Millimeter-Wave Heating in In Vitro Studies: Effect of Convection in Continuous and Pulse-Modulated Regimes

Shallow penetration of millimeter waves (MMW) and non-uniform illumination in in vitro experiments result in a non-uniform distribution of the specific absorption rate (SAR). These SAR gradients trigger convective currents in liquids affecting transient and steady-state temperature distributions. We analyzed the effect of convection on temperature dynamics during MMW exposure in continuous-wave (CW) and pulsed-wave (PW) amplitude-modulated regimes using micro-thermocouples. Temperature rise kinetics are characterized by the occurrence of a temperature peak that shifts to shorter times as the SAR of the MMW exposure increases and precedes initiation of convection in bulk. Furthermore, we demonstrate that the liquid volume impacts convection. Increasing the volume results in earlier triggering of convection and in a greater cooling rate after the end of the exposure. In PW regimes, convection strongly depends on the pulse duration that affects the heat pulse amplitude and cooling rate. The latter results in a change of the average temperature in PW regime. Bioelectromagnetics. 2019;40:553-568. © 2019 Bioelectromagnetics Society.

Microscale temperature and SAR measurements in cell monolayer models exposed to millimeter waves

Due to shallow penetration of millimeter waves (MMW) and convection in liquid medium surrounding cells, the problem of accurate assessment of local MMW heating in in vitro experiments remains unsolved. Conventional dosimetric MMW techniques, such as infrared imaging or fiber optic (FO) sensors, face several inherent limits. Here we propose a methodology for accurate local temperature measurement and subsequent specific absorption rate (SAR) retrieval using microscale thermocouples (TC). SAR was retrieved by fitting the measured initial temperature rise to the numerical solution of an equivalent thermal model. It was found that the accuracy of temperature measurement depends on thermosensor size, that is, the smaller TC, the more accurate the temperature measurement. SAR determined using TC with lead diameters of 25 and 75 μm demonstrated 98.5% and 80.4% match with computed SAR, respectively. However, both TC provided the same temperature rises in long run (> 10 min). FO probe failed to measure adequately local heating both for short and long exposures due to the relatively large size of the probe sensor (400 μm) and time constant (0.6 s). Calculated SAR in the cell monolayer was almost two times lower than that in the surrounding liquid. It was shown that the impact of the cell monolayer on heating due to its small thickness (5 to 10 μm) can be considered as negligible. Moreover, we demonstrated the possibility of accurate measurement of MMW-induced thermal pulses (up to 10 °C) using 25 μm TC. Bioelectromagnetics. 38:11-21, 2017. © 2016 Wiley Periodicals, Inc.

[Selective Heating of Membrane-forming Holes in Teflon Film Exposed to Decimeter Waves]

Calculations of heating of membrane-forming holes in Teflon film exposed to decimeter waves were performed. The dependence of the temperature increment in holes on the geometry of holes, electrolyte concentration, and decimeter wave frequency was studied. The kinetics of heating depending on the hole diameter was also obtained. It was concluded that the observed in the experiment effects of the decimeter wave on bilayer lipid membranes resulted from the elevated concentration of decimeter electromagnetic waves in membrane-forming hole that led to selective heating of electrolyte in hole and bilayer lipid membranes.

[Frequency dependence of heating of human skin exposed to millimeter waves]

In this paper we studied experimentally the frequency dependence of heating of human skin exposed to millimeter waves. Theoretical modeling of obtained data was performed using the hybrid bio-heat equation. It was found that the skin heating and SAR increased with increasing the exposure frequency. The frequency dependence of heating was entirely resulted from that of reflection from the skin. Unlike temperature, the frequency dependence of the SAR was due to the increased absorption of millimeter wave energy within the thin surface layer of the skin.

[Problems of using a thermocouple for measurements of skin temperature rise during the exposure to millimeter waves]

The possibility of using thermocouples for the artifact-free measurements of skin temperature during millimeter wave exposure was studied. The distributions of the specific absorption rate (SAR) in the human skin were calculated for different orientations of the thermocouple relative to the E-field of exposure. It was shown that, at the parallel orientation of a thermocouple relative to the E-field, SAR significantly increased at the tip of the thermocouple. This can result in an overheating of the thermocouple. At the perpendicular orientation of a thermocouple, the distortions of the SAR were insignificant. The data obtained confirm that the skin temperature can be measured with a thermocouple during exposure under the condition that the thermocouple is located perpendicular to the E-vector of the electromagnetic field. For the accurate determination of SAR from the rate of the initial temperature rise, it is necessary to fit the temperature kinetics measured with the thermocouple to the solution of the bio-heat transfer equation.

