Inositol Phospholipid Metabolism Research Articles

Effects of genistein, a tyrosine kinase inhibitor, on platelet functions: Genistein attenuates thrombin-induced Ca 2+ mobilization in human platelets by affecting polyphosphoinositide turnover

Genistein, a tyrosine kinase inhibitor, had no or only slight inhibitory effects on platelet aggregation or serotonin release induced by thrombin, while intracellular Ca 2+ concentration ([Ca 2+] i) elevation was substantially attenuated. It also inhibited the cyclooxygenase pathway, but this effect was not directly related to the suppressive effect of genistein on [Ca 2+] i elevation. In order to clarify the mechanism by which genistein suppresses Ca 2+ mobilization, its effect was examined on inositol phospholipid metabolism. The production of inositol-1,4,5-trisphosphate was inhibited by genistein in a dose-dependent manner, while 47 kDa protein phosphorylation or phosphatidic acid formation was not affected, suggesting that genistein does not inhibit phospholipase C activity. Pretreatment of unstimulated platelets with genistein increased the amount of phosphatidylinositol-4-monophosphate [PI(4)P], while that of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P 2]was reduced. Thrombin stimulation of genistein-pretreated cells intensified this tendency, i.e. a further increase in the amount of PI(4)P and a decrease in the amount of PI(4,5)P 2 in an inversely proportional manner. Taken together, these findings imply that genistein acted at the step of PI(4)P 5-kinase which produces PI(4,5)P 2 from PI(4)P. Protein tyrosine phosphorylation induced by thrombin was not affected by genistein, suggesting that the inhibitory effect of genistein on polyphosphoinositides was unrelated to tyrosine kinase inhibition.

A cAMP-linked metabotropic glutamate receptor in hippocampus.

Agonists at metabotropic glutamate receptors (mGluR) produce both pre- and postsynaptic responses in the hippocampus that can be monitored electrophysiologically. We provide evidence for the involvement of a distinct mGluR coupled to stimulation of adenosine 3',5'-monophosphate (cAMP) in the postsynaptic actions of 1-aminocyclopentane-trans-1S,3R-dicarboxylic acid (1S,3R-ACPD), a selective mGluR agonist. In contrast, the presynaptic effects of 1S,3R-ACPD are not produced by increases in cAMP, but may be mediated through a different subtype of mGluR linked to inositol phospholipid metabolism.

Depression of calcium transients after exposure to high K+ solution in Li(+)-loaded frog twitch muscle fibres and its reversal by exogenous myo-inositol.

Using Arsenazo III as a myoplasmic calcium indicator, we have studied the calcium transients evoked by voltage-clamp depolarizing pulses in frog twitch muscle fibres which had been temporarily depolarized by 80 mmol/L K+ in the absence or presence of myoplasmic Li+. After the high K+ exposure, for either a short (15 min) or long (1 h) time, the post-K+ calcium transients could gradually be restored to the level of the pre-K+ ones, if the fibres were not loaded with Li+. In contrast, the post-K+ calcium transients of Li(+)-loaded fibres could not fully recover, and were depressed in a Li+ concentration-dependent manner. The mean amplitude of the post-K+ responses recorded more than 3.5 h after 15 min high K+ exposure was reduced to 56% of pre-K+ control in the fibres which had been loaded with Li+ in 20 mmol/L Li+ Ringer's solution. This depression could be prevented or partially reversed by exogenous myo-inositol. More depression could be induced by 1 h high K+ exposure, but the presence of exogenous myo-inositol could not clearly prevent the post-K+ calcium transients from reduction. Assuming that high K+ exposure caused a depletion of myo-inositol and probably other changes in the metabolism of inositol phospholipids in Li(+)-loaded fibres, we conclude that some metabolites of phosphoinositides may play modulation roles in excitation-contraction coupling in frog twitch muscle fibres.

Platelet activation via binding of monoclonal antibodies to the Fc gamma receptor II.

