Abstract
1. This investigation was undertaken to determine the time course of development of cation transport processes between blood and the extracellular fluid compartments of the central nervous system (C.N.S.) of the foetal rhesus monkey. The development of concentration gradients within the cerebrospinal fluid (c.s.f.) system was also studied in an attempt to gain information on regional variations in transport activities.
2. Cerebrospinal fluid samples were obtained from the lateral ventricle, the cisterna magna, the cortical subarachnoid space and the lumbar region of the spinal subarachnoid space of adult monkeys. Foetal c.s.f. samples were obtained from the cisterna magna and the frontal region of the cortical subarachnoid space. Blood samples were taken within 3-10 min of the c.s.f. samples. The concentration of K, Mg and Ca were determined on appropriately diluted samples by atomic absorption spectrophotometry.
3. The concentrations of K, Mg, and Ca in the plasma and c.s.f.s of the adult monkey are similar to those of other mammalian species. The c.s.f./blood concentration gradients of these cations are in the same direction but generally of greater magnitude than those in non-primate mammals. The indicated [Mg] in the extracellular fluid of the adult monkey's cerebral cortex is high (more than 2 m-equiv/kg H2O) and the [K] is low (less than 2 m-equiv/kg H2O).
4. Accumulation of Mg into the c.s.f. system against a concentration gradient is evident in the earliest foetuses studied (50 days of intrauterine life) and by the second half of gestation the concentration gradients of this cation are similar to that of the adult.
5. In the second trimester foetus, the [K] in the cisternal fluid is near the plasma ultrafiltrate level. It then decreases gradually throughout the remainder of gestation, approximating the adult value at birth.
6. The very low [K] typical of the adult cortical subarachnoid fluid is observed by the sixth month following birth but not during foetal life or within the first few weeks of post-natal life. This indicates that the transport processes at the cortical region of the haematoencephalic interphase do not fully develop until well after birth.
7. It is concluded that the various transport processes responsible for the elaboration and maintenance of adult type of K, Ca and Mg concentrations within the extracellular fluid compartments of the monkey C.N.S. do not develop at the same time. This temporal dissociation provides strong evidence against any direct association or coupling between the haematoencephalic K and Mg transport systems.
8. Existence of normal cation concentration gradients between c.s.f. and blood may serve as a criterion for the normality of the (foetal) blood—brain barrier.
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Selected References
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