Dual effects of oleic acid on Ca2+ mobilization and protein phosphorylation in human platelets in presence or absence of platelet activating factor.

This laboratory demonstrated earlier that oleic acid inhibited platelet activating factor (PAF)-induced aggregation and serotonin release of rabbit platelets (M. Miwa, C. Hill, R. Kumar, J. Sugatani, M. S. Olson, and D. J. Hanahan, 1987, J. Biol. Chem. 262, 527-530). More recently, we reported that oleic acid caused a decrease in phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2), but did not affect the level of inositol-1,4,5-trisphosphate (IP3), in rabbit platelets (D. Nunez, J. Randon, C. Gandhi, A. Siafaka-Kapadai, M. S. Olson, and D. J. Hanahan, 1990, J. Biol. Chem. 265, 18330-18838). These results suggested that oleic acid did not stimulate phospholipase C. In contrast, PAF induced a decrease in PIP2 and an increase in PIP level and IP3. These effects were shown to be attenuated by oleic acid. In this current study, our experiments show that (a) oleic acid blocked PAF-induced rise in intracellular [Ca2+] (to provide a mechanism in agreement with our previous experiments which showed that oleic acid inhibited PAF-induced IP3 rise in platelets) and (b) oleic acid itself induced a gradual rise in [Ca2+]i, which would provide a mechanism for oleic acid-induced aggregation despite the fact that oleic acid did not cause the production of IP3 (Nunez et al., 1990). Oleic acid, in a dose-dependent manner, was shown to inhibit PAF-induced Ca2+ mobilization from intra- and extracellular sources. The inhibition was closely related to the suppressive effect of oleic acid on PAF-induced aggregation. Furthermore, oleic acid inhibited the PAF-stimulated phosphorylation of the 20- and 40-kDa proteins. At concentrations above 20 microM, oleic acid itself could induce platelet aggregation and Ca2+ mobilization, but the time sequence of these two responses in human platelets was significantly different from those obtained with PAF. Oleic acid alone, at 20 microM, caused a 1.4-fold increase in the cAMP level in platelets which was followed by a decline to a basal value at higher concentrations of this fatty acid. It seemed clear that elevation of adenylate cyclase activity was not associated with free fatty acid inhibition of platelet activation. Interestingly, both PAF and oleic acid added separately to human platelets induced protein-tyrosine phosphorylation, but oleic acid did not cause any inhibition of PAF-induced protein-tyrosine phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neomycin inhibits agonist-stimulated polyphosphoinositide metabolism and responses in human platelets

The effect of neomycin on agonist-induced changes in polyphosphoinositide metabolism was studied in human platelets. Neomycin induced no changes in the 32P-labeled phospholipids of unstimulated cells. Between 1 and 5 mM of neomycin the initial thrombin-induced decrease in [32P]phosphatidylinositol-4,5-bis-phosphate and production of [32P]phosphatidic acid were gradually inhibited. In contrast, the production of [32P]phosphatidylinositol-4-phosphate was increased. The thrombin-induced decrease in mass of phosphatidylinositol was completely inhibited at 5 mM of neomycin. Collagen- and PAF acether-induced changes in platelet phosphoinositide metabolism as well as aggregation and dense granule secretion induced by all three agonists were inhibited by neomycin. The results indicate that neomycin inhibit platelet responses by selective interference with the interconversions and hydrolysis of polyphosphoinositides upon thrombin stimulation.     

Pharmacological actions of Y-24180, a new specific antagonist of platelet activating factor (PAF): II. Interactions with PAF and benzodiazepine receptors

