L-platelet activating factor induces changes on renal vascular resistance, vascular reactivity, and renin release in the isolated perfused rat kidney

The pathophysiological effects of Russell's viper (Daboia siamensis) venom and its fractions in the isolated perfused rabbit kidney model: A potential role for platelet activating factor

The pathophysiological effects of Russell's viper venom (RVV) and its fractions, including phospholipase A₂ (RvPLA₂), metalloprotease (RvMP), L-amino acid oxidase (RvLAAO), and phosphodiesterase (RvPDE) on renal functions were investigated using the isolated perfused rabbit kidney (IPK) model. Moreover, whether their effects on renal alterations were promoted by platelet activating factor (PAF) was tested using the PAF receptor antagonist, WEB 2086. There was a marked reduction in the perfusion pressure (PP) and renal vascular resistance (RVR) 10 min after RVV administration (1.0 mg/100 ml of perfusate), thereafter both PP and RVR gradually increased and approached the control level within 90 min. These effects were abolished by pretreatment with WEB2086 (2 μg/μl). Administration with RvPLA₂ (280 μg/ml), RvMP (280 μg/ml), or RvLAAO (135 μg/ml) alone increased both the PP and RVR, whereas RvPDE (100 μg/ml) reduced both the PP and RVR. Pretreatment with WEB 2086 completely abolished the effects induced by RvMP, but not the other fractions. The RVV also caused a marked decrease in the glomerular filtration rate (GFR), urinary flow rate (UF), and osmolar clearance (Cosm), and these effects were not inhibited by pretreatment with WEB2086. Each RVV fraction also increased, to varying extents, the GFR, UF, and Cosm, and these effects induced by RvPLA₂ or RvMP, but not the other fractions, were completely blocked by WEB 2086. Changes in percent filtered Na⁺ and K⁺ excreted in the IPK by RVV, RvPDE, and RvMP were abolished by pretreatment with WEB 2086. Histological evaluation profiled mainly tubulonephrosis in the treated kidney. These results reveal that the alterations in renal functions induced by RVV and its fractions are due to the synergistic action of the different components of snake venom, instead of the action of a single component. The effects of RVV and its fractions in rabbit IPK are mediated at least in part by PAF.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Potential role of platelet activating factor in acute renal failure

The clinical condition of acute renal failure (ARF) can be caused by a diverse number of renal injuries, but it is generally characterized by a sharp reduction in the glomerular filtration rate (GFR). A lipid mediator, platelet activating factor (PAF), may be one of the entities responsible for causing the hemodynamic changes in the ARF kidney because it can act as a vasodilator or vasoconstrictor, depending upon its concentration. This review examines the action and mechanisms of PAF in experimental animal models of ischemia and nephrotoxicity, as well as renal failure associated with extrarenal disease. While further research is necessary before extrapolating our current knowledge of PAF into the prevention of renal failure of therapeutic intervention using PAF antagonists in human ARF, there is reasonable evidence to support its role as a mediator of the decrease in GFR characteristic of ARF.

Plasma and urinary platelet activating factor concentrations and leukotriene releasing activity of leukocytes in steroid sensitive nephrotic syndrome of childhood

Platelet activating factor (PAF) is synthesized and secreted by glomerular mesangial and endothelial cells. It increases glomerular basement membrane permeability and induces proteinuria. Leukotrienes (LT) are mediators released by either leukocytes or glomerular cells under the PAF effect. The possible role of PAF in steroid sensitive nephrotic syndrome (SSNS) of childhood was studied in 8 children with SSNS in the acute stage, 5 children in remission and 8 healthy controls. The PAF concentrations in urine and plasma were determined. Leukocytes were stimulated in vitro and the LT release in response to stimulation was determined. The urinary and plasma concentrations of PAF were significantly higher in the acute phase than in remission and in control patients. Children with SSNS were found to have peripheral leukocytes with increased LT releasing activity in vitro. These results are in accordance with clinical and experimental observations indicating that PAF originates in the kidney and plays a role in normal kidney physiology. Urinary PAF concentrations may be related to proteinuria because they were strongly correlated in the present study. Elevated plasma PAF concentrations in the acute stage of SSNS could result from either its secretion from the circulating leukocytes or decreased acetyl hidrolase activity needed for its hydrolysis in plasma. The increased LT release in vitro suggests that these cells might have been activated by PAF secreted from glomeruli. It is proposed that PAF and different LT in systemic and glomerular circulation are important mediators in childhood SSNS.