Enhanced absorption of millimeter wave energy in murine subcutaneous blood vessels

The aim of the present study was to determine millimeter wave (MMW) absorption by blood vessels traversing the subcutaneous fat layer of murine skin. Most calculations were performed using the finite-difference time-domain (FDTD) technique. We used two types of models: (1) a rectangular block of multilayer tissue with blood vessels traversing the fat layer and (2) cylindrical models with circular and elliptical cross-sections simulating the real geometry of murine limbs. We found that the specific absorption rate (SAR) in blood vessels normally traversing the fat layer achieved its maximal value at the parallel orientation of the E-field to the vessel axis. At 42 GHz exposure, the maximal SAR in small blood vessels could be more than 30 times greater than that in the skin. The SAR increased with decreasing the blood vessel diameter and increasing the fat thickness. The SAR decreased with increasing the exposure frequency. When the cylindrical or elliptical models of murine limbs were exposed to plane MMW, the greatest absorption of MMW energy occurred in blood vessels located on the lateral areas of the limb model. At these areas the maximal SAR values were comparable with or were greater than the maximal SAR on the front surface of the skin. Enhanced absorption of MMW energy by blood vessels traversing the fat layer may play a primary role in initiating MMW effects on blood cells and vasodilatation of cutaneous blood vessels.

Enhanced absorption of microwaves within cylindrical holes in Teflon film

Earlier publications demonstrated that 0.9 GHz microwave exposure induced notable changes of the conductivity of modified bilayer lipid membranes (BLM) formed in holes in thin Teflon film (TF). The aims of this study were: 1) to perform detailed calculations of the microwave field distributions in holes formed in TF, using the finite-difference time-domain technique and 2) to model microwave heating of the hole under the conditions used in the BLM experiments but in the absence of BLM in the hole. We found that with the E-field oriented perpendicular to the TF plane the local-specific absorption rate in holes increased significantly. The increase became larger with increasing electrolyte concentration and with decreasing diameter of the hole and frequency. The calculated temperature elevations in the hole were in good agreement with those determined experimentally. These findings allowed us to conclude that the microwave effects on BLM conductivity reported previously resulted mostly from the enhanced absorption of microwave energy by the membrane-forming holes and subsequent local temperature elevation in the holes.

Millimeter wave effects on electrical responses of the sural nerve in vivo

Millimeter wave (MMW, 42.25 GHz)-induced changes in electrical activity of the murine sural nerve were studied in vivo using external electrode recordings. MMW were applied to the receptive field of the sural nerve in the hind paw. We found two types of responses of the sural nerve to MMW exposure. First, MMW exposure at the incident power density >/=45 mW/cm(2) inhibited the spontaneous electrical activity. Exposure with lower intensities (10-30 mW/cm(2)) produced no detectable changes in the firing rate. Second, the nerve responded to the cessation of MMW exposure with a transient increase in the firing rate. The effect lasted 20-40 s. The threshold intensity for this effect was 160 mW/cm(2). Radiant heat exposure reproduced only the inhibitory effect of MMW but not the transient excitatory response. Depletion of mast cells by compound 48/80 eliminated the transient response of the nerve. It was suggested that the cold sensitive fibers were responsible for the inhibitory effect of MMW and radiant heat exposures. However, the receptors and mechanisms involved in inducing the transient response to MMW exposure are not clear. The hypothesis of mast cell involvement was discussed.

[Effect of microwaves on bilayer lipid membranes: role of a membrane-forming hole in the Teflon film]

The distributions of specific abcorption rate (SAR) and E-field in a membrane-forming hole of Teflon film and surrounding electrolyte were calculated for 0.9 GHz exposure. It was found that the specific absorption rate in the membrane-forming hole increased greatly with increasing thickness of the Teflon film, and electrolyte concentration and decreasing diameter of the hole. The previously demonstrated significant changes in the conductivity of modified bilayer lipid membranes induced by microwave exposure can be explained by a local increase in specific absorption rate and subsequent elevation of temperature in the membrane-forming hole of the Teflon film.

Influence of blood flow and millimeter wave exposure on skin temperature in different thermal models

Recently we showed that the Pennes bioheat transfer equation was not adequate to quantify mm wave heating of the skin at high blood flow rates. To do so, it is necessary to incorporate an "effective" thermal conductivity to obtain a hybrid bioheat equation (HBHE). The main aim of this study was to determine the relationship between non-specific tissue blood flow in a homogeneous unilayer model and dermal blood flow in multilayer models providing that the skin surface temperatures before and following mm wave exposure were the same. This knowledge could be used to develop multilayer models based on the fitting parameters obtained with the homogeneous tissue models. We tested four tissue models consisting of 1-4 layers and applied the one-dimensional steady-state HBHE. To understand the role of the epidermis in skin models we added to the one- and three-layer models an external thin epidermal layer with no blood flow. Only the combination of models containing the epidermal layer was appropriate for determination of the relationship between non-specific tissue and dermal blood flows giving the same skin surface temperatures. In this case we obtained a linear relationship between non-specific tissue and dermal blood flows. The presence of the fat layer resulted in the appearance of a significant temperature gradient between the dermis and muscle layer which increased with the fat layer thickness.