The inhibitory effect of Y-24180, 4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H-t hieno [3,2-f][1,2,4]triazolo [4,3-a][1,4]diazepine, on platelet activating factor (PAF)-induced platelet aggregation and the specific binding of 3H-PAF to platelets was compared with other thienodiazepine derivatives, WEB 2086 and etizolam. Y-24180 inhibited PAF-induced rabbit platelet aggregation in vitro (IC50 3.84 nM), but had little effect on adenosine diphosphate- or arachidonic acid-induced aggregation. WEB 2086 and etizolam also showed an inhibitory effect of PAF-induced aggregation (IC50 values are 456 and 6730 nM, respectively). In PAF-induced human platelet aggregation, Y-24180 (IC50 0.84 nM) was more potent than WEB 2086 (IC50 4.21 nM) and etizolam (IC50 998 nM). Y-24180, WEB 2086 and etizolam displaced 3H-PAF binding from the washed-platelets of rabbits with an IC50 value of 3.50, 9.35 and 29.5 nM, respectively. In rabbits, pretreatment with Y-24180 and WEB 2086 antagonized PAF-induced platelet aggregation dose-dependently. The significant inhibitory effect of Y-24180 (1 mg/kg, p.o.) lasted 72 hr after a single dose oral administration. WEB 2086 (10 mg/kg, p.o.) also antagonized the ex vivo response induced by PAF 1 hr after administration, but no significant effect was observed 3 hr after administration. Y-24180 displaced 3H-diazepam binding from the synaptosomal membranes of rat cerebral cortex with a Ki value of 3.68 microM. The affinity of Y-24180 for benzodiazepine(BZP) receptors was lower than those of WEB 2086 and etizolam and was about 1000 times lower than that for PAF receptors in platelets.     

Pharmacological actions of Y-24180, a new specific antagonist of Platelet Activating Factor (PAF): II. Interactions with PAF and benzodiazepine receptors

The inhibitory effect of Y-24180, 4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine, on platelet activating factor (PAF)-induced platelet aggregation and the specific binding of ³H-PAF to platelets was compared with other thienodiazepine derivatives, WEB 2086 and etizolam. Y-24180 inhibited PAF-induced rabbit platelet aggregation in vitro (IC₅₀ 3.84 nM), but had little effect on adenosine diphosphate- or arachidonic acid-induced aggregation. WEB 2086 and etizolam also showed an inhibitory effect of PAF-induced aggregation (IC₅₀ values are 456 and 6730 nM, respectively). In PAF-induced human platelet aggregation, Y-24180 (IC₅₀ 0.84 nM) was more potent than WEB 2086 (IC₅₀ 4.21 nM) and etizolam (IC₅₀ 998 nM). Y-24180, WEB 2086 and etizolam displaced ³H-PAF binding from the washed-platelets of rabbits with an IC₅₀ value of 3.50, 9.35 and 29.5 nM, respective- ly. In rabbits, pretreatment with Y-24180 and WEB 2086 antagonized PAF-induced platelet aggregation dose-dependently. The significant inhibitory effect of Y-24180(1 mg/kg, p.o.) lasted 72 hr after a single dose oral administration. WEB 2086 (10 mg/kg, p.o.) also antagonized the ex vivo response induced by PAF 1 hr after administration, but no significant effect was observed 3 hr after administration. Y-24180 displaced ³H-diazepam binding from the synaptosomal membranes of rat cerebral cortex with a Ki value of 3.68 μ M. The affinity of Y-24180 for benzodiazepine(BZP) receptors was lower than those of WEB 2086 and etizolam and was about 1000 times lower than that for PAF receptors in platelets.     

Alpha-bulnesene, a novel PAF receptor antagonist isolated from Pogostemon cablin

Alpha-bulnesene is a sesquiterpenoid isolated from the water extract of Pogostemon cablin. It showed a potent and concentration-dependent inhibitory effect on platelet-activating factor (PAF) and arachidonic acid (AA) induced rabbit platelet aggregation. In a radioligand binding assay for the PAF receptor, alpha-bulnesene competitively inhibited [(3)H]PAF binding to the PAF receptor with an IC(50) value of 17.62+/-5.68microM. alpha-Bulnesene also dose-dependently inhibited PAF-induced intracellular Ca(2+) increase in fluo-3/AM-loaded platelets (IC(50) values of 19.62+/-1.32microM). Furthermore, alpha-bulnesene inhibited AA-induced thromboxane B(2) (TXB(2)) formation and prostaglandin E(2) (PGE(2)) formation. These results indicate that the inhibitory effect of alpha-bulnesene on platelet aggregation was due to a dual activity; specifically the chemical blocked PAF-induced intracellular signal transduction and interfered with cyclooxygenase activity, which resulted in a decrease in thromboxane formation. This study is the first to demonstrate that alpha-bulnesene is a PAF receptor antagonist as well as an anti-platelet aggregation agent.     