Effect of platelet-activating factor on hepatic capillary pressure in isolated dog liver

We determined the effects of platelet-activating factor (PAF), a potent vasoactive autacoid phospholipid, on the capillary pressure and liver weight (Wt) in isolated canine livers perfused with blood bivascularly via the portal vein and hepatic artery. PAF (0.01-33 microg) administered intraportally produced dose-dependent increases in the hepatic capillary pressure, as assessed by triple vascular occlusion pressure (Pto), and Wt. An intraportal injection of 10 microg PAF produced increases in Pto by 10 mmHg and Wt by 35 g/100 g liver weight. This hepatic vasoconstriction was attributed to a threefold increase in the portal vein resistance and a fourfold increase in the hepatic vein resistance. The hepatic arterial resistance was not changed when PAF was arterially or intraportally injected. In conclusion, in isolated perfused dog livers, PAF increases the hepatic capillary pressure and liver weight due to contraction of both the portal vein and hepatic vein, but not the hepatic artery.

Renal vascular and sympathetic nerve responses to hypotension induced by platelet-activating factor in anesthetized dogs

This experiment was designed to determine renal sympathetic and renal vascular responses to platelet-activating factor (PAF)-induced hypotension in anesthetized dogs with and without systemic baroreceptor denervation. The left kidney was perfused at a constant flow, and renal perfusion pressure and efferent left renal sympathetic nerve activity were measured simultaneously. Intrarenal injection of PAF (1.25-5.0 x 10(-2) micrograms/kg, n = 6) produced a dose-dependent increase in renal perfusion pressure without any change in systemic blood pressure. An intravenous injection of PAF (10 micrograms/kg) to intact animals (n = 7) caused an initial increase in renal nerve activity (157 +/- 14%) followed by a gradual reduction below baseline (72 +/- 7%) with concomitant systemic hypotension (from 116 +/- 7 to 46 +/- 6 mmHg). Renal perfusion pressure increased significantly from 84 +/- 2 to 161 +/- 33 mmHg concomitant with an increase in renal nerve activity at 1 min and was maintained at this elevated level throughout the experiment. Similar responses of renal nerve activity and renal perfusion pressure were found in animals with complete systemic baroreceptor denervation (n = 7). These results suggest that renal vascular response during PAF-induced hypotension may presumably be mediated by a direct vasoconstrictor effect of PAF on the renal vasculature and that baroreceptor reflex is not involved in either renal sympathetic or renal vascular changes.

Vasodilator and constrictor actions of platelet-activating factor in the isolated microperfused afferent arteriole of the rabbit kidney. Role of endothelium-derived relaxing factor/nitric oxide and cyclooxygenase products

It has been suggested that platelet-activating factor (PAF) plays a prominent role in the control of glomerular hemodynamics in various physiological and pathological conditions. We examined the direct effect of PAF on rabbit glomerular afferent arterioles (Af-Arts) microperfused in vitro and tested whether endothelium-derived relaxing factor/nitric oxide (EDNO) and cyclooxygenase products are involved in its actions. In nanomolar concentrations PAF caused dose-dependent constriction of Af-Arts, with the maximum constriction being 34 +/- 10% at 4 x 10(-8) M (n = 9, P < 0.001). The constriction was blunted by cyclooxygenase inhibition (11 +/- 6%, n = 7, P < 0.05) but augmented by EDNO inhibition (76 +/- 14%, n = 8, P < 0.005). To study a possible vasodilator effect of PAF, Af-Arts were preconstricted with norepinephrine and increasing concentrations of PAF added to the lumen. At picomolar concentrations (lower than those that caused constriction), PAF produced dose-dependent vasodilation that was unaffected by cyclooxygenase inhibition but was abolished by EDNO synthesis inhibition. Both PAF-induced constriction and dilation of Af-Arts were blocked by a PAF receptor antagonist. This study demonstrates that PAF has a receptor-mediated biphasic effect on rabbit Af-Arts, dilating them at low concentrations while constricting them at higher concentrations. Our results suggest that PAF's vasodilator action may be due to production of EDNO, while its constrictor action is mediated at least in part through cyclooxygenase products.

