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BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS ARTICLE NO.
222, 374–378 (1996)
0751
A Mechanism for Action of Extremely Low Frequency Electromagnetic Fields on Biological Systems W. X. Balcavage,*,1 T. Alvager,† J. Swez,† C. W. Goff,‡ M. T. Fox,* S. Abdullyava,* and M. W. King* *Indiana University School of Medicine, at Indiana State University, †Department of Physics, and ‡Life Sciences Department, Indiana State University, Terre Haute, Indiana 47809 Received April 1, 1996 This report outlines a simple mechanism, based on the Hall Effect, by which static and low frequency (50–60 Hz) pulsed electromagnetic fields (PEMFs) can modify cation flow across biological membranes and alter cell metabolism. We show that magnetic fields commonly found in the environment can be expected to cause biologically significant interactions between transported cations and basic domains of cation channel proteins. We calculate that these interactions generate forces of a magnitude similar to those created by normal transmembrane voltage changes known to gate cation channels. Thus PEMFs are shown to have the potential of regulating flow through cation channels, changing the steady state concentrations of cellular cations and thus the metabolic processes dependent on cation concentrations. © 1996 Academic Press, Inc.
While epidemiologists continue to uncover links between PEMFs and human health (1, 2), there is no generally accepted mechanism to explain how extremely low frequency (ELF) electromagnetic fields might initiate changes in biological systems. Several mechanisms, including ion parametric resonance (3) and cyclotron resonance (4), have been proposed to explain these events. The latter involve PEMF-induced perturbation of ion transport or ion binding, with the resulting ion imbalances assumed to be responsible for PEMF-related metabolic changes. Additionally, Jacobson Resonance (5), a process based on quantum mechanics, has been proposed as a general mechanism to describe how PEMFs influence many biological processes. The present report outlines a more modest mechanism in which classical Hall-like forces (6) are shown to be capable of generating biologically significant interactions between cations and ion channel proteins in biological membranes. While our proposed mechanism is also based on interactions between ions and PEMFs, it differs from other mechanisms in that it involves PEMF-induced perturbation in the path of cations within channels, which in turn results in electrostatic interactions between cations and cation channel proteins. Such interactions could trigger functional (gating) conformation changes in channel proteins, leading to altered physiological states of the cell. This mechanism provides an easily understood yet conceptually sound link between magnetic fields and biological processes. Hall Effects in Conductors and Cation Channels The classical Hall effect is observed in electric conductors subjected to static magnetic fields oriented at right angles to the current flow. Under these circumstances, electron flow through conductors is perturbed so that a Hall voltage (VH) is generated across the conductor perpendicular to both the direction of bulk current flow and to the magnetic field. Like electrons in conventional conductors, ions flow across biomembranes in response to very large electric fields - but in this case through protein-lined channels. If ion currents through channels can be treated like electron flow through conventional conductors, it is reasonable to expect static magnetic fields to generate 1
To whom correspondence should be addressed. 374
0006-291X/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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Hall-like effects on ions passing through protein channels. Moreover, if PEMFs—purported to have bioeffects—can be treated like short lived static magnetic fields, one can easily understand how ELF fields might impact physiological processes. The premise of the following analysis is that transmembrane ion flux is regulated by voltagedependent changes in the conformation of channel proteins, and that processes that perturb ion flux will cause profound changes in the metabolism and fate of affected cells. In this report we present a mechanism by which PEMFs in the 50–60 Hz range can perturb the transmembrane movement of cations such as K+, Na+ or Ca+2 through their respective channels, thus producing biological effects. RESULTS AND DISCUSSION Figure 1, Panel A illustrates the Hall Effect in a conventional metallic-conductor. Panel B illustrates the analogous effect for cations (e.g., Na+) being conducted through membrane-localized Na+ channels. Upon application of a static magnetic field (B) to the metallic conductor, the normal