Millimeter wave reflectivity used for measurement of skin hydration with different moisturizers

BACKGROUND/AIMS

A new non-invasive method for determining the free water content in human skin has been developed. The method analyzes the reflection of millimeter (mm) wavelength electromagnetic waves. The amount of reflection of mm waves depends on an electrical property (namely, the permittivity) of the skin, and this depends upon the free water content of the various skin layers. The aim of the present study was to use the mm wave reflectometry method for determination of free water content in healthy skin treated with different hydrating substances.

METHODS

Skin lotion, pure water, glycerol, and petroleum jelly (an occlusive substance) were used for hydration of skin. The amount of free water was calculated using the permittivity values of skin layers found from fitting a three layer skin model to measured reflection data. The skin model consisted of (1) the stratum corneum (SC), (2) the viable epidermis plus the dermis, and (3) fat layers.

RESULTS

Mm wave reflection was significantly affected by the water content of the thick SC of the palm but not by the very thin SC of the forearm. Treatment of the forearm and palm skin with different hydrating substances produced notable changes of the free water content in the SC, but not in the viable epidermis or dermis. The greatest hydration was produced by pure water and skin lotion, and the lowest by petroleum jelly. However, petroleum jelly produced prolonged retention of water in the SC following its hydration by other moisturizers. The content of free water was found to return to its baseline value after removal of moisturizers in as short a time as 8.3 min.

CONCLUSION

The study shows that mm wave reflectometry can be used as a sensitive technique for the non-invasive determination of water content in living skin.

Millimeter wave dosimetry of human skin

To identify the mechanisms of biological effects of mm waves it is important to develop accurate methods for evaluating absorption and penetration depth of mm waves in the epidermis and dermis. The main characteristics of mm wave skin dosimetry were calculated using a homogeneous unilayer model and two multilayer models of skin. These characteristics included reflection, power density (PD), penetration depth (delta), and specific absorption rate (SAR). The parameters of the models were found from fitting the models to the experimental data obtained from measurements of mm wave reflection from human skin. The forearm and palm data were used to model the skin with thin and thick stratum corneum (SC), respectively. The thin SC produced little influence on the interaction of mm waves with skin. On the contrary, the thick SC in the palm played the role of a matching layer and significantly reduced reflection. In addition, the palmar skin manifested a broad peak in reflection within the 83-277 GHz range. The viable epidermis plus dermis, containing a large amount of free water, greatly attenuated mm wave energy. Therefore, the deeper fat layer had little effect on the PD and SAR profiles. We observed the appearance of a moderate SAR peak in the therapeutic frequency range (42-62 GHz) within the skin at a depth of 0.3-0.4 mm. Millimeter waves penetrate into the human skin deep enough (delta = 0.65 mm at 42 GHz) to affect most skin structures located in the epidermis and dermis.

Human skin permittivity determined by millimeter wave reflection measurements

Millimeter wave reflection from the human skin was studied in the frequency range of 37-74 GHz in steps of 1 GHz. The forearm and palm data were used to model the skin with thin and thick stratum corneum (SC), respectively. To fit the reflection data, a homogeneous unilayer and three multilayer skin models were tested. Skin permittivity in the mm-wave frequency range resulted from the permittivity of cutaneous free water which was described by the Debye equation. The permittivity increment found from fitting to the experimental data was used for determination of the complex permittivity and water content of skin layers. Our approach, first tested in pure water and gelatin gels with different water contents, gave good agreement with literature data. The homogeneous skin model fitted the forearm data well. Permittivity of the forearm skin obtained with this model was close to the skin permittivity reported by others. To fit reflection from the palmar skin with a thick SC, a skin model containing at least two layers was required. Multilayer models provided better fitting to both the forearm and palmar skin reflection data. The fitting parameters obtained with different models were consistent with each other.

Effect of cyclophosphamide and 61.22 GHz millimeter waves on T-cell, B-cell, and macrophage functions

The present study was undertaken to investigate whether millimeter waves (MMWs) at 61.22 GHz can modulate the effect of cyclophosphamide (CPA), an anti-cancer drug, on the immune functions of mice. During the exposure each mouse's nose was placed in front of the center of the antenna aperture (1.5 x 1.5 cm) of MMW generator. The device produced 61.22 +/- 0.2 GHz wave radiation. Spatial peak Specific Absorption Rate (SAR) at the skin surface and spatial peak incident power density were measured as 885 +/- 100 W/kg and 31 +/- 5 mW/cm(2), respectively. Duration of the exposure was 30 min each day for 3 consecutive days. The maximum temperature elevation at the tip of the nose, measured at the end of 30 min, was 1 degrees C. CPA injection (100 mg/kg) was given intraperitoneally on the second day of exposure to MMWs. The animals were sacrificed 2, 5, and 7 days after CPA administration. MMW exposure caused upregulation in tumor necrosis factor-alpha (TNF-alpha) production in peritoneal macrophages suppressed by CPA administration. MMWs also caused a significant increase in interferon-gamma (IFN-gamma) production by splenocytes and enhanced proliferative activity of T-cells. Conversely, no changes were observed in interleukin-10 (IL-10) level and B-cell proliferation. These results suggest that MMWs accelerate the recovery process selectively through a T-cell-mediated immune response.

