János Filep

Increased levels of platelet-activating factor in blood following intestinal ischemia in the dog

The possible role of platelet-activating factor (PAF) in superior mesenteric artery occlusion induced circulatory collapse was studied in anesthetized dogs. PAF was measured by platelet aggregation assay. Identity of PAF-like product in blood was ascertained by thin layer chromatography, high pressure liquid chromatography and alkaline treatment. Low amount of PAF was detected in the mesenteric blood under normal conditions, during reperfusion PAF levels were significantly higher. Pretreatment of the animals with BN 52021, a specific PAF receptor antagonist abolished the fall in mean arterial pressure and the rise in hematocrit due to ischemia/reperfusion. These findings suggest that PAF may play an important role in mesenteric ischemia-induced circulatory collapse.

Mechanism of vasopressin-induced platelet aggregation

The mechanism of aggregation induced by arginine vasopressin (AVP) was studied in human platelet rich plasma. AVP--over the range of 1.8-113.6 mU/ml--caused a dose-dependent aggregation with a concomitant stimulation of thromboxane B2 (TXB2) formation. d(CH2)5Tyr (Me)AVP did not by itself affect platelet aggregation or TXB2 release, but completely inhibited the action of AVP. DDAVP up to the concentration of 280 pM/ml had no effect on aggregation. Pretreatment of platelets with verapamil, trifluoroperazine or methylimidazole, a thromboxane synthetase blocker, prevented AVP-induced aggregation and TXB2 release. Neither phenidone in lower concentration nor nordihydroguaiaretic acid inhibited the ability of AVP to induce aggregation and TXB2 release. These results are consistent with the hypothesis that human platelets possess AVP receptor of the calcium-dependent vasopressor (V1) subtype and suggest that AVP-induced platelet aggregation is mediated via thromboxane release.

Prostacyclin mediates antiaggregatory and hypotensive actions of endothelin in anaesthetized beagle dogs.

The effects of endothelin on blood pressure and in vivo aggregation of platelets were studied in anaesthetized beagle dogs. Intravenous administration of endothelin (0.03‐0.3 nmol kg−1) resulted in a dose‐dependent transient hypotension followed by a long‐lasting hypertension and inhibition of platelet aggregation. These changes were accompanied by dose‐dependent elevation of plasma 6‐keto prostaglandin F1α levels. Pretreatment of the animals with acetylsalicylic acid significantly attenuated both the vascular and antiaggregatory responses to endothelin. These data provide evidence for in vivo release of prostacyclin by endothelin in anaesthetized dogs.

Effects of C-reactive protein on human neutrophil granulocytes challenged with N-formyl-methionyl-leucyl-phenylalanine and platelet-activating factor

The effects of C-reactive protein (CRP), the prototypical acute-phase reactant were studied on human polymorphonuclear leukocytes (PMNL) challenged with N-formyl-methionyl-leucyl-phenylalanine (fMLP) and platelet-activating factor (PAF). CRP at 8-64 micrograms/ml concentrations inhibited degranulation and superoxide production by PMNL in time-, and dose-dependent manner and stabilized PMNL membranes against the lytic effect of lysophosphatidylcholine. CRP was also capable of binding PAF and in lesser extent fMLP. Furthermore, CRP, 32 micrograms/ml, diminished specific binding of [3H]-fMLP and [3H]-PAF to PMNL. These findings imply that CRP may play an important protective role during the early phase of acute inflammatory reactions.

Vascular responses to leukotriene B4, C4 and D4 following FPL 55712, indomethacin, saralasin, phentolamine and verapamil in the conscious rat

1 The pressor and vascular permeability effects of leukotrienes B4 (LTB4), C4 and D4 were investigated in conscious unrestrained rats. 2 Leukotrienes C4 and D4 (3.2–51 nmol kg−1 i.v.) caused an acute dose‐dependent elevation of the mean arterial pressure, which was maximal after 2 min and returned to control levels within 14 min. Heart rate was significantly reduced by the higher doses of LTC4 and LTD4. LTB4 (up to a dose of 51 nmol kg−1) was essentially inactive. 3 These effects of LTC4 and LTD4 were abolished by FPL 55712, a putative antagonist of sulphidopeptide leukotrienes and by verapamil, a calcium channel blocker. 4 Indomethacin, phentolamine or saralasin pretreatment failed to modify the pressor response to LTC4 and LTD4. 5 LTC4 and LTD4 furthermore caused an increase in haematocrit values, which was significantly attenuated by FPL 55712, indomethacin and verapamil. 6 The present findings show that the pressor effect of LTC4 and LTD4 is not related to prostanoid release and can be reversed by calcium channel blockade; whereas the effect on vascular permeability seems to require the presence of both cyclo‐oxygenase product(s) and calcium.

