types. Phospholipase C catalyzes the breakdown of inositol lipids, notably phosphatidylinositol 4,5-bisphosphate, with the production of inositol phosphates and 1,2-diacylglycerol. Inositol 1,4,5-trisphosphate (IP3) is responsible for a rapid mobilization of intracellular Ca2+ by activating Ca2+ efflux from a subpopulation of the endoplasmic reticulum. The properties of this process are consistent with its being a ligand-activated ion channel with electrogenic Ca2+ efflux being charge-compensated by K+ influx. Sustained hormonal responses require extracellular Ca2+ and a prolonged elevation of the cytosolic free Ca2+. This is brought about by hormone-mediated changes of Ca2+ flux across the plasma membrane involving both an inhibition of Ca2+ efflux and an activation of Ca2+ influx. This review summarizes recent findings concerning the role of G proteins in receptor coupling to phospholipase C; the regulation of enzymes of phosphoinositide metabolism; the evidence for IP3 being a Ca2+-mobilizing second messenger and its mechanism of action; the formation of new inositol phosphates and their possible significance; the relation of intracellular Ca2+ mobilization and plasma membrane Ca2+ fluxes to the kinetics of the hormone-induced cytosolic free Ca2+ transient; and the possible roles of protein kinase C in influencing the hormone-mediated functional response.

Activation of phosphatidylinositol turnover by neurotensin receptors in the human colonic adenocarcinoma cell line HT29

Association of neurotensin to its receptor in HT29 cells increases the intracellular concentration of inositol phosphates. A rapid (20-30 s), transient stimulation of inositol trisphosphate (275% of the basal level) and inositol bisphosphate (420%) is first observed, followed by a slower, stable increase in inositol monophosphate (170%). Half-maximal stimulation of the three inositol phosphates was obtained with 50-100 nM neurotensin. These results indicate that neurotensin is able to regulate intracellular Ca2+ levels in HT29 cells by using inositol trisphosphate as a second messenger.

Carbachol and oxytocin stimulate the generation of inositol phosphates in the guinea pig myometrium

In the guinea pig myometrium prelabelled with myo-[2-3H]inositol, carbachol and oxytocin enhanced a concentration-dependent and rapid release of IP3 which preceded that of IP2 and IP1. The specific receptor-mediated phospholipase C activation degrading PIP2 to IP3 did not require the presence of extracellular Ca2+. The ionophore A23187 as well as K+ depolarization failed to increase inositol phosphate accumulation. It is proposed that IP3 could have a role in the contraction of uterine smooth muscle elicited by the activation of muscarinic as well as of oxytocin receptors.

myo-Inositol 1,4,5-trisphosphate mobilizes Ca2+ from isolated adipocyte endoplasmic reticulum but not from plasma membranes

The effects of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ uptake and release from isolated adipocyte endoplasmic reticulum and plasma membrane vesicles were investigated. Effects of IP3 were initially characterized using an endoplasmic reticulum preparation with cytosol present (S1-ER). Maximal and half-maximal effects of IP3 on Ca2+ release from S1-ER vesicles occurred at 20 microM- and 7 microM-IP3, respectively, in the presence of vanadate which prevents the re-uptake of released Ca2+ via the endoplasmic reticulum Ca2+ pump. At saturating IP3 concentrations, Ca2+ release in the presence of vanadate was 20% of the exchangeable Ca2+ pool. IP3-induced release of Ca2+ from S1-ER was dependent on extravesicular free Ca2+ concentration with maximal release occurring at 0.13 microM free Ca2+. At 20 microM-IP3 there was no effect on the initial rate of Ca2+ uptake by S1-ER. IP3 promoted Ca2+ release from isolated endoplasmic reticulum vesicles (cytosol not present) to a similar level as compared with S1-ER. Addition of cytosol to isolated endoplasmic reticulum vesicles did not affect IP3-induced Ca2+ release. The endoplasmic reticulum preparation was further fractionated into heavy and light vesicles by differential centrifugation. Interestingly, the heavy fraction, but not the light fraction, released Ca2+ when challenged with IP3. IP3 (20 microM) did not promote Ca2+ release from plasma membrane vesicles and had no effect on the (Ca2+ + Mg2+)-ATPase activity or on the initial rate of ATP-dependent Ca2+ uptake by these vesicles. These results support the concept that IP3 acts exclusively at the endoplasmic reticulum to promote Ca2+ release.

