Bioelectromagnetics Supplement 1:205-208 (1992)
Dosimetric Extrapolations of ExtremelyLow-Frequency Electric and Magnetic Fields Across Biological Systems Charles Polk Department of Electrical Engineering, University of Rhode Island, Kingston
BASIC PROBLEMS
Dosimetric extrapolation across biological systems implies that information gained from in vitro experiments is to be used to determine exposure parameters for a living organism, or that knowledge of effects produced by some well-defined exposure of one living system is to be used to predict what could happen to some other organism when exposure parameters are suitably scaled. In the present workshop the exposure of interest is to relatively low intensity, extremely-low-frequency (ELF) magnetic fields. Systems that are to be compared usually differ substantially in size, chemistry, and physiology. Therefore we must first understand how field generated biological effects, such as enhanced cellular transcription and altered polypeptide synthesis [Goodman et al., 1986, 1989; Wei et al., 19901, nerve regeneration [Sisken et al., 19901, or enhanced intercellular matrix synthesis [Aaron et a]., 19891 are produced, before we can predict how different systems will react to field exposure. Thus we need to discover the basic physical and chemical mechanisms responsible for the observed effects. We need to relate, in light of experimental results, known physics and chemistry to the rapidly developing knowledge of cell biology on the molecular level, including details of receptor action and enzymology. In the absence of known mechanisms, the best we can do is to assume various plausible modes of field-cell interaction and to I ) determine what physical and biological parameters must be known to allow “scaling” of dosimetric data, given each hypothetical mechanism; 2) record and control in each experiment all of these parameters-until experiments show that a given hypothesis, requiring knowledge of particular, specialized data (e.g., magnitude and direction of the ambient DC magnetic field), can be discarded; and 3) determine and control additional exposure parameters suggested by new experimental evidence (e.g., details of on-off cycle, defining a particular type of intermittent exposure). Address reprint requests to Charles Polk, Department of Electrical Engineering, University of Rhode Island, Kingston, RI 0288 I .
0 1992 Wiley-Liss, Inc.
206
Polk
At the present time we don’t even know how to define dose. We only know in the light of several experiments [Goodman et al., 1989; McLeod and Rubin, 1990; Ciombor et al., I99 11 that dose for ELF electric or magnetic fields is almost certainly not the time integral ofthe field amplitude. Litovitz et al. (pp 237-2461 show that experimental results involving amplitude windows and optimum durations of exposure can be explained, if it is assumed that the amplitude of the applied field affects synthesis and degradation rates in multistep chemical reactions. Although this is a very important observation, it still leaves open the question of whether it is the amplitude of the electric or the magnetic field that is important, or equivalently, the amplitude of the magnetic field or the magnitude of its time derivative. Furthermore, frequency “windows” [Bawin and Adey, 1976; Blackman et al., 1988; Wei et al., 19901 still lack a satisfactory explanation. Thus it is not clear, until we understand the origin of such windows, whether the frequency range over which a response is observed will be the same in different experimental systems. The discussion of the cell cycle and growth control by Stein and Lian [pp 2472651 provides a possible biological explanation of “time windows” which is not necessarily incompatible with the explanation in terms of chemical reaction rates given by Litovitz et al. [pp 237-2461. For example, Stein and Lian report that histone gene transcription has been shown to cease at the completion of proliferative activity in HL-60 cells during monocyte differentiation. Goodman et al. [ 19891 have shown that relatively weak ELF magnetic fields accelerate histone transcription in such cells in culture when applied for 20 minutes. Does this mean that the fields have an effect only during proliferative activity of cells? The validity of that conclusion would have obvious implications for the circumstances under which dosimetric data could be extrapolated between different systems. MAGNETIC VS. ELECTRIC FIELD DOSIMETRY
