Candace B. Matthew

Novel approaches to the pathophysiology of heatstroke: the energy depletion model.

De-emphasis of the role of anhydrosis as the primary cause of heatstroke has resulted in increased usage and acceptance of animal models for heatstroke research. When the total amount of work achieved by the running rat prior to exhaustion was plotted against the rate of heat storage, a heretofore unrecognized relationship emerged. These new data suggest that physical exhaustion and heat exhaustion represent opposite ends of a continuum related to the rate of heat storage. Changes in thermoregulatory and/or physical performance can be estimated by a two-dimensional shift in the work-output/thermal storage ratio. Potassium depletion reduces thermoregulatory/physical performance; a combination of atropine plus diazepam appears to improve it. The role of the cholinergic nervous system in eliciting alterations in thermoregulatory and physical ability is reviewed; endurance training, shivering, acclimatization, set-point theory, the anticholinergic syndrome, lithium intoxication, and choline deficiency are discussed.

Atropine, diazepam, and physostigmine: Thermoregulatory effects in the heat-stressed rat

We have previously reported that administration of atropine (A) to unrestrained, sedentary, heat-stressed rats resulted in a dose-dependent increase in heating rate (rate of rise of core temperature, degree C/min). Additionally, we have demonstrated that the decrements in treadmill endurance and increments in heating rate of physostigmine (PH)-treated running rats can both be restored to control levels by pretreating the animals with A and diazepam (D). Our objective in the present work was to determine if the administration of D + PH to A-treated unrestrained, sedentary, heat-stressed rats (N = 16/group, 510-530 g) could improve their thermal tolerance. The following drugs were administered singly (at 10 min intervals) via lateral tail vein: vehicle-control (C), A (200 micrograms/kg), D (500 micrograms/kg), and PH (200 micrograms/kg). After drug administration, the rats were heat-stressed (Tamb = 41.5 degrees C) until a core temperature of 42.6 degrees C was attained when they were removed to a 26 degrees C chamber. The heating rates (degrees C/min) and tolerance times (min) of the respective groups were: C- 0.02, 235; A- 0.08, 58; A D- 0.06, 94; and A + D + PH- 0.04, 143. Administration of D with A significantly decreased heating rate, and D + PH more than doubled the thermal tolerance of A-treated rats. Thus, the combination of A + D + PH not only restores PH-induced performance and thermoregulatory decrements of rats exercised in a moderate environment, but also reduces A-induced heat intolerance.

Physostigmine: dose-response effects on endurance and thermoregulation during exercise

We previously reported that the administration of 200 micrograms/kg of physostigmine (PH) to rats exercising on a treadmill resulted in decrements in both endurance (decreased running time to exhaustion) and thermoregulation. However, it was necessary to determine the dose-response effects of PH administration before PH-treated exercising rats could be used as a model with which to examine the relative anticholinergic potency of drugs. In the present work saline, 50, 100, or 200 micrograms/kg of physostigmine salicylate (0%, 40%, 50%, and 60% whole blood cholinesterase inhibition) was administered to rats (N = 12/group) prior to treadmill exercise (26 degrees C, 50% rh, 11 m/min, 6 degrees incline). The saline control group ran for 67 +/- 6 min (mean +/- SE) with a rate of rise of core temperature of 0.051 +/- 0.007 degrees C/min. The run times declined (80%, 64% and 48% of control) as rate of rise of core temperature increased (116%, 180%, and 214% of control) in a dose-dependent manner (50, 100, 200 micrograms/kg PH). Cholinergic symptoms such as salivation, tremors, and defecation were also affected in a dose-dependent manner by PH administration. Since cholinergic symptoms, thermoregulatory effects, and endurance decrements all vary in a dose-dependent manner with physostigmine administration, the exercising rat represents a useful model for examining the relative potency of cholinergic therapies.

Carbamates, atropine, and diazepam: effects on performance in the running rat.

We have reported that when rats (500 g, male) are exercised to exhaustion on a treadmill, pretreatment with the centrally acting carbamate physostigmine reduced endurance (run time, RT) and increased the rate of rise of core temperature (Tc+). Both RT and Tc+ were restored to control levels by pretreatment with either or a combination of atropine (A), and diazepam (D). Our objective in the present work was to determine whether A+D could also restore the performance and thermoregulatory decrements induced by the peripherally acting carbamate pyridostigmine (PY). After drug administration, rats were run (11 m/min, 6 degrees elevation, Ta = 26 degrees C) to exhaustion. PY treatment resulted in a reduced RT and an increased heat gain that neither A nor D alone (A+PY and D+PY) could restore to control levels. On the other hand, a combination of both A and D restored these variables to control levels. In conclusion, A+D can restore the performance and thermoregulatory decrements resulting from the administration of either a centrally or a peripherally acting carbamate.