Effect of cyclic AMP-dependent hormones and Ca2+-mobilizing hormones on the Ca2+ influx and polyphosphoinositide metabolism in isolated rat hepatocytes

The effect of the interaction between the Ca2+-mobilizing hormone adrenaline, used as alpha-adrenergic agonist, and cyclic AMP-dependent hormones, including beta-adrenergic agonists and glucagon, on the initial 45Ca2+ uptake rate and polyphosphoinositide metabolism were investigated in isolated rat hepatocytes. Each hormone alone increased the initial 45Ca2+ uptake rate. When adrenaline was added without inhibitor, it induced a rise in the initial 45Ca2+ uptake rate larger than the sum of the rises elicited by its alpha and beta components singly. Similarly, when adrenaline was used as an alpha-agonist and added together with glucagon, it enhanced the initial 45Ca2+ uptake rate synergistically. Kinetic analysis of the initial 45Ca2+ uptake rate measured at different Ca2+ concentrations suggested that the increased influx elicited by the combination of adrenaline as alpha-adrenergic agonist and glucagon reflects an activation of the rate of Ca2+ transport via a homogeneous population of Ca2+ channels or carriers. Dose-response curves for the alpha-adrenergic action of adrenaline or glucagon applied in the presence of increasing doses of glucagon or adrenaline showed that each hormone increases the maximal response to the other without affecting its ED50. Measurement of polyphosphoinositide hydrolysis and of the inositol phosphates formed in the presence of adrenaline or vasopressin and/or glucagon showed that Ca2+-mobilizing hormones and glucagon had no synergistic effects on inositol 1,4,5-trisphosphate production. It is therefore proposed that the synergistic action of glucagon and Ca2+-mobilizing hormones on Ca2+ influx occurs at a step that takes place close to the Ca2+ channels or carriers themselves. The Ca2+ gating involved might be mainly controlled by two products, one of them arising from the polyphosphoinositide metabolism, and the other from the increase in internal cyclic AMP.

Non-enzymic phosphorylation of polyphosphoinositides and phosphatidic acid is catalysed by bivalent metal ions

Phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate and phosphatidic acid undergo non-enzymic phosphorylation by ATP in the presence of bivalent metal ions. The non-enzymic reaction is more rapid in a mixture of water, chloroform and methanol than in water alone. Chemical evidence indicates that the product formed from phosphatidylinositol 4-phosphate is the corresponding 4-pyrophosphate. This product shows an RF value very close to that of phosphatidylinositol 4,5-bisphosphate on t.l.c. with an acidic solvent commonly used to characterize and measure the latter; however, it can be separated readily with an alkaline solvent. Chemical evidence indicates that the products formed from phosphatidylinositol 4,5-bisphosphate and phosphatidic acid are also pyrophosphates.

Specific vasopressin binding to rat adrenal glomerulosa cells. Relationship to inositol lipid breakdown

Cells from the zona glomerulosa of rat adrenals were isolated and maintained for 3 days in primary culture. Specific vasopressin binding was determined by using [3H]vasopressin. [3H]Vasopressin binding was time-dependent (half-time of about 2 min for 6 nM free ligand) and reversible on addition of unlabelled vasopressin (80% dissociation within 30 min). Dose-dependent [3H]vasopressin binding at equilibrium indicated that vasopressin interacted with two populations of sites: high-affinity sites (dissociation constant, Kd = 1.8 nM; maximal binding capacity = 10 fmol/10(6) cells) and low-affinity sites. Vasopressin increased the cellular content of labelled inositol mono-, bis- and tris-phosphate in cells prelabelled with myo-[3H]inositol. The vasopressin concentration eliciting half-maximal inositol phosphate accumulation was very close to the Kd value for vasopressin binding to high-affinity sites. Competition experiments using agonists and antagonists with enhanced selectivity for previously characterized vasopressin receptors indicated that vasopressin receptors from rat glomerulosa cells are V1 receptors of the vascular or hepatic subtype. The detected specific vasopressin-binding sites might represent the specific receptors mediating the mitogenic and steroidogenic effects of vasopressin on glomerulosa cells from rat adrenals.

Pertussis toxin or phorbol 12-myristate 13-acetate can distinguish between epidermal growth factor- and angiotensin-stimulated signals in hepatocytes

Epidermal growth factor (EGF) causes rapid increases in free intracellular Ca2+ and stimulates the phosphorylation of 11 cytosolic proteins in hepatocytes. Ten of the 11 cytosolic proteins altered by EGF are identical to those affected by angiotensin II, a hormone that stimulates the breakdown of phosphatidylinositol 4,5-bisphosphate. An increase in the phosphorylation of the other protein, spot c (Mr = 36,000, pI = 5.5), is observed only with EGF. Treatment of intact rats with pertussis toxin to ADP-ribosylate Ni, the inhibitory GTP-binding protein of the adenylate cyclase complex, abolished the effect of EGF on Ca2+ mobilization and on the phosphorylation of the 10 proteins affected in common with angiotensin II. This treatment had minimal effects on the ability of EGF to stimulate the phosphorylation of its unique substrate, spot c. In marked contrast, modification of Ni did not block the ability of angiotensin II to stimulate Ca2+ mobilization or protein phosphorylation. Pretreatment of normal hepatocytes with 4 beta-phorbol 12-myristate 13-acetate blocked all responses to EGF, including the increased phosphorylation of spot c, but had no effect on the responses to angiotensin II. These results imply that Ni or a similar pertussis toxin substrate may mediate the apparent effects of EGF on phosphatidylinositol breakdown and that protein kinase C may regulate a site in the transduction pathway. Angiotensin II appears to use a different signal transduction mechanism to stimulate phosphatidylinositol metabolism in hepatocytes.