Electric field dosimetry has been considered earlier i n this workshop [Kaune, pp 11-14; Bracken, pp 15-26; Hart, pp 27-42; Gandhi, pp 43-60]. Therefore n o direct reference will be made to “scaling” of electric fields in the following four papers (Polk, Litovitz et al., Stein-Lian, and Bassen), although the discussions of reaction rates (Litovitz et al.) and regulation of the cell cycle (Stein-Lian) are equally applicable to electric and magnetic fields. Biological effects of relatively large ELF electric fields, including those induced by time varying magnetic fields, are reasonably well understood. Examples are electric muscle stimulation due to electric field gradients [Reilly, 1989) or electroand magnetophosphenes [Lovsund et al., 19801. However, effects of low intensity fields (corresponding roughly to electric fields at the tissue level below I mV/m) are much more difficult to explain. Effects on calcium efflux from neural tissue [Bawin and Adey, 1976; Blackman et al., 19881, changes in cellular transcription and protein synthesis [Goodman et al., 1986, 1989; Wei et al., 19901, Ca++metabolism in lymphocytes [Yost and Liburdy, 19921, sinusoidal field effects on bone development [Ciombor et al., 19911, and reported epidemiological findings [Savitz et al., 19881 all seem to require, upon cursory examination, the detection by cells of fields which are either below thermal noise, or below the level of endogenous background fields, such as those corresponding to neural activity. The following paper on magnetic
Dosimetric Extrapolations Across Bio-Systems
207
field dosimetry gives therefore numerical examples to show how inhomogeneities in tissue composition and communication among cells could overcome this problem. That paper also emphasizes that the paths through tissue followed by currents due to magnetically induced electric fields will be substantially different from those due to directly applied electric fields. Thus even if it were to be shown that the biological effects of low amplitude, time varying magnetic fields are due to the electric fields which they induce, substantial differences between effects of directly applied electric fields and time varying magnetic fields are to be expected. Since it is not known, at the present time, whether observed effects of low level ELF magnetic fields are due to these fields themselves, or are a consequence of the electric fields which they induce, some theoretical models are reviewed that postulate direct effects of ELF magnetic fields and synergism between these and ambient static (DC) magnetic fields. Unequivocal experiments to confirm the applicability of these models have not been performed. Their validity would explain frequency windows and would therefore have clear implications for extrapolation of data between different systems. PARAMETER IDENTIFICATION
At the present state of knowledge, and in view of both reported experimental results and proposed theoretical models, the following parameters must be known if dosimetric data are to be transferred between different systems. In fact, the five parameters at the beginning of the list must also be specified if experiments on identical in uitro or in uiuo systems are to be replicated in different laboratories. 1. For in virvo exposure: exact description of the exposure vessel-that is, its shape, dimensions, and height of the enclosed liquid; also the density of cells and the electrical properties of the culture medium. 2. Physical size of an animal, organ, or cell culture and, in particular, crosssection perpendicular to the magnetic field vector (thus both size and orientation in the magnetic field of a petri dish or culture flask are essential variables). 3. Polarization and frequency, including harmonic content, of the applied field; its uniformity and direction with respect to the exposed organism and-until the question of importance of the DC field is resolved-magnitude and direction of any DC magnetic field that may be present. 4. Duration of exposure and point in time during development of organism when exposure took place. 5 . Exact description of any on-off exposure cycle if exposure was intermittent. 6. Magnetic properties of the biological substance (paramagnetic o r diamagnetic; magnetically isotropic or anisotropic; presence of ferro or ferrimagnetic enclosures). 7. Electrical properties (electrical conductivity and dielectric permittivity) in the frequency range of interest, not only of organs and tissue types, but of individual cells and even substructures of cells, such as membranes and gap junctions). 8. Presence, absence, or abundance of specialized structures, such as gap junctions in a particular tissue or organ culture, which can substantially affect electric current distribution; we also need to know whether gap junctions are confined to
208
Polk
the cells within a particular organ or whether they provide connections with adjacent tissue. 9. Mechanical properties, which can interact with forces due to induced electric fields, such as viscosity of fluids and elastic constants of membranes. 10. Presence, even if temporary, during particular physiological processes, of free radicals which could substantially influence the effects of both DC and AC magnetic fields on chemical reaction rates. 11. Presence of particular enzymes and nature of chemical processes taking place during field exposure of a cell or organ culture. 12. Existence of known mechanical or chemical oscillations in a particular cell or organ structure which could interact with a time varying field; for example Ca++oscillations of about 0.02 Hz in fibroblasts [Hartoonian et al., 19911 or 5-12 Hz oscillations generated by neurons [Silva et a]., 19911. 