Millimeter wave induced reversible externalization of phosphatidylserine molecules in cells exposed in vitro

In vitro exposure of refrigerated samples (4 degrees C) of anti-coagulated blood with millimeter waves (MMWs) at incident power densities (IPDs) between 0.55 and 1.23 W/cm2 has been found to induce clot formation. We found a small but statistically significant change in clot size with increasing IPD value. MMW exposure of blood samples starting at room temperature (22 degrees C) did not induce blood coagulation; neither did conventional heating at temperatures up to 40 degrees C. Since cell-free plasma did not clot upon MMW exposure, the role of blood cells was particularly analyzed. Experiments on various mixtures of blood cells with plasma revealed an important role of red blood cells (RBC) in the coagulation process. Plasma coagulation also developed within the MMW beam above dense keratinocyte (HaCaT) monolayers suggesting it lacked cell-type specificity. We hypothesized that alteration of the membrane surface in exposed cells might be responsible for the circumscribed coagulation. The thrombogenic role of externalized phosphatidylserine (PS) molecules is well known. Therefore, we carried out experiments for immunolabeling PS molecules with fluorescein isothiocyanate (FITC)-conjugated Annexin V on exposed cells. Fluorescence microscopy of the adherent human keratinocytes (HaCaT) and murine melanoma cells (B16F10) showed that MMW exposure at an IPD of 1.23 W/cm2 is capable of inducing reversible externalization of PS molecules in cells within the beam area without detectable membrane damage. Nonadherent Jurkat cells exposed to MMW at an IPD of 34.5 mW/cm2 also showed reversible PS externalization with flow cytometry, whether the cell temperature was held constant or permitted to rise. These results suggest that certain biological effects induced by MMWs could be initiated by membrane changes in exposed cells.

Local heating of human skin by millimeter waves: Effect of blood flow

We investigated the influence of blood perfusion on local heating of the forearm and middle finger skin following 42.25 GHz exposure with an open ended waveguide (WG) and with a YAV mm wave therapeutic device. Both sources had bell-shaped distributions of the incident power density (IPD) with peak intensities of 208 and 55 mW/cm(2), respectively. Blood perfusion was changed in two ways: by blood flow occlusion and by externally applied vasodilator (nonivamide/nicoboxil) cream to the skin. For thermal modeling, we used the bioheat transfer equation (BHTE) and the hybrid bioheat equation (HBHE) which combines the BHTE and the scalar effective thermal conductivity equation (ETCE). Under normal conditions with the 208 mW/cm(2) exposure, the cutaneous temperature elevation (DeltaT) in the finger (2.5 +/- 0.3 degrees C) having higher blood flow was notably smaller than the cutaneous DeltaT in the forearm (4.7 +/- 0.4 degrees C). However, heating of the forearm and finger skin with blood flow occluded was the same, indicating that the thermal conductivity of tissue in the absence of blood flow at both locations was also the same. The BHTE accurately predicted local hyperthermia in the forearm only at low blood flow. The HBHE made accurate predictions at both low and high perfusion rates. The relationship between blood flow and the effective thermal conductivity (k(eff)) was found to be linear. The heat dissipating effect of higher perfusion was mostly due to an apparent increase in k(eff). It was shown that mm wave exposure could result in steady state heating of tissue layers located much deeper than the penetration depth (0.56 mm). The surface DeltaT and heat penetration into tissue increased with enlarging the irradiating beam area and with increasing exposure duration. Thus, mm waves at sufficient intensities could thermally affect thermo-sensitive structures located in the skin and underlying tissue.

Millimeter wave-induced suppression of B16 F10 melanoma growth in mice: involvement of endogenous opioids

Millimeter wave treatment (MMWT) is widely used in Eastern European countries, but is virtually unknown in Western medicine. Among reported MMWT effects is suppression of tumor growth. The main aim of the present "blind" and dosimetrically controlled experiments was to evaluate quantitatively the ability of MMWT to influence tumor growth and to assess whether endogenous opioids are involved. The murine experimental model of B16 F10 melanoma subcutaneous growth was used. MMWT characteristics were: frequency, 61.22 GHz; average incident power density, 13.3 x 10(-3) W/cm2; single exposure duration, 15 min; and exposure area, nose. Naloxone (1 mg/kg, intraperitoneally, 30 min prior to MMWT) was used as a nonspecific blocker of opioid receptors. Five daily MMW exposures, if applied starting at the fifth day following B16 melanoma cell injection, suppressed subcutaneous tumor growth. Pretreatment with naloxone completely abolished the MMWT-induced suppression of melanoma growth. The same course of 5 MMW treatments, if started on day 1 or day 10 following tumor inoculations, was ineffective. We concluded that MMWT has an anticancer therapeutic potential and that endogenous opioids are involved in MMWT-induced suppression of melanoma growth in mice. However, appropriate indications and contraindications have to be developed experimentally before recommending MMWT for clinical usage.

