The Utilization of I131-Labeled Renografin as an Inulin Substitute for Renal Clearance Rate Determination

Abstract

Our previous report (1) described a technic for I¹³¹ ortho-iodo-hippuric acid (OIHA) renal clearance rate determination following a single injection of this compound and utilizing a modification of the classical Homer Smith clearance procedure (2). The OIHA clearance ratio to simultaneous para-aminohippurate clearance (PAH), varied between 0.6 and 0.8. The results were reasonably consistent on repeated examination of a given unanesthetized animal and hence probably utilizable for estimating renal tubular function and effective renal plasma flow. Due to the relatively low OIHA∕PAH ratio (0.6 to 0.8), however, a substitute for PAH giving a higher ratio than the OI¹³¹HA could be expected to give better results. It has further been our purpose to attempt to develop a radioactive technic which would parallel the inulin clearance rate. This has posed considerable difficulty because of our inability to achieve a gamma-labeled compound chemically similar to inulin. On the basis of the fact that OI¹³¹HA is handled by glomerular filtration as well as tubular transport, an initial attempt was made to paralyze the tubular transport system by employing Benemid and thus to achieve a measure of glomerular filtration. It was found that this drug affected renal clearance (PAH and inulin) for an inordinately prolonged period of time, and the results obtained with the ortho-iodo-hippuric acid (I¹³¹) under these circumstances were erratic. In a number of reviews on renal clearance, brief references have been made to other organic iodides selectively excreted by the kidney and supposedly largely excreted by glomerular filtration (3–6). These compounds have included (a) I¹³¹_ Diodrast; (b) I¹³¹-Hypaque; (c) I¹³¹_ Miokon; (d) I¹³¹-Urokon; (e) I¹³¹-Renografin. (The only difference between Hypaque and Renografin is the carrier substance methylglucamine found in Renografin.) In this series of investigations, undertaken with unanesthetized dogs, simultaneous clearance ratios were determined for inulin, para-aminohippuric acid, and radio-iodinated Renografin. These experiments were performed with the classical Homer Smith technic (2). Method of Experiment The basic technic established by Homer Smith for the determination of the clearance rate of inulin and para-aminohippuric acid was initially followed: Hydration: A stomach tube was passed and 20 c.c. of water per kilogram of body weight was instilled approximately forty-five minutes prior to the first clearance period. Priming: Six-tenths of a cubic centimeter of 20 per cent PAR solution plus 4 c.c. 10 per cent inulin solution.