MAGNESIUM CONTENT AND NET FLUXES IN SQUID GIANT-AXONS

The Mg content of axons freshly dissected from living specimens of the tropical squid Doryteulhis plei was determined by atomic absorption spectroscopy to be 4.2 ± 0.2 mmol/kg axoplasm. The axon’s ability to maintain this physiological content of total intracellular Mg ([Mg]i) was studied. Mgi was shown to be a linear function of Mg0 when Mg0 of incubating fluid was varied between 0 and 250 mM. When Mg0 = 15 mM, Mgi was found to be the same in incubated fibers as in fibers freshly dissected. Mgi levels were unaffected by depolarization of the membrane by high Ko. Stimulation resulted in an extra influx of Mg of 0.05 pmol/(cm2 · impulse) when Mg0 = 55 mM. Mgi was found to be a complicated function of the concentration of extracellular Na or Li (Xo), which was substituted for Tris. With 385 mM Li0 the Mgi level was found to be 2.5-fold larger than the level observed with 385 mM Na0 after incubation for 3 h. The function relating Mg0 to Xo was qualitatively unaffected in axons poisoned with the mitochondrial uncoupler carbonyl cyanide, p-trifluorome-thoxy-phenylhydraxone (FCCP) and the inhibitor of glycolysis, iodoacetic acid (IAA); the absolute levels of Mgi, however, were some 30% higher in the poisoned axons at all [X]0 explored. 2 h incubation of axons in a 333 mM Mgi 40 mM Li solution increased Mg, 3.5-fold in control axons and 5-fold in poisoned axons. These Mg-loaded axons were able to recover physiological levels of Mgi with a half-time of 3-5 h only if kept in a solution which contained Na (220 mM) regardless of whether the axons had been inhibited with FCCP + IAA. Therefore, it may be concluded that the physiological Mg, concentration can be maintained by the Na electrochemical gradient, even when the axon is metabolically poisoned. © 1979, Rockefeller University Press., All rights reserved.