Metabolism of inositides and the activation of platelets

Phosphoinositide signalling system in platelets of schizophrenic patients and the effect of neuroleptic therapy

Alterations in the phosphoinositide signalling system have been proposed as a possible biological marker of schizophrenia. We studied the levels of inositol 1,4,5-trisphosphate (IP3), cytosolic Ca2+ concentrations ([Ca2+]i), and the incorporation of [32P]-orthophosphate into inositol phospholipids and phosphatidic acid (PA) in blood platelets of neuroleptic-treated schizophrenics in comparison with controls. The [Ca2+]i was significantly higher in platelets of one month neuroleptic-treated patients (155+/-5.8 nM) in comparison with controls (95+/-5.4 nM). Neuroleptic therapy decreased the [Ca2+]i, but even after long-term therapy it remained significantly higher (114+/-5.7 nM) than in controls. Differences were also found in the level of IP3 between controls (30+/-4.0 pmol/10(9) platelets), drug-free schizophrenics (52+/-9.0 pmol/10(9) platelets) and treated patients (50+/-6.0 pmol/10(9) platelets). The increased turnover of PA was observed in platelets of neuroleptic-treated schizophrenic patients. The study suggests that the regulation of calcium homeostasis and pathways involved in the phosphoinositide signalling system are altered in the platelets of schizophrenics. Neuroleptic therapy did not remove the observed changes in [Ca2+]i and IP3 levels.

Receptor occupancy regulates Ca2+ entry and intracellular Ca2+ redistribution in activated human platelets

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently depleted intracellular Ca2+ stores, followed by active sequestration of released Ca2+. Ca2+ influx induced by U46619 largely relies on receptor occupancy. Removing the thromboxane analogue from its receptor by using the endoperoxide/thromboxane-receptor antagonist BM 13177 largely blunted U46619-mediated Ca2+ influx. The Ca(2+)-ATPase inhibitor thapsigargin evoked a gradual rise in intracellular Ca2+, which was potentiated by a preceding activation of platelets with the receptor agonist U46619. This agonist-sensitizing effect also depends on receptor occupancy. Removing U46619 from its receptor by addition of the endoperoxide/thromboxane-receptor antagonist BM13177 suppressed the sensitizing effect completely. Furthermore, interrupting downstream receptor signalling events by raising intracellular levels of cyclic nucleotides (cyclic AMP, cyclic GMP) again suppressed the U46619-sensitizing effect on thapsigargin-induced Ca2+ release. This study indicates that the process of Ca2+ release followed by resequestration in response to a platelet agonist by its own is not sufficient to produce the sensitizing effect. Rather, a continuously occupied receptor triggering sustained downstream signalling events seems to be required for sensitization. The presence of a receptor agonist may induce an increased cycling of Ca2+ between the agonist-responsive and the thapsigargin-dischargeable compartment, leading to faster and more intense accumulation of Ca2+ in the cytosolic compartment after inhibition of the Ca(2+) ATPase. Suggestively, receptor occupancy increases the Ca(2+)-releasing potency of thapsigargin by coupling the thapsigargin-sensitive Ca(2+)-storing compartments with an agonist-responsive compartment that exhibits a high leakage rate in stimulated platelets.

A haemorrhagic platelet disorder associated with altered stimulus-response coupling and abnormal membrane phospholipid composition

Haemorrhagic diatheses due to platelet function defects are a heterogenous and poorly understood group of conditions. We report the investigation of a female with a lifelong history of epistaxes, haemarthroses, menorrhagia and persistent iron-deficiency anaemia. Although platelet numbers and morphology were normal, platelet function was abnormal both in vivo and in vitro. Skin bleeding time was prolonged and aggregation thresholds in platelet-rich plasma to a variety of weak and strong agonists were increased. Platelet granule contents were normal and membrane glycoproteins GpIb and GpIIIa were present in normal amounts. Polyphosphoinositide metabolism and phosphatidic acid generation were diminished in thrombin-stimulated platelets, as was phosphorylation of the 47 kD substrate for protein kinase C and the 20 kD protein myosin light chain kinase, indicating impaired generation of the intracellular second messengers diacylglycerol and inositol trisphosphate due to diminished stimulated phospholipase C activity. Although intracellular free calcium, calmodulin activity and basal cAMP concentrations were normal, washed platelets showed increased cAMP accumulation following stimulation with prostaglandin E1 and forskolin. Platelet membrane lipid analysis revealed a reduction in plasmalogen phosphatidylethanolamine content. It is suggested that the membrane phospholipid abnormalities cause the abnormal platelet reactivity by interfering with signal transduction from platelet receptor, via intermediary G proteins, to phospholipase C and adenylate cylase. The bleeding tendency is likely to be a consequence of the altered stimulus-response coupling.

Ca2+ efflux from platelets. Control by protein kinase C and the filling state of the intracellular Ca2+ stores

Large amounts of Ca2+ (almost 20 nmol/10(8) cells) are released from platelets by exocytosis. This secretory-granule-associated Ca2+ does not contribute to the cytosolic free Ca2+ ([Ca2+]i), which is controlled by the much smaller agonist-sensitive Ca2+ pool, unless high (1 microM), but not low (0.04 microM) concentrations of ionomycin are present. Low concentrations of ionomycin release Ca2+ almost exclusively from the agonist-sensitive stores. In aspirinated platelets incubated in the presence of 0.5 mM EGTA the extensive depletion of the agonist-sensitive stores is obtained by the combined action of low ionomycin and the endomembrane Ca(2+)-ATPase inhibitor thapsigargin (which individually promote only a partial depletion). The subsequent decay of [Ca2+]i is increased by phorbol-myristate acetate, confirming that Ca2+ efflux from platelets is potentiated by the activation of protein kinase C [Pollock, W. K., Sage, S. O. & Rink, T. J. (1987) FEBS Lett. 210, 132-140]. A novel type of control of Ca2+ efflux appears to be exerted by the filling state of the stores. Treatment with low ionomycin or thapsigargin determines the release of a fraction of the stores-associated Ca2+; the subsequent decay of [Ca2+]i is slow. The decay rate of [Ca2+]i accelerates after extensive depletion of the stores following the addition of thapsigargin or ionomycin. If the depletion of the stores is induced by thrombin, added alone or in combination with thapsigargin, the increases of [Ca2+]i are the same and the subsequent decay rates are largely superimposable; however a large fraction of [Ca2+]i is reaccumulated into the stores in the absence, but not in the presence of thapsigargin, indicating that Ca2+ efflux is activated when the stores are empty. Ca2+ efflux can proceed against a concentration gradient. In 45Ca-loaded platelets, the thrombin-promoted 45Ca efflux is potentiated by thapsigargin. The protein-kinase-C-dependent and store-depletion-dependent stimulations of 45Ca efflux are additive. These observations indicate that, in addition to being activated by protein kinase C, Ca2+ efflux from platelets is activated by the depletion of the stores. The two activations appear to be additive.