During the last ten years a number of mouse monoclonal antibodies (mAbs) have been shown to cause platelet aggregation accompanied by a full activation response. Most commonly these mAbs recognise either the CD9 antigen or GPIIb-IIIa, two of the most abundant platelet surface glycoproteins, but more recently mAbs to other, less plentiful, surface proteins, including GPIV (CD36), CD69 and β 2 microglobulin, have been shown to give similar responses (for list see Table 1). As more data have accumulated a pattern of activation has emerged. In general, aggregation occurs following a concentration-dependant lag phase, at mAb concentrations of between 1 and 20 μg/ml. This aggregation is accompanied by an activation response which includes the metabolism of inositol phospholipids, the mobilisation of Ca2+, thromboxane synthesis and the secretion of the contents of both dense and α-granules. Characteristically the mAbs involved belong to the IgG1, subclass and are inhibited in their action by their F(ab’)2 or Fab fragments, thus implicating the Fc receptor in the activation process. The involvement of the FcγRII, the only Fcγ receptor expressed on platelets, was first demonstrated by Worthington et al. (1990) by blocking the activation caused by аnti-СD9 mAbs with a mAb, IV.3, to the FcγRII. Since that time it has become clear that this is the major route of activation.

Purification and characterization of a phosphatidylinositol 4-kinase activator in carrot cells.

A phosphatidylinositol 4-kinase activator (PIK-A49) has been purified from carrot cells grown in suspension culture. The activator was purified from a soluble fraction using DEAE-Sepharose CL-6B and S-Sepharose chromatography columns. PIK-A49 has a relative molecular mass of 49 kDa determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The A50 for the activation of the Triton X-100-solubilized phosphatidylinositol 4-kinase fraction was 0.1 microM. Maximal activation was 3-4-fold. The analysis of the sequences of seven peptide fragments containing a total of 142 amino acid residues indicated that PIK-A49 was 69% identical to an actin-binding protein (ABP-50) from Dictyostelium and > 90% identical to elongation factor-1 alpha (EF-1 alpha) from carrot, tomato, and Arabidopsis. PIK-A49 bound actin and facilitated actin polymerization. Poly(U)-directed polyphenylalanine synthesis assays indicated that PIK-A49 had EF-1 alpha activity. The EF-1 alpha activity was enhanced by rabbit EF-1 beta gamma. Activation of phosphatidylinositol 4-kinase by a protein that binds actin and that has EF-1 alpha activity provides additional complexity to the signal transduction mechanisms involving inositol phospholipid metabolism.

Cellular reactions after stimulation of receptors: research model for evaluation of effects and action mechanisms of drugs for discovery of innovative drugs

When cellular stimulants such as neurotransmitters, hormones, autacoids, cytokines and growth factors stimulate their respective specific receptors in the plasma membranes of cells, a variety of responses are elicited. GTP-binding proteins are also involved in the reactions between receptors and cellular effectors. Stimulation of receptors are subsequently coupled to the activation of ion channels, turnover of inositol phospholipid metabolism, adenylate cyclase and guanylate cyclase, inhibition of adenylate cyclase and potentiation of all proliferation. Active substances such as the so-called second messengers are produced in the cells. In this article, two findings are described: 1) Ca2+, which increases by stimulation of receptors with neurotransmitters and hormones, stimulated Ca2+/calmodulin-dependent protein kinase II in cell systems such as NG108-15 neuroblastoma x glioma hybrid cells and primarily cultured neuronal cells of rat hippocampus. 2) Coupling preferences and possible transduction mechanisms from experiments on NG108-15 cells and NL308 neuroblastoma x fibroblast hybrid cells which have been stably transfected with DNA for m1, m2, m3 and m4 muscarinic acetylcholine receptors were examined. These results may provide a useful research model for examining and evaluating the effects and mechanisms of the drugs on a living system and may help develop useful methodology for the discovery of innovative drugs.

