Relationship between the Concentrations of Calcium and Phosphorus in Blood and Cerebrospinal Fluid*

Abstract

The integrity of the blood-brain barrier [BBB] is disrupted in certain clinical conditions, such as hyperparathyroidism, resulting in an increase in brain Ca content. To examine the mechanism(s) of this phenomenon, studies were undertaken to evaluate the regulation of Ca and P by the blood-CSF interface of the BBB in humans and animals [dog]. The relationship between the concentrations of total Ca (CaT) and its ionized fraction (CaI) and inorganic P [iP] in blood and CSF were studied during acute and chronic marked variations in their blood levels and in the presence and absence of parathyroid hormone. The concentration of CaT in CSF was stable [4.78 .+-. 0.05 (SE) mg/100 ml] when the blood CaT ranged between 5.1-9.5 mg/100 ml and rose only slightly (to 5.37 .+-. 0.06 mg/100 ml) when blood CaT ranged between 9.5-14.8 mg/100 ml. CaI constituted only 50-65% CaT in CSF. The level of CSF Pi ranged between 1.0-1.9 mg/100 ml despite wide variations in blood iP (1.2-21.1 mg/100 ml). The CSF contained lactate and citrate in concentrations of 1.87 .+-. 0.33 and 0.29 .+-. 0.04 mM. The concentrations of CaT, CaI and iP in CSF vary only slightly despite marked variations in their blood values, the level of CaI in CSF is approximately 1/2 that of the CaT in CSF, a difference that may be accounted for by the presence of complexed Ca with lactate and citrate, the Ca-iP product in CSF was maintained within a narrow range in the face of 3- to 4-fold variations in the values of the product in blood. The blood-CSF interface of the BBB apparently maintains a stable level of Ca and iP in CSF regardless of alterations in their blood concentrations or the presence or absence of parathyroid hormone.