INTRACELLULAR CONCENTRATIONS OF MAJOR IONS IN RAT MYELINATED AXONS AND GLIA - CALCULATIONS BASED ON ELECTRON-PROBE X-RAY MICROANALYSES

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.