The Single Injection Technique for Determination of Renal Clearance: V. A Comparison with the Continuous Infusion Technique in the Dog and in Man

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Abstract
The clearance of radioactive iothalamate and iodohippurate was investigated in dogs and in man using concomitant single injection and continuous infusion of the test substances. For iothalamate the renal single injection clearance, calculated on arterial plasma or blood values, was equal to the continuous infusion clearance. The total plasma clearance, as calculated from a two-hour curve, was higher than the continuous infusion clearance; this difference was more pronounced in the dog. The different methods for iothalamare clearance showed a high degree of correlation (r < 0.95) in the dog as well as in man. For iodohippurate the arterial single injaction clearance decreased by 3.9% per period in the dog and by 6.5% per period in man. The decreas: of the venous clearance values was even more pronounced. The total clearance as calculated on arterial plasma samples was 22% higher in the dog than, but in man equal to, the continuous infusion clearance. All changes were more pronounced when the calculations were performed on whole blood values. The correlations between the different iodohippurate clearances were excellent (r <0.95). It is concluded that for iothalamate the rsnal single injection technique gives the same clearance values as the continuous infusion technique when arterial blood is used, and nearly so when venous blood is used. For iodohippuratc the renal single injection method and the cmtinuous infusion method do not give the same result, since after single injection the renal extraction falls continuously, due partly to re-entry of iodohippurate into the blood and partly to a relative accumulation of free iodine from the test substance.