The single injection technique for determination of renal clearance. I. Clearance of iothalamate and iodohippurate in dogs

Experimental Studies Comparing Iohexol and 51Cr-Edta for Glomerular Filtration Rate Measurements

The total plasma clearance as well as the renal clearance of iohexol were evaluated for determination of the glomerular filtration rate (GFR) in 16 anesthetized pigs. The iohexol levels in urine and plasma were measured by X-ray fluorescence. The total plasma clearance of 1 and 4 ml/kg b.w. of iohexol 300 mg I/ml was compared to the simultaneously as well as nonsimultaneously calculated total plasma clearance of 51Cr-EDTA. The total plasma clearance of 51Cr-EDTA and of iohexol turned out to be equal and independent of the injected dose of iohexol. The injected dose did not have any effect on the renal clearance of iohexol either. It is concluded that iohexol and 51Cr-EDTA are similar as marker substances for determination of the GFR.

Clearance and renal extraction of technetium-99m-L,L-ethylenedicysteine, I-131-ortho-iodohippurate and I-125-iothalamate in pigs

99mTc-L,L-ethylenedicysteine (99mTc-EC) has been proposed as a 99mTc-labelled alternative to radio-iodinated ortho-iodohippurate (OIH) for renal imaging and evaluation of renal function. The kinetics of this new renal function agent were studied by a single-injection plasma clearance technique in pigs. 99mTc-EC, 131I-OIH and 125I-iothalamate were injected and the plasma concentration of the three tracers was followed for 240 min. Renal, hepatic and total plasma clearance were calculated. There was no difference between the renal plasma clearance of 99mTc-EC and 131I-OIH (175 +/- 9 versus 178 +/- 8 ml min-1, P = 0.43), whereas the difference between the total plasma clearance of 99mTc-EC and 131I-OIH was highly significant (268 +/- 16 versus 185 +/- 9 ml min-1, P = 0.0001). 99mTc-EC had a significant hepatic clearance of 83 +/- 10 ml min-1 whereas the hepatic clearance of 131I-OIH was negligible. Renal plasma extraction of both 99mTc-EC and 131I-OIH decreased significantly between 2 and 240 min post-injection from 0.85 to 0.45% for 99mTc-EC and from 0.93 to 0.57% for 131I-OIH. Red blood cell binding of 99mTc-EC and 131I-OIH was 6.1% and 20%, respectively. The protein binding of 99mTc-EC and 131I-OIH was 32% for both tracers. We conclude that 99mTc-EC is not a suitable tracer for measuring renal function by the single-injection plasma clearance technique in pigs. This is due to a decreasing renal extraction and a significant hepatic clearance.

Pharmacokinetic aspects of measurement of glomerular filtration rate in the dog: a review

Glomerular filtration rate (GFR) is estimated by means of clearance, defined as the volume of plasma that has been cleared of a particular substance per unit time. Glomerular filtration rate may be estimated by measuring the renal clearance of a filtration marker using data from both urine and plasma or by plasma clearance using only plasma data. Several alternative pharmacokinetic models are used for the calculation of clearance using various filtration markers with slightly different pharmacokinetic properties. The purpose of this article is to discuss how the choice of marker and pharmacokinetic model may influence estimated GFR values and to elucidate commonly used methods and reported GFR values in the dog.

Clearance of iohexol, 51Cr-EDTA and endogenous creatinine for determination of glomerular filtration rate in pigs with reduced renal function: a comparison between different clearance techniques

In order to simplify and/or improve determination of glomerular filtration rate (GFR) the clearances of iohexol, 51Cr-EDTA and endogenous creatinine were simultaneously determined with different techniques in 21 anesthetized landrace pigs. Their GFR had been reduced to about 1/3 or less of normal GFR. After an intravenous bolus of the GFR markers, their plasma concentration curves were followed for 6 hours with 16 plasma samples. A bladder catheter collected urine during six 60-min periods. The plasma clearance was calculated by dividing "dose of marker" with "area under the plasma concentration curve" (AUC) from the time of injection to infinity using a one- (Clprovisional) and a three-compartment (ClAUC-3comp) model. The renal clearance of iohexol and 51Cr-EDTA was calculated by dividing the amount of marker excreted in the urine in a period by AUC in the same period. The AUC was for iohexol and 51Cr-EDTA determined by integrating the total area in the period (Clren adv)-our reference method representing the "true" GFR and for creatinine determined by using the arithmetic mean of the plasma concentration of the marker at the start and at the end of the urine collection period (Clren simple). Renal clearance of creatinine was significantly lower than renal clearance of iohexol (p = 0.0019) and 51Cr-EDTA (p = 0.0001). There were no significant differences between the renal clearances (Clren adv) of iohexol and 51Cr-EDTA or between their plasma clearances (ClAUC-3comp). For iohexol the median overestimation of the "true" GFR with Clprovisional was higher when "early" plasma samples (30-120 min) were used (4.5 ml min-1 10 kg-1) than when late samples (180-360 min) were used (1.9 ml min-1 10 kg-1). Subtraction of the median extrarenal clearance (known from a study of nephrectomized pigs) from the plasma clearances (ClAUC-3comp) of iohexol and 51Cr-EDTA in pigs with reduced renal function decreased the median overestimation of the "true" GFR from 1.9 to 1.0 ml min-1 10 kg-1 with iohexol and from 1.7 to 0.9 ml min-1 10 kg-1 with 51Cr-EDTA. The plasma clearance technique may be improved in pigs with reduced GFR by (i) including a "late" plasma sample in three- and one-compartment models, which tends to increase the AUC; (ii) introducing a correction formula by normalizing the GFR values of the one-compartment model to those of the three-compartment model, thereby compensating for the rapid early changes in plasma concentration of marker after the bolus injection of the marker; or (iii) subtracting a median (or mean) extrarenal clearance of the marker in pigs from the plasma clearance [according to (i) or (ii)]. The plasma clearance one-compartment technique may be improved in pigs with various levels of GFR values by normalizing the plasma clearance values to the renal clearance values, thereby compensating for both the early changes in plasma concentration of marker and the extrarenal clearance of marker.