Mechanisms of interaction of a plasmalogenic analog of platelet-activating factor with human platelets.

The interaction of a plasmalogenic analog of platelet-activating factor (1-O-alk-1;-enyl-2-acetyl-sn-glycero-3-phosphocholine; 1-alkenyl-PAF) with human platelets was studied. 1-Alkenyl-PAF induced an increase in intracellular Ca2+ concentration and inhibition of adenylate cyclase at significantly higher concentrations than PAF. 1-Alkenyl-PAF inhibits PAF-induced platelet aggregation but has no effect on ADP- or thrombin-induced aggregation of human platelets. In contrast to PAF, 1-alkenyl-PAF increases [3H]PGE1 binding with human platelets. The properties of 1-alkenyl-PAF as an agonist or antagonist of PAF receptors apparently depend on its concentration in the cell medium. Under physiological conditions 1-alkenyl-PAF might be a natural PAF antagonist acting in the human cardiovascular system.     

Identification of a second putative receptor of platelet-activating factor from human polymorphonuclear leukocytes

Due to multiple molecular species of platelet-activating factor (PAF) and the existence of high affinity binding sites in a variety of cells and tissues, possible existence of PAF receptor subtypes has been suggested. This report shows differences between specific PAF receptors in human leukocytes and platelets. Human polymorphonuclear leukocyte membranes showed high affinity binding sites for PAF with an equilibrium dissociation constant (KD) of 4.4 (+/- 0.3) x 10(-10) M. We compared the relative potencies of several PAF agonists and receptor antagonists between human platelet and human leukocyte membranes. One receptor antagonist (Ono-6240) was found to be 6-10 times less potent in inhibiting the specific [3H]PAF receptor binding, PAF-induced GTPase activity, as well as the PAF-induced aggregation in human leukocytes than in human platelets. Mg2+, Ca2+, and K+ ions potentiated the specific [3H]PAF binding in both systems. Na+ and Li+ ions inhibited the specific [3H]PAF binding to human platelets but showed no effects in human leukocytes. K+ ions decreased the Mg2+-potentiated [3H]PAF binding in human leukocytes but showed no effects in human platelets. PAF stimulates the hydrolysis of [gamma-32P] GTP with an ED50 of about 1 nM, whereas the biological inactive enantiomer shows no activity even at 10 microM in both human platelets and human leukocytes. The PAF-stimulated GTPase in human leukocytes can be abolished by the pretreatment of membranes with pertussis toxin and cholera toxin. However, the PAF-stimulated activity of GTPase in human platelets is insensitive to pertussis toxin and cholera toxin. These results suggest that there exists a second type of PAF receptor in human polymorphonuclear leukocytes, which is structurally different from the one characterized in human platelets, and that the guanine nucleotide-binding protein coupled to PAF receptors in human leukocytes is also different from the one in human platelets.     

Repair of potentially lethal and sublethal damage induced by neocarzinostatin in normal and ataxia-telangiectasia skin fibroblasts

Changes in 32P labeling of phosphatidylinositol-4,5-bisphosphate (PIP2) were examined during ADP-induced aggregation of washed rabbit platelets prelabeled with [32P]phosphate. ADP caused a significant decrease in the amount and 32P labeling of PIP2 at 10 and 60 sec. The decrease in labeling persisted at 2.5 min when the platelets were still aggregated, but disappeared by 10 min. Platelets refractory to ADP showed no further significant change in 32P in PIP2 when exposed to ADP; a decrease in PIP2 labeling could be induced, however, after platelets had recovered their disc shape and sensitivity to ADP. These data indicate that PIP2 may play a role in the response of platelets to ADP.     

