Renal blood flow measurement

In addition to its content in the glomerular filtrate, the marker is secreted by the renal tubule. Its clearance rate is used to measure renal blood flow but it overestimates the GFR. The aniline derivatives,p-amino hippurate (PAH) or radiolabeled o-iodohippuxan (OIH), represent this group of renal markers. Note that PAH is partly filtered by the glomerular membrane, but molecules that are exclusively excreted by the renal tubules would be ideal for renal blood flow measurements. 

Evans, R.G., et al.. Chronic renal blood flow measurement in dogs by transittime ultrasound flowmetry, /. Pharmacol. Toxicol. Meih., 1997,38 33-39. 

Potassium-42 Reactor Brain and breast tumour localization determination of total exchangeable K-t- renal blood 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.)... 

Renal function Measurement of effects on urine excretion in saline loaded rats Renal dynamics Measurement of renal blood flow, GFR and clearance... 

In addition to their diuretic activity, loop agents appear to have direct effects on blood flow through several vascular beds. Furosemide increases renal blood flow. Furosemide and ethacrynic acid have also been shown to reduce pulmonary congestion and left ventricular filling pressures in heart failure before a measurable increase in urinary output occurs, and in anephric patients. 

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.

Age-dependent, receptor-mediated drug effects have been observed with dopamine. The effect of dopamine on blood flow (estimated by measuring the pulsatility index by ultrasonography) was assessed in the right renal, superior mesenteric, and middle cerebral arteries in sick premature infants (52). Renal blood flow increased during the dopamine infusion, but mesenteric and cerebral blood flow were not altered. In adults, dopamine does increase blood flow to the intestine, indicating that the lack of response in preterm neonates is related to the immaturity of the mesenteric vascular bed. 

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. 

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%. 

This ingenious method gave a rough index of blood flow through the kidneys. It was subsequently greatly developed by my old friend and former associate, Homer Smith. Measurements of discrete function of glomerular filtration rate and renal blood flow using mannitol, creatinine, and p-aminohippurate were developed later. 

FPL55712, in the endotoxaemic rat as measured by improvement in renal blood flow and absence of haemoconcentration. Additionally, pretreatment with LY171883 improved survival and indices of tissue injury in rats subjected to traumatic shock. ... 

Renal blood flow (RBF) can be measured either directly or indirectly. Direct measurement requires the placement of a flow probe around the renal artery, which is technically difficult in rats due to their small size. Therefore, renal hemodynamic studies are rather conducted in large animal species such as dogs, (mini-)pigs, and nonhuman primates, due to the easy surgical accessibility of... 

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. 

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