FIG. 1. Panel A: Development of Hall potentials (VH) in conventional conductors. Application of a static magnetic field (heavy arrows labeled B) is perpendicular to current flow represented by dashed arrows parallel to the conductor’s long axis. Circles labeled e− represent the path of electrons in response to B, leading to an asymmetric distribution of electrons, and development of VH. Panel B: The analogous curvilinear path taken by sodium ions (Na+) in a sodium channel due to application of the field B. The lightly shaded areas surrounding the circles, labeled Na+, represent electrostatic fields associated with cations as they transit cation channels. The radial displacement of Na+ is a diagrammatic representation of the value r calculated in equation 1. 375
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current flow (e) is perturbed and electrons trace a curvilinear path through the conductor, accumulating at the lower surface. The result is that a Hall voltage (VH) is developed between the lower (electron rich) and upper, (electron poor) surfaces of the conductor. In conventional conductors, VH develops and saturates rapidly upon application of magnetic fields, and little or no effect is noted on net current flow through the conductor. In a cation channel (Panel B), different results can be expected. Specificity for cation flow through pores, such as Na+ channels, is determined in large part by an ion’s physical size. Potassium (K+), with a diameter 30% greater than that of Na+, is highly impermeable to sodium channels; while Li+, with a diameter 30% smaller than Na+, permeates Na+ channels substantially faster than Na+ (7). The implications of the latter are that the diameter of the Na+ channel is not much greater than that of a sodium ion and that Na+ moves through the channels in single file. Assuming that biological cation currents behave similarly to electron currents in conventional conductors, it is reasonable to propose that static magnetic fields can alter the path of cations flowing through cation channels (Fig 1, Panel B), thereby generating a potential across the channel analogous to a Hall voltage. It is also known that ion flow through sodium and other cation channels is voltage gated, and that the voltage sensor(s) is a set of four symmetrically arranged, basic, transmembrane helical domains. Each domain is designated S4 (8). Site-directed mutation studies indicate that the charge on S4 domains is a key determinant of the magnitude of the transmembrane gating voltage (9, 10, 11). Since the conformation of ion channel proteins (open or closed) appears to depend on electrostatic interaction between S4 domains and the transmembrane voltage, it can also be expected that magnetic fields inducing Hall-like events in cation channels will modify normal interactions between S4 domains and the membrane potential. Such events are likely to modulate net ion flow through the channels as calculated below. Since S4 domains are common to many cation channels (12, 13, 14), the principles we outline here have wide applicability. Calculation of PEMF Forces Generated on Cation Channel Domains To evaluate the biological impact of Hall-like effects generated by low intensity static magnetic fields, we calculate the force such fields generate when they act on a sodium ion moving through a sodium channel—and we compare it with the force generated on an S4 domain by potentials known to gate cation channels. The force generated by a low intensity magnetic field operating on a sodium ion in a channel depends on the magnitude of the radial deflection (r) of the ion as it moves through the channel. This deflection, calculated from Eq. 1, depends on the strength of the magnetic field, the speed, charge, mass, distance traveled and transit time of the ion as it traverses a channel. For Na+, the charge q 4 +1 (or 1.6 × 10−19 Coulombs) and the mass m 4 3.8 × 10−26 Kg. Transit time through the Na+ channel is in the vicinity of 10−6 to 10−7) s (10), resulting in a velocity (v) of 7.5 × 10−3 m/s to 7.5 × 10−2 m/s through the 7.5 nm long channel. Using the latter, and an epidemiologically relevant value of 100 × 10−6 Tesla (T) for the magnetic field (B), Eq. 1 yields a r ranging from 0.16 to 1.6 pm.