Parathyroid hormone and calcitonin modify inositol phospholipid metabolism in fetal rat limb bones

Inositol-containing phospholipids are believed to be intimately involved in the first steps of cellular signalling by certain hormones and neurotransmitters. We examined whether parathyroid hormone (PTH) and calcitonin (CT), two hormones that affect bone physiology, would elicit changes in inositol-phospholipid metabolism in cultured bone. [3H]inositol readily entered into the tissue phospholipid pool in fetal rat limb bones, and incorporated into phosphatidylinositol (92.9%), phosphatidylinositol-4-P (4.5%), and phosphatidylinositol-4,5-P2 (2.6%). PTH enhanced the incorporation of inositol into PtdIns in limb bones following 2- or 24-h hormone treatments. The effect of PTH was dose dependent (EC50 of 0.3-0.4 nM) and occurred in a concentration range similar to that for hormone-stimulated bone resorption. In contrast, 24-h treatment with CT-inhibited inositol incorporation, also in a dose-dependent manner. Two-hour CT treatment had variable effects on labeling. CT inhibited the stimulatory effect of PTH at both 2 and 24 h. The effects induced by PTH and CT were specific for PtdIns and were independent of the [3H]inositol pool size. These results indicate that inositol-phospholipid turnover can be modified during the action of these hormones on bone tissue. Although the time course of hormone-stimulated inositol incorporation observed here is slower than that found in other tissues, the change in phosphatidylinositol metabolism could mediate delayed effects of PTH or CT. Alternatively, alterations induced by PTH and CT in bone cell membranes, cell populations, or in the mineralized matrix could conceivably result in secondary changes in phosphatidylinositol metabolism.

Muscarinic stimulation of inositol phosphate accumulation and acid secretion in gastric fundic mucosal cells

The muscarinic agonist, carbachol (CCh), was shown to stimulate the production of inositol phosphates (IP) in isolated cells from rabbit fundic mucosa. This stimulatory effect was time- and dose-dependent: EC50 values for IP1, IP2 and IP3 accumulation were not statistically different. The mean value was 30 +/- 8 microM (n = 6). The corresponding maximal stimulation (% of basal value) observed after 20 min incubation in the presence of 100 microM CCh was 160 +/- 15%. CCh-induced IP accumulation was abolished by atropine (Ki = 0.32 +/- 0.18 nM (n = 3)). The CCh concentrations leading to half-maximal inhibition of N-[3H]methylscopolamine binding and half-maximal IP accumulation were similar. The half-maximal value for CCh-induced aminopyrine accumulation was 8-times lower. These results indicate that IP3-mediated mobilization of intracellular Ca2+ might be involved in CCh-induced acid secretion by parietal cells.

Platelet-activating factor mobilizes intracellular calcium in vascular smooth muscle cells

The effect of platelet-activating factor (PAF) on polyphosphoinositide metabolism and 45Ca2+ efflux was examined in a vascular smooth muscle cell line (A7r5). PAF stimulated a rapid but transient production of inositol trisphosphate and inositol bisphosphate which, in the presence of lithium, resulted in an accumulation of inositol monophosphate. In addition, PAF induced a rapid efflux of 45Ca2+ from preloaded cells, an effect which was concentration-dependent. These data suggest that PAF mobilizes intracellular Ca2+ via the production of inositol trisphosphate.

Antigen-stimulated metabolism of inositol phospholipids in the cloned murine mast-cell line MC9

Cells of the murine mast-cell clone MC9 grown in suspension culture were sensitized with an anti-DNP (dinitrophenol) IgE and subsequently prelabelled by incubating with [32P]Pi. Stimulation of these cells with DNP-BSA (bovine serum albumin) caused marked decreases in [32P]polyphosphoinositides (but not [32P]phosphatidylinositol) with concomitant appearance of [32P]phosphatidic acid. Whereas phosphatidylinositol monophosphate levels returned to baseline values after prolonged stimulation, phosphatidylinositol bisphosphate levels remained depressed. Stimulation of sensitized MC9 cells with DNP-BSA increased rates of incorporation of [32P]Pi into other phospholipids in the order: phosphatidylcholine greater than phosphatidylinositol greater than phosphatidylethanolamine. In sensitized cells prelabelled with [3H]inositol, release of inositol monophosphate, inositol bisphosphate and inositol trisphosphate, was observed after stimulation with DNP-BSA. When Li+ was added to inhibit the phosphatase activity that hydrolysed the phosphomonoester bonds in the sugar phosphates, greater increases were observed in all three inositol phosphates, particularly in inositol trisphosphate. The IgE-stimulated release of inositol trisphosphate was independent of the presence of extracellular Ca2+. In addition, the Ca2+ ionophore A23187 caused neither the decrease in [32P]polyphosphoinositides nor the stimulation of the release of inositol phosphates. These results demonstrate that stimulation of the MC9 cell via its receptor for IgE causes increased phospholipid turnover, with effects on polyphosphoinositides predominating. These data support the hypothesis that hapten cross-bridging of IgE receptors stimulates phospholipase C activity, which may be an early event in stimulus-secretion coupling of mast cells. The results with the Ca2+ ionophore A23187 indicate that an increase in intracellular Ca2+ alone is not sufficient for activation of this enzyme.