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.

Effects of YM264, a novel PAF antagonist, on puromycin aminonucleoside-induced nephropathy in the rat

We investigated the effects of YM264, WEB2086, methylprednisolone and ticlopidine on puromycin-induced nephropathy in the rat. Puromycin produces marked proteinuria, hypercholesterolemia, and hypoalbuminemia. The structurally differing PAF antagonists YM264 and WEB2086 inhibited proteinuria and improved hypercholesterolemia and hypoalbuminemia. Methylprednisolone also exhibited a beneficial effect on these variables. However, ticlopidine, a platelet inhibitor, showed no inhibitory effect on nephropathy. These results indicate that PAF may play a major role in puromycin-induced nephropathy in the rat, and that PAF antagonists may prove of therapeutic value in the treatment of nephropathy in humans.

Vasorelaxant effect of C16-PAF and C18-PAF on renal blood flow and systemic blood pressure in the anesthetized rat

The renal vasoactive and systemic hypotensive effects of platelet activating factor (C16:0-PAF and C18:1-PAF) were examined in anesthetized male Wistar rats. Bolus injections of C16-PAF (0.5-25 ng/kg) and C18-PAF (2.5-200 ng/kg) into the arterial circulation of the kidney produced increases in renal blood flow (6-15%) before causing dose-dependent systemic hypotension (2-64 mmHg). The dose-response curves for renal blood flow and systemic blood pressure generated by intrarenal C18-PAF administration were approximately 7 fold to the right of the dose-response curves generated by C16-DPAF. Intrarenal injections of vehicle or the biologically inactive enantiomer C16-DPAF (25-200 ng/kg) did not affect renal blood flow or systemic blood pressure. These results suggest that C16:0-PAF is a more potent renal vasodilator and hypotensive lipid than C18:1-PAF.

Reproducible vasodilatation by platelet-activating factor in blood- and Krebs-perfused rat kidneys is albumin-dependent

The vasodilator effect of platelet-activating factor (PAF) and its reproducibility was determined in rat kidneys perfused in situ with blood or in isolated kidneys perfused with Krebs solution or Krebs plus 0.25% bovine serum albumin (BSA) at 3 ml/min. Dilatation was measured as inhibition of the increase in perfusion pressure produced by stimulation of the renal nerves or by infusion of noradrenaline. PAF, in saline and 0.25% BSA, was infused into the perfusate at 0.05 ml/min to produce eight incremental consecutive concentrations from 3 x 10(-11) to 10(-7). Two sets of PAF infusions were made during nerve-stimulated responses followed by one set during noradrenaline infusion. With blood perfusion, PAF consistently produced dose-dependent dilatation and 3 x 10(-9) M reduced nerve-stimulated responses to 52% of control. This effect was reproduced by a second infusion. However in Krebs-perfused kidneys the effect of the first PAF infusion was not consistent, with responses either not affected or reduced only at the lower doses so that the mean maximum decrease was only 10% and the vasodilatation was not dose-dependent. The second PAF infusion invariably had no effect. On the other hand perfusion with Krebs and 0.25% BSA produced consistent and dose-dependent inhibition of vasoconstriction which was reproduced by a second infusion. PAF was effective at 10-fold lower doses in Krebs-albumin perfused compared to blood-perfused kidneys. These results indicate that exogenous PAF is a potent and reproducible dilator of the rat renal vasculature perfused with blood or Krebs-albumin solution but not albumin-free Krebs solution.

