PAF antagonists: different effects on platelets, neutrophils, guinea pig ileum and PAF-induced vasolidation in isolated rat lung

The effects of structurally different PAF receptor blockers were investigated in platelets, neutrophils, guinea pig ileum, rat isolated lung and rat isolated pulmonary artery. PAF caused serotonin release from platelets and a characteristic shape change and adhesion of neutrophils. The antagonists (CV 3988, alprazolam, 48740 RP and Merck-Sharp and Dohme L-652, 731) inhibited platelet serotonin release but not neutrophil shape change adhesion or lysosomal enzyme release. The antagonists in high concentrations (10⁻⁵ −10⁻⁴M) inhibited nonspecifically the PAF-induced (10⁻⁸M) guinea pig ileum contraction, but were ineffective at concentrations which inhibited platelet responses. In the rat lung the compounds, in high concentrations, partially inhibited the low dose PAF-induced pulmonary vasodilation and the high dose PAF induced pulmonary vasoconstriction and edema. Our data indicate that some platelet PAF antagonists may be ineffective in blocking the action of PAF on neutrophils and smooth muscle preparations and suggest either PAF-receptor independent actions of PAF or different classes of PAF receptors.     

Comparative effects of platelet activating factor, leukotriene D4 and histamine on guinea pig trachea, bronchus and lung parenchyma

The myotropic effect of platelet activating factor (PAF), leukotriene D4 (LTD4) and histamine were compared on guinea pig pulmonary tissues. The initial administration of PAF induced a contraction of strips of trachea, bronchus and lung parenchyma. However subsequent injections were characterized by relaxation of trachea and bronchus and a highly reduced (if any) contraction of the parenchyma. The three tissues of the guinea pig respiratory system contracted strongly to leukotriene D4 and histamine. Indomethacin blocked PAF-induced relaxation of the trachea and bronchus and reduced the contraction of the lung parenchyma. The injection of PAF in the pulmonary circulation stimulated the release of substance(s) causing the contraction of the trachea, bronchus and parenchyma. This study suggests that PAF is not a direct agonist of bronchoconstriction.     

Comparative effects of platelet activating factor, leukotriene D4 and histamine on guinea pig trachea, bronchus and lung parenchyma

The myotropic effect of platelet activating factor (PAF), leukotriene D₄ (LTD₄) and histamine were compared on guinea pig pulmonary tissues. The initial administration of PAF induced a contraction of strips of trachea, bronchus and lung parenchyma. However subsequent injections were characterized by relaxation of trachea and bronchus and a highly reduced (if any) contraction of the parenchyma. The three tissues of the guinea pig respiratory system contracted strongly to leukotriene D₄ and histamine. Indomethacin blocked PAF-induced relaxation of the trachea and bronchus and reduced the contraction of the lung parenchyma. The injection of PAF in the pulmonary circulation stimulated the release of substance(s) causing the contraction of the trachea, bronchus and parenchyma. This study suggests that PAF is not a direct agonist of bronchoconstriction.     

A pharmacologic analysis of the action of platelet-activating factor in the induction of hindpaw edema in the rat

Platelet-activating factor (PAF), a phospholipid product of neutrophils, alveolar macrophages, monocytes, and platelets and an important mediator of inflammatory reactions, was studied for its ability to evoke hindpaw edema in the rat. PAF caused edema, peaking at 1 hr and gradually declining over the next 2 hr. The H1 and H2 antihistamines, mepyramine and cimetidine, the serotonin/histamine antagonist, cyproheptadine, and the serotonin antagonist, methysergide, were ineffective in reducing PAF-induced paw edema. Indomethacin, acetylsalicylic acid, and dexamethasone did not inhibit the peak edematous response but significant reduction was noted with only dexamethasone at 3 hr. Prazosin and propranolol did not prevent PAF-induced edema, whereas, yohimbine, phentolamine, rauwolscine, verapamil and theophylline partially inhibited edema. Clonidine and guanfacine did not induce edema when injected into the rat hindpaw. These results suggest that PAF elicits edema at vascular sites of the rat hindpaw which are partially dependent on extracellular Ca2+ movement, are not due to alpha-1 or alpha-2-adrenoreceptor stimulation, histamine, serotonin, or prostaglandin activity, and demonstrates variable sensitivities to agents blocking Ca2+ entry. Inhibition of specific PAF-sensitive receptors await the discovery of specific PAF antagonists.     

