Jerry W. Highfill

Acute effects of diisopropyl fluorophosphate (DFP) on autonomic and behavioral thermoregulatory responses in the Long-Evans rat☆

Experiments were designed to assess the mechanisms of diisopropyl fluorophosphate (DFP)-induced changes in thermoregulation of the rat. In one study, male rats of the Long-Evans strain were injected with DFP (s.c.) at doses ranging from 0 to 2.0 mg/kg while maintained at an ambient temperature (Ta) of 20--24 degrees C. Body (Tb) and tail skin (Tt) temperatures were recorded for 5 h post-injection. DFP doses of greater than or equal to 1.0 mg/kg resulted in significant decreases in Tb lasting up to 5 h and increases in Tt lasting up to 1 h post-injection. In a second study, metabolic rate (MR), evaporative water loss (EWL), motor activity (MA), Tb, and Tt were measured at 2 h post-injection of 0, 0.5, 1.0, and 1.5 mg/kg DFP (s.c.) at Ta values of 10, 20, and 30 degrees C. DFP treatment resulted in hypothermia at all three Ta values, but the effect was attenuated at 30 degrees C. MR was significantly reduced at a Ta of 20 degrees C following 1.5 mg/kg, unaffected by DFP at a Ta of 30 degrees C, and stimulated at 10 degrees C following 0.5 mg/kg DFP. EWL was significantly elevated at 30 degrees C following 1.5 mg/kg DFP. MA was significantly reduced following greater than or equal to 1.0 mg/kg DFP at 20 and 30 degrees C and 1.5 mg/kg at 10 degrees C. Tt was elevated and reduced by DFP at Ta values of 30 and 10 degrees C, respectively. In a third study, rats were injected with DFP and placed in a temperature gradient for 1 to 2 h post-injection while selected Ta and Tb were monitored. While both control and DFP-treated rats remained in the cool end of the gradient, rats administered DFP at doses of 1.0 and 1.5 mg/kg were significantly hypothermic. It was also found that Ta values of 10, 20, and 30 degrees C had no effect on DFP-induced inhibition of cholinesterase activity of plasma and erythrocyte fractions of whole blood. Overall, these data support the hypothesis that acute DFP may lower the set-point for the control of body temperature in the rat and demonstrates that the toxicity of DFP is modified by changes in Ta.

Impact of the Hypothermic Response in Inhalation Toxicology Studies

Previous studies from this laboratory showed that the decreases in Tco and associated functional parameters often observed in rodents following exposure to xenobiotic agents are capable of modulating the subsequent toxic response and that the magnitude of this induced hypothermic response may itself be modified by a number of experimental conditions. A moderate hypothermic response, characterized by a temperature drop of approximately 2 degrees C, appears to afford the optimal protection. Studies in which exposures occur through inhalation of harmful gases or particles present a special set of problems. In such studies, the dose of the toxic agent to which the animal is exposed is a function of the concentration of the agent in the atmosphere and the minute ventilation of the animal. Although ambient concentrations is generally held constant in laboratory studies, minute ventilation varies directly with metabolism, and both of these parameters may change significantly across experimental conditions. Thus, at low Tas, metabolism and minute ventilation are relatively high and uptake of inhalable toxic agents is increased. However, the development of the hypothermic response during the exposure entails a directly correlated reduction in these parameters and, presumably, in dose. For the most part, inhalation toxicological studies are conducted using resting animals or exercising humans. Animals are sometimes concurrently exposed to CO2 to simulate the increased ventilation of exercise and more closely mimic human studies. The experimental protocols employed in the above inhalation studies permitted examination of (1) the impact of species, size, handling stress, and changes in Ta on both the induced hypothermic response and the concomitant pulmonary toxicity; (2) the additive impact of exercise stress on O3 toxicity; and (3) the toxicity of ambient-derived particulate matter in normal rats and in rats with preexisting pulmonary inflammation. The results of these studies demonstrate that the magnitude of the induced hypothermic response is directly proportional to the uptake of the toxic agent by the lung and inversely proportional to the mass of the animal and the ambient temperature at which the exposure is conducted. The hypothermic response is sensitive to a number of experimental stresses including handling and changes in cage conditions. Exercise attenuates the hypothermic response, whereas CO2-stimulated increases in ventilation employed as an exercise surrogate may potentiate the response. Toxic exposures conducted in animals with lung disease or compromised pulmonary function may induce a severe hypothermic response while comparable exposures in normal animals produce only mild or moderate responses. In general, the development of the hypothermic response in the presence of ambient pollutants serves to decrease the minute ventilation of the animal and therefore limits the uptake and dose of the airborne toxicant. The results of these inhalation studies support our previous conclusions concerning the impact of the hypothermic response on toxicity and emphasize the need to monitor and incorporate these changes in functional parameters into analyses of toxicological data. Furthermore, because humans do not demonstrate a robust hypothermic response following exposure to toxic agents, extrapolation of the results obtained from animal studies and comparisons with data from human studies are considerably more complicated.

