Defective glomerular [3H]lysoPAF metabolism in the autologous phase of rabbit nephrotoxic nephritis.

Glomerular infiltration of blood-derived mononuclear cells contributes to the glomerular injury in the autologous phase of nephrotoxic nephritis (NTN). LysoPAF has recently been shown to be chemotactic for human monocytes, thus its accumulation might account for monocyte recruitment. We investigated [3H]lysoPAF metabolism in isolated glomeruli from normal and NTN rabbits studied both in the heterologous and in the autologous phases of the disease. [3H]lysoPAF was converted to [3H]1-O-alkyl-glycerol and [3H]1-O-alkyl-2-acyl-GPC by phospholipase C and acyltransferase, respectively, both in normal and NTN glomeruli. Glomerular metabolism of [3H]lysoPAF was normal during the heterologous phase of NTN. By contrast, in isolated glomeruli from NTN rabbits studied in the autologous phase of the disease, a significantly lower [3H]lysoPAF degradation occurred with respect to normal ones. This defective degradation resulted in a significantly reduced formation of [3H]1-O-alkyl-glycerol. The apparent Km for enzymatic conversion of [3H]lysoPAF to [3H]1-O-alkyl-glycerol, determined at 15 minutes as a function of [3H]lysoPAF concentration, was doubled in glomeruli from rabbits studied in the autologous phase of NTN as compared to normal ones, while Vmax values were similar in the two groups. These results show a defective glomerular lysoPAF degradation in the autologous phase of NTN, likely due to a decreased affinity of phospholipase C to lysoPAF. Altered lysoPAF metabolism results in glomerular accumulation of lysoPAF in the autologous phase of NTN, as shown by significantly higher levels of lysoPAF measured in nephritic glomeruli as compared to normal ones.

Renal metabolism and urinary excretion of platelet-activating factor in the rat.

The origin of platelet-activating factor (PAF) in the urine remains ill defined. The present study documents that [3H]PAF (3.5 mu Ci) injected into the renal artery of isolated control rat kidney preparations perfused at constant pressure with a cell-free medium containing 1% bovine serum albumin (BSA) was excreted in negligible amounts (0.034%) in the urine, whereas 6% was retained by the kidney. When kidneys were perfused with a BSA-free medium, 0.029 and 71% of the total radioactivity added to the perfusate was recovered in the urine and in the renal tissue, respectively. [3H]PAF urine excretion in proteinuric kidneys from adriamycin-treated rats was still negligible (0.015%). Analysis of the renal tissue-retained radioactivity in control and proteinuric kidneys perfused with 1% BSA indicated metabolism into long chain acyl-sn-glycero-3-phosphorylcholine species, lyso-PAF, glycerols, and intact PAF. Thin layer chromatography analysis of [3H]glycerol fraction in these renal extracts showed two major components comigrating with 1-O-alkylglycerol and 1-O-alkyl-2-fatty acylglycerol. Isolated proximal tubules, but not glomeruli from nephrotic rats exposed to increasing concentrations of BSA (0-4%), had a higher PAF uptake than control tubules for BSA concentrations ranging from 0 to 0.1%. Our findings in the isolated perfused kidneys indicate that, in normal conditions, circulating PAF is excreted in the urine in negligible amounts and that the altered glomerular permeability to proteins does not affect this excretion rate. Moreover, analysis of renal tissue radioactivity documented that the renal metabolism of PAF is comparable in control and nephrotic kidneys.