Aminoglycosides Induced Activation of Saponin-permeabilized Platelets

Neomycin, an aminoglycoside antibiotic, binds more strongly to inositol polyphospholipid than to other membrane phospholipids. Therefore, this antibiotic is commonly used as a relatively specific inhibitor of inositol phospholipid metabolism in studies regarding the involvement of inositol phospholipids in various cell functions. The mechanism of action of neomycin is presumed to be inhibition of phospholipase C. However, the mode of inhibition remains obscure. Other aminoglycosides, including gentamicin and streptomycin, may also possess phospholipid binding affinity and platelet activity, but their effects are less potent than those of neomycin. The present study investigated both the direct action of aminoglycoside antibiotics and the activation of platelets rendered semi-permeable with saponin, which allows polar aminoglycoside antibiotics access to the innerleaf let of the plasma membrane, where the phosphoinositides are located. With semi-permeabilized platelets, relatively low concentrations of aminoglycoside antibiotics stimulated secretion, aggregation, arachidonic acid release and generation of inositol trisphosphate. All of the above-mentioned effects on semi-permeabilized platelets were fully inhibited by high concentrations of the aminoglycoside antibiotics. By acetylsalicylic acid pretreatment, aggregation, secretion and generation of inositol trisphosphate were fully inhibited. In contrast, arachidonic acid release was only slightly inhibited by this treatment. These data therefore suggest that the mechanism of action of aminoglycosids antibiotics on semipermeabilized platelets is not related to phospholipase C, but to phospholipase A2. These results provide evidence that aminoglycoside antibiotics activate phospholipase A2, which leads to the release of arachidonic acid in semi-permeabilized platelets. But whether themechanism of activation of phospolipase A2 by aminoglycoside antibiotics is direct or not is yet unclear.

Histamine, actin-gelsolin binding, and polyphosphoinositides in human umbilical vein endothelial cells.

Histamine activates inositol phospholipid metabolism, increases calcium, and causes a change in shape of human umbilical vein endothelial (HUVE) cells. Changes in endothelial cell shape are determined, in part, by changes in the actin cytoskeleton. Gelsolin is an actin-binding protein with the potential to alter the actin cytoskeleton in response to changes in cell calcium and/or changes in polyphosphoinositides. Therefore, we examined the interactions of actin and gelsolin in HUVE cells in which inositol phospholipid metabolism was activated with histamine. In HUVE cells exposed to histamine we estimated actin-gelsolin binding by quantitating actin and gelsolin, immunoprecipitated with anti-gelsolin Sepharose. We estimated the relative amount of filamentous actin in the histamine-exposed HUVE cells by quantitating the amount of actin that was Triton soluble. We also measured the amount of phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) in the HUVE cells before and after exposure to histamine. We found that histamine decreased the amount of actin that was immunoprecipitated with gelsolin, decreased the fraction of cell actin that was Triton soluble, and increased PIP and PIP2. These results demonstrate that histamine promotes actin filament formation in HUVE cells and that histamine-mediated changes in actin-gelsolin binding in these cells are better predicted by changes in polyphosphoinositides than by increases in cell calcium.

The influence of indomethacin, theophylline, and propranolol on ethanol augmentation of glucose-induced insulin secretion

The effect of indomethacin, theophylline, and progranolol on ethanol augmentation of insulin secretion after intravenous glucose stimulation was studied. Intravenous glucose tolerance tests were performed with and without pretreatment with oral ethanol. The role of indomethacin was evaluated in six healthy subjects; the effect of theophylline and propranolol on ethanol augmentation in insulin secretion was studied in five and six subjects, respectively. Ethanol pretreatment was followed by increased insulin ( P < .01) and C-peptide areas ( P < .01) after intravenous glucose (Area, 0 to 20 minutes). Indomethacin suppressed the ability of ethanol to augment insulin ( P < .01) and C-peptide ( P < .01) responses. Neither theophylline nor propranolol affected ethanol augmentation of insulin secretion. In conclusion, indomethacin probably interferes with the mechanism for ethanol augmentation of glucose-induced insulin secretion. It is suggested that inositol phospholipids take part in the effect of ethanol to augment insulin secretion, and that indomethacin interferes with the metabolism of inositol phospholipids.

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation.

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.