A pharmacologic analysis of the action of platelet-activating factor in the induction of hindpaw edema in the rat

Platelet-activating factor (PAF), a phospholipid product of neutrophils, alveolar macrophages, monocytes, and platelets and an important mediator of inflammatory reactions, was studied for its ability to evoke hindpaw edema in the rat. PAF caused edema, peaking at 1 hr and gradually declining over the next 2 hr. The H₁ and H₂ antihistamines, mepyramine and cimetidine, the serotonin/histamine antagonists, cyrpoheptadine, and the serotonin antagonist, methysergide, were ineffective in reducing PAF-induced paw edema. Indomethacin, acetylsalicylic acid, and dexamethasone did not inhibit the peak edematous response but significant reduction was noted with only dexamethasone at 3 hr. Prazosin and propranolol did not prevent PAF-induced edema, whereas, yohimbine, phentolamine, rauwolscine, verapamil and theophylline partially inihibited edema. Clonidine and guanfacine did not induce edema when injected into the rat hindpaw. These results suggest that PAF elicits edema at₂₊ vascular sites of the rat hindpaw which are partially dependent on extracellular Ca movement, are not due to -1 or -2,-adrenoreceptor stimulation, histamine, serotonin, of photaglandin activity, and demonstrates variable sensitivities to agents blocking Ca²⁺ entry. Inhibition of specific PAF-sensitive receptors await the discovery of specific PAF antagonists.     

Pulmonary vascular reactivity: effect of PAF and PAF antagonists.

We investigated the effects of two different platelet-activating factor (PAF) antagonists, SRI 63-441 and WEB 2086, on PAF-, angiotensin II-, and hypoxia-induced vasoconstrictions in isolated rat lungs perfused with a physiological salt solution. Bolus injection of PAF (0.5 micrograms) increased pulmonary arterial and microvascular pressures and caused lung edema. Both SRI 63-441, a PAF-analogue antagonist, and WEB 2086, a thienotriazolodiazepine structurally unrelated to PAF, completely blocked PAF-induced vasoconstriction and lung edema at 10(-5) M. At a lower concentration (10(-6) M), WEB 2086 was more effective than SRI 63-441. WEB 2086 also blocked the pulmonary vasodilation induced by low-dose PAF (15 ng) in blood-perfused lungs preconstricted with hypoxia. SRI 63-441 and CV 3988 (another PAF analogue antagonist), but not WEB 2086, caused acute pulmonary vasoconstriction at 10(-5) M and severe lung edema at a higher concentration (10(-4) M). PAF-induced but not SRI- or CV-induced pulmonary vasoconstriction and edema were inhibited by WEB 2086. In addition, SRI 63-441 potentiated angiotensin II- and hypoxia-induced vasoconstrictions. This effect of SRI 63-441 is not due to PAF receptor blockade because 1) addition of PAF (1.6 nM) to the perfusate likewise potentiated angiotensin II-induced vasoconstriction and 2) WEB 2086 did not cause a similar response. We conclude that both SRI 63-441 and WEB 2086 are effective inhibitors of PAF actions in the rat pulmonary circulation. However, antagonists with structures analogous to PAF (SRI 63-441 and CV 3988) can have significant pulmonary vasoactive side effects.     

L-NAME augments PAF-induced venoconstriction in isolated perfused livers of rat and guinea pig, but not mouse

Platelet-activating factor (PAF), one of vasoconstrictive lipid mediators, is involved in systemic anaphylaxis. On the other hand, nitric oxide (NO) is known to attenuate anaphylactic venoconstriction of the pre-sinusoids in isolated guinea pig and rat livers. However, it is not known whether NO attenuates PAF-induced hepatic venoconstriction. We therefore determined the effects of L-NAME, a NO synthase inhibitor, on PAF-induced venoconstriction in blood- and constant flow-perfused isolated livers of mice, rats and guinea pigs. The sinusoidal pressure was measured by the double occlusion pressure (Pdo), and was used to determine the pre- (Rpre) and post-sinusoidal (Rpost) resistances. PAF (0.01-1 microM) concentration-dependently caused predominant pre-sinusoidal constriction in all livers of three species studied. The guinea pig livers were the most sensitive to PAF, while the mouse livers were the weakest in responsiveness. L-NAME pretreatment selectively increased the basal Rpre in all of three species. L-NAME also significantly augmented the PAF-induced increases in Rpre, but not in Rpost, in rat and guinea pig livers. This augmentation was stronger in rat livers than in guinea pig livers at the high concentration of 0.1 microM PAF. However, L-NAME did not augment PAF-induced venoconstriction in mouse livers. In conclusion, in rat and guinea pig livers, NO may be released selectively from the pre-sinusoids in response to PAF, and then attenuate the PAF-induced pre-sinusoidal constriction. In mouse liver, PAF-induced venoconstriction is weak and not modulated by NO.     