The effect of long-term consumption of fish oil on platelet-activating factor synthesis in rat renal microsomes

The effect of consuming eicosapentaenoic acid (EPA) plus docosahexaenoic acid (DHA) on platelet-activating factor (PAF) biosynthesis by renal acetyl-CoA: 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) acetyltransferase activity was compared in rats fed diets containing fish oil (FO) or sunflower oil (SO) for 11 months. Microsomes from the FO group exhibited a significantly lower formation of [14C]PAF from [1-14C]acetyl-CoA in the absence of exogenously-added lysoPAF. There were no differences between the two groups in the generation of radioactive acylacetyl-glycerophosphocholine (GPC) upon incubation of microsomes without added lysoPAF. Upon addition of lysoPAF, the microsomal synthesis of PAF, but not acylacetyl-GPC, rose considerably. Furthermore, PAF synthesis via microsomal acetyltransferase activity in the FO-fed animals was only 40% that of the SO groups. These findings may bear potential relevance in terms of reducing PAF synthesis in association with renal pathophysiology.

Inhibition of vasoconstriction by platelet activating factor in the in situ blood perfused rat mesentery

1. Perfusion pressure was measured in the in situ mesentery of anaesthetized rats perfused with blood at a constant 2 mL/min. 2. Increases in perfusion pressure were produced by mesenteric peri-arterial nerve stimulation at 10 Hz for 5 s at 2 min intervals and by bolus intra-arterial injections of the vasoconstrictors noradrenaline, angiotensin II and 5-hydroxytryptamine. 3. The intra-arterial infusion of platelet-activating factor (PAF) to produce a blood concentration of 3 X 10(-10) mol/L inhibited all responses to a similar extent. Intra-arterial prazosin (1-5 X 10(-9) mol/L), however, preferentially reduced responses to nerve stimulation and noradrenaline. 4. PAF at concentrations from 3 X 10(-11) to 10(-9) mol/L produced increasing inhibition of vasoconstrictor responses to nerve stimulation. The dose-response to PAF was shifted to the right by the concomitant intra-arterial infusion of the PAF antagonist SRI 63-441. 5. PAF at very low concentrations in vivo inhibits mesenteric vasoconstriction, produced by sympathetic nerve stimulation or various agonists, by a PAF-receptor mediated vasodilatation of the mesenteric vasculature.

Nonimmunological alterations of glomerular filtration by s-PAF in the rat kidney

Rat kidneys were isolated and perfused with a cell-free perfusion buffer containing 4% albumin. Infusion of platelet activating factor (s-PAF) into the isolated perfused kidney caused a dose-dependent fall in renal vascular resistance (RVR): 12 +/- 6% at 10 nM s-PAF, 18 +/- 3% at 100 nM s-PAF and 20 +/- 7% at 1 microM s-PAF. Glomerular filtration rate fell by 32 +/- 5% at 10 nM, 38 +/- 6% at 100 nM, and 52 +/- 10% at 1 microM. s-PAF (50 nM) increased urinary protein excretion after 20 minutes. Because GFR fell to a greater extent than RVR, possible changes in glomerular permeability after s-PAF treatment were assessed morphologically using native ferritin. After s-PAF treatment (100 nM), the number of ferritin particles/micron2 increased from 1.2 +/- 0.9 (control) to 795 +/- 69 in the glomerular basement membrane (GBM) and from 0.2 +/- 0.06 (control) to 98 +/- 29 in lamina rara externa (LRE). To quantitate changes in fixed anionic charges, polyethylenimine (PEI) was quantitated morphologically in GBM. No significant change between s-PAF treated and untreated kidneys was seen. s-PAF did not alter the sialoglycoprotein pattern in the perfused kidney as assessed by lysozyme staining. These results are in contrast to findings with s-PAF in vivo where in addition to increased glomerular permeability, a reduction of fixed anionic charges is seen.(ABSTRACT TRUNCATED AT 250 WORDS)