Extracellular ATP activates polyphosphoinositide breakdown and Ca2+ mobilization in Ehrlich ascites tumor cells

The effects of extracellular ATP on phosphoinositide metabolism and intracellular Ca2+ homeostasis were studied in Ehrlich ascites tumor cells. Cytosolic [Ca2+] was measured using either quin 2 or the recently described indicator fura 2. Addition of 0.5-25 microM extracellular ATP to intact cells results in a rapid mobilization of Ca2+ from a nonmitochondrial, intracellular Ca2+ store. Likewise, direct addition of 0.2-2 microM myo-1,4,5-inositol trisphosphate (IP3) to digitonin-permeabilized Ehrlich cells induces a rapid and reversible release of Ca2+ from a nonmitochondrial pool. Under the same conditions which facilitate intracellular Ca2+ mobilization, extracellular ATP also triggers a rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and accumulation of IP3. A maximal 18% decrease of the polyphosphoinositide is observed 40-60 s after the addition of 25 microM ATP; within 5 min PtdIns(4,5)P2 returns to or exceeds the original, prestimulus level. These conditions also trigger a rapid accumulation of phosphatidic acid (1.7-fold increase within 5 min). Paralleling these ATP-induced changes in phospholipid levels is a substantial accumulation of the mono-, bis-, and trisphosphate derivatives of inositol; most significantly, a 2-fold increase in the IP3 level is observed within 30 s after ATP addition. These results suggest that in these tumor cells, extracellular ATP elicits changes in phosphoinositide metabolism similar to those produced by a wide variety of Ca2+-mobilizing hormones and growth factors.

Desensitization and antagonism of vasopressin-induced phosphoinositide metabolism and elevation of cytosolic free calcium concentration in human platelets

The receptor mechanisms underlying vasopressin-induced human platelet activation were investigated with respect to stimulation of phosphoinositide metabolism and changes in the cytosolic free Ca2+ concentration ([Ca2+]i). Vasopressin stimulated phosphoinositide metabolism, as indicated by the early formation of [32P]phosphatidic acid ([32P]PtdA) and later accumulation of [32P]phosphatidylinositol ([32P]PtdIns). In addition, vasopressin elicited a transient depletion of [glycerol-3H]PtdIns and accumulation of [glycerol-3H]PtdA. The effects of vasopressin on phosphoinositide metabolism were concentration-dependent, with half maximal [32P]PtdA formation occurring at 30 +/- 15 nM-vasopressin. In the presence of 1 mM extracellular free Ca2+, vasopressin induced a rapid, concentration-dependent elevation of [Ca2+]i in quin2-loaded platelets: half-maximal stimulation was observed at 53 +/- 20 nM-vasopressin. The V1-receptor antagonist [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine,8-arginine]-vasopressin selectively inhibited vasopressin (100 nM)-induced [32P]PtdA formation [I50 (concn. giving 50% inhibition) = 5.7 +/- 2.4 nM] and elevation of [Ca2+]i (I50 = 3 +/- 1.5 nM). Prior exposure of platelets to vasopressin rendered them unresponsive, in terms of [32P]PtdA formation and elevation of [Ca2+]i, to a subsequent challenge with vasopressin, but responsive to a subsequent challenge with U44069, a thromboxane-A2 mimetic. These results indicate that vasopressin-induced human platelet activation is initiated by combination with specific V1 receptors on the platelet, and that the sequelae of receptor occupancy (stimulation of phosphoinositide metabolism and elevation of [Ca2+]i) are equally susceptible to inhibition by receptor antagonists and by receptor desensitization.