C-reactive protein inhibits binding of platelet-activating factor to human platelets

Serum concentration of C-reactive protein (CRP), a prototypical acute-phase protein rises dramatically in response to tissue injury or inflammation. We report here that CRP (1-20 micrograms/ml) inhibited platelet-activating factor (PAF)-induced aggregation of human platelets in time-, and dose-dependent manner. This inhibitory action of CRP was nearly completely removed by treatment with anti CRP antiserum. At higher concentrations (20-100 micrograms/ml), CRP stabilized platelet membrane against the detergent-like effect of beta-deoxy-lysolecithin. Furthermore, CRP (10 micrograms/ml) diminished specific [3H]PAF binding to platelets and displaced previously bound labeled PAF from platelets. These results suggest that by depressing the bioavailability of PAF, CRP may be an important modulator of platelet activation during acute inflammatory reactions.

Dexamethasone-induced gastric mucosal damage in the rat: possible role of platelet-activating factor

1 The aim of the present experiments was to study the possible role of platelet‐activating factor (PAF) in mediating gastric mucosal damage induced by dexamethasone in the rat by measuring gastric tissue levels of PAF during dexamethasone‐treatment and by investigating the effects of specific PAF receptor antagonists on dexamethasone‐induced gastric lesions. PAF‐like bioactivity extracted from the rat glandular stomach was determined by a platelet aggregation assay. 2 Dexamethasone treatment (0.4– 4 mg kg−1, daily for 1– 6 days) produced time‐ and dose‐dependent damage to the glandular mucosa of the stomach as characterized by extensive, uniform hyperaemia with multiple, focal petechiae and erosions. 3 These changes were accompanied by a time‐, and dose‐dependent increase in PAF content of the glandular stomach. Control rat stomach contained small amounts of PAF (0.14 ± 0.04 ng per g wet weight), which increased over 40 fold in response to dexamethasone treatment (4 mg kg−1, daily for 6 consecutive days). The presence of PAF‐like material in the stomach extract was ascertained by thin‐layer chromatography, high performance liquid chromatography and by alkaline hydrolysis. 4 Pretreatment of the animals with one or other of the structurally unrelated PAF receptor antagonists, BN 52021 (10 mg kg−1, i.p.) or BN 50727 (1 mg kg−1, i.p.) significantly reduced dexamethasone‐induced gastric damage. In these animals neither petechiae nor erosions were observed. 5 These observations suggest that PAF is a likely endogenous mediator of glucocorticoid‐induced gastric mucosal damage in the rat.

Significance of platelet-activating factor in mesenteric ischemia-reperfusion

Reperfusion of the ischemic mesenterium is frequently followed by acute circulatory collapse. This review focuses on the possible role of platelet-activating factor (PAF) in ischemia-induced damage. It provides evidence that (i) PAF concentrations are elevated in the mesenteric circulation following temporary ischemia; (ii) administration of exogenous PAF into the superior mesenteric vein mimics many events observed during reperfusion; and (iii) pretreatment of the experimental animals with specific PAF receptor antagonists prevent the circulatory collapse. These findings suggest that PAF may play an important role in the development of circulatory collapse caused by mesenteric ischemia-reperfusion.

Inhibition by calcium channel blockers of the binding of platelet-activating factor to human neutrophil granulocytes

The inhibitory action of calcium channel blockers on platelet-activating factor (PAF)-induced activation of human polymorphonuclear granulocytes (PMNL) and on the binding of [3H]PAF to neutrophils was studied. Verapamil and diltiazem inhibited PAF (10(-8)-10(-15) M)-induced degranulation and superoxide production in a dose-dependent manner, with pA2 values of 5.6 and 6.1 for verapamil and 5.9 and 6.2 for diltiazem, respectively. Both channel blockers inhibited the specific binding of [3H]PAF to PMNL in dose-dependent fashion, with Ki values of 3.9 +/- 0.6 X 10(-5) M and 3.2 +/- 0.4 X 10(-5) M for verapamil and diltiazem, respectively. Scatchard analysis of the binding data revealed that both calcium channel blockers decreased the receptor binding affinity and slightly increased the number of high-affinity PAF receptors, whereas they did not affect the binding affinity and number of low-affinity receptors. These results indicate that calcium channel blockers can inhibit neutrophil responses elicited by PAF by a mechanism other than inhibition of calcium influx and suggest that the PAF receptor may be closely associated with calcium channels.

Platelet-activating factor may mediate dexamethasone-induced gastric damage in the rat

The possible role of platelet-activating factor (PAF) in dexamethasone-induced gastric mucosal damage was studied in rats. PAF was measured by a platelet aggregation assay. The identity of the PAF-like product recovered from gastric tissues was ascertained by thin-layer chromatography and high-pressure liquid chromatography. Low levels of PAF were detected in the normal rat stomach, while in dexamethasone-treated animals PAF levels were significantly higher. Pretreatment of the animals with BN 52021, a specific PAF receptor antagonist, significantly attenuated dexamethasone-induced mucosal injury. These findings suggest that PAF may be a mediator of mucosal damage induced by glucocorticoids.