Phosphoinositide-generated second messengers in cardiac signal transduction

Hokin and Hokin were the first to demonstrate that tissue inositol phospholipid (phosphoinositide, PI) turnover was increased by hormone treatment. Twenty years later, Michell published a seminal review in which he suggested a relationship between stimulated inositol phospholipid metabolism and Ca 2+ mobilization. The biochemical link between these two events was subsequently identified by Berridge and colleagues as inositol trisphosphate (InsP 3), a Ca 2+-mobilizing ligand that is formed by the breakdown of phosphatidylinositol bisphosphate (PIP 2). The other product of inositol phospholipid hydrolysis, diacylglycerol, activates a Ca 2+-sensitive phospholipid-dependent protein kinase, protein kinase C, which has been considered as a potential regulator of cardiac ion channels, inotropic state, and gene expression. This review summarizes our current state of knowledge concerning the formation of phosphoinositide-generated second messengers in cardiac cells and their potential role in mediating functional responses in the myocardium.

Adenovirus E1A proteins stimulate inositol phospholipid metabolism in PC12 cells.

To study the influence of nuclear oncogenes on inositol phospholipid metabolism, we examined the various parameters of inositol phospholipid metabolism in PC12 cells expressing adenovirus type 12 or adenovirus type 5 E1A. Although the inositol 1,4,5-trisphosphate content was increased only slightly, the diacylglycerol content was 2.4-fold higher in E1A-expressing PC12 cells. Furthermore, we found that the activity of phospholipase C, one of the key enzymes in inositol phospholipid metabolism, was increased at least five- to eightfold. Diacylglycerol kinase activity in the membrane fraction was 10 to 15% of that in parental PC12 cells. Overall protein kinase C activities in E1A-expressing PC12 cells were decreased, but the activity of membrane-bound protein kinase C was significantly increased. These observations clearly indicate that inositol phospholipid metabolism is stimulated in cells producing E1A and suggest that nuclear oncogene E1A has the ability to stimulate inositol phospholipid metabolism.

Alcohol and brain development: The interaction of ethanol with the metabolism of inositol phospholipids

Alcohol and brain development: The interaction of ethanol with the metabolism of inositol phospholipids

Tyrosine protein kinase is involved in anti-IgM-mediated signaling in BAL17 B lymphoma cells.

BAL17 B lymphoma cells, representing mature B lymphocytes, were used to analyze the role of tyrosine kinase in B cell activation. Anti-IgM-induced tyrosine phosphorylation was inhibited by preincubation of cells with tyrosine kinase inhibitor herbimycin A. Enzymatic activity of lyn protein was also inhibited by this drug, accompanied by down-regulation of p53lyn and p56lyn. However, a protein kinase C-mediated event was intact in the herbimycin A-pretreated cells, suggesting that the inhibitor acts selectively on tyrosine kinase. Anti-IgM failed to stimulate herbimycin A-pretreated cells to induce increases in inositol phospholipid metabolism or increased [Ca2+]i, whereas aluminum fluoride-induced metabolism was not altered. Moreover, membrane IgM density as revealed by flow cytometry was not changed by herbimycin A. These results indicate that tyrosine kinase(s) participates in the coupling of an Ag receptor cross-linkage to phospholipase C activation through a phosphorylation event in B lymphoma cells.

Phospholipid metabolism and second messenger system after brain ischemia.

To evaluate possible involvement of phospholipid metabolism and related second messenger systems in the selective neuronal damage after ischemia, we measured changes of polyphosphoinositides (PPIs) and free fatty acids (FFAs) in a model of 5-min or 10-min ischemia and reperfusion in gerbils. The binding activity of 3H-phorbol 12,13-dibutyrate (PDBu) for protein kinase C (PKC) and 3H-inositol 1,4,5-triphosphate (IP3) for IP3 receptors was demonstrated autoradiographically. Induction of 70 KDa heat shock protein (HSP70) mRNA and amyloid precursor protein (APP) mRNA was also examined using Northern blot analysis. In the parietal cortex (an area resistant to transient ischemia), PPIs decreased during ischemia and recovered rapidly after reperfusion. However, recovery did not occur in the hippocampal CA1 area (an area more vulnerable to transient ischemia). In the cortex, arachidonic acid (AA) increased during ischemia and returned to baseline by 7 days after reperfusion; in the CA1 area, the AA level remained elevated even after 7 days of reperfusion. PDBu binding decreased in CA1 cells after 2 days of reperfusion. IP3 binding began to decrease at 5 hr of reperfusion, which is far earlier than either the onset of decreased PDBu binding or the observation of neuronal damage by light microscopy. The induction of HSP70 mRNA occurred, but the induction of APP mRNA did not. Regional differences in the induction of HSP70 mRNA were found; CA1 cells produced less HSP70 mRNA than cortical cells 8 hr after transient ischemia. These results suggest that CA1 cell membranes may not recover after transient ischemic attack, and that the membranes of the endoplasmic reticulum, which have IP3 receptors, may undergo alterations earlier than cytoplasmic membranes. The variable induction of HSP70 mRNA may be related to regional differences in vulnerability in cortical and hippocampal CA1 cells after transient ischemia. Involvement of excitatory neurotransmission in the induction of HSP70 has been suggested. The combined data may support a role for inositol phospholipid metabolism, changes in related second messenger systems, and induction of HSP70 in the excitotoxic mechanism of hippocampal CA1 neuronal damage, death, and repair.

