Ornithine decarboxylase activity in tissues from rats exposed to 60 Hz magnetic fields, including harmonic and transient field characteristics.

Ornithine decarboxylase (ODC) activity is used widely as a biomarker for tumor promotion in animal model systems. Several previous studies have reported increases in ODC activity in tissues of rats exposed to 60 Hz magnetic fields. The goals of this study were to confirm these findings and to determine whether ODC activity is increased in tissues of animals exposed to magnetic fields containing complex metrics. Three experiments were conducted in male F344 rats. Each study included a sham control group and a group exposed to pure continuous 60 Hz fields (0.2 mT). Additional groups included animals exposed to randomly time-varying 60 Hz fields (range of 0.02 to 0.2 mT); intermittent 60 Hz fields (2 mT) with on-off cycles ranging from 5 s to 5 min; pure continuous 180 Hz fields (2 mT); 60 Hz fields with a superimposed 3rd harmonic (total field strength, 2 mT); 60 Hz fields with superimposed third, fifth, and seventh harmonics (total field strength, 2 mT); 60 Hz fields (2 mT) with superimposed transients; and randomly time-varying 60 Hz fields (range of 0.02 to 0.2 mT) with superimposed transients. After 4 weeks of exposure (18.5 h/day), eight animals per group were euthanized within 1 h of magnetic field deactivation. Homogenates of liver, kidneys, spleen, and brain were prepared from each animal, quick-frozen, and shipped for analysis by four independent laboratories. No consistent pattern of differences in the ODC activity among experimental groups was found either within a laboratory or among laboratories. The results do not support the hypothesis that exposure to extremely low frequency magnetic fields stimulates ODC activity.

Ornithine decarboxylase activity in developing chick embryos after exposure to 60-hertz magnetic fields.

Fertilized white leghorn eggs were exposed to a 4 micro-Tesla (microT) 60 Hz horizontal magnetic field for 15, 18, 23 and 28 h. After exposure to the magnetic field, the embryos were isolated and assayed for ornithine decarboxylase (ODC) activity. ODC activity in magnetic field-exposed embryos was compared to ODC activity in sham-exposed embryos. ODC activity in magnetic field-exposed embryos was not statistically elevated above sham-exposed embryos.

Ornithine decarboxylase activity in L929 cells following exposure to 60 Hz magnetic fields.

To determine whether there is a biological basis for epidemiological studies which suggest an association between exposure to magnetic fields and cancer, we have attempted to replicate earlier findings on cellular enzymes related to cell proliferation. Here we report on an effort to replicate the doubling of ornithine decarboxylase (ODC) activity in L929 murine fibroblasts following exposure to 60 Hz magnetic fields reported by Litovitz et al. Efforts were made to reproduce the methods and exposure conditions used by the original investigators. Positive controls showed that our assay system responded to other known stimuli of ODC activity. We extended the previously reported investigations by testing a number of exposure conditions and other associated variables. Initial results suggested that cells exposed in the original investigators' laboratory demonstrated an enhanced enzyme activity, whereas cells exposed in our laboratory did not. Experiments in our laboratory using the most important elements of the original investigators' exposure system did not demonstrate any enhancement of ODC activity. Finally, a series of magnetic field exposure and sham exposure experiments conducted in the original investigators' laboratory failed to demonstrate an effect of magnetic fields on ODC activity.

Effects of bufalin on norepinephrine turnover in canine saphenous vein.

Abundant experimental data suggest that an endogenous digitalislike factor is responsible for some essential hypertension. Some forms of hypertension have also been associated with increased levels of catecholamines. We therefore designed experiments to investigate the role of digitalislike factors in the regulation of norepinephrine turnover in the neurovascular junction. We chose bufalin, an amphibian-derived compound that shares many of the physiological properties postulated as characteristic of digitalislike compounds, as a model of the mammalian compound. In vitro experiments in canine saphenous veins showed that, in addition to inhibiting norepinephrine uptake, bufalin increased norepinephrine overflow by an amount larger than could be explained solely by uptake inhibition. The effect of bufalin on norepinephrine overflow is inhibited by tetrodotoxin, which suggests a dependence of this response on Na+ influx through the neuronal membranes. We propose that Na+,K(+)-ATPase inhibition resulting in neuronal depolarization is responsible for the augmented norepinephrine turnover caused by bufalin and that these indirect effects of norepinephrine on the cardiovascular system may play a role in the etiology of hypertension.

Epidural anaesthesia attenuates the catecholamine response to hypoventilation.

The effect of a high epidural block on the catecholamine response to hypoventilation was studied in six unanesthetized dogs given intravenous sufentanil (15 micrograms.kg-1). Sufentanil alone resulted in a increase of norepinephrine (NE) concentration from 108 +/- 73 pg.ml-1 to 843 +/- 399 pg.ml-1 and epinephrine (E) from 279 +/- 80 pg.ml-1 to 2010 +/- 1416 pg.ml-1. At least one week later, an epidural block to T1 was achieved using 8-10 ml, two per cent lidocaine. Plasma NE and E decreased after EA to about 50 per cent of resting baseline measurements. The addition of sufentanil increased NE and E levels to reach approximately the resting base-line levels. In all dogs intravenous sufentanil resulted in bradypnoea, bradycardia, hypoxaemia, and hypercarbia. Intravenous lidocaine infusions had no significant effect on plasma catecholamine levels when plasma lidocaine levels ranged from 1.7 micrograms.ml-1 to 5.3 micrograms.ml-1. We conclude that a high two per cent lidocaine epidural block attenuates the catecholamine response to hypoventilation in dogs, but the persistence of baseline plasma levels of NE and E suggests that the efferent sympathetic block by high EA is incomplete.