Repetitive transient rises in cytoplasmic free calcium in hormone-stimulated hepatocytes

In the stressed animal, the vasoactive hormones vasopressin and angiotensin-II and the neurotransmitter noradrenaline induce liver cells to release glucose from glycogen. The intracellular signal that links the cell-surface receptors for noradrenaline (alpha 1) and vasoactive peptides to activation of glycogenolysis is known to be a rise in the cytoplasmic concentration of free calcium ions (free Ca). The receptors for these agonists induce the hydrolysis of phosphatidylinositol 4,5-bisphosphate, a minor plasmalemma lipid, to produce inositol trisphosphate and diacylglycerol. Inositol trisphosphate has been shown to mobilize intracellular calcium in hepatocytes. We show here, by means of aequorin measurements in single, isolated rat hepatocytes, that the free Ca response to these agonists consists of a series of transients. Each transient rose within 3 s to a peak free Ca of at least 600 nM and had a duration of approximately 7 s. The transients were repeated at intervals of 0.3-4 min, depending on agonist concentration. Between transients, free Ca returned to the resting level of approximately 200 nM. Clearly, the mechanisms controlling free Ca in hepatocytes are more complex than hitherto suspected.

Studies of endogenous polyphosphoinositide hydrolysis in human platelet membranes. Evidence that polyphosphoinositides remain inaccessible to phosphodiesterase in the native membrane

Human platelet plasma membranes incubated in the presence of [gamma-32P]ATP and 15 mM MgCl2 incorporated radioactivity mostly into phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-phosphate (PIP), which represented together over 90% of the total lipid radioactivity. After washing, reincubation of prelabelled membranes revealed some hydrolysis of the two compounds by phosphomonoesterase(s), as detected by the release of radioactive inorganic phosphate (Pi) from the two phospholipids. This degradation attained 40%/30 min for PIP in the presence of 2 mM calcium and cytosol. The effect of calcium was observed at concentrations equal to or greater than 10(-4) M. In no case did calcium alone facilitate the formation of inositol 1,4,5-trisphosphate (IP3) and inositol 1,4-bisphosphate (IP2). In contrast, simultaneous addition of 2 mM calcium and 2 mg/ml sodium deoxycholate promoted the formation of IP3 and IP2, indicating phosphodiesteratic cleavage of PIP2 and PIP. Phospholipase C activity was detected at calcium concentrations as low as 10(-7) M, in which case PIP2 hydrolysis was slightly more pronounced compared to PIP. Addition of cytosol increased to some extent the phospholipase C activity, suggesting that the low amount of enzyme remaining in the membrane is sufficient to promote submaximal degradation of PIP2 and PIP. We conclude that platelet polyphosphoinositides are present in the plasma membrane in a state where they remain inaccessible to phospholipase C, which is still fully active even at basal calcium concentrations, i.e., 10(-7) M. These results support the view that phosphodiesteratic cleavage of PIP2 promotes and thus precedes calcium mobilization brought about by IP3. The in vitro model presented here may prove very useful in future studies dealing with the mechanism rendering polyphosphoinositides accessible to phospholipase C attack upon agonist-receptor binding.

Activation of membrane phospholipase C by vasopressin. A requirement for guanyl nucleotides

Vasopressin stimulates the liberation of labelled inositol phosphate in partially purified plasma membranes prepared from myo-[3H]inositol prelabelled WRK1 cells. This stimulatory effect was very rapid (165% stimulation of inositol trisphosphate accumulation after a 10 s incubation period in the presence of 1 microM vasopressin), concentration dependent (EC50 = 12 nM) and was abolished by an antagonist of the vasopressor response to vasopressin. GTP, even at high concentrations (0.1 mM), did not increase inositol phosphate release: it was found to be absolutely necessary for hormonal stimulation of phospholipase C activity. Non-hydrolysable analogues of GTP may also stimulate this enzyme activity.

Basal phosphatidylinositol turnover controls aortic Na+/K+ ATPase activity

To determine whether basal phosphoinositide turnover plays a role in metabolic regulation in resting rabbit aortic intima-media incubated under steady state conditions, we used deprivation of extracellular myo-inositol as a potential means of inhibiting basal phosphatidylinositol (PI) synthesis at restricted sites and of depleting small phosphoinositide pools with a rapid basal turnover. Medium myo-inositol in a normal plasma level was required to prevent inhibition of a specific component of basal de novo PI synthesis that is necessary to demonstrate a discrete rapidly turning-over [1,3-14C]glycerol-labeled PI pool. Medium myo-inositol was also required to label the discrete PI pool with [1-14C]arachidonic acid (AA). The rapid basal turnover of this PI pool, when labeled with glycerol or AA, was not attributable to its utilization for polyphosphoinositide formation, and it seems to reflect basal PI hydrolysis. Depleting endogenous free AA with medium defatted albumin selectively inhibits the component of basal de novo PI synthesis that replenishes the rapidly turning-over PI pool. A component of normal resting energy utilization in aortic intima-media also specifically requires medium myo-inositol in a normal plasma level and a free AA pool; its magnitude is unaltered by indomethacin, nordihydroguaiaretic acid, or Ca2+-free medium. This energy utilization results primarily from Na+/K+ ATPase activity (ouabain-inhibitable O2 consumption), and in Ca2+-free medium deprivation of medium myo-inositol or of free AA inhibits resting Na+/K+ ATPase activity to a similar degree (60%, 52%). In aortic intima-media basal PI turnover controls a major fraction of resting Na+/K+ ATPase activity.