Modulating effects of adenosine on the process of human platelet activation]

The inhibitory effect of adenosine on aggregation of human platelets activated by platelet activating factor (PAF), ADP and serotonin (5-HT) were examined using native platelets from blood of volunteers. Platelet aggregation was determined by Born's method. Effective adenosine concentrations (IC50) which had inhibited platelet aggregation were found to be 0.63 +/- 0.11, 1.47 +/- 0.31 and 0.64 +/- 0.18 microM, respectively. It was shown that 10 microM adenosine inhibited PAF-induced platelet aggregation completely. The same adenosine concentration blocked ADP- and 5-HT-induced aggregation only partially. Adenosine is physiological inhibitor of human platelet aggregation in administration of PAF, ADP and 5-HT. Specific characteristics of adenosine modulating effect on these ligands was elicited.     

(+/-)-trans-2-(3-Methoxy-5-methylsulfonyl-4-propoxyphenyl)-5-(3,4,5- trimethoxyphenyl)tetrahydrofuran (L-659,989), a novel, potent PAF receptor antagonist

The title compound, L-659,989, is a highly potent, competitive, and selective antagonist of the binding of [3H]PAF to its receptors in platelet membranes from rabbits and humans. It exhibits equilibrium inhibition constants for PAF binding of 1.1 nM (rabbit) to 9.0 nM (human), values that are at least 1-2 orders of magnitude lower than those of other PAF antagonists tested. L-659,989 potently inhibits PAF-induced aggregation of rabbit platelets and degranulation of rat (ED50 4.5 nM) and human (ED50 10 nM) neutrophils. L-659,989 inhibits PAF-induced extravasation and lysosomal enzyme release in rats, and is active orally in female rats (ED50 0.2 mg/kg) with an extraordinary oral duration of action of 12 to 16 hours at 1.0 mg/kg p.o.     

Spiramine N-6,一种新的抗血小板和抗血小板-中性粒细胞相互作用的物质

Spiramine N-6属粉花秀线菊植物中提取分离的二十碳二萜生物碱。本实验采用Born,Shen和Hamburger等方法分别观察了spiramine N-6在体外和体内对兔血小板聚集功能的影响。应用荧光分光光度法测定其对血小板5-羟色胺释放反应的作用,同时评价spiramine N-6对激活的血小板与中性粒细胞之间粘附反应的影响。结果表明：spiramine N-6在体外选择性抑制血小板活化因子(PAF)诱导的血小板聚集,并呈量效关系,其IC50=26μmol／L,对花生四烯酸(AA)或腺苷二磷酸(ADP)引起的血小板聚集无明显作用;spiramine N-6静注后明显抑制PAF、AA和ADP诱导的血小板聚集。Spiramine N-6呈浓度依赖性减少AA和PAF引起血小板5-羟色胺的释放,其IC50分别为64．7和33．5μmol／L。Spiramine N-6明显阻抑激活的血小板与中性粒细胞间的粘附率,其IC50为78．6μmol／L。结果提示spiramine N-6作为二十碳二萜生物碱具有较强的抗血小板和阻抑血小板一中性粒细胞相互作用的生物活性。     

Role of platelets in contraction of canine tracheal muscle elicited by PAF in vitro

To elucidate mechanisms of platelet-activating factor (PAF)-induced contraction, we studied the effect of PAF on 203 canine tracheal smooth muscle (TSM) strips from 45 dogs in vitro in the presence and absence of platelets. PAF (10(-11) to 10(-7) M) alone caused no contraction of TSM even in the presence of airway epithelium. In the presence of 2 x 10(5) platelets/microliter, PAF was an extremely potent contractile agonist (threshold 10(-11) M). This response was inhibited by the PAF antagonist, CV-3988 (10(-6) M), and reversed by the serotonin antagonist, methysergide (EC50 = 3.7 +/- 0.79 x 10(-9) M). Neither atropine nor chlorpheniramine (10(-9) to 10(-6) M) attenuated the response to PAF + platelets. In the presence of platelets, 10(-7) M PAF caused an increase in perfusate concentration of serotonin from 0.93 +/- 0.037 x 10(-8) to 1.7 +/- 0.046 x 10(-8) M (P less than 0.001). Tachyphylaxis, previously demonstrated to be irreversible, was shown to be a platelet-dependent phenomenon; contraction could be repeated in the same TSM after addition of fresh platelets. We demonstrate that PAF-induced contraction of canine TSM is caused by the release of cellular intermediates such as serotonin from platelets. We also demonstrate the site of PAF-induced tachyphylaxis in airway smooth muscle contraction.     

