Volume adjustment by renal medullary cells in hypo- and hyperosmolal solutions containing permeant and impermeant solutes.

1. The changes in the volumes of cells in slices (thickness 0-3-0-4 mm) of rat renal outer and inner medulla have been investigated during aerobic incubation for 20 min at 37 degrees C in Krebs phosphate-bicarbonate Ringer modified by the addition of urea or sucrose in order to produce a range of media hypo- and hyperosmolal with respect to the calculated tissue fluid osmolalities in these regions. 2. On the assumption that under these conditions the measured inulin space approximates to the true extracellular space (ECS), it was found that osmotic swelling or shrinkage of cells was not accompanied by any significant variation in the absolute size of the ECS. 3. Calculated cell volume changes in both regions were minimal when slices were incubated in urea-containing media iso-osmolal with tissue fluids in that region. In sucrose-containing media minimal cell volume changes occurred when media were hypo-osmolal in relation to tissue fluids by a factor of approximately 0-68. 4. In all except the most hypo-osmolal media studied, calculated cell volume changes (as percentage of initial volume) were linearly related to the reciprocal of the incubation media osmolalities. The points of interception of the regression lines on the cell volume axis were dependent upon both the region studied and the composition of the incubation medium (urea or sucrose). 5. These changes were accompanied by variations in slice solute concentrations. Slice [Na] was greatest, and slice [K] least, following incubation in those media producing the greatest percentage changes in cell volume. 6. The volume of distribution [14-C]sucrose within the inner medulla was 61-7 plus or minus 2-5 mul./100 mg wet weight of tissue (mean plus or minus S.E., n equals 6) after 10 min incubation. The increase to 70-8 plus or minus 4-2 mul./100 mg (n equals 6) after 100 min was not significant (0-1 greater than P greater than 0-05). The volume of distribution within the outer medulla rose markedly during this period, from 38-1 to 58-2 mul./100 mg.