COMPARTMENTATION OF THE INULIN SPACE IN MOUSE BRAIN SLICES
Authors: COHEN S. R. , BLASBERG R. , LEVI G. , LAJTHA A. 
Published: October 2006

Abstract— (1) Mouse cerebrum slices swell in tris‐buffered Krebs‐Ringer medium. Swelling is rapid at first, then slows to a more or less constant rate. Even after 3 hr incubation, water content/g of tissue dry wt. shows no sign of an asymptotic limit. Swelling is the same at 37° and at 0°.

(2) Tissue water measured by incubation with tritiated water is equal to total tissue water measured by drying slices. Equilibration between tritiated water and tissue water is complete within 2 min.

(3) Tissue liquid can be divided into three phenomenologically distinguishable compartments: first inulin space, which is the compartment permeable to inulin at both 0° and 37°; second inulin space, which is the compartment permeable to inulin at 37° but not at 0°; and 37°non‐inulin space, which is the compartment impermeable to inulin at both 0° and 37°. The evidence for this is:

(a) Penetration of inulin into tissue is greater at 37° than at 0°. After the first 20 min the rate of penetration at 0° is approximately equal to the rate of penetration at 37°, and only slightly less than the rate of increase of total tissue water. Therefore the smaller inulin space observed at 0° cannot be due to slower entry of inulin.

(b) The inulin content of slices incubated in inulin‐containing medium at 37° and cooled to 0° in the same medium is the same as the inulin content of tissue incubated at 37° without subsequent cooling. In contrast, the inulin content of tissues preincubated in inulin‐free medium at 37° and then incubated in inulin‐containing medium at 0° is the same as the inulin content of tissues incubated in inulin‐containing medium at 0° without preincubation at 37°. Therefore the smaller inulin space at 0° than at 37°can be due neither to a reversible temperature‐dependent change in the size of one single inulin space nor to an irreversible, greater swelling of a single inulin space at the higher temperature, but is due to some portion of the 37° inulin space becoming impermeable to inulin at 0°.

(c) Some inulin is retained by tissue incubated with inulin at 37°, then transferred to inulin‐free medium at 0°; the amount of retained inulin is equal to the difference between inulin content of tissue incubated with inulin at 37° and tissue incubated with inulin at 0°. This confirms 3b above and in addition shows that inulin which has entered the second inulin space at 37° is trapped there when this space becomes impermeable to inulin at 0°.

(4) The penetration of the amino acids, L‐lysine and D‐glutamate at 0° is equal to the penetration of inulin at 37°. This confirms the real existence of the 37° inulin space at 0°, and shows that the barrier at 0° between the first and second inulin spaces does not exist for these substances.

(5) The amino acids L‐leucine and glycine penetrate total tissue water at 0°. L‐leucine is actively transported at this temperature.

(6) The amino acids α‐aminoisobutyric acid, L‐leucine, and L‐lysine at 2 mm have no effect at 37° on either the inulin space or the non‐inulin space.

(7) The inulin space is insensitive at 37° to physiologically significant changes in the medium. In contrast, the non‐inulin space is quite sensitive to these changes. Addition of D‐glutamate greatly increases the non‐inulin space; addition of ouabain or cyanide, or omission of glucose, increases the non‐inulin space slightly; and replacement of Na+ ion by choline+ ion greatly decreases this space. These changes are independent and roughly additive.