|
| Other Studies - Free Oxygen Radicals A free radical is a molecule or ion that has an unpaired electron in its outer orbit. Radical particles are unstable and are very reactive. Most commonly free radicals are produced from oxygen metabolism, and are called reactive oxygen species (ROS) (Yurekli, 2006). Cells stressed by oxidation react by producing ROS. Chain reactions can start as the unpaired electron is transferred from one molecule to another. Various consequences can arise, one of which is lipid peroxidation. One of the markers of this is the production of malondialdehyde (MDA). Based on oxidant products, 3 main classes of prooxidant enzymes can be designated: (1) nitric oxide symthases (NOS) produce nitric oxide (NO); (2) cyclooxygenases (COX), xanthine oxidase (XO) and NADPH oxidase generate superoxide anion as the main oxidant in various cell types; (3) myeloperoxidase (MAO) and monoamine oxidase generate hypochlorous acid and hydrogen peroxide in leukocytes and in parenchymal cells respectively. Some enzymatic systems can counterbalance the production of ROS. These include superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) (Irmak, 2002). ). Balance between the prooxidants and the antioxidant enzymes can be upset by an increase in free radicals or by a decrease in antioxidants. Some research studies have explored this area. Moustafa (2001) reported an increase in plasma lipid peroxide and a decrease in erythrocyte antioxidants in humans exposed to a 900 MHz field for up to 4 hours. However, there are no details of SAR levels, and there was no sham exposure. Dasdag (2004) found that MDA levels were increased in the brains of rats exposed to RFR from a 900 MHz source at SAR of 0.52 W/kg for 20 minutes a day, 7 days a week, for 1 month. However, there was no histological alteration or change in brain phospholipid fatty acid composition. Irmak (2002) did not find any changes in oxidant or antioxidant levels in brains of rabbits exposed to RFR at 900 MHz for 30 minutes per day for 1 week. Serum SOD levels were increased and serum NO levels decreased. Ilhan (2004) exposed rats to RFR at 900 MHz for 1 hour a day for 1 week. They found increased MDA and NO levels in brain tissue, decrease in SOD and GSH-Px levels, and increase in XO and adenosine deaminase levels. These changes were prevented when the rats were treated with Ginkgo biloba, which has antioxidant activity. Zmyslony and colleagues (2004) found that ROS levels were increased in rat lymphocytes exposed to RFR at 930 MHz (SAR 1.5 W/kg) for either 5 or 15 minutes. The lymphocytes had been treated in vitro with FeCl2 to induce oxidated stress. Ozguner (2005) reported that antioxidant enzyme levels were reduced and levels of ROS increased in the kidneys of rats exposed to 900 MHz frequency RFR for 30 minutes a day for 1 month. The whole body SAR was 4 W/kg. Yurekli (2006) reported that in rats exposed to far-field exposure (power density 3.67 W/m², SAR 11.3 mW/kg) for 8 days and 7 hours a day, MDA and SOD levels increased and glutathione levels decreased. However, there were baseline differences in mean weight between the exposed and the sham rats, and the experiment does not appear to have been a randomized, double-blind study. Meral (2007) found increased levels of MDA and decreased levels of CAT and GSH in brain tissue of guinea pigs exposed to GSM RFR at 890-915 MHz for 12 h/day for 30 days. Blood levels of MDA, vitamins A, D3, and E, and CAT increased, and GSH decreased. Friedman (2007) reported that exposure of rat or HeLa cells to RFR at 835 MHz was associated with activation of extracellular-signal-regulated kinase (ERK), which results from initial activation of NADH oxidase, leading to generation of ROS, and then to the activation of the ERK cascade. Arthur (2007) reviews Friedman's paper in an accompanying commentary. There have
been a number of negative studies in this area. Hook et al.
(2005) reported no evidence of oxidative stress in mouse macrophage
cells exposed to RFR at either 835.62 MHz (FMCW) or 847.74 MHz
(CDMA) at a SAR of 0.8 W/kg for 20-22 hours. Lantow (2006a)
also found no significant difference in production of free radicals
after exposure of human leukemia cells to RFR at 1800 MHz at
SARs of 0.5-2.0 W/kg for 45 minutes. The same group (Lantow
2006b) did not find induction of ROS release (or of HSP70 expression)
in human lymphocytes or monocytes with RF exposure at SAR of
2W/kg for 45 minutes. However, they did find a decrease in ROS
production in monocytes
during sham exposure when GSM-DTX exposure was being tested.
The reason for this is not clear. Simko (2006) reported no increase
in free radicals after exposure of human monocytes to RFR at
1800 MHz for 60 minutes. Ferreira (2006) did not find any evidence
of lipid or protein oxidative damage in the brains of rats exposed
to 834 MHz frequency RFR at SAR of 0.98 W/kg for 6 days, 7 and
1/2 hours a day. Authors
Authors Authors Authors Authors
Authors Authors Authors Authors Authors
Authors Authors |
|