INFLUENCE OF THE KIDNEY ON THE DISAPPEARANCE RATE OF LABELLED CORTICOTROPHIN FROM THE BLOOD STREAM

Abstract

The concentration of radioactivity (RA) in the blood and kidney of normal and kidney-ligated rats was studied at various times after injection. The question whether the disappearance rate of RA from the blood after administration of I131-labelled corticotropin was influenced by a previous injection of this agent was also investigated. RA in the kidney and liver reaches a maximum about 5 minutes after injection. During this period there is a sharp fall in RA in the blood. There is a difference in RA in the blood of kidney-ligated and control rats from 5 minutes after injection onwards. This difference remains constant during the next 15 minutes. There is no difference between the disappearance rate of RA from the blood after the 1st and 2d injection. The amount 15 and 40 minutes after injection was about the same. The kidney/blood ratio 20 minutes after the 1st and 2d injection showed no essential difference. There was, however, a significant difference between the kidney /blood ratio 20 and 40 minutes after injection. A still greater difference was observed at 20 and 60 minutes after injection. Hence RA in the blood and kidney does not decline at a constant ratio. The rate of decrease of RA in the blood during the 1st period after injection cannot be used as an index for the rate at which the administered corticotropin is metabolized by the organism. Corticotropin has not been fully distributed during this period. It also seems clear that injected labelled corticotropin is not merely excreted but metabolized.