Cyclic nucleotides and intracellular-calcium homeostasis in human platelets

The relationship between agonist-sensitive calcium compartments and those discharged by the Ca(2+)-ATPase inhibitor thapsigargin were studied in human platelets. In this context, calcium mobilization from intracellular pools and manganese influx was investigated in relation to the effect of altered cyclic-nucleotide levels. For maximal calcium release from intracellular stores, thapsigargin, compared to a receptor agonist like thrombin, requires the platelet's self-amplification mechanism, known to generate thromboxane A2. With this lipid mediator formed, thapsigargin released calcium and stimulated manganese influx in a manner similar to thrombin. Blocking the thromboxane receptor by addition of sulotroban (BM13.177) or, alternatively, increasing platelet cAMP or cGMP using prostacyclin or sodium nitroprusside, dramatically reduced the ability of thapsigargin to release calcium from intracellular compartments. The same experimental conditions significantly reduced the rate of manganese influx initiated by thapsigargin compared to thrombin. The experiments indicate that thapsigargin-sensitive compartments play only a minor role in inducing manganese influx compared to the receptor-sensitive compartment. Cyclic nucleotides accelerate the redistribution of an agonist-elevated platelet calcium into the thapsigargin-sensitive compartment, from which calcium can be released by inhibition of the Ca(2+)-ATPase. In human platelets, thapsigargin-induced calcium increase and influx were responsible for only part the calcium release resulting from inhibition of the corresponding ATPase; another part results from the indirect effect of thapsigargin acting via thromboxane-A2-receptor activation. Cyclic nucleotides are therefore an interesting regulatory device which can modify the thapsigargin response by not allowing the self-amplification mechanism of platelets to operate.

Studies on inhibition of human platelet response by diltiazem

1. Aim of the present investigation was to investigate the effects of calcium blocking agent diltiazem on human platelet response to aggregating agents. 2. Results showed that diltiazem inhibits platelet aggregation induced by ADP, arginine vasopressin, adrenaline, collagen, Na arachidonate, thrombin and phorbol ester PMA in a dose-dependent way. 3. Diltiazem decreased also beta-thromboglobulin release and Thromboxane B2 production from stimulated platelets. 4. Intraplatelet cyclic AMP levels were not modified by the substance. 5. Data provide evidence that the modulation of human platelet function by diltiazem could be also related to inhibition of protein kinase C.

Thrombin-induced activation of calcium transport pathways and their role in platelet functions

In human platelets thrombin-induced calcium release from intracellular stores, the consequent influx of extracellular calcium, as well as their role in the aggregation and ATP-secretion reactions were examined. In indo-1-loaded platelets intracellular calcium release was studied in the presence of excess EGTA in the incubation medium, while calcium influx was followed after a rapid repletion of external calcium. After thrombin-stimulation both calcium release and calcium influx produced about the same peak levels of cytoplasmic free calcium but in the first case it was only a transient response, while in the latter one a sustained calcium signal was observed. Increased calcium influx could be evoked for several minutes after the addition of thrombin, it was selectively inhibited by Mg2+ (20 mM) and Ni2+ (1 mM) ions, by neomycin and by PCMB, a non-penetrating SH-group reagent. This calcium influx was practically insensitive to organic calcium channel blockers. Thrombin-induced platelet aggregation was only partial in the absence of external calcium, even if excess magnesium was present in the media, while the aggregation response became complete if external calcium was repleted. A significantly reduced aggregation could be seen in calcium-containing media if calcium influx was selectively inhibited. Platelet ATP-secretion under the same conditions did not depend on external calcium or on calcium influx. These data indicate that in thrombin-stimulated platelets the opening of specific plasma membrane calcium channels can be selectively modulated and these channels play a major role in the development of a full-scale aggregation.

Possible role of phospholipase C in the regulation of cell division in normal and neoplastic cells

It is proposed that a phosphatidylinositol-specific phospholipase C (PLC) enzyme may be present in abnormally high concentrations in certain cancer cells, and that the elevated activity may explain many, if not all, of the neoplastic characteristics of the cancer cells. There have thus far, been two reports in which PLC activity has been found to be elevated several fold in neoplastic cells. The products of the action of PLC on the phosphoinositides, including diglycerides and inositol phosphates, have been shown to activate the process of cell division by elevating the intracellular concentration of calcium ions and by stimulating the activity of protein kinase C. An elevated content of PLC in at least certain neoplastic cells could thus explain uncontrolled proliferative processes in those cells.

