Efflux mechanism contributing to the stability of the potassium concentration in cerebrospinal fluid.

1. The clearance of (42)K from c.s.f. has been separated into two components by means of ventriculo-cisternal perfusion in the rabbit. At 2 hr the largest fraction of radioactivity is recoverable from brain. A smaller fraction passes into the bloodstream and this loss can be expressed as a barrier clearance.2. The clearance into brain was largely independent of potassium concentrations in the perfusion fluid of 15 m-equiv/l. and below. It was depressed by ouabain, 10(-2) mM.3. The barrier clearance was small, about 9% of the total, when the perfusion fluid contained potassium (1.5 m-equiv/l. or below). Above this concentration it increased steeply reaching 32 mul./min or 37% of the total at 10 m-equiv/l. A similar high barrier clearance was caused by replacing 84% of the sodium in the perfusion fluid with choline. Ouabain, 10(-2) mM, abolished the increased barrier clearance due to potassium (10 m-equiv/l.).4. The clearance of [(14)C]urea into both brain and blood was unaffected by the potassium concentration in c.s.f. The barrier clearance of [(14)C]urea was, if anything, increased by 10(-2) mM ouabain.5. Perfusion of the low sodium fluid caused a net loss of potassium from c.s.f.6. The influx of (42)K into c.s.f. from blood was the same, when the perfusion fluid contained potassium (2.9 or 10 m-equiv/l.).7. The potential between c.s.f. and blood of about 4 mV (c.s.f. positive) was little affected by the potassium or sodium concentration in the perfusion fluid.8. These observations indicate that the net flux of potassium ions from c.s.f. to blood begins to increase very steeply with the potassium concentration in c.s.f., when the latter is between 2 and 3 m-equiv/l. This relation, taken together with the variation of influx with the potassium concentration in blood plasma, can largely explain the known stability of the potassium concentration in the c.s.f. of the rabbit at 2.8-2.9 m-equiv/l.9. The increased flux of potassium from c.s.f. at raised concentrations of potassium in this fluid appears to depend on a sodium-potassium pump inhibitable by ouabain.