Local heating of human skin by millimeter waves: A kinetics study

Heating rates of human skin exposed locally to 42.25 GHz mm waves, coming from a waveguide (WG) opening or a YAV device designed for therapeutic application, were studied in vivo using infrared (IR) thermography. For both radiators, the power density distribution was described by a circularly symmetrical Gaussian type function on the exposed skin surface. Insertion of a small thermocouple (d = 0.1 mm) in the exposed area did not produce any significant artifact, either in the power density distribution or kinetics measurement, providing it was perpendicular to the E vector. The heating kinetics in the skin exposed with either the WG opening or the YAV device were well fitted to solutions of the 2-D bio-heat transfer equation for homogeneous tissue. Changes in irradiating beam size (1-8 mm) had no detectable effect on the initial (0.3-3.0 s) phase of the heating kinetics. However, the amplitude of the kinetics decreased substantially with decreasing the beam size. As the temperature rise in the time interval necessary for reliable measurement of the initial temperature rise rate was very small, an accurate experimental determination of specific absorption rate (SAR) becomes practically impossible at the low intensities normally used in our experiments. The correct SAR values may be found from fitting of the model to the heating kinetics. Bioelectromagnetics 24:571-581, 2003.

Distortion of millimeter-wave absorption in biological media due to presence of thermocouples and other objects

Specific absorption rate (SAR) distributions in the vicinity of a thermocouple or air bubble in water and in the presence of hair or sweat duct in skin were calculated using analytical and two-dimensional impedance methods. The objects were exposed to uniform 42.25 GHz plane electromagnetic fields. Insertion of a 0.1-mm thermocouple or similarly sized air bubble into water produced a strong localized disturbance of the otherwise uniform SAR distribution. However, the average of SAR values immediately surrounding the thermocouple was close to the undisturbed uniform average SAR. This allows measuring the average SAR during exposure of both unbounded and bounded media using calibrated small thermocouples (up to 0.1 mm). The SAR distribution in the vicinity of a hair was qualitatively similar to that produced by an air bubble. The maximal value of SAR was more than three times higher than the overall average SAR value in the skin. Sweat ducts produced a smaller disturbance of the millimeter-wave (mm-wave) field.

Millimeter wave power density in aqueous biological samples

Power density distribution inside a water sample placed between two parallel lossy dielectric plates (Polystyrene) was calculated using Fresnel equations for the frequency range of 42.25-53.57 GHz. Due to the multiple internal reflections from the sample boundaries, the distribution of the power density within the thin sample is more uniform than that within a semi-infinite medium. The power density in a sample depends on the thicknesses of the sample and the adjacent dielectric plates. For the given frequency range the sample thickness optimal for power density uniformity varies between 0.28 and 0.33 mm. The front plate has a significant effect on the magnitude of the power density within the sample but little effect on the power density distribution. The thicker the rear plate, the greater is the non uniformity of the power density distribution within the sample. Based on the calculated data, we determined the dimension of an exposure chamber providing the optimal power density distribution uniformity for mm-wave irradiation.

Peripheral neural system involvement in hypoalgesic effect of electromagnetic millimeter waves

In a series of blind experiments, using the cold water tail-flick test (cTFT) as a quantitative indicator of pain, the hypoalgesic effect of a single exposure of mice to low power electromagnetic millimeter waves (MW) was studied. The MW exposure characteristics were: frequency = 61.22 GHz; incident power density = 15 mW/cm2; and duration = 15 min. MW treatment was applied to the glabrous skin of the footpad. Exposure of an intact murine paw to the MW resulted in a statistically significant hypoalgesia as measured in the cTFT. These mice were able to resist cold noxious stimulation in the cTFF more than two times longer than animals from the sham-exposed group. A unilateral sciatic nerve transection was used to deafferent the area of exposure in animals from one of the experimental groups. This surgery, conducted six days before the MW treatment, completely abolished the hypoalgesic effect of the exposure to MW. The results obtained support the conclusion that the MW-skin nerve endings interaction is the essential step in the initiation of biological effects caused by MW. Based on our past and present results we recommend that in order to obtain a maximum therapeutic effect, densely innervated skin areas (head, hands) need to be used preferentially for exposure to MW in clinical practice.

Millimeter waves thermally alter the firing rate of the Lymnaea pacemaker neuron

The effects of millimeter waves (mm-waves, 75 GHz) and temperature elevation on the firing rate of the BP-4 pacemaker neuron of the pond snail Lymnaea stagnalis were studied by using microelectrode techniques. The open end to a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates (SARs), measured in physiological solution at the radiator outlet, ranged from 600 to 4,200 W/kg, causing temperatures rises from 0.3 to 2.2 degrees C, respectively. Irradiation at an SAR of 4200 W/kg caused a biphasic change in the firing rate, i.e., a transient decrease in the firing rate (69 +/- 22% below control) followed by a gradual increase to a new level that was 68 +/- 21% above control. The biphasic changes in the firing rate were reproduced by heating under the condition that the magnitude (2 degrees C) and the rate of temperature rise (0.96 degrees C/s) were equal to those produced by the irradiation (for an SAR of 4,030 W/kg). The addition of 0.05 mM of ouabain caused the disappearance of transient responses of the neuron to the irradiation. It was shown that the rate of temperature rise played an important role in the development of a transient neuronal response. The threshold stimulus for a transient response of the BP-4 neuron found in warming experiments was a temperature rise of 0.0025 degrees C/s.