Phosphoinositide metabolism in intestinal smooth muscle: preferential production of Ins(1,4,5)P3 in circular muscle cells.

The pattern of inositol phospholipid metabolism in response to stimulation by contractile agonists was examined in muscle cells isolated separately from circular and longitudinal muscle layers of guinea pig intestine. Addition of cholecystokinin octapeptide (CCK-8) to circular muscle cells caused a prompt decrease in phosphatidylinositol 4,5-bisphosphate (PtdInsP2) (50 +/- 6%) and PtdInsP (38 +/- 9%), which reached a nadir in 15 s. Addition of CCK-8 to longitudinal muscle cells caused a decrease in PtdInsP only (42 +/- 6%); PtdInsP2 levels, which were three times lower in this cell type, did not change throughout the period of stimulation. Hydrolysis of PtdInsP2 in circular muscle cells was accompanied by a concentration-dependent increase in inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]; the maximal increase was 10 to 16 times greater in circular muscle cells as measured by radioreceptor assay and by ion-exchange chromatography. The small amounts of InsP3 produced in longitudinal muscle cells did not result from more rapid degradation, since the rates of disappearance of exogenous Ins(1,4,5)P3 in both cell types were similar. Within 5 s after addition of CCK-8, InsP3 comprised 73 +/- 5% of total inositol phosphates produced in circular muscle cells and 2.0 +/- 0.2% in longitudinal muscle cells. These divergent patterns indicate that inositol phospholipid metabolism is channeled toward generation of InsP3 in circular muscle cells only; the metabolic pattern in these cells parallels the selective presence of high-affinity InsP3 receptors and the pattern of intracellular Ca2+ mobilization.

PAF effects on transmembrane signaling pathways in rat kupffer cells

Platelet activating factor (PAF) was found to stimulate the metabolism of inositol phospholipids via deacylation and phospholipase C in Kupffer cells, the resident macrophages in liver. PAF-induced phosphoinositide metabolism occurred in two phases. Within seconds after stimulation, in the absence of extracellular Ca++, platelet activating factor caused the phosphodiester hydrolysis of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate with the release of inositol 1,4,5-trisphosphate and inositol 1,4-bisphosphate. This was followed by an extracellular Ca(++)-dependent release of glycerophosphoinositol, inositol monophosphates and inositol bisphosphates. Various Ca(++)-mobilizing agonists failed to evoke hydrolysis of phosphoinositides. Platelet activating factor also stimulated the synthesis and release of prostaglandins from these cells. Platelet activating factor-stimulated phosphodiester metabolism of phosphoinositides and prostaglandin synthesis was inhibited by treatment with pertussis toxin and cholera toxin. Pertussis toxin also inhibited platelet activating factor-induced glycerophosphoinositol release. Cholera toxin, in contrast, stimulated platelet activating factor-induced glycerophosphoinositol release and prostaglandin synthesis and synergistically stimulated the effect of platelet activating factor on these processes. The results suggest that platelet activating factor-induced metabolism in the Kupffer cells occurs via specific receptors and may be mediated through the activation of different G-proteins.