Nicotinic-type unitary currents in Helix neurons

1. The platelet aggregation response to several known platelet agonists was evaluated in four Asian elephants. The platelets were highly responsive to stimulation with platelet-activating factor (PAF) and collagen, less responsive to adenosine diphosphate (ADP) and non-responsive to arachidonic acid, serotonin and epinephrine. 2. Arachidonic acid (1 x 10(-4) M), while inducing no aggregation, caused the release of 1248 +/- 1147 pg/ul (mean +/- SD) of thromboxane B2 (TXB2), the stable metabolite of thromboxane A2 from stimulated platelet. The addition of 1 x 10(-4) M ADP to platelets caused suboptimal aggregation and the release of only 25 +/- 10 pg TXB2/microliters. 3. The calcium channel blocker, verapamil, produced a dose-dependent inhibition of PAF-induced but not collagen-induced aggregation. The cyclooxygenase inhibitor, acetylsalicylic acid, produced no inhibition of either collagen- or PAF-induced aggregation.     

Pharmacological effects of oral E6123, a novel PAF antagonist, on biological changes induced by PAF inhalation in guinea pigs.

The effects of a newly synthesized PAF antagonist E6123, (S)-(+)-6-(2-chlorophenyl)-3-cyclopropanecarbonyl-8,11-dimethyl-2, 3,4,5- tetrahydro-8H-pyrido[4',3':4,5]thieno[3,2-f][1,2,4]triazolo [4,3-a][1,4]diazepine, on in vivo inhaled PAF-induced pulmonary changes were investigated. E6123 inhibited PAF inhalation-induced bronchoconstriction in guinea pigs with an ED50 value (p.o.) of 1.3 micrograms/kg which was lower than those of other PAF-antagonists such as WEB2347 (ED50 = 26 micrograms/kg) and Y-24180 (ED50 = 12 micrograms/kg). E6123 significantly inhibited PAF inhalation-induced eosinophil infiltration into the bronchiole and trachea, and bronchial hyperreactivity in guinea pigs after oral administration at 1 and 10 micrograms/kg, respectively. E6123 inhibited the PAF-induced increase in intracellular free calcium ion concentration ([Ca2+]i) in guinea pig eosinophils with an IC50 value of 14 nM. The present results suggest that E6123 may be beneficial for the treatment of asthma, in which PAF is assumed to be involved.     

Dextran sulfate and heparin sulfate inhibit platelet-activating factor-induced pulmonary edema.

We tested the hypothesis that dextran sulfate and heparin sulfate inhibit platelet-activating factor- (PAF) induced pulmonary edema in the isolated perfused guinea pig lung via a charge-dependent mechanism. Dextran sulfate prevented the changes in pulmonary capillary pressure (Ppc, 7.8 +/- 0.9 vs. 14.0 +/- 0.7 cmH2O), lung weight gain (dW, +0.48 +/- 0.29 vs. +8.41 +/- 2.07 g), and pulmonary edema formation or wet-to-dry weight ratio [(W-D)/D, 6.5 +/- 0.3 vs. 13.2 +/- 2.6] occurring 60 min after PAF infusion (10(-11) M) into an isolated lung. The unsulfated form of dextran had no protective effect [Ppc, dW, and (W-D)/D, 11.9 +/- 1.4 cmH2O, +5.33 +/- 2.18 g, and 11.2 +/- 3.2, respectively]. The unrelated anionic compound, heparin sulfate, also inhibited the PAF response [Ppc, dW, and (W-D)/D, 7.0 +/- 0.5 cmH2O, +0.61 +/- 0.32 g, and 6.1 +/- 0.2, respectively], whereas the partially desulfated form of heparin was not effective in inhibiting PAF-induced edema [Ppc, dW, and (W-D)/D, 15.1 +/- 0.7 cmH2O, +6.07 +/- 1.58 g, and 10.0 +/- 1.2, respectively]. When the metachromatic dye crystal violet was used as an indicator of charge interactions, the sulfated compounds interacted with PAF in vitro. The data indicate that PAF-induced pulmonary edema is inhibited by sulfated polysaccharides, possibly via a charge interaction between negatively charged compounds and PAF.     