Effects of millimeter waves on ionic currents of Lymnaea neurons

The effects of mm-waves 60.22-62.22 GHz and 75 GHz on A-type K+ currents and the effects of 61.22 GHz on Ca2+ currents of Lymnaea neurons were investigated using a whole-cell voltage-clamp technique. The open end of a rectangular waveguide covered with a thin Teflon film served as a radiator. Specific absorption rates at the waveguide outlet, inserted into physiological solution, were in the range of 0-2400 W/kg. Millimeter wave irradiation increased the peak amplitudes, activation rates, and inactivation rates of both ion currents. The changes in A-type K+ current were not dependent on the irradiation frequency. It was shown that the changes in the amplitudes and kinetics of both currents resulted from the temperature rise produced by irradiation. No additional effects of irradiation on A-type K+ current other than thermal were found when tested at the phase transition temperature or in the presence of ethanol. Ethanol reduced the thermal effect of irradiation. Millimeter waves had no effect on the steady-state activation and inactivation curves, suggesting that the membrane surface charge and binding of calcium ions to the membrane in the area of channel locations did not change.

Reflection and absorption of millimeter waves by thin absorbing films

Reflection, transmission, and absorption of mm-waves by thin absorbing films were determined at two therapeutic frequencies: 42. 25 and 53.57 GHz. Thin filter strips saturated with distilled water or an alcohol-water solution were used as absorbing samples of different thicknesses. The dependence of the power reflection coefficient R(d) on film thickness (d) was not monotonic. R(d) passed through a pronounced maximum before reaching its steady-state level [R(infinity)]. Similarly, absorption, A(d), passed two maximums with one minimum between them, before reaching its steady-state level [A(infinity)]. At 42.25 GHz, A(d) was compared with absorption in a semi-infinite water medium at a depth d. When d < 0.3 mm, absorption by the film increased: at d = 0.1 mm the absorption ratio for the thin layer sample and the semi-infinite medium was 3.2, while at d = 0.05 mm it increased up to 5.8. Calculations based on Fresnel equations for flat thin layers adequately described the dependence of the reflection, transmission, and absorption on d and allowed the determination of the refractive index (n), dielectric constant (epsilon), and penetration depth (delta) of the absorbing medium for various frequencies. For water samples, epsilon was found to be 12.4-19.3j, delta = 0.49 mm at 42.25 GHz, and epsilon = 9.0-19.5j, delta = 0.36 mm at 53.57 GHz. The calculated power density distribution within the film was strongly dependent on d. The measurements and calculations have shown that the reflection and absorption of mm-waves by thin absorbing layers can significantly differ from the reflection and absorption in similar semi-infinite media. The difference in reflection, absorption, and power density distribution in films, as compared to semi-infinite media, are caused by multiple internal reflections from the film boundaries. That is why, when using thin phantoms and thin biological samples, the specifics of the interaction of mm-waves with thin films should be taken into account.

Hypoalgesic effect of millimeter waves in mice: dependence on the site of exposure

Based on a hypothesis of neural system involvement in the initial absorption and further processing of the millimeter electromagnetic waves (MW) signal, we reproduced, quantitatively assessed and compared the analgesic effect of a single MW treatment, exposing areas of skin possessing different innervation densities. The cold water tail flick test (cTFT) was used to assess experimental pain in mice. Three areas of exposure were used: the nose, the glabrous skin of the right footpad, and the hairy skin of the mid back at the level of T5-T10. The MW exposure characteristics were: frequency = 61.22 GHz; incident power density = 15mW/cm2; and duration = 15 min. The maximum hypoalgesic effect was achieved by exposing to MW the more densely innervated skin areas--the nose and the footpad. The hypoalgesic effect in the cTFT after MW exposure to the murine back, which is less densely innervated, was not statistically significant. These results support the hypothesis of neural system involvement in the systemic response to MW.

Effects of alcohols on A-type K+ currents in Lymnaea neurons

The effects of short-chain alcohols (methanol, ethanol and n-propanol) on the fast-inactivating, A-type, potassium current of Lymnaea neurons were examined using macroscopic recording techniques. Alcohols produced a blockade of the current and modified its inactivation mechanism. The extracellular concentrations of methanol, ethanol and n-propanol causing 50% suppression of the current were 2970, 830 and 230 mM, respectively. The main effects of alcohols on inactivation were a decrease in the amplitude of the fast component and a simultaneous increase in the amplitude of the slow component of inactivation. In a model, the suppression of the fast component could be reproduced by an increase of the backward rate constant related to the dissociation of the inactivation particle from its binding site. The blockade and modification of inactivation reveal similar dependences on ethanol concentration, indicating that the same type of interaction of ethanol with the channel underlies both of these events. Ethanol was effective only in extracellular applications. The data support an action of alcohols at a hydrophobic site near the extracellular portion of the channel.

