The role of the cat choroid plexus in regulating cerebrospinal fluid magnesium.

1. The regulation of c.s.f. Mg concentration was studied using the cat choroid plexus isolated in a chamber in situ. 2. An increase in plasma Mg concentration was accompanied by the usual reciprocal decrease in plasma Ca concentration. Chamber fluid Ca concentration was unaffected. 3. Hypermagnesemia (plasma Mg concentration greater than 6 m-equiv/1.) caused relatively small increases in c.s.f. Mg concentration (delta plasma [Mg]/ delta c.s.f. [Mg] = 4). 4. Various chamber fluid Mg concentrations (0, 2.4 or 4.8 m-equiv/1.) were rapidly (within 30--60 min) returned to near the control value of 1.83 m-equiv/1. 5. When plasma and chamber fluid Mg concentrations were altered simultaneously, the final chamber fluid Mg concentration was returned towards normal with or against a concentration gradient. 6. The data indicate that the choroid plexus is involved in maintaining the constancy of the c.s.f. Mg concentration by sensing changes in the normal c.s.f. Mg concentration and altering approximately its rate of active secretion of Mg.