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Inulin, sodium, potassium, and chloride were determined on identical samples of serum and fluid collected from the kidney tubules of Necturus under urethan anesthesia. Inulin fluid-serum ratios obtained were as follows: highest ratios for proximal tubules approximated 2, indicating a reabsorption of about 50%, of filtered water; highest ratios for distal tubules were near 3.4; ratios for sodium remained near 1 in proximal tubules, tending to be lowest near the ends of the segments. There was an abrupt drop in sodium concentration in the early part (thin segment) of the distal tubules, which may be made possible by the better oxygen supply in this region. Chloride ratios rose to 1.12 in the proximal tubules and chloride concentrations fell over the same region, as did sodium. Potassium ratios remained close to 1 in the proximal tubules, indicating that up to one-half of the filtered potassium may be reabsorbed there. Distal tubule potassium ratios showed great variability, some being higher and some lower than 1.

During the course of studies of the properties of the renal tubule of amphibia we had occasion to search for a substance which, introduced into the tubule, would not be expected to be absorbed from it into the blood, either actively or by diffusion. Among those tried was the plysaccharide inulin, chosen because of its high molecular weight and because it is not hydrolyzed by enzymes or tissues of vertebrates. After having found that inulin is much less rapidly diffusible than either creatinine or glucose ; that it is filterable through collodion membranes which are impermeable to protein and through the glomerular membranes of amphibia; and also that it is not excreted by the aglomerular kidney (toadfish) after intravenous injection, it seemed desirable to study its rate of excretion in mammals. Given intravenously to dogs or rabbits it is excreted in the urine rapidly, and, insofar as a few experiments show, completely. Concentration ratios, U/P, as high as 150 have k n observed. It became obvious that some of the considerations developed by Jolliffe, Shannon and Smith' upon which they based their advocacy of plasma clearance of xylose in preference to that of creatinine as a true measure of the volume of glomerular filtration could equally well be made the basis of similar advocacy of inulin clearance for the same purpose; with this advantage, that the low diffusibility of inulin, much lower than that of creatinine, of xylose and of the other 2 non-metabolized sugars tested by them (sucrose, raffinose) could be expected to minimize a difference between plasma clearance and rate of glomerular filtration, provided such a difference exists and is the result of back diffusion of the substance studied. Accordingly, a series of preliminary experiments has been made, admittedly not perfect, in which the simultaneous plasma clearances of inulin and creatinine have been determined in unanesthetized female dogs. The results, which seem sufficiently convincing to put on record, indicate that the plasma clearance of inulin given intravenously is slightly higher than, but of the same order as that of creatinine.

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