Two types of A-channels in Lymnaea neurons

The gating mechanism of A-channels of Lymnaea neurons and the effect of tetraethylammonium (TEA) on these channels were studied using macroscopic recording techniques. Along with the fast-inactivating A-current (Iaf) described earlier we found a slow-inactivating A-current (Ias) in some neurons of the visceral ganglion. Both currents have revealed similar activation kinetics, but differ in the inactivation kinetics and mechanisms. The inactivation kinetics of Ias were satisfactorily described by a sum of two exponentials with rate constants (tau -1) of 28 s-1 and 4.5 s-1 at V = -20 mV. Intracellular TEA reduced the peak amplitudes of Iaf and Ias and slowed the rate of the fast phase of inactivation of Iaf. This resulted in a crossover of the current traces in the presence and absence of TEA, as though it competes with the binding of the inactivating particle. The mechanism of the fast phase of inactivation of Iaf is similar to that of fast inactivation of the Shaker K+ channels which appears to be due to a ball-and-chain mechanism. The slow phases of inactivation of Iaf and Ias reveal properties characteristic of C-type inactivation shown in Shaker K+ channels. A partially coupled model including three pathways for transition of a channel from the closed to open states accurately reproduces all of the experimental data. It has voltage-independent transitions to the inactivation states indicating that inactivation of A-current is not associated with charge movement through the membrane. The results suggest that Lymnaea A-channels seem to be heteromultimeric.

[Effect of radiofrequency range electromagnetic radiation on chemoreceptor structure]

The data concerning the effects of radio frequency electromagnetic radiation on chemoreceptor systems available in current literature were reviewed. These effects were systematized by the level of organization of the systems including organisms, cell and subcell preparations and membrane structures. The different mechanisms which could underlie electromagnetic radiation effects were analyzed.

Millimeter microwave effect on ion transport across lipid bilayer membranes

The effects of millimeter microwaves in the frequency range of 54-76 GHz on capacitance and conductance of lipid bilayer membranes (BLM) were studied. Some of the membranes were modified by gramicidin A and amphotericin B or by tetraphenylboron anions (TPhB-). The millimeter microwaves were pulse-modulated (PW) at repetition rates ranging from 1 to 100 pps, PW at 1000 pps, or unmodulated continuous waves (CW). The maximum output power at the waveguide outlet was 20 mW. It was found that CW irradiation decreased the unmodified BLM capacitance by 1.2% +/- 0.5%. At the same time, membrane current induced by TPhB- transport increased by 5% +/- 1%. The changes in conductance of ionic channels formed by gramicidin A and amphotericin B were small (0.6% +/- 0.4%). No "resonance-like" effects of mm-wave irradiation on membrane capacitance, ionic channel currents, or TPhB- transport were detected. All changes in membrane capacitance and currents were independent of the modulation employed and were equivalent to heating by approximately 1.1 degrees C.

Influence of microwaves on different types of receptors and the role of peroxidation of lipids on receptor-protein shedding

The effects of a continuous wave or pulse-modulated, 900 MHz microwave field were studied by in vitro assays of rat chemoreceptors. The pulsed field was modulated as rectangular waves at rates of 1, 6, 16, 32, 75, or 100 pps. The pulse-period to pulse-duration ratio was 5 in all cases, and specific absorption rates (SARs) ranged from 0.5 to 18 W/kg. Binding of ligands to cell membranes was differentially affected by exposure to microwaves. For example, binding of H3-glutamic acid to hippocampal cells was not altered by a 15 min exposure to a continuous wave field at 1 W/kg, but binding of H3-dihydroalprenolol to liver-cell membranes of neonates underwent a fivefold decrease under the same field conditions. This effect was not dependent on modulation or on a change in the constant of stimulus-receptor binding but depended on a shedding of the membrane's receptor elements into solution. The magnitude of inhibition correlated with the oxygen concentration in the exposed suspension. Antioxidants (dithiothreitol and ionol) inhibited the shedding of receptor elements. The microwave exposure did not cause an accumulation of products from the peroxidation of lipids (POL). Ascorbate-dependent or non-enzymatic POL was not responsible for the inhibition, and POL was not found in other model systems. However, enzymatic POL mechanisms in localized areas of receptor binding remain a possibility.

