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 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 of 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 less than or equal to 0.53 mumol/ml, maximal Ca influx for the saturable component between 27 and 67 x 10(-5) mumol.g-1.s-1, and the passive component of Kcsf 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].
Murphy
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Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, MD 20892.
Smith
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Rapoport
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SI
D000818:Animals; D001835:Body Weight / Q000187:drug effects; D002118:Calcium / Q000134:cerebrospinal fluid*; D002132:Calcium Radioisotopes; D002136:Calcium, Dietary / Q000493:pharmacokinetics / Q000494:pharmacology; D002713:Chlorine; D002831:Choroid Plexus / Q000378:metabolism; D004032:Diet; D006934:Hypercalcemia / Q000134:cerebrospinal fluid*; D006996:Hypocalcemia / Q000134:cerebrospinal fluid*; D008297:Male; D011868:Radioisotopes; D051381:Rats; D011916:Rats, Inbred F344
D002132:Calcium Radioisotopes; D002136:Calcium, Dietary; D011868:Radioisotopes; D002713:Chlorine; D002118:Calcium