Platelet activating factor and inflammation in atherogenesis: Targets for drug development

PAF-acetylhydrolase activity and PAF levels in pancreas and plasma of well-fed, diabetic and fasted rat

PAF-AH activity was determined in pancreas homogenates. The enzyme activity was moderately stable upon storage at -20 degrees C. PAF and lyso-PAF were identified in rat pancreas and their concentrations were determined. PAF levels and PAF-AH activity were compared in the pancreatic tissue and plasma of three different groups of animals: well-fed, STZ-induced diabetic and fasted rats. The concentration of PAF in the pancreas of fasted rats was ten fold lower as compared with that of the well-fed or the diabetic animals. The last two groups had similar pancreatic PAF concentration. PAF levels in the plasma of fasted rats were seven fold lower than those of well-fed or diabetic rats, which were found to be similar. The enzyme PAF-AH had the highest activity in the pancreas of well-fed rats. On the contrary, the enzyme seems to be more active in the plasma of fasted as compared with diabetic and well-fed animals.

Platelet-activating factor in cardiovascular stress situations

Since the elucidation of its chemical structure two decades ago, platelet-activating factor (PAF) has emerged as an important mediator of various cardiovascular stress situations. Most notably, PAF was implicated as a key factor in the septic shock syndrome, based on the similarities between endotoxin and PAF biological effects, the elevation of circulating and tissue levels of PAF during endotoxemia, and the protective effect of PAF antagonists in the septic state. In addition, accumulating data suggest the involvement of PAF in the pathophysiological processes associated with ischemia, hemorrhage and trauma, where PAF exerts its effects directly on cells and blood elements or indirectly through interactions with other mediators such as cytokines and prostaglandins. Nevertheless, the relative contribution of PAF to the pathophysiological processes in endotoxemia is still unknown and should await further investigations. The primary aims of this chapter are: to delineate the effects of PAF on the cardiovascular system, to summarize the data which suggest the involvement of PAF in stress situations of the cardiovascular system, and to identify areas where future experimental efforts should be focused.

Differential effect of nicotinic acid derivatives on smooth muscle and endothelial cell proliferation

The pathogenesis of atherosclerosis is a multifactorial process. A possible anti-atherosclerotic drug should therefore interfere with different targets that are important during the development of an atherosclerotic lesion. Two of the early events are the activated migration and proliferation of arterial smooth muscle cells. Here we investigated in several in vivo and in vitro experiments the effect of two nicotinic acid derivatives L44 and L44-0, on smooth muscle cell migration and proliferation. Balloon catheter de-endothelialization was used as an animal model for intimal lesion formation. Migration was subsequently quantified in vitro using the explant outgrowth technique. Subcultured smooth muscle and endothelial cells were used to test the effect of the drugs on proliferation. Time-lapse video microscopy was applied to differentiate between smooth muscle cell migration and proliferation on the level of individual cells. We showed that L44 and L44-0 are very effective in decreasing smooth muscle cell proliferation and migration. Endothelial cell proliferation, important to re-establish endothelial integrity was, however, not affected.

The degradation of platelet-activating factor in serum and its discriminative value in atherosclerotic patients

Platelet-activating factor (PAF) is transformed in vivo rapidly into the biologically inactive lyso-PAF. This reaction as well as lipid parameters were quantified in serum from 40 survivors of myocardial infarction and 36 healthy controls matched for age and body weight. The PAF-degrading capacity was 23% (p less than 0.001) higher in patients compared with the control group. Using the degradation of PAF as an univariate discriminator more than 70% of subjects were classified correctly. This is comparable with the discriminatory value of the best lipid variables, apolipoprotein B and HDL-cholesterol. Statistically significant differences in the degradation of PAF were found also by comparing subgroups which were matched for plasma levels of total cholesterol, VLDL/LDL-cholesterol or apolipoprotein B. The ratio by 48% (p less than 0.0001) in the case group was identified as an additional good discriminator between both groups. In contrast, platelet aggregation tests which were performed in acetylsalicyclic acid treated platelet-rich plasma discriminated poorly between patients and controls.