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.     

Activation of guinea pig peritoneal macrophages by platelet activating factor (PAF) and its agonists

The platelet activating factor (PAF: 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine) and its analogs were examined to determine their effects on guinea pig peritoneal macrophages. PAF activated macrophages, but its effect on macrophages was much weaker than that observed on platelets: the concentration required for 50% maximum activation was 8.5 X 10(-6) M for macrophages and 2.9 X 10(-10) M for platelets. Three PAF agonists, 1-O-octadecyl-2-O-(N,N-dimethylcarbamoyl)-glycero-3-phosphocholine (Compound I), 1-O-octadecyl-2-acetamido-2-deoxy-glycero-3-phosphocholine (Compound II), and 1-O-octadecyl-2-O-methyl-glycero-3-phosphocholine (Compound III), showed higher activity in stimulating macrophage function than PAF. The abilities of these non-metabolizable PAF agonists to activate macrophage paralleled their relative potency to induce platelet activation. The sn-3 enantiomers of PAF and Compound III exhibited activity, while the sn-1 did not. By comparing the activities of derivatives of Compound III, it was shown that the long-chain alkyl-ether group in the glycerol-1 position, a relatively small size of the substituent on the hydroxy group at the sn-2 position, and the choline moiety in the glycerol-3 position must play critical roles in the process of macrophage activation. A specific PAF antagonist, CV3988, which inhibits PAF-induced platelet activation and hypotension, inhibited the activation of macrophages caused by PAF and its agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intrathoracic accumulation of 111In-labeled neutrophils in guinea pigs in response to PAF.

The intrathoracic content of neutrophils, labeled with 111In-oxine has been measured in the anesthetized guinea pig by using an automated isotope-monitoring system. Intravenous infusion of platelet-activating factor (PAF; 5.6, 10, or 18 ng.kg-1.min-1 over 5 min) caused a dose-related abrupt intrathoracic accumulation of neutrophils, which dispersed from the thorax within 20 min. Repetition of this procedure after 1 h gave responses of comparable magnitude and duration. Anti-platelet antiserum pretreatment did not influence the response of neutrophils to PAF. Iloprost infusion (10 ng.kg-1.min-1 over 15 min) did not affect the response of neutrophils to PAF, whereas accumulation of radiolabeled platelets in the lung was totally suppressed by this dose. Intrathoracic accumulation of neutrophils in response to PAF can be considered to be independent of platelet activation.     

Pharmacologic profile of BN 50739, a new PAF antagonist in vitro and in vivo

The effect of a new PAF antagonist BN 50739 was studied on PAF-induced [3H]-serotonin release from washed rabbit platelets in vitro and on PAF-induced hypotension in vivo. BN 50739 competitively inhibited PAF-induced [3H]-serotonin release from the platelets in a dose-dependent manner. In the presence of 4, 10 and 50 nM of BN 50739, the concentration of PAF inducing 50% maximal [3H]-serotonin release from the platelets (EC50) increased from 2.15 nM to 5.10, 45.10 and 900 nM, respectively. The IC50 of BN 50739 for PAF (10 nM) induced [3H]-serotonin release was 3.67 nM. Under the same experimental condition, the IC50s of BN 50726, BN 50730, BN 50741, WEB 2086, SRI 63-441 and BN 52021 were 5.40, 4.61, 6.88, 5.98, 40.90 nM and 14.90 microM, respectively. PAF-induced hypotension in conscious rats was also inhibited dose-dependently by i.p. pretreatment of BN 50739 (3 and 10 mg/kg). PAF-induced hypotension was diminished both in magnitude and duration in rats pretreated with BN 50739. These data taken together indicate that BN 50739 is a most potent PAF antagonist in vitro and in vivo.     

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.     

Immunoreactive calmodulin in supernatant fraction of platelet homogenates from humans, guinea pigs, rats, and rabbits

Immunoreactive Calmodulin (CM) was measured in the supernatant fraction of homogenates of platelets obtained from humans, rats, guinea pigs and rabbits, using a commercial RIA kit. The levels (microgram/g wet weight of platelets) of immunoreactive CM were 6.8 +/- 0.5, 6.9 +/- 0.4, 8.6 +/- 1.8 and 9.7 +/- 3.1 (mean +/- SEM) for rat, human, rabbit and guinea pig, respectively. There was no statistically significant difference in values between these four species. The refractoriness of rat platelets to aggregate to certain agonists such as platelet activating factor (PAF) cannot be explained on differences in amount of immunoreactive CM.