Inulin as an Indicator for the Extracellular Space

Summary. A number of results are quoted from the literature, showing that the available volumes for Cl‐and Na+are larger than the extracellular volume; the author gives a tabulated summary of literature references to the size of the available volume for these substances and the volumes of distribution for these and SCN‐, Br‐, SO–4, sucrose and Mg++. In 13 nephrectomized rabbits and one nephrectomized dog the decline in the concentrations of inulin and SCN‐in plasma was traced after intravenous injection of a single dose of inulin + NaSCN. It was found that: The experimentally determined curve of distribution for SCN‐(the volumes of distribution plotted against time) was found to be placed well above a theoretical curve of distribution for SCN‐, constructed from the assumptions that inulin and SCN are finally distributed in the same volume and that this distribution proceeded exclusively by diffusion. The quantity of fluid in the erythrocytes is entirely insufficient to explain this discordance; it its presumable that SCN‐penetrates other cells of the organism. The distribution after a very long experimental period (12 hours) suggests that the ultimate distribution in rabbits will be such that inulin distributes itself in slightly less than 20 % of the body weight and SCN– in a little over 30 %. In a few of the experiments, when the distribution of SCN‐and inulin was well advanced, 100–120 ml. isotonic sulphate solution was infused into the rabbits. From the resulting dilution of plasma‐chloride the author calculated the “volume of dilutable chlorides”; at a certain time after the sulphate infusion it was found to be less than the volume of distribution for SCN‐at the same period of time after the NaSCN injection. Whereas the available spaces for SCN‐and Cl‐must be supposed to be larger than the extracellular space, inulin must be assumed to distribute itself in a volume that is equal to the extracellular space. Tests showed that inulin is not excreted in rabbit bile, and experiments with blockade of the reticulo‐endothelial system seem to indicate that inulin is not stored by this system. Inulin's slow distribution, especially in sparsely vascularized tissue such as tendons, is, however, a serious drawback to its practical application for the determination of the extracellular space, even in nephrectomized animals.