Inhibition of [3H]platelet activating factor (PAF) binding by Zn2+: a possible explanation for its specific PAF antiaggregating effects in human platelets

Zinc ions in the micromolar range exhibited a strong inhibitory activity toward platelet activating factor (PAF)-induced human washed platelet activation, if added prior to this lipid chemical mediator. The concentration of Zn2+ required for 50% inhibition of aggregation (IC50) was inversely proportional to the concentration of PAF present. The IC50 values (in microM) for Zn2+ were 8.8 +/- 3.9, 27 +/- 5.8, and 34 +/- 1.7 against 2, 5, and 10 nM PAF, respectively (n = 3-6). Zn2+ exhibited comparable inhibitory effects on [3H]serotonin secretion and the IC50 values (in microM) were 10 +/- 1.2, 18 +/- 3.5, and 35 +/- 0.0 against 2, 5, and 10 nM PAF, respectively (n = 3). Under the same experimental conditions, aggregation and serotonin secretion induced by ADP (5 microM), arachidonic acid (3.3 microM), or thrombin (0.05 U/ml) were not inhibited. Introduction of Zn2+ within 0-2 min after PAF addition not only blocked further platelet aggregation and [3H]serotonin secretion but also caused reversal of aggregation. Analysis of [3H]PAF binding to platelets showed that Zn2+ as well as unlabeled PAF prevented the specific binding of [3H]PAF. The inhibition of [3H]PAF specific binding was proportional to the concentration of Zn2+ and the IC50 value was 18 +/- 2 microM against 1 nM [3H]PAF (n = 3). Other cations, such as Cd2+, Cu2+, and La3+, were ineffective as inhibitors of PAF at concentrations where Zn2+ showed its maximal effects. However, Cd2+ and Cu2+ at high concentrations exhibited a significant inhibition of the aggregation induced by 10 nM PAF with IC50 values being five- and sevenfold higher, respectively, than the IC50 for Zn2+, and with the IC50 values for inhibition of binding of 1 nM [3H]PAF being 5 and 19 times higher, respectively, than the IC50 for Zn2+. The specific inhibition of PAF-induced platelet activation and PAF binding to platelets suggested strongly that Zn2+ interacted with the functional receptor site of PAF or at a contiguous site.     

Collagen-induced binding to human platelets of platelet-derived growth factor leading to inhibition of P43 and P20 phosphorylation

Platelet-derived growth factor (PDGF) is known to inhibit collagen-induced platelet aggregation. Collagen-induced binding of 125I-PDGF to human washed platelets was therefore investigated. It was found 1) to be time-dependent, reaching a plateau at 20 degrees C after 30 min, 2) collagen concentration-dependent, 3) specifically inhibited by unlabeled PDGF, and 4) saturable. Scatchard plot analysis showed a single class of sites with 3000 +/- 450 molecules bound/cell and an apparent KD of 1.2 +/- 0.2 10(-8) M. The effects of PDGF on collagen-induced phosphoinositide breakdown and protein phosphorylation were also investigated. At 50 ng/ml PDGF, a concentration which completely inhibited collagen-induced aggregation, the breakdown of [32P]phosphatidylinositol 4,5-biphosphate (PIP2) and [32P]phosphatidylinositol 4-phosphate (PIP) was observed, but the subsequent replenishment of [32P]PIP2 was inhibited. The same PDGF concentration totally inhibited collagen-induced phosphatidic acid formation. PDGF also completely prevented phosphorylation of P43 and P20, as a result of protein kinase C activation consecutive to phosphoinositide metabolism. These results suggest that (i) a specific PDGF receptor can be induced by collagen, and (ii) PDGF can effect the early events of collagen-induced platelet activation by inhibiting PIP2 resynthesis and P43 and P20 phosphorylation. It is concluded that PDGF might be involved in a negative feed-back control of platelet activation.     

CV-3988 - a specific antagonist of platelet activating factor (PAF)

CV-3988, rac-3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl 2-thiazolioethyl phosphate was shown to be a specific inhibitor of platelet activating factor (PAF). This compound in concentrations of 3 x 10(-6) to 3 x 10(-5)M inhibited aggregation of rabbit platelets induced by PAF (3 x 10(-8)M), while it had no effect on the aggregation induced by arachidonic acid, ADP, collagen or A-23187. CV-3988 alone even at a concentration of 10(-3)M had no effect on platelet aggregation. The inhibitory action of CV-3988 on the PAF-induced aggregation was independent of the formation of micelles. The PAF (0.1 to 1.0 micrograms/kg, i.v.)-induced hypotension in anesthetized rats was also inhibited dose-dependently by the i.v. administration of CV-3988 (1 and 10 mg/kg), while the hypotensive actions induced by the i.v. administration of acetylcholine (1 micrograms/kg), arachidonic acid (1 mg/kg), bradykinin (10 micrograms/kg), isoproterenol (1 microgram/kg) and histamine (100 micrograms/kg) were not altered by CV-3988 (10 mg/kg, i.v.). All these findings indicate that CV-3988 specifically inhibits the action of PAF in vitro and in vivo. This is the first report of a PAF antagonist which can specifically inhibit the PAF-induced hypotension as well as the PAF-induced platelet aggregation.     

