Tritiated water space in gastrocnemius of rat.

Abstract

IF a muscle is heated at 105° C until its weight is constant, the loss in weight (70–80 per cent of the original weight) represents the total amount of water in the muscle. More refined methods of estimating water contained in the tissue—normally applied to a whole animal in vivo—have been evolved; these methods depend on the equilibration of a marker molecule within the fluid spaces of the tissue or tissues under examination, followed by subsequent estimation of its concentration therein. The use of various markers, notably urea, thiourea and sulphanilamide, and their degree of correspondence with whole body water content as obtained by desiccation, has been comprehensively reviewed by Keys and Brozeck1. Antipyrine spaces have been found to agree with tissue deuterium oxide spaces2 and tritiated water spaces3. Antipyrine, however, is slowly metabolized4, and, like its N-acetyl-4-amino-derivative5, equilibrates relatively slowly with tissue fluids. Neither of these disadvantages is found in the case of deuterium oxide6 or tritiated water3, although a small rapid exchange has been noted between the isotopic hydrogen atoms and the hydrogen atoms in the organic molecules of the tissues3,7.