Angiotensin II AT 1 receptors in rat superior cervical ganglia: characterization and stimulation of phosphoinositide hydrolysis

Angiotensin II receptor number was higher in superior cervical ganglia of 2-week-old when compared to 8-week-old rats. In both young and adult rats, specific binding of [ 125I][Sar 1]angiotensin II was displaced competitively by the AT 1-receptor antagonist DuP 753 but not by the AT 2-receptor competitor PD 123177. In ganglia from adult rats. DuP 753 competed with an IC 50 of 113 nM. The stable guanine nucleotide GTPγS inhibited binding of [ 125I][Sar 1]angiotensin II in young and adult rats by approximately 50% with IC 50 values of 105 and 120 nM, respectively, suggesting that the angiotensin receptor is G-protein linked. Angiotensin II at a dose of 1 μM stimulated inositol phosphate formation 58% over control values in superior cervical ganglia from 8-week-old rats. This effect was totally blocked by 10 μM DuP 753 but not by 10 μM PD 123177. Our findings demonstrate that rat superior cervical ganglia contain AT 1-type angiotensin receptors that are probably G-protein linked, and their stimulation results in increased inositol phospholipid metabolism.

Gamma-interferon, retinoic acid, and cytosine arabinoside induce neuroblastoma differentiation by different mechanisms.

1. The effects of gamma-interferon (gamma-IFN), retinoic acid (RA), and cytosine arabinoside (ARA-C) on the growth, morphology, and phenotype of the human neuroblastoma (NB) cell lines, LAN-1 and GI-ME-N, have been extensively tested. 2. RA, gamma-IFN, and ARA-C induced a dose-dependent morphological differentiation and growth inhibition, without affecting cell viability. Cells exposed to 10(-6) M RA or 1000 U/ml gamma-IFN significantly decreased their growth rate within the first 24 and 48 hr of culture, respectively. Cells became smaller and polygonal and sprouted long cellular processes with varicosities along their courses. In contrast, ARA-C-differentiated cells were larger and flattened, with few elongated dendritic processes. 3. Analysis of membrane and cytoskeletal markers by immunofluorescence and Western blot showed several changes in NB-specific antigen expression after 5 days of treatment with all inducing agents. Analysis of labeled phosphatidylinositol metabolites from prelabeled cells showed, within 1 min of treatment with RA, a rapid decrease in inositol 1,4,5-trisphosphate and of 1,2-diacylglycerol levels. No changes in inositol phospholipid metabolism were observed in gamma-IFN- or ARA-C-treated cells. 4. We conclude that RA-induced decrease in phosphatidylinositol (PI) hydrolysis is not likely to be a consequence of the acquisition of a different phenotype, as its changes precede the acquisition of neuronal markers. In addition, gamma-IFN and ARA-C, both inducing a mature phenotype, did not affect PI hydrolysis. 5. Decreased PI hydrolysis seems to be sufficient, although not necessary, to commit NB cells to neuronal differentiation. Analysis of molecular mechanisms associated with NB cell differentiation may be helpful to clarify the potential of various biological agents in affecting the development of the neural cell.

ATP-stimulated inositol phospholipid metabolism and surfactant secretion in rat type II pneumocytes.

Extracellular ATP (10(-3) M) stimulated [3H]phosphatidylcholine secretion approximately 3.4-fold in rat type II pneumocytes prelabeled overnight with [3H]choline. The same concentration of ATP caused a rapid increase in [3H]inositol trisphosphate (IP3) and a decrease in [3H]phosphatidylinositol bisphosphate (PIP2) in [3H]inositol-prelabeled cells. ATP also caused a biphasic increase in 1,2-[3H]diacylglycerol in cells prelabeled with [3H]arachidonic acid: a rapid increase that peaked at 10 s followed by a larger increase that peaked at 5-10 min. The first peak in diacylglycerol and the increase in IP3 are consistent with phospholipase C action on PIP2 and generation of second messengers that promote mobilization of intracellular Ca2+ and activation of protein kinase C. However, at the level of phosphatidylcholine secretion the stimulatory effects of ATP and of direct activators of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1,2-dioctanoyl-sn-glycerol, were at least additive, suggesting that activation of protein kinase C may not be the major signal transduction mechanism in ATP action or alternatively that ATP activates a different isoform of protein kinase C. Pretreatment of type II cells with TPA for 30 min led to a subsequent 40% diminution in the stimulatory effects of ATP on both phosphatidylcholine secretion and IP3 generation.