Kinetic study of A-type current inactivation in Lymnaea neurons

Macroscopic inactivation of A-current was studied in internally perfused Lymnaea neurons under voltage clamp conditions. Inactivation kinetics were satisfactorily described by the sum of two exponentials, suggesting the presence of two type inactivation. The kinetics of recovery from inactivation were exponential. The rate constants of the fast phase of inactivation gamma f(V) rose steeply with depolarization exposing the pronounced plateau in the range from -30 to 0 mV. The time course of inactivation in this potential range was more closely approximated with the sum of three exponentially decaying components. Calcium and hydrogen ions strongly affected the fast phase of inactivation. Calcium gave a positive shift of a part of the gamma f(V) curve on the left of the plateau. Raising the pH caused a negative shift of the right-hand branch of the gamma f(V) curve. It was shown that these effects are associated with Ca2+ and H+ binding to some specific sites of the channel protein. Two models give good fits with the experimental data. They include two pathways for fast inactivation. Calcium and hydrogen ions are assumed to selectively affect the voltage-dependent transitions related to these pathways of inactivation.

[Effect of SHF-radiation on spontaneous impulse activity of cerebral cortex slices in vitro]

The effect of microwaves field (900 MHz), pulse modulated at frequencies of 7, 16 and 30 Hz, on spontaneous electrical activity (SEA) of cerebral cortex slices of guinea-pig has been studied. Microwaves were shown to induce an irreversible decrease in repetitive rate of SEA. Conventional heating with the temperature rise above that during irradiation produced the same but reversible effect. It is suggested that the microwave effect on SEA is complex consisting of thermal and athermal components.

Interaction of sulfhydryl reagents with A-type channels of Lymnaea neurons

The effect of sulfhydryl reagents on macroscopic inactivation of A-current in internally perfused Lymnaea neurons under voltage-clamp conditions was investigated. It was found that the binding of Hg2+ rather than PHMB with channel proteins resulted in a strong decrease of the peak current and the inactivation rate. Hg2+ markedly influenced the steady-state inactivation but did not change the rate of recovery from inactivation. It was found that both reagents reacted with the same groups of the channel protein and that those are most likely sulfhydryl groups. These groups seemed not to be involved in the gating charge movement. Hg2+ ions can immobilize some part of the gating charge thereby resulting in strong changes of the steady-state inactivation.

Bursting responses of Lymnea neurons to microwave radiation

Microelectrode and voltage-clamp techniques were modified to record spontaneous electrical activity and ionic currents of Lymnea stagnalis neurons during exposure to a 900-MHz field in a waveguide-based apparatus. The field was pulse-modulated at repetition rates ranging from 0.5 to 110 pps, or it was applied as a continuous wave (CW). When subjected to pulsed waves (PW), rapid, burst-like changes in the firing rate of neurons occurred at SARs of a few W/kg. If the burst-like irregularity was present in the firing rate under control conditions, irradiation enhanced its probability of occurrence. The effect was dependent on modulation, but not on modulation frequency, and it had a threshold SAR near 0.5 W/kg. CW radiation had no effect on the firing rate pattern at the same SAR. Mediator-induced, current activation of acetyl-choline, dopamine, serotonin, or gamma-aminobutyric-acid receptors of the neuronal soma was not altered during CW or PW exposures and, hence, could not have been responsible for the bursting effect.

Microwave effect on camphor binding to rat olfactory epithelium

Microwave radiation decreased specific camphor binding to a membrane fraction of rat epithelium but not to a Triton X-100 extract of this fraction. Inhibition of the ligand binding did not depend on the modulation frequency of the microwave field in the region 1-100 Hz and was not a linear function of specific absorption rate (SAR). The decreased ligand binding was due to a shedding or release of the specific camphor-binding protein from the membrane into solution. It is highly probable that several other membrane proteins may be shed into solution during microwave exposure.

[Changes in the electrical activity of pacemaker neurons of the snail depending on the heating rate]

Rapid temperature increases (1 degree C for 5-10 sec) cause inhibition of firing rate of L. stagnalis pacemakers due to fast activation of the sodium pump. Slow warming to 22 degrees-24 degrees C has an opposite effect; it increases the firing rate. Different responses of the pacemakers to the heating rate explain the features of the microwave effect on the neuron electrical activity.

[Effect of electromagnetic radiation in a decimeter wave-length range on the calcium current of molluscan neurons]

Microwave effect on calcium current of the dialysed snail neurons was investigated. Isolated neurons were irradiated by unmodulated and modulated microwaves (900 MHz) with modulation frequency 0,5 divided by 1000 Hz and SAR = 0,1 divided by 20 W/kg. Calcium current increase was shown to be induced by microwave heating. The current increments were proportional to SAR. Microwave effects on the charge distribution of the cell membrane surface and other non-thermal special microwave effects were not detected.

[Interaction of rhodopsin with quinone]

The reaction of rodopsin extracted with cetyltrimethylammonium bromide with p-benzoquinone was studied with spectrophotometric and ESR methods. In the dark quinone reacted with three SH-groups of rodopsin, in the light with three more SH- and two unknown groups of decolourized rodopsin. The interaction of quinone with SH-groups liberated under light effect on rodopsin proceeds with the formation of intermediate free-radical products.