Membrane response to static magnetic fields: effect of exposure duration.

The time-course for the reversible alteration in presynaptic membrane function associated with exposure to a 123 mT static magnetic field was examined in an attempt to help define the mechanism whereby these fields influence biomembranes. Miniature endplate potentials (MEPPs) were recorded in the isolated murine neuromuscular junction preparation, maintained at a temperature of 35.5 degrees C. A minimum field duration of 50 s was found to be necessary for MEPP inhibition, with the efficacy of the field in inducing further inhibition being a function of its duration, but only for periods up to 150 s. Longer durations were not associated with additional inhibition. The time required for MEPP frequency to return to baseline, following deactivation of the field, was found to be linear for field durations up to 150 s. At and above this limit, recovery time remained constant at 135 s. These findings are consistent with the slow reorientation of diamagnetic molecular domains within the membrane and suggest tight coupling to the mechanism responsible for neurotransmitter release. The limits on this effect are compatible with the mechanical constraints imposed by the membrane's cytoskeleton.