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Commercial inulin available at present is not a homogeneous product (1). It consists of two major fractions the proportions of which vary: (a) a portion of relatively large molecular weight resistant to heating in alkali; and (b) a portion of smaller molecular weight susceptible to destruction with alkali. In addition, most samples contain a small percentage of fructose, which also yields color in the analytical methods for inulin unless removed by treatment with yeast or hot alkali.Two recent reports (2, 3) indicate that the clearance of the alkali-stable and alkali-labile fractions of inulin may not be equal. Since some commercial preparations of inulin (such as the one used in this study) may contain as much as 50 per cent of the alkali-labile material, considerable errors in estimation of glomerular filtration rate would occur if a difference in clearance of the two fractions exists.The present report indicates that the clearance of the two fractions is identical, within the limits of error of analysis, in confirmation of the preliminary report by Cotlove (1). METHODSSubjects. The subjects were patients of the National Heart Institute at the Clinical Center. One was a normal volunteer. The others had renal or endocrine diseases. In most cases studies were performed in the fasting state. The comparisons were made during clearance determinations performed for other purposes.Procedure. Before each clearance determination, the ampoules of inulin to be used were mixed together and a portion withdrawn for analysis.' A priming injection and a sustaining infusion of inulin were administered intravenously. The plasma level of the alkali-stable fraction was approximately 13 mg. per cent. After a 30 to 40-minute equilibration period, 3 to 8 urine collection periods were obtained, each of 20 to 40 minutes' duration.

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A B S T R A C T Glomerulotubular balance was investigated in isolated, perfused rabbit proximal tubules in vitro in order to evaluate some of the mechanisms proposed to account for the proportionate relationship between glomerular filtration rate and fluid absorption generally observed in vivo. The rate of fluid transport from lumen to bath in proximal convoluted tubules in vitro was approximately equal to the estimated normal rate in vivo. The absorption rate in proximal straight tubules however was approximately one-half as great. If the mechanism responsible for maintenance of glomerulotubular balance is intrinsic to the proximal tubule, as has been proposed on the basis of micropuncture studies, the rate of fluid absorption in vitro should be directly related to the perfusion rate and/or tubule volume. In the present studies absorption rate was only minimally affected when perfusion rate was increased or the tubule distended. Thus, glomerulotubular balance is not mediated by changes in velocity of flow of the tubular fluid or tubular diameter and therefore is not an intrinsic property of the proximal tubule. It has also been proposed that glomerulotubular balance results from a humoral feedback mechanism in which angiotensin directly inhibits fluid absorption by the proximal convoluted tubule. In the present experiments, angiotensin was found to have no significant effect on absorption rate.

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