Saturable transport of Ca into CSF in chronic hypo‐ and hypercalcemia

To further characterize possible saturable transport of Ca into CSF during chronic plasma [Ca] changes, weanling rats were fed diets differing in Ca for 10 weeks. Transfer coefficients for unidirectional uptake of 45 Ca and 36 Cl into CSF (K csf ) were determined 3 or 10 min after intravenous tracer injection in unanesthetized animals. In rats fed low Ca diet, K csf S for 45 Ca and 36 Cl were elevated above control, but the 45 Ca/ 36 Cl ratio for K csf , a more specific measure of Ca transport, was also increased. In animals fed high Ca diet, K csf S both radiotracers were not statistically different from control, but the 45 Ca/ 36 Cl ratio was decreased. Injection of CaCl 2 into hypocalcemic rats elevated plasma [Ca], depressed 45 Ca K csf , and returned the 45 Ca/ 36 Cl ratio to the control value. The inverse relationship between plasma ionized [Ca] and 45 Ca/ K csf was fitted to saturation kinetics with K m ≤ 0.53 μmol/ml, maximal Ca influx for the saturable component between 27 and 67 × 10 −5 μmol·g −1 ·s −1 , and the passive component of K csf ≤ 15 × 10 −5 ml·g −1 ·s −1 . We conclude that Ca transport into CSF is saturable and this transport is important is the regulation of CSF [Ca].