Effects of phosphoinositides on calcium movements in human platelet membrane vesicles

In a mixed endoplasmic and surface-type membrane vesicle preparation from human platelets the polyphosphoinositides PIP and PIP2, similarly to IP3, were found to induce a rapid calcium release reaction. At physiological (resting) cytoplasmic calcium concentrations (0.1-0.3 microM) the PIP2 and IP3 concentrations producing half-maximum calcium release were similar (0.7 microM) and both agents could mobilize about 30-40% of the intravesicular calcium. However, the phosphodiesteric degradation of PIP2 in the membrane vesicles was found to be negligible and the ion- and drug-sensitivities of the calcium release reactions were different. The IP3-induced calcium release was selectively inhibited by micromolar calcium concentrations and by cinnarizine, while the PIP2-induced release was blocked by magnesium ions and neomycin. The calcium release evoked by either agent was inhibited by low concentrations of lanthanum but, in contrast to the ATP-dependent calcium pump, it was insensitive to vanadate, quercetin and to the lowering of the incubation temperature. When added simultaneously or in a rapid succession, maximum effective IP3 and PIP2 concentrations produced an additive calcium release reaction. Based on these data we suggest that IP3 and PIP2, respectively, induce rapid transmembrane calcium movements involving different transport pathways and/or membrane calcium pools, which are not related to the active calcium transport systems.

The alpha 1-adrenergic transduction system in hamster brown adipocytes. Release of arachidonic acid accompanies activation of phospholipase C

Previous studies of brown adipocytes identified an increased breakdown of phosphoinositides after selective alpha 1-adrenergic-receptor activation. The present paper reports that this response, elicited with phenylephrine in the presence of propranolol and measured as the accumulation of [3H]inositol phosphates, is accompanied by increased release of [3H]arachidonic acid from cells prelabelled with [3H]arachidonic acid. Differences between stimulated arachidonic acid release and formation of inositol phosphates included a requirement for extracellular Ca2+ for stimulated release of arachidonic acid but not for the formation of inositol phosphates and the preferential inhibition of inositol phosphate formation by phorbol 12-myristate 13-acetate. The release of arachidonic acid in response to phenylephrine was associated with an accumulation of [3H]arachidonic acid-labelled diacylglycerol, and this response was not dependent on extracellular Ca2+ but was partially prevented by treatment with the phorbol ester. The release of arachidonic acid was also stimulated by melittin, which increases the activity of phospholipase A2, by ionophore A23187, by lipolytic stimulation with forskolin and by exogenous phospholipase C. The arachidonic acid response to phospholipase C was completely blocked by RHC 80267, an inhibitor of diacylglycerol lipase, but this inhibitor had no effect on release stimulated with melittin or A23187 and inhibited phenylephrine-stimulated release by only 40%. The arachidonate response to forskolin was additive with the responses to either phenylephrine or exogenous phospholipase C. These data indicate that brown adipocytes are capable of releasing arachidonic acid from neutral lipids via triacylglycerol lipolysis, and from phospholipids via phospholipase A2 or by the sequential activities of phospholipase C and diacylglycerol lipase. Our findings also suggest that the action of phenylephrine to promote the liberation of arachidonic acid utilizes both of these reactions.

Heterogeneous defects of platelet secretion and responses to weak agonists in patients with bleeding disorders

Eleven patients with mild bleeding disorders had as a common abnormality, impaired platelet aggregation and secretion with low concentrations (0.5-1.0 micrograms/ml) of collagen and, in most cases, an absence of second phase aggregation with epinephrine. Platelet granule contents were normal, ruling out storage pool deficiency. To characterize further the platelet abnormalities, we measured aggregation, 14C-5HT secretion, and TxB2 formation induced by a variety of platelet agonists. In eight of the 11 patients we observed decreased initial rates as well as extents of aggregation with one or more weak agonists (ADP, epinephrine, thromboxane A2 and the endoperoxide analogue U44069), i.e. agonists which induced secretion only as a result of aggregation, but normal responses to strong agonists such as arachidonate and high (10 micrograms/ml) concentrations of collagen, which can induce secretion in the presence or absence of aggregation. In all of these patients, TxB2 formation with arachidonate and all concentrations of collagen was normal. The platelet defects in these eight patients have been designated as weak agonist response defects (WARDs). In contrast, the initial aggregation responses to all weak agonists were normal in the three other patients, while secretion and TxB2 formation induced by strong agonists were impaired. Thus, in contrast to the eight patients above, the platelet defects in these three patients were characteristic of defects in the secretion response per se. The results obtained in the 11 patients studied indicate that these types of platelet disorders, previously referred to as primary secretion defects, include defects in the initial platelet responses which precede secretion (WARD) as well as defects in the secretory mechanism per se. Both groups of defects appear to be heterogeneous in nature.

Effects of activation of protein kinase C on the agonist-induced stimulation and inhibition of cyclic AMP formation in intact human platelets

Jakobs, Bauer & Watanabe [(1985) Eur. J. Biochem. 151, 425-430] reported that treatment of platelets with phorbol 12-myristate 13-acetate (PMA) prevented GTP- and agonist-induced inhibition of adenylate cyclase in membranes from the platelets. This was attributed to the phosphorylation of the inhibitory guanine nucleotide-binding protein (Gi) by protein kinase C. In the present study, the effects of PMA on cyclic [3H]AMP formation and protein phosphorylation were studied in intact human platelets labelled with [3H]adenine and [32P]Pi. Incubation mixtures contained indomethacin to block prostaglandin synthesis, phosphocreatine and creatine kinase to remove ADP released from the platelets, and 3-isobutyl-1-methylxanthine to inhibit cyclic AMP phosphodiesterases. Under these conditions, PMA partially inhibited the initial formation of cyclic [3H]AMP induced by prostaglandin E1 (PGE1), but later enhanced cyclic [3H]AMP accumulation by blocking the slow decrease in activation of adenylate cyclase that follows addition of PGE1. PMA had more marked and exclusively inhibitory effects on cyclic [3H]AMP formation induced by prostaglandin D2 and also inhibited the action of forskolin. Adrenaline, high thrombin concentrations and, in the absence of phosphocreatine and creatine kinase, ADP inhibited cyclic [3H]AMP formation induced by PGE1. The actions of adrenaline and thrombin were attenuated by PMA, but that of ADP was little affected, suggesting differences in the mechanisms by which these agonists inhibit adenylate cyclase. sn-1,2-Dioctanoylglycerol (diC8) had effects similar to those of PMA. The actions of increasing concentrations of PMA or diC8 on the modulation of cyclic [3H]AMP formation by PGE1 or adrenaline correlated with intracellular protein kinase C activity, as determined by 32P incorporation into the 47 kDa substrate of the enzyme. Parallel increases in phosphorylation of 20 kDa and 39-41 kDa proteins were also observed. Platelet-activating factor, [Arg8]vasopressin and low thrombin concentrations, all of which inhibit adenylate cyclase in isolated platelet membranes, did not affect cyclic [3H]AMP formation in intact platelets. However, the activation of protein kinase C by these agonists was insufficient to account for their failure to inhibit cyclic [3H]AMP formation. Moreover, high thrombin concentrations simultaneously activated protein kinase C and inhibited cyclic [3H]AMP formation. The results show that, in the intact platelet, the predominant effects of activation of protein kinase C on adenylate cyclase activity are inhibitory, suggesting actions additional to inactivation of Gi.