Arginine-vasopressin stimulates inositol phospholipid metabolism in rat hippocampus

The hippocampal vasopressin receptors have been characterised by measuring the stimulated accumulation of inositol monophosphate in the presence of 10 mM LiCl after hippocampal slices were prelabelled with [3H]inositol. Arginine-vasopressin caused a dose-dependent increase in inositol monophosphate accumulation (ED50 = 7.1 nM). The response was unchanged in the absence of Ca2+ and significantly reduced in the presence of a V1-receptor antagonist. Equimolar oxytocin was ineffective as a stimulus. This suggests that the hippocampal receptors are of the V1 type.

Requirement of Ca2+ for the production and degradation of inositol 1,4,5-trisphosphate in macrophages

The requirement of Ca2+ for the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) or the accumulation of inositol 1,4,5-trisphosphate (InsP3) in macrophages stimulated with fMet-Leu-Phe was examined using [32P]Pi or [3H]inositol-labeled cells. The dependence on Ca2+ of inositol-trisphosphate phosphatase was also examined. The application of 1 X 10(-8) M fMet-Leu-Phe caused a rapid decrease in the amount of PtdInsP2 to 70% of the control within 10 s, and the decrease was reverted to the control level by prolonged incubation. The decrease in the amount of PtdInsP2 accompanied the accumulation of phosphatidic acid and of InsP3, indicating that the loss of PtdInsP2 is due to phosphodiesteric breakdown. The dose-dependence of fMet-Leu-Phe or its analog on the hydrolysis of PtdInsP2 was much the same as that of the increase in intracellular free Ca2+ concentration in macrophages. The loss of PtdInsP2 as induced by fMet-Leu-Phe was similarly observed in macrophages treated with ionophore A23187 in the absence of external Ca2+ for 10 min. InsP3 was degraded by the particulate or cytosol fraction prepared from macrophages, and the activity of inositol-trisphosphate phosphatase in the particulate fraction was higher than that in the cytosol fraction. The enzyme in the cytosol fraction required Mg2+ for activity, and was activated by free Ca2+ concentrations ranging from 10(-7) to 10(-6) M in the presence of 1 mM MgCl2.

Mobilization of intracellular calcium by glucagon and cyclic AMP analogues in isolated rat hepatocytes

Separate or combined addition of cyclic AMP-dependent and Ca2+-linked hormones to isolated rat hepatocytes suspended in a low Ca2+ medium reduced the total cellular Ca. When the hormones were administered together, their effects were not additive. This suggests that both types of hormones mobilize Ca2+ from a common intracellular pool. In the presence of 1.8 mM extracellular Ca2+, the Ca2+ influx counterbalanced or even exceeded the hormone-induced Ca2+ loss, depending on the ability of the hormones to stimulate the Ca2+ influx.

Mobilization of different pools of glucose-incorporated calcium in pancreatic beta-cells after muscarinic receptor activation

Muscarinic receptor activation resulted in a biphasic mobilization of Ca2+ from isolated pancreatic islets. Glucose was essential for preparing the beta-cells to respond with the initial stimulatory phase. This effect seems to depend on the ability of the sugar to promote active sequestration of Ca2+ in the endoplasmic reticulum.

Is prostaglandin E2 involved in the pathogenesis of fever? Effects of interleukin-1 on the release of prostaglandins

Interleukin-1 (IL-1) induces the formation of PGE2 from monocytes, fibroblasts, muscle cells, and brain tissue by increasing the intracellular concentrations of CA2+; this cation, in turn, activates a phospholipase which cleaves arachidonic acid from either diacylglycerol or a membrane phospholipid. In addition, IL-1 increases the synthesis of cyclooxygenase, as evidenced by the increased conversion of arachidonic acid into prostaglandins after fibroblasts are pre-incubated with IL-1. Evidence is also presented that fever is caused by interleukin-1-induced prostaglandin E2.

Control of phenylalanine and tyrosine metabolism by phosphorylation mechanisms

A system for the parallel determination of enzyme phosphorylation and expressed activity in rat liver cells, and its application to studies of phenylalanine hydroxylase and tyrosine aminotransferase, is described. Phenylalanine hydroxylase is phosphorylated by agents which stimulate cyclic AMP- and Ca2+-dependent protein kinase activity. The phosphorylation site(s) appear to be the same for both kinases. Phosphorylation is accompanied by increased metabolic flux at low, physiologically relevant, substrate concentrations. Insulin and spermine both inhibit the phosphorylation of the enzyme, possibly by increasing dephosphorylation. Tyrosine aminotransferase is phosphorylated in liver cell incubations but the rate is slow and insensitive to additions to the medium. No parallel changes in flux could be detected. Both enzymes are subject to complex regulatory mechanisms, short- and long-term. Their activities may be coordinated in vivo by control exerted at the level of the plasma membrane where both amino acids share the same transport processes. Determination of the control coefficients for the several components indicates that membrane transport may be a major limitation on flux.

Inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate formation in Ca2+-mobilizing-hormone-activated cells

The inositol trisphosphate liberated on stimulation of guinea-pig hepatocytes, pancreatic acinar cells and dimethyl sulphoxide-differentiated human myelomonocytic HL-60 leukaemia cells is composed of two isomers, the 1,4,5-trisphosphate and the 1,3,4-trisphosphate. Inositol 1,4,5-trisphosphate was released rapidly, with no measurable latency on hormone stimulation, and, consistent with its proposed role as an intracellular messenger for Ca2+ mobilization, there was good temporal correlation between its formation and Ca2+-mediated events in these tissues. There was a definite latency before an increase in the formation of inositol 1,3,4-trisphosphate could be detected. In all of these tissues, however, it formed a substantial proportion of the total inositol trisphosphate by 1 min of stimulation. In guinea-pig hepatocytes, where inositol trisphosphate increases for at least 30 min after hormone application, inositol 1,3,4-trisphosphate made up about 90% of the total inositol trisphosphate by 5-10 min. In pancreatic acinar cells, pretreatment with 20 mM-Li+ caused an increase in hormone-induced inositol trisphosphate accumulation. This increase was accounted for by a rise in inositol 1,3,4-trisphosphate; inositol 1,4,5-trisphosphate was unaffected. This finding is consistent with the observation that Li+ has no effect on Ca2+-mediated responses in these cells. The role, if any, of inositol 1,3,4-trisphosphate in cellular function is unknown.

Differential effects of lithium on muscarinic receptor stimulation of inositol phosphates in rat cerebral cortex slices

The accumulation of labelled inositol mono-, bis-, and trisphosphate in rat cerebral cortex slices was examined following preincubation with [3H]inositol. The muscarinic receptor agonist carbachol produced a rapid and sustained increased accumulation of each labelled inositol phosphate both in the presence and absence of 5 mM lithium. Lithium potentiated carbachol-stimulated accumulation of inositol monophosphate (EC50 0.5 mM) and inositol bisphosphate (EC50 4 mM) in a concentration-dependent manner. However, exposure to lithium in the presence of the muscarinic agonist produced a concentration- and time-dependent inhibition of inositol trisphosphate accumulation that was not related to receptor desensitisation. Although the present data do suggest that polyphosphoinositides are substrates for agonist-stimulated phospholipase C in brain, these results may not be entirely consistent with the production of inositol mono- and bisphosphate through inositol trisphosphate dephosphorylation. Furthermore, these data suggest site(s) additional to inositol monophosphatase that are affected by lithium.

P2-purinergic control of liver glycogenolysis

Purinergic agonists cause a dose-dependent activation of glycogen phosphorylase in isolated rat hepatocytes. Half-maximally effective concentrations are 5 X 10(-7)M for ATP, 2 X 10(-6)M for ADP, and about 5 X 10(-5) M for AMP and adenosine. This potency series indicates the presence of P2-purinergic receptors. The mode of action of ATP appears to be identical with that of the Ca2+-dependent glycogenolytic hormones angiotensin, vasopressin and alpha 1-adrenergic agonists. (1) They all require Ca2+ for phosphorylase activation; (2) they do not increase cyclic AMP levels; (3) they are susceptible to heterologous desensitization by vasopressin and phenylephrine; (4) they lower cyclic AMP concentrations in hepatocytes stimulated by glucagon, most probably mediated by an enhanced phosphodiesterase activity.

Hormone-mediated inositol lipid breakdown in hepatocytes and WRK1 cells: relationship to receptor function

All hormones and neurotransmitters which provoke their intracellular effects by increasing the cytosolic concentration of Ca2+ in their target cells also stimulate the breakdown of inositol phospholipids. Much evidence suggests that this breakdown is intimately involved in the mechanism which couples cell-surface receptor activation to intracellular Ca2+ mobilization. Recent results indicate that the primary, receptor-mediated event in stimulated cells is a phosphodiesteric hydrolysis of phosphatidylinositol 4,5-bisphosphate to yield inositol trisphosphate and diacylglycerol. It is likely that both products of this reaction fulfill 'second messenger' roles within stimulated cells.

