MATERIALS AND METHODS
Forty-five rabbits of both sex, weighing 2.4±0.6 kg were selected. They were anaesthetized with 3% sodium pentobarbitorium, then the right femoral vein and artery were both cannulated for blood taking and a continuous monitoring of MAP. The changes of heart rates (HR) and respiratory rates (RR) of the rabbits were also carefully observed. Cannulation, being connected with the hemodynamic analysis system via the right carotid artery, leads to left ventricle. The observed cardiac functional parameters includes: left ventricular systolic pressure (Lvsp), the maximum increasing and decreasing rates of left intraventricular pressure (±dp/dtmax). PAF levels were determined by a bioassay method via the release of 5-HT from the stimulated rabbit platelet. Animals grouping: (1) Saline controls (n=5), normal saline intravenous infusion alone. (2)Endotoxin-treated alone group (n=15), endotoxin (purchased from Sigma Co., NO.O₅₅₅ B₅ ) was administrated iv. with a dose of 200μg/kg.bw. (3) Endotoxin combined with BN52021 pretreatment group (n=11), PAF receptor antagonist BN52021 with a dose of 5 mg/kg.bw.ip was injected about 10 minutes before endotoxin treatment. (4) Exogenous PAF-treated alone group (n=8), PAF (purchased from Sigma Co.) was administrated with a dose of 0.1 μg/kg.bw.iv. (5) PAF combined with BN52021 pretreatment group (n=6), being treated the same way as the above groups.

The obtained data were expressed as mean±SD, and were further analyzed using student's t-test. It is significant when P<0.05, and is quite significant when P<0.01.

RESULTS
Changes of RR and HR of the rabbits
An obvious increase of RR and HR could be observed in rabbits after being infused with exogenous endotoxin. The changes of RR showed much more remarkably, which gradually stabilized about 5 hours after infusion with the drugs. Remarkable depression of RR and HR was seen in rabbits before death. BN52021 could markedly improve RR of the rabbits, but had little effect on HR.

Changes of MAP of the rabbits.
MAP of the rabbits decreased remarkably after endotoxin bolus injection, which showed much lower than that of control and baseline (P<0.01).About three hours later, MAP slightly increased but still with a significant difference (P<0.01). After being pretreated with BN52021, MAP levels of rabbits lowered with a relatively small degree (but P<0.05 as compared with group of endotoxin alone). MAP decreased rapidly to its peak value nearly 3-5 minutes after exogenous PAF bolus injection, and gradually returned back to its baseline about 30 minutes later. Such an effect could be completely reversed by the pretreatment of BN 52021 (Table 1).

Table 1 The changes of MAP in Rabbiys with Endotoxemia (kPa)

Changes of cardiovascular function of the rabbits
Both endotoxin and PAF could induce a remarkable decrease of Lvsp (P<0.01 as compared with baseline), which could be obviously reversed by BN52021 pretreatment. However, the depressive effect of PAF on Lvsp lasted only a few minutes, and eventually recovered in about 30 minutes.

Endotoxin could induce a remarkable and long lasting decrease of ±dp/dtmax. While the effect of PAF was very transient, peaking at 3-5 minutes after PAF injection and gradually reversed about 1 hour later. PAF induced a relatively stronger depressive effect on -dp/dtmax. The effects of both endotoxin and PAF could be effectively reversed by BN52021 pretreatment. (Table 2)

Table 2 Changes of Lvsp, ±dp/dtmax in rabbits with endotoxemia

Changes of blood PAF
Circulating PAF increased remarkably (significantly higher than the control) 5-30 minutes right after endotoxin being administrated, and showed an increasing trends as time being prolonged. Pretreatment with BN52021 did not affect the changes of PAF levels. (Table 3).

Table 3 Changes of blood PAF level in rabbits with endotoxemia (ng/ml)

DISCUSSION
Previous researches have evidenced that PAF levels of the blood and tissues increase in endotoxic shock, and that might be obviously improved through reducing the production of PAF by PLA₂ inhibitors (such as chloroquine phosphate and dexamethasone).³ Our experiment revealed that a marked depression of MAP, Lvsp and ±dp/dtmax was induced 5 minutes after endotoxin bolus injection, and these parameters would return to their baseline level about an hour later with an exception of -dp/dtmax. The intoxication effects of endotoxin could last longer time and might eventually turn to worse. The depressive effect of PAF, as a contrast, acted transiently and much more fiercely, lasting no more than an hour with the above items being recovered. BN52021 showed a better protective and preventive effects on the cardiofunctional perturbations caused by either endotoxin or PAF. PAF levels in the blood during endotoxemia increased rapidly and it does not declined until 8 hours later. BN52021, as a PAF receptor antagonist could remarkably reverse the cardiofunctional disorders induced by endotoxin, but has little effect on the increasement of circulating PAF.

It is reported that PAF may cause a dose-dependent hypotension in mouse, rats, rabbits, guinea pigs and dogs. Moreover, the direct cause of death of the animals may probably be the severe hypotension.^1，2 In our experiment, there were three rabbits appeared with severe cardiac functional disorders, MAP was just about 1kPa, Lvsp <4kPa, +dp/dtmax<200kPa/s, and their surviving time was less than an hour. At the mean time, the RR changed rapidly from short of breath to deep depression (might be less than 10 times/min within three minutes after PAF being administrated). Kenzora and his colleagues⁴ have demonstrated that dogs being bolus injected with exogenous PAF may manifest a marked depression of dp/dtmax, an obvious lowering of MAP that accompanied by the increase of systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR). Toyfuku et al.⁵ have also proved that a continuous intravenous injection with endotoxin 1 μg/kg.bw in 30 minutes to a conscious sheep could induce an obvious decrease of left atrium mean pressure (LAMP), MAP and CQ, and a remarkable increase of pulmonary artery mean pressure (PAMP), and SVR. The above observed parameters could be significantly improved by PAF receptor antagonist ONO-6240. Qian et al.⁶ found that various doses of PAF (5-10 ng/kg.bw) for a slowly and continuously intravenous injection (may lasting an hour) to guinea pigs could induce dose-dependent depression of cardiac output and the contractility of myocardium. A single dose of BN52021 would effectively reverse these changes, and this result was in agreement with our experiment. It is also suggested that the effects of PAF mainly acted through increasing the afterload and decreasing the preload, besides its direct negative inotropic effect.⁴ The direct and powerful depressive effects of PAF on heart may be the substantial causes of PAF-induced hypotension and PAF-induced death. The depressive mechanisms of PAF on heart may be related to the released free radicals and other lipid mediators from the aggregated PMNL. Sympathetic nervous system may also participated in the effects of PAF induced cardiac functional disorders.^7～11

The time phases of cardiac functional depression were in accordance with increase of circulating blood PAF levels in endotoxic shock. PAF receptor antagonist BN52021 could effectively improve the cardiovascular functional disorders in the early phases of rabbits with endotoxic shock. The results indicated that PAF might play an important role in the phases of cardiovascular perturbations in endotoxic shock. However, our experiment and others have revealed that there were eicosanoid products and other important inflammatory mediators increased besides the high level of PAF.^8～10 Although PAF receptor antagonist effectively antagonizes PAF, its effect on the other inflammatory mediators such as PGs, LTs and TNF, etc., the interactions between PAF and other inflammatory mediators and the pathophysiologic significance of its interactions in endotoxic shock still need to be further investigated.

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