Pk. Stys et al., INTRACELLULAR CONCENTRATIONS OF MAJOR IONS IN RAT MYELINATED AXONS AND GLIA - CALCULATIONS BASED ON ELECTRON-PROBE X-RAY MICROANALYSES, Journal of neurochemistry, 68(5), 1997, pp. 1920-1928
Electron probe x-ray microanalysis (EPMA) was used to measure water co
ntent (percent water) and dry weight elemental concentrations (in mill
imoles per kilogram) of Na, K, Cl, and Ca in axoplasm and mitochondria
of rat optic and tibial nerve myelinated axons, Myelin and cytoplasm
of glial cells were also analyzed. Each anatomical compartment exhibit
ed characteristic water contents and distributions of dry weight eleme
nts, which were used to calculate respective ionized concentrations, F
ree axoplasmic [K+] ranged from approximate to 155 mM in large PNS and
CNS axons to approximate to 120-130 mM in smaller fibers. Free [Na+]
was approximate to 15-17 mM in larger fibers compared with 20-25 mM in
smaller axons, whereas free [Cl-] was found to be 30-55 mM in all axo
ns. Because intracellular Ca is largely bound, ionized concentrations
were not estimated. However, calculations of total (free plus bound) a
queous concentrations of this element showed that axoplasm of large CN
S and PNS axons contained approximate to 0.7 mM Ca, whereas small fibe
rs contained 0.1-0.2 mM, Calculated ionic equilibrium potentials were
as follows (in mV): in large CNS and PNS axons, E-K = -105, E-Na = 60,
and E-Cl = -28; in Schwann cells, E-K = -107, E-Na = 33, and E-Cl = -
33; and in CNS glia, E-K = -99, E-Na = 36, and E-Cl = -44. Calculated
resting membrane potentials were as follows (in mV, including the cont
ribution of the Na+,K+-ATPase): large axons, about -80; small axons, a
bout -72 to -78; and CNS glia, -91. E-Cl is more positive than resting
membrane potential in PNS and CNS axons and glia, indicating active a
ccumulation. Direct EPMA measurement of elemental concentrations and s
ubsequent calculation of ionized fractions in axons and glia offer fun
damental neurophysiological information that has been previously unatt
ainable.