Involvement of phosphoinositide metabolism in potentiation by adrenaline of ADP-induced aggregation of rabbit platelets.

Changes in phosphoinositide metabolism were examined in washed rabbit platelets stimulated with 0.5 microM-ADP, 50 microM-adrenaline, or ADP and adrenaline in combination. Adrenaline does not stimulate platelet aggregation when used alone, but does potentiate aggregation stimulated by ADP. In platelets prelabelled with [32P]Pi and [3H]glycerol, adrenaline was found to potentiate the ADP-induced changes in platelet phospholipids, causing larger increases in the amount and labelling of phosphatidylinositol 4-phosphate (PIP) and phosphatidic acid than was observed with ADP alone. The combination of ADP and adrenaline did not produce a greater decrease in phosphatidylinositol 4,5-bisphosphate (PIP2) than was produced by ADP alone. In platelets prelabelled with [3H]inositol, adrenaline potentiated the increases in labelling of inositol phosphate and inositol bisphosphate stimulated by ADP; no increase in inositol trisphosphate labelling was detected with ADP alone or with the combination of ADP and adrenaline. Phentolamine, an alpha-adrenergic-receptor antagonist, blocked potentiation by adrenaline of ADP-induced changes in phosphoinositide metabolism. Propranolol and sotalol, beta-adrenergic-receptor antagonists, augmented the potentiation; this is consistent with the concept that the effect of adrenaline is mediated by beta-adrenergic receptors. The effect of adrenaline on phosphoinositide metabolism appears to be to potentiate the mechanisms by which ADP causes turnover of PIP and possibly degradation of PI, rather than the mechanism by which PIP2 is decreased.     

The benzodiazepine receptor ligands RO 5-4864 and RO 15-1788 do not block the inhibition of PAF-induced platelet aggregation seen with the hetrazepine WEB2086

The hypothesis was tested that the hetrazepine WEB 2086 acts as an inhibitor of PAF-induced platelet aggregation via interaction with the platelet benzodiazepine receptor(BDZR). WEB 2086 is a potent inhibitor of rabbit platelet aggregation and ATP secretion induced by 370 nM PAF. The two BDZR ligands RO 5-4864 and RO 15-1788 (7-96 microM) are inactive as PAF antagonists. When platelets were pretreated with either BDZR ligand, and then exposed to various concentrations of WEB 2086, there was no alteration of the dose-response relationship of the hetrazepine on PAF-induced aggregation, as reflected by threshold concentration, ED50, or maximum inhibition seen with WEB 2086. Pretreatment of platelets with the BDZR ligands also failed to block the inhibitory action of WEB 2086 on PAF-induced ATP release. The data are consistent with the notion that WEB 2086 acts as a PAF antagonist through its action at a specific PAF receptor, and is dissociated from, and independent of, interaction with the benzodiazepine receptor.     

Synergism between thrombin and adrenaline (epinephrine) in human platelets. Marked potentiation of inositol phospholipid metabolism.

We have studied synergism between adrenaline (epinephrine) and low concentrations of thrombin in gel-filtered human platelets prelabelled with [32P]Pi. Suspensions of platelets, which did not contain added fibrinogen, were incubated at 37 degrees C to measure changes in the levels of 32P-labelled phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 4-phosphate (PIP) and phosphatidate (PA), aggregation and dense-granule secretion after stimulation. Adrenaline alone (3.5-4.0 microM) did not cause a change in any parameter (phosphoinositide metabolism, aggregation and dense-granule secretion), but markedly enhanced the thrombin-induced responses over a narrow range of thrombin concentrations (0.03-0.08 units/ml). The thrombin-induced hydrolysis of inositol phospholipids by phospholipase C, which was measured as the formation of [32P]PA, was potentiated by adrenaline, as was the increase in the levels of [32P]PIP2 and [32P]PIP. The presence of adrenaline caused a shift to the left for the thrombin-induced changes in the phosphoinositide metabolism, without affecting the maximal levels of 32P-labelled compounds obtained. A similar shift by adrenaline in the dose-response relationship was previously demonstrated for thrombin-induced aggregation and dense-granule secretion. Also, the narrow range of concentrations of thrombin over which adrenaline potentiates thrombin-induced platelet responses is the same for changes in phosphoinositide metabolism and physiological responses (aggregation and dense-granule secretion). Our observations clearly indicate that adrenaline directly or indirectly influences thrombin-induced changes in phosphoinositide metabolism.     

