Platelet-Activating Factor Mediates Angiotensin TI-Induced Proteinuria in Isolated Perfused Rat Kidney NORBERTO PERICO,*t RADOSLAW LAPINSKI,* KRZYSZTOF KONOPKA,* SISTIANA AIELLO,* MARINA NORIS,* and GIUSEPPE REMUZZI*t *Mario Negri Institute for Pharmacological Research and *Division of Nephrology and Dialysis, Ospedali Riuniti di Bergamo, Bergamo, Italy. Abstract. Isolated kidney preparations (IPK) from male Spra- gue Dawley rats perfused at constant pressure were used to evaluate the effect of angiotensin II (All) and platelet-activat- ing factor (PAF) on renal function and urinary protein excre- tion. Compared with basal, intrarenal infusion of All at 8 ng/min caused a progressive increase in protein excretion ( 11 ± 6 versus 73 ± 21 pg/min) in parallel with a decline in renal perfusate flow (RPF) (29 ± 3 versus 18 ± 3 ml/min). Addition to the perfusate of PAF at 50 nM final concentration also induced proteinuria (9 ± 4 versus 55 ± 14 j.g/min) but did not change RPF (29 ± 3 versus 30 ± 3 mI/mm). Pre- exposure of isolated kidneys to the PAF receptor antagonist WEB 2086 prevented the increase in urinary protein excretion induced by All infusion (basal: 13 ± 6; post-All: 12 ± 7 tg/min) but failed to prevent the vasoactive effect of All (RPF, basal: 30 ± 2; post-All: 21 ± 3 ml/min). In additional exper- iments, dexamethasone reduced the proteinuric effect of PAF remarkably. These results indicate that in isolated kidney prep- aration: (I) All infusion induced proteinuria and decreased RPF; and (2) the effect of All in enhancing urinary protein excretion was completely prevented by a specific PAF receptor antagonist, which, however, did not influence the All-induced fall in RPF. It is suggested that PAF plays a major role in All-induced changes in the permselective function of the gb- merular capillary barrier. (J Am Soc Nephrol 8: 1391-1398, 1997) In the past few years, evidence has been provided that control- ling systemic BP with various drugs, especially angiotensin- converting enzyme (ACE) inhibitors, reduced the urinary pro- tein excretion rate and preserved renal function in experimental animals and humans with progressive chronic renal diseases (1-1 2). This, together with more recent findings that in animals and humans (13-15) angiotensin II (All) receptor antagonists shared antiproteinuric and renoprotective properties of ACE inhibitors, has suggested that All enhanced the movement of proteins across the gbomerular capillary barrier. However, the biochemical nature of the effect of All on glomerubar perms- elective function remains elusive. It is possible that proteinuria induced by All is entirely of hemodynamic origin. This would be consistent with data showing that intra-aortic infusion of All in rats with early passive Heymann nephritis induced an ebe- vation of gbomerular capillary pressure, lowered gbomerular plasma flow and ultrafiltration coefficient, and accentuated the pattern of urinary protein excretion ( 16). On the other hand, saralasin, an Al! receptor antagonist that normalized glomer- ular capillary pressure and flow in a rat model of partial renal vein constriction, also limited the abnormal urinary protein Received October 28, 1996. Accepted March 28, 1997. Correspondence to Dr. Norberto Perico, Mario Negri Institute for Pharmaco- logical Research. Via Gavazzeni 1 1. 24125 Bergamo. Italy. Dr. Timothy Meyer served as Guest Editor and supervised the review and final disposition of this manuscript. 1046-6673/0809- 1 39 1 $03.OO/O Journal of the American Society of Nephrology Copyright 0 1997 by the American Society of Nephrology excretion (1 7). In isolated perfused rat kidney, we recently found that All increased urinary protein excretion in a dose- dependent manner with variable effects on filtration fraction, which were dependent on the rate of infusion of the hormone ( 18). When the sieving properties of the gbomerular capillary barrier were evaluated in the same model by fractional clear- ance of pobydisperse Ficoll, chosen because of its albumin-like globular configuration (19), we found that All enhanced the filtration of tracer molecules of radii >34A, an event com- pletely prevented by a selective All type 1 receptor antagonist (1 8). Thus, All can directly modulate glomerular permselective function, even independently of the hemodynamic effects, at least in isolated kidney preparations (IPK). So far, however, no studies have addressed the mechanism(s) of such an effect. Besides All, platelet activating factor (PAF), a lipid medi- ator of inflammation (20,2 1 ), also enhances protein excretion and glomerular permeability to dextran macromolecules in IPK (22,23). Of interest, All can promote PAF formation, deliver- ing through specific receptors intracellular signals that activate phospholipase A2 (24), the indispensable initial step in the sequence of biochemical events leading to PAF synthesis (20,25). In vitro All induces PAF synthesis in rat mesangial cells in a dose-dependent manner and with a rapid time course (26). This has also been observed in rat isolated gbomeruli exposed to the vasoactive peptide (27). The fact that All can stimulate PAF formation invites further speculation on All- induced proteinuna in isolated perfused kidney. Because PAF enhances gbomerular permeability to macromolecules (22,28), one may wish to address whether the proteinuric effect of All