Observations on the early renal uptake and later tubular metabolism of radiolabelled aprotinin (Trasylol) in man: theoretical and practical considerations.

1. The novel method recently developed to measure renal tubular degradation of filtered proteins in man using radiolabelled aprotinin (Trasylol) has been modified to allow the fate and the significance of the renal catabolism of radiolabelled aprotinin to be determined beyond 24h. 2. Ten renal patients with normal kidney function and variable proteinuria each received two separate intravenous injections of radiolabelled aprotinin, 5.0 mg of 99mTc-labelled aprotinin (40MBq) and 0.5mg of 131I-labelled aprotinin (5MBq). Chromatography (Sephadex-G-25-M) was used to separate undegraded radiolabelled aprotinin from the free isotope in urine and plasma. Renal uptake from gamma-camera images (24h for 99mTc-labelled aprotinin and up to 96h for 131I-labelled aprotinin) and urinary activity (48 and 96h, respectively) were measured. 3. The renal handling of radiolabelled aprotinin was similar with the two isotopes. Chromatography showed that all plasma activity was undegraded radiolabelled aprotinin, and urine activity was only the free isotopic label. 4. Kidney uptake of 131I-labelled aprotinin was prompt, reaching a cumulative maximum of 37.1 +/- 3.0% of dose at 24h, but falling exponentially thereafter to 5.6 +/- 1.0% of dose at 96h. 5. The rate of excretion of the free label in urine, i.e. the metabolic rate of radiolabelled aprotinin, was relatively constant over the first 24h (1.6 +/- 0.09% of dose/h), but then fell in parallel with the diminishing activity over the kidney, i.e. to 1.0 +/- 0.1% of dose/h over 24-48h and to only 0.4 +/- 0.08% of dose/h over 72-96h.(ABSTRACT TRUNCATED AT 250 WORDS)