REGULATION OF BRAIN AND CEREBROSPINAL-FLUID CALCIUM BY BRAIN BARRIER MEMBRANES FOLLOWING VITAMIN D-RELATED CHRONIC HYPOCALCEMIA AND HYPERCALCEMIA IN RATS

Male Fischer-344 rats, 21 days old, were fed diets containing 0 (LOD), 2,200 (CONT), or 440,000 (HID) international units of vitamin D3 per kilogram for 12 weeks. [Ca] was measured in plasma, CSF brain, and choroid plexus. In addition, 45Ca and 36Cl transfer coefficients (KCa and KCl) for uptake from blood into CSF and brain were determined. Although plasma ionized [Ca]s in LOD and HID rats were 50% an 136%, respectively, of values in CONT animals, CSF and brain [Ca]s ranged from only 85% to 110% of respective CONT values. Choroid plexus [Ca] was increased by 37% after HID diet, but was decreased only 10% after LOD. KCa values at CSF, parietal cortex, and pons-medulla were negatively correlated with plasma ionized [Ca], whereas KCl values at CSF and brain were not fifferent between the diet groups. The findings demonstrate that central nervous system [Ca] is maintained during chronic hypo- or hyper-calcemia by saturable transport of Ca at brain barrier membranes. This transport does not seem to involve modulation by 1,25-dihydroxyvitamin D3.