S D
1 qvB 2 1 t 4 0.16 to 1.6 picometers (pm) r 4 at2 4 2 2 m
[1]
Since the S4 domains of sodium channels are symmetrically arranged around the central ion pathway, a sodium ion deflected by a distance r will generate a Coulomb force due to interaction of the charge on the sodium ion (q) with charges on S4 subunits (Q). This force depends on the magnitude of the charges Q and q and the distance between them (rQ−q). Based on the structure of cation channels (8), the distance rQ−q is conservatively taken as two a helix diameters (1 nm). Although the effective charge on S4 is not critical to the comparisons made in this report, for computational purposes we assume it to have a value of 1.75 C (15). With these considerations, the 376
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force (C) generated by a 100 mT magnetic field (FCmag acting via a sodium ion on an S4 domain of a sodium channel is given by Coulomb’s Law (Eq. 2). FC
mag
4k
S D qQ
(rQ1q)2
[2]
FCmag can be evaluated, as shown in Eq. 3, for the range of small changes in radial deflection (i.e., r) that were calculated in Eq. 1 FC
mag
4 −2
S D kqQ
~r Q1q)3
r 4 −1.3 pN to −1.3 pN
[3]
The latter force can be viewed as a repulsive force directed against an S4 domain of a cation channel, leading to an altered conformation of the channel protein. Calculation of Forces Generated on Channel Domains by Gating Voltages A key question at this point is whether forces of the latter magnitude have any significance to the biological function of ion channels. It is known that changes in membrane potential of about 30 mV gate cation channels (8). Since these voltage changes generate their effect by acting on S4 domains, we can calculate (Eq. 4) that a force of 1.1 pN is generated by a 30 mV transmembrane voltage change (V) acting on an S4 domain (Q 4 1.75 C) in a 7.5 nm (d) biological membrane. This is the force normally required to interconvert channels between open and closed states. FCoul 4 V V
SD Q d
4 1.1 pN
[4]
Comparison of results from Eq. 3 and 4 clearly illustrates that low intensity static magnetic fields can generate forces on channel proteins that are in the range of electrostatic forces known to gate ion channels. Regarding the magnitude of the repulsive force (negative sign) generated by the magnetic field, it is important to emphasize that r of an ion, calculated from equation 1, is critically related to the velocity of the ion in the channel. Consequently, we calculated the forces generated by a 100 m Tesla field acting on a sodium ion over a reasonable range of velocities. From these considerations, it appears that there is little need to invoke complex arguments based on quantum mechanical principles, or novel resonance modes of ions in magnetic fields, to explain how electromagnetic fields might generate forces capable of having significant impact on biological systems. Comparison of Effects Due to Static and Pulsed Electromagnetic Fields While the foregoing analysis seems clear for static fields, epidemiological studies suggest that PEMFs in the 50–60 Hz range are important environmental hazards. Therefore it is important to consider if PEMFs might induce the same effects as static fields. If the lifetime of the biological event is sufficiently short and if the combination of PEMF intensity and frequency is sufficiently great, then PEMFs will generate the same effects as static fields. In the case of Na+ transport, the transit time of a single Na+ is about 0.1 − 1.0 ms (10). The time of a single 60 Hz field oscillation is 1.7 ms, or about 104 − 105 greater than the time required for a sodium ion to transit the sodium channel. Consequently, the change in PEMF potential experienced by a cation during its brief transit through a channel protein is negligible, and PEMF-generated Hall-like effects can be treated almost identically to those generated by static fields. For example, assume that induction of a biological effect on sodium channels requires a static magnetic field with intensity equal to that generated by a direct current of 100 ± 5 V. With this 377
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constraint, the application of a PEMF generated by a 60 Hz source of 100 V generates a field with the same magnetic intensity range for 3.4 msec during the positive and negative phase of each 60 Hz oscillation. In this example, 3.4 msec is very long compared to the transit time of Na+ through a sodium channel (less than 1 ms). This makes it reasonable to expect that Hall-like effects calculated for static fields will also be observed in ion channels exposed to oscillating fields (PEMFs). One of the most curious issues stemming from the discussions presented here is that static electromagnetic fields do not appear to be extremely bioactive, as is apparent from the fact that humans are often subjected to very high intensity magnetic fields with no obvious biologic effects. For example, in one recent NMR study subjects were regularly exposed to static magnetic fields about 104 greater than those assumed in our calculations, with no apparent related pathology (16). Although we cannot explain this apparent incongruity, one possibility is that biological effects of static fields saturate at very low field intensities. The calculations presented above support this idea. Additionally, the orientation required for magnetic fields to generate biologically significant Halllike forces leads to the expectation that in three-dimensional cells and tissues, only cation channels perpendicular to an applied field will be influenced by the field. Thus the biological consequences of static fields will generally be very weak except in unusual situations where most of the channels are suitably oriented in the field. The same considerations apply to PEMFs, since we have shown that PEMFs are much like static fields in their effects on rapidly moving charged particles. Depending on the cation channels involved, metabolic results stemming from the effect of electromagnetic fields on channel conductivity might be as innocuous as a transient change in transmembrane potential (due to changes in cation flow). Alternatively, an appropriate combination of magnetic field and biological conditions could initiate an event as significant as oncogenesis. However, it should be noted that the efficiency of the proposed mechanism in modulating the activity of cation channels and initiating a significant biological event is likely to be low, and highly dependent on the long term spatial orientation of channels in the applied fields. Epidemiological data are already available correlating long-term occupational exposure to PEMFs with biological abnormalities where the individuals, and thus many tissue-specific ion channels, remained in relatively constant orientation in the electromagnetic fields for much of the duration of their exposure (1). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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