Relationship between cholinesterase inhibition and thermoregulation following exposure to diisopropyl fluorophosphate in the rat

This study examined the relationship between inhibition of cholinesterase activity (CA) and thermoregulatory response in the rat following exposure to the organophosphate (OP), diisopropyl fluorophosphate (DFP). Male Long-Evans rats were injected with DFP dissolved in peanut oil in doses ranging from 0 to 1.5 mg/kg (s.c.). Colonic (Tcol) and tail skin temperature (Ttail) were recorded at 0, 1, 2 and 3 h post-injection. At 3 h post-injection the rat was sacrificed and a blood sample was taken by cardiac puncture and analyzed for CA. There was a biphasic dose effect of DFP on Tcol with slight but significant elevation in Tcol in the dose range of 0.01-0.5 mg/kg and a significant depression in Tcol at doses of 1.0 and 1.5 mg/kg. There was a dose-dependent fall in CA with DFP administration in the erythrocyte, plasma, and whole blood fractions. Hypothermia was associated with 80-87% inhibition in CA, whereas the elevation in Tcol was associated with 20-70% inhibition in CA. DFP also elicited significant elevations in Ttail. Overall, the data fail to demonstrate any clear relationship between inhibition of blood CA and thermoregulatory response following exposure to DFP. However, the elevation in Tcol following relatively low doses of DFP may be of relevance to the frequently reported symptom of fever in humans exposed to OP agents.

Thermoregulation in mice following acute administration of lead acetate

Several reports in the literature suggest a relationship between lead intoxication and thermoregulatory capacity. To investigate the effects of lead on the control of body temperature, mice of the BALB/c strain were injected intraperitoneally with lead acetate (0 to 100 mg/kg) while colonic temperature was measured 30, 60, and 90 min post-injection at ambient temperatures (Ta) of 20 and 30 degrees C. Lead acetate caused a transient hypothermia, an effect which was augmented at cooler Tas. In a second experiment, mice were injected with 100 mg/kg lead acetate and placed in a longitudinal temperature gradient to measure their preferred Ta. Lead acetate significantly reduced the preferred Ta during the first 30 min post-injection which augmented the lead-induced hypothermia. In a third experiment it was found that lead acetate-induced lethality was potentiated with increasing Ta. Hence, the hypothermic response to acute lead acetate treatment may be beneficial to survival.

Thermoregulatory Considerations Affecting Both Acute and Prolonged Exposures to Ozone in Rodents

Our laboratory has been investigating the thermoregulatory effects of xenobiotic agents in rodents for a number of years [1,2]. The general hypothesis which has evolved as a result of these studies can be stated as follows: In response to exposure to a wide variety of toxic agents, rodents physiologically and behaviorally lower their body core temperature; the resulting moderate hypothermia confers specific advantages with respect to decreased toxic response and increased survival. If measures are employed to block or exacerbate the moderate hypothermic response, toxicity and lethality may be potentiated. More recent work which examined pulmonary and extrapulmonary effects of air pollutants has provided additional support for the above hypothesis. In these studies [3, 4], exposure to near environmental levels of ozone (O3) triggered a profound hypothermia in the rat which was accompanied by significant decreases in heart rate (HR) and activity, as well as increases in selected biochemical indices of pulmonary damage. The hypothermic response may precede the other observed responses and appears to play an important role in determining the extent of O3-induced toxicity. The present studies investigated the effects of both acute and prolonged exposures to O3 in rats maintained at normal ambient temperatures (Ta), as well as the modulatory influence on the induced toxicity exerted by altering the Ta at which the exposures were conducted.