Cyclic GMP formation and inositol phosphate accumulation do not share common origins in rat brain slices

Cyclic GMP formation and inositol phospholipid hydrolysis were studied in rat brain slices to determine if the two processes have common origins. Muscarinic cholinergic stimulation enhanced [3H]inositol phosphate ([ 3H]IP) accumulation from slices prelabelled with [3H]inositol but did not affect cyclic GMP formation in the cortex, striatum, or cerebellum. An elevated level of extracellular K+ stimulated accumulation of both cyclic GMP and [3H]IP in cortex slices. The former, but not the latter, was reduced by lipoxygenase and phospholipase A2 inhibition. Calcium channel activation enhanced and blockade reduced K+-stimulated [3H]IP formation without affecting the cyclic GMP level, and there were differences in the Ca2+ requirements for the two responses. Thus, there is no support for the concept that guanylate cyclase activation inevitably accompanies inositol phospholipid breakdown, and the evidence presented demonstrates that K+ stimulation promotes cyclic GMP and [3H]IP accumulation by different transducing pathways.

Alkylacetylglycerophosphocholine stimulates Na+-Ca2+ exchange, protein phosphorylation and polyphosphoinositide turnover in rat ileal plasmalemmal vesicles

The novel ether phospholipid, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC), isometrically contracted helically cut rat ileal smooth muscle strips in a dose- and time-dependent manner. Utilizing an enriched plasma membrane vesicular preparation from rat ileal longitudinal smooth muscle, AGEPC specifically stimulated Na+-Ca2+ exchange in a dose- and time-dependent manner. Concomitant with the AGEPC stimulation of Na+-dependent Ca2+ influx in plasma membrane vesicles is an enhanced turnover of the polyphosphoinositides, an elevated concentration of phosphatidic acid and also an enhanced phosphorylation of an Mr 40,000 plasmalemmal protein. The mechanisms by which AGEPC may regulate ileal plasmalemmal Ca2+ flux and contractility are considered.

Acute desensitization to angiotensin II: evidence for a requirement of agonist-induced diacylglycerol production during tonic contraction of rat aorta

A possible role for protein kinase C during the tonic phase of arterial contraction was examined in rat aorta by observing the effects of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), on angiotensin II (AII)-induced responses. The ability of AII and phenylephrine (PE) to induce diacylglycerol (DAG) production was monitored as agonist-stimulated 32P-labelling of phosphatidic acid (PA). AII (5 X 10(-7) M) causes only a transient contractile response, while PE (10(-5) M) causes a sustained tonic contraction. 32P-labelling studies showed that AII caused an initial increase of PA synthesis equal to PE, however, AII failed to sustain this increase at 5 and 10 min while PE was able to do so. This indicates a failure of AII to provide DAG to sustain protein kinase C activation. Activation of protein kinase C with TPA prior to and during AII exposure converted the normally transient contraction to a more sustained, tonic pattern. These results suggest that the capacity of neuroendocrine agonists to maintain tension is due to their ability to produce DAG continuously and thereby activate protein kinase C.

Effect of common agonists on cytoplasmic ionized calcium concentration in platelets. Measurement with 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin

Because of controversy regarding the relationship of cytoplasmic ionized calcium concentration ([Cai2+]) to platelet activation, we studied the correlation of platelet aggregation and ATP secretion with [Cai2+] as determined by 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin in response to ADP, epinephrine, collagen, the Ca2+ ionophore A23187, and thrombin. Both indicators showed a concentration-dependent increase in [Cai2+] in response to all agonists except epinephrine when gel-filtered platelets were suspended in media containing 1 mM Ca2+. With epinephrine, a rise in [Cai2+] was indicated by aequorin, but not by quin2; [Cai2+] signals, aggregation, and secretion were suppressed by EGTA. ADP [0.5 microM] produced a rise in [Cai2+] that was registered by both aequorin and quin2 in platelets in Ca2+-containing media; addition of EGTA to the medium raised the threshold concentration of ADP to 5.0 microM for both indicators. Collagen produced progressive concentration-related increases in [Cai2+] and aggregation in aspirin-treated aequorin-loaded platelets. Quin2 failed to indicate a rise in [Cai2+]at lower collagen concentrations with EGTA or aspirin. [Cai2+] response to A23187 and thrombin was reduced by addition of EGTA to platelets loaded with either aequorin or quin2. With all five agonists in all conditions tested, aequorin [Cai2+] signals occurred at the same agonist concentration as that or lower than that which produced platelet shape change, aggregation, or secretion. Platelet activation was better correlated with changes in [Cai2+] indicated by aequorin than with the response of quin2, possibly because aequorin is more sensitive to local zones of [Cai2+] elevation.

