Plasma volume determination by use of enzyme dilution in the dog

The use of enzymes for plasma volume determination in the dog was investigated. Plasma volume was determined from dilution of intravenously injected enzymes and compared with results obtained by high molecular weight fluorescein-labelled dextran. Aspartate aminotransferase and alanine aminotransferase from liver tissue gave good results, but the activities of creatine kinase and lactate dehydrogenase from heart tissue were inhibited in plasma, resulting in an overestimation of plasma volume. Enzymes offer a useful alternative to methods presently used since no radioactive isotopes are needed. In laboratory animals the enzyme preparations are readily obtainable and there is minimal risk of immunological complications arising after repeated determinations.