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

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Abstract
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 45Ca and 36Cl into CSF (Kcsf) were determined 3 or 10 min after intravenous tracer injection in unanesthetized animals. In rats fed low Ca diet, KcsfS for 45Ca and 36Cl were elevated above control, but the 45Ca/36Cl ratio for Kcsf, a more specific measure of Ca transport, was also increased. In animals fed high Ca diet, KcsfS both radiotracers were not statistically different from control, but the 45Ca/36Cl ratio was decreased. Injection of CaCl2 into hypocalcemic rats elevated plasma [Ca], depressed 45Ca Kcsf, and returned the 45Ca/36Cl ratio to the control value. The inverse relationship between plasma ionized [Ca] and 45Ca/ Kcsf was fitted to saturation kinetics with Km ≤ 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 Kcsf ≤ 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].