Adhesion of human platelets to collagen in the presence of prostacyclin, indomethacin and compound BW 755C

Prostacyclin (1 ng to 2 micrograms per ml), which effectively inhibits platelet secretion and aggregation, does not affect adhesion of a proportion of platelets (10-38%) to collagen (50-100 micrograms/ml). Adhesion is not detectable by changes of light transmission (as measured in the optical aggregometer) and is not affected by inhibitors of cyclooxygenase and lipoxygenase enzymes such as indomethacin and compound BW 755C. This adhesion is independent of the collagen concentration (50-400 micrograms/ml) and the incubation time (5-20 min). This suggests that adhesion to collagen is related to a specific platelet population. Adhesion in the presence of prostacyclin, indomethacin and BW 755C occurs in parallel with the formation of a limited amount of phosphatidic acid. Under those conditions it is also possible to observe some phosphorylation of a 40,000 dalton protein which is a substrate for protein kinase C activity. Phosphorylation of the 20,000 dalton protein, or myosin light chain, is less evident. Chlorpromazine (25-100 micrograms/ml) inhibited the adhesion of platelets to collagen, but propanolol (0.5-4 microM) was inactive. The adhesion of platelets to collagen in these experiments parallels the formation of a fraction of phosphatidic acid and 40,000 dalton protein phosphorylation, which are independent of the increased levels of platelet cyclic-AMP induced by high concentrations of prostacyclin. It is also independent of the formation of cyclooxygenase or lipoxygenase products.

Hypothesis: control of intracellular calcium level

It is proposed that cells store calcium in the hydrogen belt of their membranes, on the cytoplasmic side, with the Ca2+ ion captive in cages formed by the phosphate and carbonyl oxygens of two acidic phospholipid molecules; for instance, phosphatidylinositol and phosphatidylserine. Evidence for the existence of such Ca-cages is adduced from the properties of the [Ca(phosphatidate)2] complex. Cytoplasmic Ca2+ concentration, approx. 10(-7) M, corresponds to the calcium cage dissociation constant. The high stability of the cages is the result of multiple hydrogen bonds between inositol and serine, or inositol and inositol. Phosphorylation of the inositol in position 4 and 5 opens the calcium cage by breaking the inter-headgroup hydrogen bonds and by introducing electrostatic and steric hindrance. This allows the escape of Ca2+ into the cytosol. The mono in equilibrium with di in equilibrium with triphosphoinositide shuttle serves as a regulator of Ca2+ concentration in the cytoplasm: phosphorylation of the lipids will raise, dephosphorylation lower the level of free Ca2+. The inositide shuttle may be linked to a stimulus-induced inositide cycle in which inositol triphosphate is generated, and to Ca(phosphatidate)2 cross-membrane transport.

KF4939, a new anti-platelet agent, inhibits activation of phospholipase C and A2 in rabbit platelets

Effect of KF4939 on activation of phospholipase C and A2 was examined by using rabbit platelets prelabelled with [14C]-arachidonic acid. The results showed that KF4939 inhibited the activation of the two phospholipases, separately. The inhibition of phospholipase C activation is thought to be a possible mechanism of KF4939 to inhibit aggregation and secretion which are independent of the synthesis of cyclooxygenase products such as thromboxane A2.

Alpha 1-adrenoceptor-mediated inositol phospholipid hydrolysis in rat cerebral cortex: relationship between receptor occupancy and response and effects of denervation

The characteristics of catecholamine-mediated breakdown of inositol phospholipids in rat cerebral cortex slices have been examined using a direct assay involving prelabeling with [3H]inositol and examining the production of labelled inositol phosphates in the presence of lithium. Noradrenaline produced a marked stimulation of inositol phosphate accumulation and this response could be potently and competitively antagonised by the alpha 1-adrenoceptor antagonist prazosin. The alpha 2-antagonist yohimbine was almost 1000-fold less potent at antagonising noradrenaline inositol phospholipid response. Noradrenaline and adrenaline were full agonists at alpha 1-adrenoceptors but phenylephrine and methoxamine were only partial agonists in their ability to stimulate inositol phospholipid metabolism. There was a significant correlation between the ability of a variety of agonists and antagonists to activate or inhibit [3H]inositol phosphate accumulation and their ability to displace the alpha 1-adrenoceptor selective ligand [3H]prazosin from specific binding sites when assays were performed on rat cerebral cortical slices under identical conditions. The similarity of EC50 values of agonists stimulating inositol phosphate accumulation and their IC50 values in [3H]prazosin binding experiments suggested a close relationship between receptor occupancy and alpha 1-mediated inositol phosphate accumulation. Further experiments were performed to examine this directly by inactivating alpha 1-adrenoceptors with the alkylating antagonist phenoxybenzamine. After washing out unbound antagonist, [3H]prazosin binding was reduced to a very similar proportion to that observed on the maximal noradrenaline-stimulated accumulation of [3H]inositol phosphates in the slices. The EC50 values for noradrenaline-stimulated inositol phosphate accumulation was unaltered and the affinity of [3H]prazosin for the remaining sites was equally unaffected. In rats treated 14 days previously with i.c.v. 6-hydroxydopamine (2 X 250 micrograms) there was a small increase in alpha 1-adrenoceptor binding sites but a parallel shift to the left in the noradrenaline [3H]inositol phosphate accumulation dose-response curve. On the other hand, the partial agonist phenylephrine induced a larger maximal response in denervated animals without a change in the EC50 values. When slices from 6-hydroxydopamine treated animals were preincubated with phenoxybenzamine, the loss in alpha 1-adrenoceptor binding sites was greater than the reduction in the maximal response to noradrenaline. This may indicate the development of a small receptor reserve after denervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of phosphodiesterase inhibitors on bradykinin-mediated prostaglandin E2 and cyclic AMP synthesis in renal papillary collecting tubule cells

Isolated rabbit renal papillary collecting tubule cells were used to examine the effects of phosphodiesterase inhibitors on intracellular cyclic AMP and prostaglandin synthesis. Experiments performed on confluent primary tissue cultures demonstrated that bradykinin increases intracellular cyclic AMP by a prostaglandin-dependent mechanism. Phosphodiesterase inhibitors induced a dose-dependent decrease in bradykinin-stimulated prostaglandin synthesis. Fifty percent inhibition occurred with approximately 0.7 mM 3-isobutyl-1-methylxanthine (IBMX). Inhibition was found to be reversible. IBMX did not inhibit bradykinin-induced prostaglandin synthesis as a result of increased intracellular cyclic AMP. The nonmethylxanthine phosphodiesterase inhibitor RO 20-1724 also reduced bradykinin-stimulated prostaglandin synthesis. IBMX inhibited calcium-ionophore-A23187-induced prostaglandin synthesis but did not inhibit arachidonic acid stimulation of prostaglandin synthesis. The data demonstrate that bradykinin increased renal papillary collecting tubule cell cyclic AMP in a prostaglandin-dependent manner. Based on the data presented, phosphodiesterase inhibitors act to decrease arachidonic acid availability for prostaglandin synthesis, independent of changes in cellular cyclic AMP content.

