AJP - Renal Physiology, Vol 271, Issue 2 269-F274, Copyright © 1996 by American Physiological Society
Renal 131I-hippurate clearance overestimates true renal blood flow in the instrumented conscious dog
C. A. Visscher, D. De Zeeuw, G. Navis, A. K. Van Zanten, P. E. De Jong and R. M. Huisman
Department of Medicine, Groningen Institute for Drug Studies, University Hospital, The Netherlands.
We evaluated renal 131I-hippurate clearance (ERPFhip) as a measure of renal
blood flow (RBF) in chronically instrumented conscious dogs. When adjusted
for renal hippurate extraction (Ehip, 0.77 +/- 0.01) and hematocrit (Hct,
39.7 +/- 1%), calculated RBFhip (656 +/- 37 ml/min) markedly exceeded renal
blood flow measured with renal artery blood flow probes (RBFprobe, 433 +/-
27 ml/min). The discrepancy could not be explained by flow probe
calibration, because in vivo comparison of flow probe values with renal
venous outflow showed only a slight underestimation 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 underestimation of Ehip by 4 +/- 1% and thus
could only explain a small part of the difference. Extrarenal hippurate
clearance was excluded, because the amount of 131I-hippurate cleared from
plasma equaled that appearing in the urine (303 +/- 17 and 307 +/- 17
ml/min). Applying these corrections, we found that RBFhip still exceeded
RBFprobe by 37 +/- 3%. These data indicate that renal blood flow measured
by the hippurate clearance technique markedly overestimates true renal
blood flow. Because other errors were excluded, a combination of sampling
of nonrenal blood and intrarenal hippurate extraction from erythrocytes
might play a role.