13. Differences in metabolic pathways among species which are being compared. REFERENCES Aaron RK, Ciombor D McK, Jolly G ( I 989): Stimulation of experimental endochondral ossification by low-energy pulsing electromagnetic fields. J Bone Miner Res 4:227-233. Bawin SM, Adey WR (1976): Sensitivity of calcium binding in cerebral tissue to weak environmental electric fields oscillating at low frequency. Proc Natl Acad Sci USA 73: 1999-2003. Blackman CF, Benane SG, Elliott DC, House DC, Pollock MM (1988): Influence of electromagnetic fields on the efflux of calcium ions from brain tissue i n vitro: A three-model analysis consistent with the frequency response up to 510 Hz. Bioelectromagnetics 9:215-227. Ciombor McK, Aaron RK,Polk C (1991): Sine wave interaction with an in vivo model of bone development. Transactions, 1 Ith Annual Meeting of the Bioclcctric Repair and Growth Society, p 1I. Goodman R, Henderson A ( 1986). Sine waves enhance cellular transcripton. Bioelectromagnetics 7:2329. Goodman R, Wei L-X, Xu J-C, Henderson A (1989): Exposure of human cells to low-frequency electromagnetic fields results in quantitative changes in transcripts. Biochim Biophys Acta l009:216220. Hartoonian AT, Kaojpy, Paranjape S , Tsien RY (1991): Generation of calcium oscillations i n fibroblasts by positive feedback between calcium and IP,. Science 251 :75-78. Lovsund P, Oberg PA, Nilsson SEG, Reuter T (1980): Magnetophosphenes: A quantitative analysis of thresholds. Med Biol Comput 18:326. McLeod KJ, Rubin CT (1990): Frequency specific modulation of bone adaptation by induced electric fields. J Theor Biol 145:385-396. Reilly JP ( 1989): Peripheral nerve stimulation by induced electric currents: Exposure to time-varying magnetic fields. Medical and Biological Engineering and Computing 27: 101-1 10. Savitz DA, Wachtel HW, Barnes FA, J o h n EM, Tvrdik JG (1988): Case-control study of childhood cancer and exposure to 60 Hz magnetic fields. Am J Epidemiol 128:21-38. Silva LR, Amital Y, Connors BW (1991): Intrinsic oscillations of neocortex generated by layer 5 pyramidal neurons. Science 25 1:432-435. Sisken BF, Kanje M, Lundborg G, Kurtz W (1990): Pulsed electromagnetic fields stimulate nerve regeneration i n vitro and in vivo. Restorative Neurology and Neuroscience 1:303-309. Wei L-Z, Goodman R, Henderson A ( I 990): Changes in levels of c-myc and histone H2B following exposure of cells to low-frequency sinusoidal electromagnetic fields: Evidence for a windoweffect. Bioelectromagnetics 1 1 :269-272. Yost MG, Liburdy RP (1992): Time-varying and static magnetic fields act in combination to alter calcium signal transduction i n the lymphocyte. FEBS 296-2: 1 17-1 22.
Dosimetric Extrapolations of ExtremelyLow-Frequency Electric and Magnetic Fields Across Biological Systems Charles Polk Department of Electrical Engineering, University of Rhode Island, Kingston
BASIC PROBLEMS
Dosimetric extrapolation across biological systems implies that information gained from in vitro experiments is to be used to determine exposure parameters for a living organism, or that knowledge of effects produced by some well-defined exposure of one living system is to be used to predict what could happen to some other organism when exposure parameters are suitably scaled. In the present workshop the exposure of interest is to relatively low intensity, extremely-low-frequency (ELF) magnetic fields. Systems that are to be compared usually differ substantially in size, chemistry, and physiology. Therefore we must first understand how field generated biological effects, such as enhanced cellular transcription and altered polypeptide synthesis [Goodman et al., 1986, 1989; Wei et al., 19901, nerve regeneration [Sisken et al., 19901, or enhanced intercellular matrix synthesis [Aaron et a]., 19891 are produced, before we can predict how different systems will react to field exposure. Thus we need to discover the basic physical and chemical mechanisms responsible for the observed effects. We need to relate, in light of experimental results, known physics and chemistry to the rapidly developing knowledge of cell biology on the molecular level, including details of receptor action and enzymology. In the absence of known mechanisms, the best we can do is to assume various plausible modes of field-cell interaction and to I ) determine what physical and biological parameters must be known to allow “scaling” of dosimetric data, given each hypothetical mechanism; 2) record and control in each experiment all of these parameters-until experiments show that a given hypothesis, requiring knowledge of particular, specialized data (e.g., magnitude and direction of the ambient DC magnetic field), can be discarded; and 3) determine and control additional exposure parameters suggested by new experimental evidence (e.g., details of on-off cycle, defining a particular type of intermittent exposure). Address reprint requests to Charles Polk, Department of Electrical Engineering, University of Rhode Island, Kingston, RI 0288 I .
0 1992 Wiley-Liss, Inc.