Effects of neuronal activity on inositol phospholipid metabolism in the rat autonomic nervous system

The effect of nerve stimulation on inositol phospholipid hydrolysis in autonomic tissue was assessed by direct measurement of [3H]inositol phosphate production in ganglia that had been preincubated with [3H]inositol. Within minutes, stimulation of the preganglionic nerve increased the [3H]inositol phosphate content of the superior cervical sympathetic ganglion indicating increased hydrolysis of inositol phospholipids. This effect was blocked in a low Ca2+, high Mg2+ medium. It was also greatly reduced when nicotinic and muscarinic antagonists were present together in normal medium. However, neither the nicotinic antagonist nor the muscarinic antagonist alone appeared to be as effective as both in combination. In other experiments, stimulation of the vagus nerve caused dramatic increases in [3H]inositol phosphate in the nodose ganglion but did not increase [3H]inositol phosphate in the nerve itself. This effect was insensitive to the cholinergic antagonists. Thus, neuronal activity increased inositol phospholipid hydrolysis in a sympathetic ganglion rich in synapses, as well as in a sensory ganglion that contains few synapses. In the sympathetic ganglion, synaptic stimulation activated inositol phospholipid hydrolysis and this was primarily due to cholinergic transmission; both nicotinic and muscarinic pathways appeared to be involved.

Interdependent effects of the ionophore A23187 and serum on the serotonin N-acetyltransferase activity of cultured chick pineal glands

Marked effects of the ionophore A23187 on the cycle of N-acetyltransferase (NAT) activity in cultured chick pineal glands were observed under three conditions of illumination. However, the effects were qualitatively and quantitatively dependent on the batch of fetal calf serum used in the medium and time of explanation into culture. Ionophore increased the level of NAT activity remaining in glands exposed prematurely to light regardless of the serum used. The ionophore suppressed the "spike" in cyclic GMP content of glands cultured in the dark, and extended the period of maximum cyclic GMP content of glands under diurnal illumination.

Receptor-effector coupling in platelets: roles of guanine nucleotides

Platelet-activating factor (PAF), which is thought to cause platelet aggregation and degranulation via a receptor-mediated activation of phospholipase C, had no direct action on PGE1-stimulated cyclic AMP formation in intact human platelets, although it caused a GTP and Na+-dependent inhibition of the adenylate cyclase activity of human platelet particulate fractions. Studies with PAF analogues indicated that the receptors mediating this inhibition of adenylate cyclase had structural specificity very similar or identical to that of the receptors mediating platelet aggregation. These results suggest that the PAF receptors linked to the activation of phospholipase C in intact platelets may, in membrane preparations, become coupled to the inhibition of adenylate cyclase via the guanine nucleotide-binding protein, Gi. Studies with permeabilized human platelets that secrete 5-HT on addition of low concentrations of Ca2+ showed that addition of either PAF or a guanine nucleotide decreased the Ca2+ required for secretion. When added together, PAF and GTP promoted secretion synergistically at low Ca2+ concentrations. Enhanced secretion of 5-HT was associated with increased formation of diacylglycerol. These results show that PAF can stimulate phospholipase C by both GTP-dependent and independent mechanisms. In intact human platelets, PAF receptors may interact preferentially with a guanine nucleotide-binding protein that promotes phosphoinositide breakdown by phospholipase C, rather than with Gi.

Platelet-activating factor phosphatidate, but not platelet-activating factor, is a powerful calcium ionophore in the human red cell

Platelet-activating factor (1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, PAF) is a potent inducer of shape-change, aggregation and secretion in platelets. PAF causes a rapid increase in intracellular calcium, but has no calcium gating effect in intact lipid bilayers. Human red cells (RBC) did not metabolize either PAF or PAF-phosphatidate (PAF-PA). While PAF (10 microM) was devoid of calcium ionophoretic activity, PAF-PA (1-5 microM) stimulated calcium influx into intact human RBC. In addition, PAF-PA (1-10 microM), but not PAF (10 microM), elicited a series of satellite effects related to the rise of intracellular calcium: 1) increased efflux of intracellular potassium (Gàrdos effect); 2) alkalinization of unbuffered RBC suspensions; 3) stimulation of ATP consumption and production, and enhancement of glycolytic flux with crossover at the glyceraldehyde 3-phosphate dehydrogenase step. These effects exactly duplicate those brought about by the calcium ionophore A23187. The ionophoretic potency of PAF-PA was about half that of A23187. Approximately the same concentrations of PAF-PA as those that stimulate calcium influx into RBC elicit full aggregatory response in human platelets. It is possible that transformation of PAF into PAF-PA by the combined action of phospholipase C and diacylglycerol kinase contributes to the increase of calcium influx in platelets.

