Inulin Space as a Function of Equilibration Time

¹ From the Department of Physiology, Washington University School of Medicine, Saint Louis, Missouri

Recovery of alkali-stable inulin injected intravenously into nephrectomized rats is complete at 30 minutes and at 3 hours, but falls progressively to 73% at 72 hours. Whole body inulin space is less than Cl space at 30 minutes and 3 hours, equals Cl space somewhere between 15 and 24 hours, and exceeds total body water at 72 hours. Inulin in cells is still in alkali-stable form. Alkali-stable inulin space equals total inulin space. Inulin space at 72 hours greatly exceeds water content in three tissues, liver, skin and spleen, rich in macrophages, while it is less than water content in a tissue, skeletal muscle, poor in macrophages. It is postulated that macrophages actively ingest and concentrate inulin. Rise in inulin space follows a straight semilog line rather closely, but this cannot be taken as proof that a single exponential process is operating. Since inulin space never levels off, the period of equilibration for the best measurement of ECF is uncertain.