Carrier-mediated transport of calcium into the in vitro choroid plexus: inhibitory effects of sodium, potassium, and pharmacological agents.

To test the hypothesis that sodium modifies calcium influx into the choroid plexus, the volume of distribution of ('45)Ca in rat choroid plexus was measured after various incubation periods in synthetic CSF solutions in which sodium and potassium concentrations were altered. The initial slope of the curve of ('45)Ca volume of distribution versus time is equal to the rate of calcium entrance into the choroid plexus tissue (I(,Ca)). The results indicate that calcium influx into the rat choroid plexus is carrier mediated with a I(,max) of 0.222 liter/kg wet tissue/min at 37(DEGREES)C and a Km equal to 0.275 mM. Sodium and potassium competitively inhibit calcium influx with Km's of 54.1 and 13.4 mM, respectively. Calcium influx is also correlated with the calculated membrane potential. The ('45)Ca volume of distribution at ten minutes, which is approximately equal to the steady state volume and reflects the amount of calcium in the cell cytoplasm, significantly correlates with choroid plexus water content. Total tissue water decreases as tissue calcium increases indicating the possibility of a negative feedback mechanism for calcium in regulating cell water and/or CSF secretion. Both amiloride (10('-3)M) and ouabain (10('-3)M) significantly (P < 0.05) inhibited calcium influx to 41% and 60% respectively of controls. Verapamil, a voltage-dependent calcium channel blocker, had no effect on calcium influx; this indicates that changes in calcium influx due to alterations in membrane potential caused by ouabain probably can not be attributed to opening or closing of voltage-dependent calcium channels. It is postulated that calcium, sodium and potassium compete for the inward arm of a cation/cation antiport system in the choroid plexus and that amiloride and ouabain influence calcium influx by directly affecting this membrane carrier. While not excluding membrane potential as a factor which affects calcium influx, extracellular sodium and potassium concentration appear to be dominant factors influencing calcium influx into the lateral ventricle rat choroid plexus.