Osmoregulation in surviving slices from the kidneys of adult rats

Abstract

The total amount of water in slices of the renal cortex of adult rats was determined after they had been exposed to saline solutions with concentrations from 0.58 to 0.03 os-mol./l.: (a) When metabolism was suppressed by lowering the temperature to 0 to 4 degrees C. (b) When the slices were respiring in oxygen at 38.5 degrees C. (c) When respiration was inhibited by cyanide at 38.5 degrees C. (d) When metabolism was reduced by cooling to temperatures between 38 and 16 degrees C. The amount of water in slices at 0 to 4 degrees C varied inversely with the concentration of the medium, and when this concentration was less than 0.58 os-mol./l. the slices contained more water than the tissue in vivo. Slices respiring at 38.5 degrees C in solutions more dilute than 0.58 os-mol./l. contained considerably less water than slices in the same solutions at 0 to 4 degrees C, and dilution of the medium from 0.58 to 0.19 os-mol./l. produced a much smaller increase in the amount of water in the slices when they were respiring than when they were at 0 to 4 degrees C. The uptake of oxygen at 38.5 degrees C was independent of the concentration of the medium between 0.45 and 0.12 os-mol./l. Slices whose respiration was inhibited by cyanide in 'isotonic' (0.30 OSM) solutions at 38.5 degrees C contained more water than slices respiring freely in concentrations as low as 0.06 os-mol./l. The changes produced by cyanide in oxygen uptake and in water content were both reversible. When the uptake of oxygen was reduced by cooling to 30 degrees C, the amount of water in the slices decreased. Evidence is presented that all the changes in water content were almost complete in 2 min., and were then maintained for several hours. These results suggest that respiration is more important than the osmotic pressure of the external medium in determining the amount of water in cells. They cannot by explained by the orthodox theory that mammalian cells are in osmotic equilibrium with their surroundings, and indeed they suggest that the osmotic pressure of the cell fluids is normally 50 to 100% greater than that of the extracellular fluids. All the observations can be explained if energy derived from respiration is used to expel water from the cells, so that a steady state is maintained in which the higher internal osmotic pressure causes water to diffuse into the cells as fast as it is pumped out. The energy required to maintain the observed amount of water in the respiring slices was calculated by a simple theoretical treatment. It was found to be proportional to the observed oxygen uptake and to be a small percentage of its energy equivalent.