206
Polk
At the present time we don’t even know how to define dose. We only know in the light of several experiments [Goodman et al., 1989; McLeod and Rubin, 1990; Ciombor et al., I99 11 that dose for ELF electric or magnetic fields is almost certainly not the time integral ofthe field amplitude. Litovitz et al. (pp 237-2461 show that experimental results involving amplitude windows and optimum durations of exposure can be explained, if it is assumed that the amplitude of the applied field affects synthesis and degradation rates in multistep chemical reactions. Although this is a very important observation, it still leaves open the question of whether it is the amplitude of the electric or the magnetic field that is important, or equivalently, the amplitude of the magnetic field or the magnitude of its time derivative. Furthermore, frequency “windows” [Bawin and Adey, 1976; Blackman et al., 1988; Wei et al., 19901 still lack a satisfactory explanation. Thus it is not clear, until we understand the origin of such windows, whether the frequency range over which a response is observed will be the same in different experimental systems. The discussion of the cell cycle and growth control by Stein and Lian [pp 2472651 provides a possible biological explanation of “time windows” which is not necessarily incompatible with the explanation in terms of chemical reaction rates given by Litovitz et al. [pp 237-2461. For example, Stein and Lian report that histone gene transcription has been shown to cease at the completion of proliferative activity in HL-60 cells during monocyte differentiation. Goodman et al. [ 19891 have shown that relatively weak ELF magnetic fields accelerate histone transcription in such cells in culture when applied for 20 minutes. Does this mean that the fields have an effect only during proliferative activity of cells? The validity of that conclusion would have obvious implications for the circumstances under which dosimetric data could be extrapolated between different systems. MAGNETIC VS. ELECTRIC FIELD DOSIMETRY
Electric field dosimetry has been considered earlier i n this workshop [Kaune, pp 11-14; Bracken, pp 15-26; Hart, pp 27-42; Gandhi, pp 43-60]. Therefore n o direct reference will be made to “scaling” of electric fields in the following four papers (Polk, Litovitz et al., Stein-Lian, and Bassen), although the discussions of reaction rates (Litovitz et al.) and regulation of the cell cycle (Stein-Lian) are equally applicable to electric and magnetic fields. Biological effects of relatively large ELF electric fields, including those induced by time varying magnetic fields, are reasonably well understood. Examples are electric muscle stimulation due to electric field gradients [Reilly, 1989) or electroand magnetophosphenes [Lovsund et al., 19801. However, effects of low intensity fields (corresponding roughly to electric fields at the tissue level below I mV/m) are much more difficult to explain. Effects on calcium efflux from neural tissue [Bawin and Adey, 1976; Blackman et al., 19881, changes in cellular transcription and protein synthesis [Goodman et al., 1986, 1989; Wei et al., 19901, Ca++metabolism in lymphocytes [Yost and Liburdy, 19921, sinusoidal field effects on bone development [Ciombor et al., 19911, and reported epidemiological findings [Savitz et al., 19881 all seem to require, upon cursory examination, the detection by cells of fields which are either below thermal noise, or below the level of endogenous background fields, such as those corresponding to neural activity. The following paper on magnetic
Dosimetric Extrapolations Across Bio-Systems
207
field dosimetry gives therefore numerical examples to show how inhomogeneities in tissue composition and communication among cells could overcome this problem. That paper also emphasizes that the paths through tissue followed by currents due to magnetically induced electric fields will be substantially different from those due to directly applied electric fields. Thus even if it were to be shown that the biological effects of low amplitude, time varying magnetic fields are due to the electric fields which they induce, substantial differences between effects of directly applied electric fields and time varying magnetic fields are to be expected. Since it is not known, at the present time, whether observed effects of low level ELF magnetic fields are due to these fields themselves, or are a consequence of the electric fields which they induce, some theoretical models are reviewed that postulate direct effects of ELF magnetic fields and synergism between these and ambient static (DC) magnetic fields. Unequivocal experiments to confirm the applicability of these models have not been performed. Their validity would explain frequency windows and would therefore have clear implications for extrapolation of data between different systems. PARAMETER IDENTIFICATION