Wortmannin对凝血酶诱导的人血小板聚集和磷脂酰肌醇三磷酸累积的影响

 Ｗ ｏｒｔｍ ａｎｎｉｎ 是肌醇磷脂 ３ 激酶的不可逆抑制剂．用比浊法分析血小板聚集;肌醇磷脂用３２ Ｐ 磷酸钠标记,用氯仿和甲醇抽提,用 Ｔ Ｌ Ｃ和放射自显影分析,研究了 Ｗ ｏｒｔｍ ａｎｎｉｎ 对凝血酶诱导的人血小板聚集和磷脂酰肌醇三磷酸（ Ｐ Ｉ Ｐ３）累积的影响．结果显示, Ｗ ｏｒｔｍ ａｎｎｉｎ 对凝血酶（５００ Ｕ／ Ｌ）诱导的人血小板聚集有抑制作用,这种抑制作用在一定范围内呈剂量依赖关系（２０～８０μｍ ｏｌ／ Ｌ）．凝血酶（５００ Ｕ／ Ｌ）诱导人血小板 Ｐ Ｉ Ｐ３ 的累积, Ｗ ｏｒｔｍ ａｎｎｉｎ 对此累积有抑制作用,这种抑制作用在一定范围内呈剂量依赖关系（４０～１６０ μｍ ｏｌ／ Ｌ）．结果提示： Ｗ ｏｒｔｍ ａｎｎｉｎ 可能是潜在的抗血小板药物,抑制凝血酶诱导的人血小板聚集主要与其抑制 Ｐ Ｉ Ｐ３ 的累积有关．结果也提示,肌醇磷脂 ３ 激酶在血小板活化中起重要作用．     

VIP elevates platelet cyclic AMP (cAMP) levels and inhibits in vitro platelet activation induced by platelet-activating factor (PAF)

Platelet-activating factor (PAF), a potent endogenous phospholipid released by a variety of mammalian cells, induces platelet activation in vivo and in vitro. Little is known, however, about the physiological modulation of its actions. We have examined the ability of two naturally occurring compounds which stimulate cAMP production, vasoactive intestinal peptide (VIP) and prostacyclin (PGI2), to inhibit PAF-induced platelet aggregation and secretion in vitro. Washed, [3H]serotonin-labeled, rabbit platelets were incubated 60 sec in the presence of VIP, PGI2 or 3-isobutyl-1-methylxanthine (IBMX) and subsequently stimulated with PAF. In separate studies, cAMP levels were determined in similar aliquots of platelets incubated for 30 sec with VIP, PGI2 or IBMX. VIP, PGI2 and IBMX inhibited platelet aggregation and secretion in a dose-dependent manner. Fifty percent inhibition was achieved at final concentrations of 1.7 X 10(6) M VIP, 3.6 X 10(6) M PGI2 and 6.5 X 10(5) M IBMX. IBMX potentiated the inhibitory effects of VIP and PGI2 on PAF-induced platelet activation. VIP and PGI2 elevated platelet cAMP levels four-fold and 50-fold, respectively, in the presence of 10(3) M IBMX. These findings demonstrate that VIP inhibits PAF-induced platelet activation, with a potency comparable to that of PGI2.     