Plasma from atherosclerotic patients exerts an increased degradation of platelet-activating factor

Platelet-activating factor (PAF) is a naturally occurring phospholipid that exerts diverse biological activities. In the present study the degradation of PAF as well as lipid concentrations were measured both in plasma from 28 patients suffering from peripheral vascular disease and 18 healthy volunteers of comparable age. Beside some changes of the lipoprotein pattern it was also found that the capacity to degrade PAF is significantly elevated in the patient group. In view of this finding the question arises whether there is any link between the degradation of PAF and the development of atherosclerosis.

Biological properties of the antagonist SRI 63-441 in the PAF and endotoxin models of hypotension in the rat and dog

Intravenous administration of platelet-activating factor (PAF) produces dose-dependent hypotension in several species. We have evaluated a recently developed PAF antagonist, SRI 63-441, for its ability to inhibit the hypotensive effect of PAF in the rat and dog. In the rat, 100 ng/kg PAF produced a 38.6 +/- 5.1% decrease in carotid mean arterial pressure (MAP), followed by a 3.2 +/- 0.7 min recovery period for MAP to return to baseline values. SRI 63-441 reduced the hypotension response in the rat, where the ED50 values for inhibition of MAP were 0.16 mg/kg i.v. and 0.19 mg/kg i.v. for the recovery period. Dogs challenged with 1.5 micrograms/kg PAF i.v. demonstrated a 52 +/- 8% decrease in MAP that persisted for at least 15 min. The ED50 for inhibition of MAP by SRI 63-441 was 0.20 mg/kg i.v. Following injection of tritium-labeled SRI 63-441, 56.8 +/- 2.4% of the dose was recovered in the urine and 43.2 +/- 8.9% in the feces in the rats while in dogs 38.7 +/- 5.6% and 60.9 +/- 23.5% of the dose was excreted in the urine and feces, respectively. In the rat model of endotoxin-induced hypotension, SRI 63-441 given 1 min after a 5 mg/kg endotoxin challenge (which produced a 52 +/- 7% decrease in MAP), reversed the systemic effects, with an ED50 of 0.18 mg/kg i.v. The ED50 for reversal 6 min after endotoxin injection was 0.01 mg/kg. These results of inhibition and reversal by SRI 63-441 strongly implicate PAF as a pivotal mediator of hypotension and shock.

Inhibition and reversal of endotoxin-, aggregated IgG- and paf-induced hypotension in the rat by SRI 63-072, a paf receptor antagonist

Platelet activating factor (paf) given intravenously produces systemic hypotension in the rat. Similar effects can be induced using endotoxin or heat-aggregated IgG challenges, which are thought to involve endogenous paf release. Extending this concept, we have examined the ability of the paf antagonist SRI 63-072 to inhibit or reverse systemic hypotension induced with paf, heat-aggregated IgG or endotoxin 0111-B4 in rats. At 100 ng kg-1 paf, there occurred a 38.6 +/- 5.1% decrease in carotid mean arterial pressure (MAP) followed by a 3.2 +/- 0.7 min recovery period (RP) to return to normal pressure values. The ED50 of SRI 63-072 was 0.16 mg kg-1 i.v. (MAP) and 0.25 mg kg-1 (RP) when given 1-5 min before the paf challenge. Endotoxin (15 mg kg-1 i.v.) produced a hypotensive response (54 +/- 8% decrease in MAP) and a corresponding 80% decrease in mesenteric artery blood flow. When given 2-8 min after endotoxin, 1.0 mg kg-1 i.v. SRI 63-072 totally restored blood pressure and artery blood flow. SRI 63-072 similarly reversed heat-aggregated IgG (10 mg kg-1) induced reduction of MAP, with an ED50 of 0.05 mg kg-1 i.v. The observations that SRI 63-072 can inhibit or reverse systemic vascular effects produced from paf and other provocators of endogenous paf release strongly implicates paf as a common final mediator of hypotension and shock. As SRI 63-072 is a competitive receptor antagonist, the hypotensive effects of these provocators appear to be mediated by vascular receptors for paf.