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

The integrity of the blood-brain barrier is disrupted in certain clinical conditions, such as hyperparathyroidism, resulting in an increase in brain calcium content. To examine the mechanism(s) of this phenomenon, studies were undertaken to evaluate the regulation of calcium and phosphorus by the blood-cerebrospinal fluid (CSF) interface of the blood-brain barrier in humans and animals. The relationship between the concentrations of total calcium (CaT) and its ionized fraction (CaI and inorganic phosphorus (P) 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. Ca1 constituted only 50–65% CaT in CSF. The level of CSF P ranged between 1.0–1.9 mg/100 ml despite wide variations in blood P (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 mmol/liter. In summary, the data reveal that: 1) the concentrations of CaT, CaI, and P in CSF vary only slightly despite marked variations in their blood values, 2) the level of CaI in CSF is approximately one half that of the CaT in CSF, a difference that may be accounted for by the presence of complexed Ca with lactate and citrate, 3) the calcium-P 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 results suggest that the blood-CSF interface of the blood-brain barrier maintains a stable level of calcium and P in CSF regardless of alterations in their blood concentrations or the presence or absence of parathyroid hormone.