DEVELOPMENTAL-CHANGES IN BLOOD-BRAIN-BARRIER POTASSIUM PERMEABILITY IN THE RAT - RELATION TO BRAIN GROWTH

1. The potassium permeability of the blood-brain barrier (BBB) was det ermined in anaesthetized rats aged between 21 days gestation and adult using Rb-86(+) as a marker for potassium. 2. The brain influx rate co nstant for Rb-86(+) was high in fetal cortex at 21 days gestation (42. 5 +/- 4.3 mu l g(-1) min(-1)) but had decreased markedly by just after birth (12.2 +/- 0.6 mu l g(-1) min(-1)). There was a further, gradual , postnatal decline to 7.0 +/- 0.3 mu l g(-1) min(-1) by 50 days after birth. 3. Developmental changes in passive BBB permeability were exam ined over the same age range using (14)[C]urea. These studies showed s imilar developmental changes in influx rate to those found for Rb-86(). Specifically, a marked perinatal decline followed by a more gradual postnatal fall. Thus, the changes in potassium permeability probably reflect a decrease in the BBB paracellular leak during development. 4. The changes in BBB permeability coincide with changes in the rate of brain growth and the associated rate of brain potassium accumulation. As the potassium permeability properties of the adult BBB would provid e insufficient potassium influx to meet the requirement associated wit h fetal brain growth, it is suggested that need for potassium may be t he reason for the greater BBB permeability early in development.