Regulation of brain water during acute hyperosmolality in ovine fetuses, lambs, and adults

In adult rats, when plasma osmolality increases, water flows across the blood-brain barrier down its concentration gradient from brain to plasma, and brain volume deceases. The brain responds to this stress by gaining osmotically active solutes, which limit water loss. This phenomenon is termed brain volume (water) regulation. We tested the hypothesis that brain volume regulation is more effective in young lambs and adult sheep than in fetuses, premature lambs, and newborn lambs. Brain water responses to acute hyperosmolality were measured in the cerebral cortex, cerebellum, and medulla of fetuses at 60 and 90% of gestation, premature ventilated lambs at 90% of gestation, newborn lambs, young lambs at 20–30 days of age, and adult sheep. After exposure of the sheep to increases in systemic osmolality with mannitol plus NaCl, brain water content and electrolytes were quantified. The ideal osmometer is a system in which impermeable solutes do not enter or leave in response to an osmotic stress. There were si...