At the present state of knowledge, and in view of both reported experimental results and proposed theoretical models, the following parameters must be known if dosimetric data are to be transferred between different systems. In fact, the five parameters at the beginning of the list must also be specified if experiments on identical in uitro or in uiuo systems are to be replicated in different laboratories. 1. For in virvo exposure: exact description of the exposure vessel-that is, its shape, dimensions, and height of the enclosed liquid; also the density of cells and the electrical properties of the culture medium. 2. Physical size of an animal, organ, or cell culture and, in particular, crosssection perpendicular to the magnetic field vector (thus both size and orientation in the magnetic field of a petri dish or culture flask are essential variables). 3. Polarization and frequency, including harmonic content, of the applied field; its uniformity and direction with respect to the exposed organism and-until the question of importance of the DC field is resolved-magnitude and direction of any DC magnetic field that may be present. 4. Duration of exposure and point in time during development of organism when exposure took place. 5 . Exact description of any on-off exposure cycle if exposure was intermittent. 6. Magnetic properties of the biological substance (paramagnetic o r diamagnetic; magnetically isotropic or anisotropic; presence of ferro or ferrimagnetic enclosures). 7. Electrical properties (electrical conductivity and dielectric permittivity) in the frequency range of interest, not only of organs and tissue types, but of individual cells and even substructures of cells, such as membranes and gap junctions). 8. Presence, absence, or abundance of specialized structures, such as gap junctions in a particular tissue or organ culture, which can substantially affect electric current distribution; we also need to know whether gap junctions are confined to
208
Polk
the cells within a particular organ or whether they provide connections with adjacent tissue. 9. Mechanical properties, which can interact with forces due to induced electric fields, such as viscosity of fluids and elastic constants of membranes. 10. Presence, even if temporary, during particular physiological processes, of free radicals which could substantially influence the effects of both DC and AC magnetic fields on chemical reaction rates. 11. Presence of particular enzymes and nature of chemical processes taking place during field exposure of a cell or organ culture. 12. Existence of known mechanical or chemical oscillations in a particular cell or organ structure which could interact with a time varying field; for example Ca++oscillations of about 0.02 Hz in fibroblasts [Hartoonian et al., 19911 or 5-12 Hz oscillations generated by neurons [Silva et a]., 19911. 13. Differences in metabolic pathways among species which are being compared. REFERENCES Aaron RK, Ciombor D McK, Jolly G ( I 989): Stimulation of experimental endochondral ossification by low-energy pulsing electromagnetic fields. J Bone Miner Res 4:227-233. Bawin SM, Adey WR (1976): Sensitivity of calcium binding in cerebral tissue to weak environmental electric fields oscillating at low frequency. Proc Natl Acad Sci USA 73: 1999-2003. Blackman CF, Benane SG, Elliott DC, House DC, Pollock MM (1988): Influence of electromagnetic fields on the efflux of calcium ions from brain tissue i n vitro: A three-model analysis consistent with the frequency response up to 510 Hz. Bioelectromagnetics 9:215-227. Ciombor McK, Aaron RK,Polk C (1991): Sine wave interaction with an in vivo model of bone development. Transactions, 1 Ith Annual Meeting of the Bioclcctric Repair and Growth Society, p 1I. Goodman R, Henderson A ( 1986). Sine waves enhance cellular transcripton. Bioelectromagnetics 7:2329. Goodman R, Wei L-X, Xu J-C, Henderson A (1989): Exposure of human cells to low-frequency electromagnetic fields results in quantitative changes in transcripts. Biochim Biophys Acta l009:216220. Hartoonian AT, Kaojpy, Paranjape S , Tsien RY (1991): Generation of calcium oscillations i n fibroblasts by positive feedback between calcium and IP,. Science 251 :75-78. Lovsund P, Oberg PA, Nilsson SEG, Reuter T (1980): Magnetophosphenes: A quantitative analysis of thresholds. Med Biol Comput 18:326. McLeod KJ, Rubin CT (1990): Frequency specific modulation of bone adaptation by induced electric fields. J Theor Biol 145:385-396. Reilly JP ( 1989): Peripheral nerve stimulation by induced electric currents: Exposure to time-varying magnetic fields. Medical and Biological Engineering and Computing 27: 101-1 10. Savitz DA, Wachtel HW, Barnes FA, J o h n EM, Tvrdik JG (1988): Case-control study of childhood cancer and exposure to 60 Hz magnetic fields. Am J Epidemiol 128:21-38. Silva LR, Amital Y, Connors BW (1991): Intrinsic oscillations of neocortex generated by layer 5 pyramidal neurons. Science 25 1:432-435. Sisken BF, Kanje M, Lundborg G, Kurtz W (1990): Pulsed electromagnetic fields stimulate nerve regeneration i n vitro and in vivo. Restorative Neurology and Neuroscience 1:303-309. Wei L-Z, Goodman R, Henderson A ( I 990): Changes in levels of c-myc and histone H2B following exposure of cells to low-frequency sinusoidal electromagnetic fields: Evidence for a windoweffect. Bioelectromagnetics 1 1 :269-272. Yost MG, Liburdy RP (1992): Time-varying and static magnetic fields act in combination to alter calcium signal transduction i n the lymphocyte. FEBS 296-2: 1 17-1 22.
