THEORETICAL LIMITS ON THE THRESHOLD FOR THE RESPONSE OF LONG CELLS TOWEAK EXTREMELY-LOW-FREQUENCY ELECTRIC-FIELDS DUE TO IONIC AND MOLECULAR FLUX RECTIFICATION

Understanding exposure thresholds for the response of biological syste ms to extremely low frequency (ELF) electric and magnetic fields is a fundamental problem of long-standing interest. We consider a two-state model for voltage-gated channels in the membrane of an isolated elong ated cell (L-cell = 1 mm; r(cell) = 25 mu m) and use a previously desc ribed process of ionic and molecular flux rectification to set lower b ounds for a threshold exposure. A key assumption is that it is the abi lity of weak physical fields to alter biochemistry that is limiting, n ot the ability of a small number of molecules to alter biological syst ems. Moreover, molecular shot noise, not thermal voltage noise, is the basis of threshold estimates. Models with and without stochastic reso nance are used, with a long exposure time, t(exp) = 10(4) s. We also d etermined the dependence of the threshold on the basal transport rate. By considering both spherical and elongated cells, we find that the l owest bound for the threshold is E-min approximate to 9 x 10(-3) V m(- 1) (9 x 10(-5) V cm(-1)). Using a conservative value for the loop radi us r(loop) = 0.3 m for induced current, the corresponding lower bound in the human body for a magnetic field exposure is B-min approximate t o 6 x 10(-4) T (6 G). Unless large, organized, and electrically amplif ying multicellular systems such as the ampullae of Lorenzini of elasmo branch fish are involved, these results strongly suggest that the biop hysical mechanism of voltage-gated macromolecules in the membranes of cells can be ruled out as a basis of possible effects of weak ELF elec tric and magnetic fields in humans.