Measurement of extracellular fluid volume using 131I labelled inulin

• 1.1. Using in vitro preparations of rat hemi-diaphragm, comparison was made of volumes of distribution of inulin and allyl inulin labelled with ¹³¹I. Hemi-diaphragms were equilibrated in 0.9% saline at room temperature.
• 2.2. Iodide space, measured in 10 rat hemi-diaphragms, did not differ significantly from the volume of tissue water, suggesting that iodide is freely diffusible into cell water. Contaminating free iodide, ¹³¹I, will thus inflate the apparent volume of distribution of labelled inulin.
• 3.3. In 58 comparisons, volumes of distribution of unlabelled and labelled inulin, corrected for such overestimation, were close (19.0 and 20.6 ml/100 g wet tissue respectively). The slight, but significant difference was attributed to differences in molecular dispersion in the samples of inulin. It is likely that samples of allyl inulin contain a greater proportion of smaller inulin polymers than the inulin from which they were prepared.
• 4.4. It is concluded that allyl inulin labelled with ¹³¹ I can be used to measure, in vitro, the volume of extracellular fluid in pieces of tissue, but the desirability is emphasised of standardised preparations of inulin with known molecular dispersion.
• 5.5. The need, too, is emphasised either to remove all contaminating free iodide from samples of labelled inulin, or to define precisely the conditions under which free iodide does not diffuse into cell fluid or when the space it measures is identical with total water. Measured in vitro, iodide spaces, of slices of rat anterior tibial and various whole muscles, always exceeded the volume of extracellular fluid, but did not always equal the total volume of water in tissue.