SATURABLE TRANSPORT OF CA INTO CSF IN CHRONIC HYPOCALCEMIA 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 Ca-45 and Cl-36 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 Ca-45 and Cl-36 were elevated above control, but the Ca-45/Cl-36 ratio for K(csf), a more specific measure of Ca transport, was also increased. In animals fed high Ca diet, K(csf)s of both radiotracers were not statistically different from control, but the Ca-45/Cl-36 ratio was decreased. Injection of CaCl2 into hypocalcemic rats elevated plasma [Ca], depressed Ca-45 K(csf), and returned the Ca-45/Cl-36 ratio to the control value. The inverse relationship between plasma ionized [Ca] and Ca-45 K(csf) was fitted to saturation kinetics with K(m) less-than-or-equal-to 0.53-mu-mol/ml, maximal Ca influx for the saturable component between 27 and 67 x 10(-5)-mu-mol.g-1.s-1, and the passive component of K(csf) less-than-or-equal-to 15 X 10(-5) ml.g-1.s-1. We conclude that Ca transport into CSF is saturable and this transport is important in the regulation of CSF [Ca].