Renal plasma flow measurement

A substance that fulfills these criteria is para-aminohippuric acid (PAH). All of the PAH not filtered at the glomerulus is secreted by the proximal tubule. The net effect is that all of the plasma flowing through the nephrons is completely cleared of PAH. It is important to note that about 10 to 15% of the total renal plasma flow supplies regions of the kidneys that are not involved with filtration or secretion. Consequently, this plasma cannot be cleared of PAH. Therefore, the plasma clearance of PAH provides a measurement of the effective renal plasma flow, that is, the volume of plasma that actually flows through the nephrons. The ERPF is normally about 625 ml/ min. (This value is based on a renal blood flow of about 1.1 1/min and a hematocrit of about 42.)... 

If a marker is extracted from the blood exclusively by the kidney resulting in a renal venous concentration of 0% (i.e. the arterio-venous extraction fraction is 100%), then the calculated value of the clearance of the marker (Cx) is equal to renal plasma flow. In practice, a compound, such as para-amino hippurate (PAH) with an extraction fraction of about 87%, is used. To acknowledge the fact that there is discrepancy between the PAH clearance and renal plasma flow, the term effective renal plasma flow is used when the extraction factor is not measured. In sum, renal plasma flow = effective renal plasma flow + extraction factor and renal blood flow = effective renal plasma flow + the hematocrit. 

PAH transport. Thus, the PAH clearance cannot be considered a reliable measure of renal plasma flow, unless the extraction factor of PAH is measured simultaneously. This requires that a sample of renal venous blood be obtained. 

Vaughan ED, Buhler FR, Laragh JH. Renovascular hypertension Renin measurements to indicate hypersecretion and contralateral suppression, estimate renal plasma flow, and score for surgical curability. Am J Med 1973 55 402-14. 

Measurement of renal plasma and blood flow is usually reserved for research settings to evaluate hemodynamic changes related to disease or drug therapy. The kidneys receive approximately 20% of cardiac output and representative values of renal blood flow in men and women of about 1200 250 and 1000 180 mL/min per 1.73 w have been reported, respectively. Renal plasma flow (RPF) can be estimated to be 60% of renal blood flow if it is assumed that the average hematocrit is 40%. 

Effective renal plasma flow (ERPE)—The flow of plasma through the kidneys often measured by para-amino hippurate (PAH) clearance and expressed in volume per unit of time (mL/min). The ERPF is less than the true renal plasma flow (RPF) because plasma flow through renal connective and adipose tissue is not measured and the extraction of PAH, although high (>0.9), is not complete. 

Para-aminohippurate (PAH)—A small molecule which is completely secreted from the tubules into urine, so that blood leaving the kidney is virtually free of PAH a marker that is often used to measure renal plasma flow (RPF). 

A decrease in the PAH clearance might be due to either an actual decline in renal plasma flow or a decrease in the extraction factor of PAH. The latter occurs when the tubular secretion of PAH in proximal tubules is impaired due to tubular disease or the presence of substances, which compete with transceUular PAH transport. Thus, the PAH clearance cannot be considered a reliable measure of renal plasma flow, unless the extraction factor of PAH is measured simultaneously. This requires that a sample of renal venous blood be obtained. 

In humans, approximately 25% of renal plasma flow (450-600 ml/min) is filtered. Glomerular filtration rate (GFR), defined as the volume of filtrate formed per unit time, is commonly used to measure kidney function. GFR is measured as the clearance of a marker... 

Silkalns GI, Jeck D, Eaton J, Edelmann CM Jr, Chervu LR, Blaufox MD, and A Spitzer (1973). Simultaneous measurement of glomerular filtration rate and renal plasma flow using plasma disappearance curves. Journal of Pediatrics 83 749-757. 

An indirect measurement of RBF can be made using para-aminohippuric acid (PAH) clearance. This molecule is an ideal marker of the effective renal plasma flow (eRPF), as it freely filters through the glomerulus, and any amotmt remaining in the peritubular capillary plasma is secreted into the proximal tubule. Therefore, essentially aU PAH passing through the kidneys appears in the urine. For this reason, the PAH clearance is directly proportional to the rate of plasma flow through the kidneys. If the hematocrit is known, the total renal blood flow can be easily calculated from the eRPF value. 

Laroute V, Lefebvre HP, Costes G, Toutain PL (1999) Measurement of glomerular filtration rate and effective renal plasma flow in the conscious beagle dog by single intravenous bolus of iohexol and p-aminohippuric acid. J Pharmacol Toxicol Methods 41 17-25 LeBel CP, Foss JA (1996) Use of a rodent neurotoxicity screening battery in the preclinical safety assessment of recombinant-methionyl human brain-derived neurotrophic factor. Neurotoxicology 17 851-863... 

A substance used in the assessment of renal function. At low plasma levels, estimation of its clearance can be used to measure renal plasma flow whereas at higher plasma levels it can give an indication of the tubular secretory capacity. 

A substance which can be used in a similar way to p-aminohip-puric acid for the investigation of renal function. At low blood levels, measurement of its clearance gives an indication of renal plasma flow, whereas, at higher blood levels its clearance corresponds more to the tubular secretory capacity. 

Measurement of the renal plasma flow by measuring the clearance of low levels of p-aminohippuric acid. 

Both prerenal factors (dehydration, blood loss, altered vasomotor tone, age-related decreases in renal blood flow in rats) and postrenal factors (obstruction or extravasation of urine to the peritoneal cavity) may cause elevations of the commonly measured analytes that do not reflect primary kidney injury. Plasma analytes also cannot be used to determine the location of renal injury (glomerular versus tubular, or tubular segment affected) (Baum et al. 1975 Corman and Michel 1987 Finco 1997 Newman and Price 1999). 

Fig. 7. Renal plasma clearance of 125I-cystatin C (Ccy) compared to that of 51 Cr-EDTA (Cq-—edta) in rats with normal glomerular filtration ( ), and in rats with renal blood flow reduced to 25-50% of control by constricting the aorta above the renal arteries (o). The clearance measurements were completed 2.5-6.0 min after tracer injection. Ccy = 0.944 x Ccr-EDTA f = 0.989.

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