Potentiation by thrombin of the secretion of serotonin from permeabilized platelets equilibrated with Ca2+ buffers. Relationship to protein phosphorylation and diacylglycerol formation

After human platelets have been rendered permeable to small molecules by high voltage electric discharges, addition of buffered micromolar concentrations of Ca2+ causes an ATP-dependent secretion of dense granule serotonin [Knight & Scrutton (1980) Thromb. Res. 20, 437-446]. In the present study, platelets permeabilized by this technique were found to show an up to 10-fold increase in their sensitivity to Ca2+ after exposure to thrombin. In permeabilized platelets, as in the intact cells, release of serotonin was associated with the Ca2+-dependent phosphorylation of 47 000 and 20 000 Da polypeptides (P47 and P20). Thrombin markedly increased the phosphorylation of P47 in the presence of 0.1-1.0 microM-Ca2+ free but had a much smaller effect on phosphorylation of P20. Thrombin also stimulated the formation of 1,2-diacylglycerol in the presence of 0.1 microM-Ca2+ free and was even more effective with 1.0 microM-Ca2+ free, suggesting that receptor-activated hydrolysis of phosphoinositides to 1,2-diacylglycerol was preserved in permeabilized platelets and was potentiated by low intracellular concentrations of Ca2+. The increase in phosphorylation of P47 on addition of thrombin may therefore be accounted for by the stimulatory action of 1,2-diacylglycerol on Ca2+-activated, phospholipid-dependent protein kinase. However, in both the presence and absence of thrombin, higher Ca2+ concentrations were required for optimal secretion than for maximal phosphorylation of both P47 and P20, indicating that additional actions of Ca2+ and thrombin, perhaps also mediated by 1,2-diacylglycerol formation, may be involved in the release of serotonin.

Tight coupling of thrombin-induced acid hydrolase secretion and phosphatidate synthesis to receptor occupancy in human platelets

Human platelets incubated with [32P]Pi and [3H]arachidonate were transferred to a Pi-free Tyrode's solution by gel filtration. The labile phosphoryl groups of ATP and ADP as well as Pi in the metabolic pool of these platelets had equal specific radioactivity which was identical to that of[32P]phosphatidate formed during treatment of the cells with thrombin for 5 min. Therefore, the 32P radioactivity of phosphatidate was a true, relative measure for its mass. The thrombin-induced formation of[32P]-phosphatidate had the same time course and dose-response relationships as the concurrent secretion of acid hydrolases. 125I-alpha-Thrombin bound maximally to the platelets within 13s and was rapidly dissociated from the cells by hirudin; readdition of excess 125I-alpha-thrombin caused rapid rebinding of radioligand. This binding-dissociation-rebinding sequence was paralleled by a concerted start-stop-restart of phosphatidate formation and acid hydrolase secretion. [3H]Phosphatidylinositol disappearance was initiated upon binding but little affected by thrombin dissociation and rebinding. ATP deprivation caused similar changes in the time courses for [32P]-phosphatidate formation and acid hydrolase secretion which were different from those of [3H]phosphatidylinositol disappearance. The metabolic stress did not alter the magnitude (15%) of the initial decrease in phosphatidylinositol-4,5-bis[32P]phosphate, but did abolish the subsequent increase of phosphatidylinositol-4,5-bis[32P]-phosphate in the thrombin-treated platelets. It is concluded that in thrombin-treated platelets (1) phosphatidate synthesis, but not phosphatidylinositol disappearance, is tightly coupled to receptor occupancy and acid hydrolase secretion in platelets, (2) successive phosphorylations to phosphatidylinositol-4,5-bisphosphate is unlikely to be the main mechanism for phosphatidylinositol disappearance, and (3) only a small fraction (15%) of phosphatidylinositol-4,5-bisphosphate is susceptible to hydrolysis.

Prostaglandin synthesis linked to phosphatidylinositol turnover in isolated rat glomeruli

Prostaglandins produced by the glomerulus are important factors in controlling glomerular function. The controlling step, i.e., the release of arachidonic acid from the phospholipids by either phospholipase A2 and/or C, remains poorly defined. The present studies were designed to determine which factors control arachidonic acid turnover and prostaglandin synthesis in glomeruli. As tools we used the calcium ionophore A23187, mepacrine, a phospholipase inhibitor, trifluoperazine, a calmodulin antagonist, and angiotensin II. A23187 (2 microM) caused a significant stimulation of both prostaglandin E2 and prostaglandin F2 alpha synthesis (measured by radioimmunoassay), which was associated with increased phosphatidylinositol turnover (measured by [14C]arachidonic acid and [32P]orthophosphate incorporation). Surprisingly, trifluoperazine (10-100 microM) also progressively increased synthesis of both prostaglandins, which was accompanied by increased phosphatidic acid/phosphatidylinositol turnover and decreased phosphatidylinositol content. In contrast, phosphatidylcholine and phosphatidylethanolamine turnover were significantly inhibited by trifluoperazine and their total content remained unaffected. Mepacrine (1 mM) decreased prostaglandin synthesis and both phosphatidylcholine and phosphatidylethanolamine turnover, and had no consistent effect on phosphatidylinositol turnover in control glomeruli. Mepacrine did, however, inhibit both A23187 or trifluoperazine-induced increase in phosphatidylinositol turnover. Angiotensin II increased turnover of phosphatidylinositol and also phosphatidylcholine, as determined by incorporation of [14C]arachidonic acid. Thus, all agents that increased prostaglandin synthesis also enhanced phosphatidylinositol turnover. The exact pathway of arachidonic acid release remains to be determined.

RHC 80267 inhibits thyrotropin-stimulated prostaglandin release from rat thyroid lobes

In the present report, we studied the effect of the diglyceride (DG) lipase inhibitor, RHC 80267 on basal and thyrotropin (TSH)-stimulated prostaglandin (PG) release from rat thyroid lobes Further, we tested the effect of RHC 80267 on phosphatidylinositol phospholipase C (PIPLC), DG lipase, and arachidonate cyclo-oxygenase activities in rat thyroid cytosol, plasma membrane, and whole homogenate preparations, respectively. Whereas RHC 80267 inhibited DG lipase activity in a dose-related manner from 0.5-10 microns (17-80% inhibition), it failed to inhibit either PIPLC or arachidonate cyclo-oxygenase activities by more than 9% when tested at 5 and 10 microns (n = 3). RHC 80267 reduced TSH-stimulated 6-keto-PGF1alpha and PGF2alpha release by 100 +/- 14% and 57 +/- 12%, respectively (means + S.E.; p less than 0.01 for both; n = 10-12); the diglyceride lipase inhibitor did not reduce basal release of either PG. These data provide additional evidence which implicate a PIPLC-DG lipase pathway in TSH-stimulated PG synthesis in thyroid.