Reversible shape change in platelets is regulated by phosphatidylinositol metabolism

Reversible platelet activation was studied after mechanical activation by low speed centrifugation (600 xg). Immediately following centrifugation platelets exhibited no shape change response to low doses of thrombin, collagen and ADP. After incubation at 37 degrees C a time-dependent recovery of the shape change response was observed. This was accompanied by a 50% decrease of 32P-incorporation into phosphatidic acid (PA) phosphatidylinositol-monophosphate (PIP), but not PIP2, relative to levels observed immediately after centrifugation. After 60 minutes platelet relaxation was complete: [32]PA and [32]PIP reached lowest levels and the shape change response to low doses of thrombin was completely restored.     

Platelet phosphoinositide turnover in streptozotocin-induced diabetes

Increased platelet aggregation and secretion in response to various agonists has been described in both diabetic humans and animals. Alterations in the platelet membrane fatty acid composition of phospholipids and changes in the prostacyclin and thromboxane formation could only partly explain the altered platelet function in diabetes. In the present study, we have examined the role of phosphoinositide turnover in the diabetic platelet function. We report alterations in 2-[³H] myo-inositol uptake, phosphoinositide turnover, inositol phosphate and diacylglycerol (DAG) formation, phosphoinositide mass, and phospholipase C activity in platelets obtained from streptozotocin (STZ)-induced diabetic rats. There was a significant increase in the 2-[³H) myo-inositol uptake in washed platelets from diabetic rats. Basal incorporation of 2-[³H] myo-inositol into phosphatidylinositol 4,5-bisphosphate (PIP₂), phosphatidylinositol 4-phosphate (PIP) or phosphatidylinositol (PI) in platelets obtained from diabetic rats was, however, not affected. Thrombin stimulation of platelets from diabetic rats induced an increase in the hydrolysis of [³²P]PIP₂ but indicated no change in the hydrolysis of [³²P]PIP and [³²P]PI as compared to their basal levels. Thrombin-induced formation of [³H]inositol phosphates was significantly increased in both diabetic as well as in control platelets as compared to their basal levels. This formation of [³H]inositol phosphates in diabetic platelets was greater than controls at all time intervals studied. Similarly, there was an increase in the release of DAG after thrombin stimulation in the diabetic platelets. Based on these results, we conclude that there is an increase in the transport of myoinositol across the diabetic platelet membrane and this feature, along with alterations in the hydrolysis of PIP₂, inositol phosphates and DAG in the diabetic platelets, may play a role in increased phosphoinositide turnover which could explain the altered platelet function in STZ-induced diabetes.     

Mechanisms of platelet activating factor-induced aggregation and secretion in human platelets

The role of TXA2 in PAF-induced aggregation and secretion of human platelets is unclear. We have studied the relationship between aggregation, synthesis of TXA2 and release of 5-HT during the time course of aggregation induced by PAF and collagen. For PAF-induced aggregation there was strong aggregation and secretion with minimal production of TXA2 in contrast to collagen in which a surge in TXA2 synthesis preceded both aggregation and secretion. To determine the role of calcium flux in PAF-induced aggregation we have similarly studied the temporal relationships between aggregation, secretion and TXA2 synthesis for calcium ionophore A23187 induced aggregation but found these to be distinctly different from those determined for PAF. A method for measuring absolute amounts of 5HT released from platelets in small volumes of plasma is described. We conclude that TXA2 is not important in the mechanism of PAF induced aggregation and that an increase in the level of intraplatelet calcium per se is not sufficient to explain the mediation of PAF-induced aggregation.     

Human platelet electrorotation change induced by activation: inducer specificity and correlation to serotonin release

Electrorotation of single platelets was compared with [14C]serotonin release, aggregation and electron microscopy. Activation of washed and degranulated platelets was induced by thrombin, arachidonic acid, collagen, adrenaline, platelet activation factor (PAF), ADP and A23187. A strong correlation between electrorotation decrease and serotonin release was found. Electrorotation did not correlate with aggregation. It was concluded that an increase of the specific conductivity of the platelet membrane by three orders of magnitude (approx. 1.0.10(-7) S.m-1 to 1.0.10(-4) S.m-1) upon activation was responsible for the observed decrease of anti-field rotation and the shift of the first characteristic frequency towards higher values. Electrorotation allowed for time-dependent measurements of activation. Characteristic activation times in the order of minutes were found. There was the following sequence of activators classified by increasing activation time constants: A23187 was the fastest followed by thrombin, collagen, PAF, arachidonic acid, adrenaline, and ADP.