Electrolytes and water in the brain and cerebrospinal fluid of the foetal sheep and guinea-pig.

1. Samples of cisternal cerebrospinal fluid (c.s.f.) and blood plasma have been obtained nearly simultaneously from foetal sheep of different ages, the foetus having been exteriorized and maintained in a normal state with respect to its blood gases and arterial pH. The brains were removed from these foetuses and also from foetal guinea-pigs after exsanguination.2. A comprehensive study has been made of the concentrations of water, chloride, sodium, potassium, calcium and magnesium in blood plasma, c.s.f. and brain from foetal sheep and in brain from foetal guinea-pigs during development in utero. Maternal arterial blood plasma, cisternal c.s.f. and brain from the sheep and brain from the guinea-pig have been analysed for comparison.3. Concentrations of the ions analysed in foetal blood plasma are similar to those found by others in the sheep and other species. In the case of calcium, the results suggest an active maintenance of the concentration of this ion in foetal plasma by the placenta.4. The chloride concentration in c.s.f. at ages between 65 days and term (147 days) averaged 1.19 times that in foetal plasma, but only 1.08 at 45-50 days; the sodium concentration in c.s.f. was also slightly reduced at this time. The increase in the c.s.f./plasma ratios for chloride and sodium appears to coincide with the first development of the blood-brain and blood-c.s.f. barriers to non-electrolytes.5. Magnesium was at a slightly higher concentration in c.s.f. than in plasma at all foetal ages and did not vary with age. The concentrations of potassium and calcium in c.s.f. were high at early ages and fell to reach adult concentrations after birth: the mechanisms determining the concentrations of the various ions in c.s.f. develop at very different, largely independent, rates.6. The water content of the cerebral hemispheres of the foetal sheep was stable at 90% of the wet weight till 105 days and fell thereafter. The contents of chloride, sodium and potassium followed paraboloid relations with age. Chloride and sodium reached a peak of 62 and 81 m-equiv/kg respectively between 89 and 105 days in the sheep. Potassium was at a minimum of 65 m-equiv/kg at the same time. The content of water and these electrolytes in the cerebral hemispheres of the foetal guinea-pig underwent similar changes, the maxima and minimum occurring as in the sheep at two-thirds of the total length of the pregnancy, namely 65 days in the guinea-pig. At 46 days in the guinea-pig, chloride in brain reached 53 m-equiv/kg, sodium was 68 m-equiv/kg and potassium was 75 m-equiv/kg. In contrast to the sheep, no reversal of the sodium-potassium ratio occurred. These changes in water and electrolytes probably represent a rise, a peak and a decrease in the volume of the extracellular space of cerebral cortex, but changes in the volume occupied by a cell-type, containing much intracellular chloride and sodium, could also contribute to this phenomenon.7. The calcium content in the cerebral hemispheres of the foetal sheep remained at about 3.0 m-equiv/kg throughout pregnancy. Magnesium in the cerebral hemispheres of both the foetal sheep and the guinea-pig showed a trough in concentration during pregnancy which corresponded approximately in position to the minimum in potassium content and the maxima in chloride and sodium contents. Lowest values were 9.6 m-equiv/kg at 105 days in the sheep and 9.5 m-equiv/kg at 51 days in the guinea-pig.