Prostacyclin inhibition of phosphatidic acid synthesis in human platelets is not mediated by protein kinase C

The activation of protein kinase C in human platelets by phorbol-12, 13- dibutyrate (PDBu) results in the phosphorylation of a 40,000 dalton protein. This phosphorylation is time- and concentration-dependent. Maximal phosphorylation is rapid and is not affected by indomethacin or prostacyclin. PDBu does not promote activation of the phosphodiesteratic cleavage (phospholipase C) of the inositol phospholipids and the subsequent formation of 1,2-diacylglycerol or its phosphorylated product, phosphatidic acid. If platelets exposed to PDBu are subsequently stimulated with thrombin, this stimulus does not initiate further 40,000 dalton protein phosphorylation but will promote the formation of phosphatidic acid and also the phosphorylation of a 20,000 dalton protein (myosin light chain). However, prostacyclin will prevent the subsequent stimulation of phosphatidic acid synthesis by thrombin in a concentration-dependent manner. The fact that prostacyclin can affect the response to thrombin, even in the presence of phorbol ester, supports the idea that the enzymes related to the formation of phosphatidic acid or inhibition of its synthesis are not related to the phosphorylated 40K protein.

Effect of using calcium-free Krebs' solution on basal and A23187-stimulated prostaglandin output from the Day 15 guinea-pig uterus superfused in vitro

The basal outputs of prostaglandin (PG) F2 alpha and PGE2 from the Day 15 guinea-pig uterus superfused in vitro were unaffected by omitting Ca2+ from the Krebs' solution. In contrast, this omission of Ca2+ reduced the basal output of 6-oxo-PGF1 alpha (which reflects PGI2 production) from the uterus by an average of 50%. Spontaneous and A23187-stimulated contractions of, and the stimulation by A23187 of PGF2 alpha, PGE2 and 6-oxo-PGF1 alpha outputs from the Day 15 guinea-pig uterus were all abolished by superfusing the tissue with Krebs' solution lacking Ca2+. It is concluded that the basal output of 6-oxo-PGF1 alpha, the occurrence of spontaneous contractions, and the effects of A23187 on PG output and contractility of the Day 15 guinea-pig uterus are dependent on extracellular Ca2+. However, the increase in PGF2 alpha output from the guinea-pig uterus on Day 15 compared to days much earlier in the cycle is apparently not dependent upon extracellular Ca2+. The implications of these findings regarding the biochemical mechanisms involved in the increased synthesis of PGF2 alpha (the uterus luteolytic hormone) by the guinea-pig endometrium during the last one-third of the cycle are discussed.

Receptor-mediated metabolism of the phosphoinositides and phosphatidic acid in rat lacrimal acinar cells

The metabolism of the inositol lipids and phosphatidic acid in rat lacrimal acinar cells was investigated. The muscarinic cholinergic agonist methacholine caused a rapid loss of 15% of [32P]phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] and a rapid increase in [32P]phosphatidic acid (PtdA). Chemical measurements indicated that the changes in 32P labelling of these lipids closely resembled changes in their total cellular content. Chelation of extracellular Ca2+ with excess EGTA caused a significant decrease in the PtdA labelling and an apparent loss of PtdIns(4,5)P2 breakdown. The calcium ionophores A23187 and ionomycin provoked a substantial breakdown of [32P]PtdIns(4,5)P2 and phosphatidylinositol 4-phosphate (PtdIns4P); however, a decrease in [32P]PtdA was also observed. Increases in inositol phosphate, inositol bisphosphate and inositol trisphosphate were observed in methacholine-stimulated cells, and this increase was greatly amplified in the presence of 10 mM-LiCl; alpha-adrenergic stimulation also caused a substantial increase in inositol phosphates. A23187 provoked a much smaller increase in the formation of inositol phosphates than did either methacholine or adrenaline. Experiments with excess extracellular EGTA and with a protocol that eliminates intracellular Ca2+ release indicated that the labelling of inositol phosphates was partially dependent on the presence of extracellular Ca2+ and independent of intracellular Ca2+ mobilization. Thus, in the rat lacrimal gland, there appears to be a rapid phospholipase C-mediated breakdown of PtdIns(4,5)P2 and a synthesis of PtdA, in response to activation of receptors that bring about an increase in intracellular Ca2+. The results are consistent with a role for these lipids early in the stimulus-response pathway of the lacrimal acinar cell.

The potentiation of arterial contraction with platelet activating factor

The incubation of porcine carotid arteries with platelet activating factor induces a potentiation of contraction when the artery is subsequently challenged with 100 mM KCl compared with control preparations. This potentiation of arterial contraction is not observed when extracellular Ca2+ influx or prostaglandin metabolism is inhibited.

The role of phosphatidylinositol 4,5 bisphosphate breakdown in cell-surface receptor activation

The activation of Ca2+-mobilising receptors on hepatocytes and many other cells leads to a prompt reduction in the cellular content of inositol phospholipids. The primary event which underlies these changes is, most probably, a phospholipase C-catalysed attack upon phosphatidylinositol 4,5 bisphosphate. The receptor-mediated breakdown of this lipid in stimulated cells is: (i) not mediated by an increase in cytosol [Ca2+] and (ii) closely coupled to receptor occupation. Phosphatidylinositol 4,5 bisphosphate degradation may be studied by measuring the appearance of the water-soluble product, inositol trisphosphate (and its metabolites: inositol bisphosphate and inositol monophosphate), in stimulated cells. Recent evidence indicates that inositol trisphosphate and the lipid soluble product of phosphatidylinositol 4,5 bisphosphate breakdown, 1,2 diacylglycerol, may act as 'second messengers' which mediate the effects of many extracellular signals in stimulated cells.