AJP - Regulatory, Integrative and Comparative Physiology, Vol 248, Issue 5 621-R626, Copyright © 1985 by American Physiological Society
Differences between CSF and plasma Na+ and K+ activities and concentrations
D. C. Johnson, L. Orlowitz and B. M. Hitzig
[Na+] and [K+] determined by ion-selective electrodes (ISE) by direct
potentiometric determination were compared with those obtained by flame
photometry in plasma and in cisternal cerebrospinal fluid (CSF) obtained
from pentobarbital-anesthetized dogs. Comparisons of pre- and
postanesthesia values showed no change in plasma [Na+] but a 5% fall in
plasma [K+]. CSF and plasma Na+ activities were identical, indicating that
Na+ does not contribute to the potential difference between them. CSF K+
activity was 74% of that in plasma. The CSF-to-plasma molar and molal
ratios differed from the activity (ISE) ratios. Concentrations of Na+ and
K+ were also determined in an electrolyte solution and in the same solution
with albumin added. Flame values differed from ISE values in plasma, CSF,
and solutions. Albumin lowered flame values to a greater extent than ISE
values. Bicarbonate lowered ISE values. ISE ratios, which equal activity
ratios, rather than molar or molal ratios should be used in thermodynamic
equilibrium equations, whereas molar concentrations should be used in
electroneutrality equations or in determination of the strong ion
difference.
