RENAL I-131 HIPPURATE CLEARANCE OVERESTIMATES TRUE RENAL BLOOD-FLOW IN THE INSTRUMENTED CONSCIOUS DOG

We evaluated renal I-131-hippurate clearance (ERPF(hip)) as a measure of renal blood flow (RBF) in chronically instrumented conscious dogs. When adjusted for renal hippurate extraction (E(hip), 0.77 +/- 0.01) a nd hematocrit (Hct, 39.7 +/- 1%), calculated RBF(hip) (656 +/- 37 ml/m in) markedly exceeded renal blood flow measured with renal artery bloo d flow probes (RBF(probe), 433 +/- 27 ml/min). The discrepancy could n ot be explained by flow probe calibration, because in vivo comparison of flow probe values with renal venous outflow showed only a slight un derestimation of renal blood flow (slope 0.93, 95% confidence interval 0.89-0.97). Redistribution of hippurate from erythrocytes into renal venous plasma during or shortly after blood sampling led to an underes timation of E(hip) by 4 +/- 1% and thus could only explain a small par t of the difference. Extrarenal hippurate clearance was excluded, beca use the amount of I-131-hippurate cleared from plasma equaled that app earing in the urine (303 +/- 17 and 307 +/- 17 ml/min). Applying these corrections, we found that RBF(hip) still exceeded RBF(probe) by 37 /- 3%. These data indicate that renal blood flow measured by the hippu rate clearance technique markedly overestimates true renal blood flow. Because other errors were excluded, a combination of sampling of nonr enal blood and intrarenal hippurate extraction from erythrocytes might play a role.