The effect of angiotensin II on arachidonate metabolism in adrenal glomerulosa cells

The effect of angiotensin II on arachidonate metabolism was examined in rat adrenal glomerulosa cells. Incorporation of both [3H]arachidonate and [32P]phosphate into phosphatidylinositol (PI) were significantly stimulated by angiotensin II. These effects were abolished by lithium, a cation, which was found suitable to prevent increased synthesis of PI in our previous study (T. Balla et al., FEBS Letters 171, 179, 1984). On the other hand, the phospholipase A2 inhibitor mepacrine failed to inhibit the increased labelling of PI. These observations suggest that the increased 3H labelling of PI occurs via CDP-diacylglycerol, and not via enhanced deacylation-reacylation cycle. The validity of this assumption was further supported, since angiotensin II failed to stimulate the formation of lyso-PI, as examined by both [32P]phosphate incorporation and pulse-chase techniques. Angiotensin II decreased the incorporation of [3H]arachidonate into phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Considering that we did not find arachidonate release either from phospholipids or from other possible arachidonate sources this decrease may not be due to dilution of the tracer. Thus we assume that angiotensin II may induce a shift in phospholipid synthesis from PC and PE to phosphoinositides. These observations indicate that the enhanced hydrolysis and synthesis of PI in response to angiotensin II is not associated with increased phospholipase A2 activity in adrenal glomerulosa cells.

Adrenergic regulation of gluconeogenesis: possible involvement of two mechanisms of signal transduction in alpha 1-adrenergic action

We have previously suggested that the effects of alpha 1-adrenergic agents on hepatocyte metabolism involve two mechanisms: (i) a calcium-independent insulin-sensitive process that is modulated by glucocorticoids and (ii) a calcium-dependent insulin-insensitive process that is modulated by thyroid hormones. We have studied the effect of epinephrine (plus propranolol) on gluconeogenesis from lactate and dihydroxyacetone. It was observed that the adrenergic stimulation of gluconeogenesis from lactate seemed to occur through both mechanisms, whereas when the substrate was dihydroxyacetone the action took place exclusively through the calcium-independent insulin-sensitive process. This effect was absent in hepatocytes from adrenalectomized rats, suggesting that it is modulated by glucocorticoids.

Effects of ATP and adenosine addition on activity of oxoglutarate dehydrogenase and the concentration of cytoplasmic free Ca2+ in rat hepatocytes

Addition of ATP (100 microM) to hepatocytes from starved rats incubated with 5 mM [1-14C]glutamine caused a stimulation of glucose formation; the magnitude of the concomitant increases in 14CO2 production and glutamine consumption indicate that flux from glutamine to glucose was increased. ATP also caused a simultaneous decrease in the cell content of oxoglutarate; together with the increased flux this is consistent with an activation of oxoglutarate dehydrogenase. In corroboration of this, a stimulation by ATP of gluconeogenesis and a decrease in oxoglutarate was also observed with 5 mM proline as substrate. ATP caused an increase in hepatocyte cytoplasmic free Ca2+ concentration, [Ca2+]c, as indicated by the increase in the fluorescence of cytoplasmically trapped quin2, from a resting value of about 0.2 microM to greater than 1 microM. The mechanism of oxoglutarate dehydrogenase activation may be via an increase in mitochondrial Ca2+ content as a consequence of the increase in [Ca2+]c. The effects of 100 microM adenosine were also investigated. An increase in flux from glutamine to glucose was observed together with a decrease in the cell oxoglutarate, thus indicating that adenosine addition to hepatocytes could also activate oxoglutarate dehydrogenase. The activation by adenosine was less than that produced by ATP. Adenosine caused a small apparent increase in [Ca2+]c to 0.3-0.4 microM; it remains to be established if this effect, which is small relative to that of ATP, is sufficient to elicit the activation of oxoglutarate dehydrogenase: alternative mechanisms may exist.

Platelet-derived growth factor stimulates rapid polyphosphoinositide breakdown in fetal human fibroblasts

The addition of human platelet-derived growth factor (PDGF) to confluent, quiescent cultures of human diploid fibroblasts induced the rapid breakdown of cellular polyphosphoinositides. The levels of 32P-labeled phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol (PI) decreased by 30 to 40% within 1 min after exposure of the cells to PDGF. The levels of PIP and PIP2 returned to their initial values within 3 and 10 min, respectively, after PDGF addition. The level of PI continued to increase after it had returned to control values and was up threefold within 30 min after PDGF addition. In cells prelabeled with myo-[3H]inositol PDGF caused an eightfold increase in the levels of inositol trisphosphate (IP3) within 2 min. Lesser increases, twofold and 1.3-fold, respectively, were seen in levels of inositol bisphosphate (IP2) and inositol monophosphate (IP). Within 10 min after PDGF addition the levels of all three inositol phosphates had decreased to control values. The levels of IP3 measured 2 min after PDGF addition depended on the PDGF concentration and were maximal at 5-10 ng/ml of PDGF. Similar concentrations of PDGF stimulate maximal cell growth and DNA synthesis in these cells.