Renal and non-renal clearances of iothalamate

An evaluation of the literature indicated that certain aspects of the disposition kinetics of iothalamate, important to the accurate determination of glomerular filtration rate in dogs and humans, remain to be resolved. The simultaneous clearances of iothalamate and inulin in 5 dogs were determined at three steady-state iothalamate plasma levels (2, 10, and 40 micrograms ml-1) following various rates of intravenous infusion. The iothalamate clearances, both renal and non-renal, were concentration-independent. The overall mean non-renal clearance was 18 per cent (ranging from 9 to 25 per cent) of its plasma clearance. The mean iothalamate/inulin renal clearance ratio was about 0.84 with individual values ranging from 0.72 to 0.95. The significant (4-26 per cent) plasma protein binding of iothalamate in these dogs was the main reason for the lower-than-unity clearance ratios obtained. The literature indicates the existence of up to 25 per cent of non-renal elimination in humans with normal renal function; this is comparable to the present results obtained with dogs but contrary to the assumption, sometimes reported in the literature that non-renal elimination is essentially absent in humans. Binding of iothalamate to plasma proteins from humans was not found in the present study. The above results suggest that for accurate glomerular filtration rate determination in humans and dogs, especially for those with renal impairment, renal clearance rather than plasma clearance should be used, and in the case of dogs it should also be corrected for plasma protein binding. Iothalamate in plasma and urine was analysed by a simple, micro high-performance liquid chromatographic method with UV detection.

The effect of experimental hydronephrosis on clearance data

Partial obstruction of one ureter was created in newborn rats and its effects were studied in the adult rat. The obstructed renal pelvis was found to be enlarged 2-3 times. The weight of the kidney on this side was 85% of the contralateral one. Inulin clearance was measured in a way, which allowed comparison between the conventional technique and a modified technique, designed to circumvent the dilution in the enlarged pelvis. With the conventional technique, filtration was 32% of that in the contralateral kidney, whereas with the new technique true filtration of the hydronephrotic kidney was 82% (p less than 0.001). The modified technique is evaluated and the implications of the functional results are discussed.

The influence of dihydralazine and angiotensin-II blockade on renal extraction of [131I]hippuran

The renal extraction of [131I]ortho-iodohippuran ([131I]OIH) was measured during the first minutes after the bolus injection during renal vein catheterization of two groups of hypertensive patients. Concomitantly the hippuran clearance (effective renal plasma flow (ERPF) was determined with a quantitative renographic technique based on net kidney uptake between 1 1/2 and 2 1/2 min and plasma radioactivity 2 min after the bolus injection of hippuran. The study was repeated 30 min after intravenous administration of dihydralazine (0.1 mg/kg body weight) in 14 subjects (group A) and during continuous infusion of the angiotensin II blocking agent Saralasin (5 micrograms/kg . min) in 10 subjects (group B). Before dihydralazine was given, the rate of [131I]OIH extraction was (Mean +/- S.D.) 74.0 +/- 16.7% in the 2 min sample after bolus injection of hippuran, 67.5 +/- 17.3% after 10 min, falling to 53.0 +/- 15.3% after 30 min. There was no difference after dihydralazine administration, as the extraction values were 75.6 +/- 17.5 and 65.6 +/- 21.4% 2 and 10 min after the bolus injection of hippuran. The extraction values before Saralasin infusion were 74.2 +/- 15.7%, 59.8 +/- 18.2% and 47.1 +/- 16.6% at 2, 10 and 25 min after bolus injection of hippuran. Saralasin elicited no change in the extraction, as the values were 74.1 +/- 18.6% and 64.3 +/- 23.2%, 2 and 10 min after hippuran injection. For kidneys with ERPF above 100 ml/min . 1.73 m2 the extraction 2 min after hippuran injection was at the level of 82%, whereas the extraction fell rapidly with further reduction in ERPF. It is concluded that ERPF determined with the renographic technique, reflects the renal plasma flow when the clearance for the individual kidney exceeds 100 ml/min . 1.73 m2. This is the case also after dihydralazine and Saralasin administration.