Distribution Kinetics of Intravenous Radiopotassium

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Abstract
Specific activities of blood and tissues of male albino rats were detd. at frequent intervals following rapid intraven. admn. of radioactive potassium, K42. Four major organ exchange rate groups are indicated in the intact rat. The kidney, lung and intestine exchange rapidly and equilibrate with the plasma in 10 min. Liver, skin and spleen comprise an intermediate rate group which equilibrates with the plasma in 100 min. Muscle and testes constitute a slow group which equilibrates in about 600 min., and brain and erythrocyte a very slow group which is not equilibrated by 24 hrs. after injection. The K+ in all the organs studied seems to be completely exchangeable. The plasma specific activities were fitted to a 5 term exponential equation of the form developed by Sheppard and Householder for a multi-compartment mamillary exchange system. This equation has been interpreted as representing a 6 compartment system; a central compartment consisting of the total extracellular K+, 4 peripheral compartments which may represent the organ exchange groups discussed above, and a 6th compartment representing the continuous loss of the injected isotope from the animal via normal excretory routes for K+. Some failures of agreement between the experimental data and the equations derived from the model system, and a number of practical limitations to the applicability of this approach to the study of the intact animal are considered in the text.