Chronic vs acute carbamate administration in exercising rats

Physostigmine (PH), alone, and pyridostigmine (PY), in combination with atropine and 2-PAM, have been shown to protect animals against organophosphate poisoning. While acute administration of either of these carbamates increased heating rates and decreased endurance of exercising rats, chronically administered PY did not induce these decrements, and we hypothesized that chronic administration of PH could also result in similar attenuation of these effects. Thus, PH was administered acutely (iv) or chronically (osmotic mini-pump) in the following 4 groups (510-530g, male, N = 10/group): C (control, saline iv), AC-200 (acute, 200 ug/kg, 58% whole blood cholinesterase (ChE) inhibition), CH-7 (chronic, 125 ug/hr, 7 days, 60% inhib.), and CH-14 (chronic, 125 ug/hr, 14 days, 56% inhib.). Rats were run (11 m/min, 26 degrees C) to exhaustion. The run times and heating rates (% of control) were: AC-200 - 47, 213%; CH-7 - 60, 157%; CH-14 - 92, 109%. Additionally, ultrastructural changes noted in diaphragms of acutely treated animals were less evident in chronically treated animals. Thus, the decremental effects of acute PH administration on endurance, thermoregulation, and ultrastructure were attenuated with chronic administration at similar levels of ChE inhibition.

Ambient temperature effects on thermoregulation and endurance in anticholinesterase-treated rats

In sedentary animals, physostigmine (PH) administration resulted in a decreased core temperature that is ambient temperature (Ta) dependent. PH administration in rats exercising on a treadmill (26 degrees C, 50% rh, 11m/min, 6 degrees incline) decremented endurance and increased rate of rise of core temperature (heating rate, HR). This study was undertaken to examine the effects of Ta on the endurance and thermoregulatory decrements of PH-treated running rats. Adult male rats (510-530g) were given either 0.2ml saline (C) or 100 ug/kg physostigmine salicylate in 0.2 ml saline via tail vein 15 min prior to the start of running to exhaustion at 10, 15, 26, or 30 degrees C. In both C- and PH-treated groups, endurance decreased and HR increased with increasing Ta from 15 to 30 degrees C. At 15 and 26 degrees C the C rats ran significantly (p < .05) longer and had significantly lower HR than the PH rats: C15 = 90 +/- 8 min, 0.022 +/- 0.006 degrees C/min; C26 = 67 +/- 6, 0.051 +/- 0.007; PH15 = 57 +/- 5, 0.052 +/- 0.008; and PH26 = 43 +/- 6, 0.092 +/- 0.007. At 10 and 30 degrees C there were no significant differences between C and PH-treated rats. A Ta of 30 degrees C was too high for effective cooling in either group, and at 10 degrees C both groups were able to dissipate heat despite the increased metabolic rate of the PH-treated rats. The PH-treated rat model of cholinergic drug effect is useful at a Ta of 15 and 26 degrees C.

Cholinergic drug interactions and heat tolerance

Acute, subchronic and chronic exposures to cholinergic compounds may result in differing effects. The efficacy of pyridostigmine bromide (PY) prophylaxis against organophosphorus poisoning depends on post exposure atropine (AT) administration. AT induces a dose-dependent increase in rate of rise of core temperature in heat exposed humans and rats. To determine whether ATs anticholinergic potency is altered following PY administration, we examined ATs effects following acute or subchronic (2 weeks) PY administration in the sedentary heat-stressed rat. Four groups of rats were acutely (a, i.v.) treated with saline (SAL) or PY (100 ug/kg) followed by SAL or AT (200 ug/kg), and 4 groups were subchronically (c, osmotic pump) treated with SAL or PY (20 ug/hr) followed by SAL or AT (200 ug/kg). Fifteen minutes following the final injection, rats were subjected to an ambient temperature of 41.5 degrees C until a core temperature of 42.6 degrees C was attained. Heat tolerance times were significantly improved for cPY+SAL over aPY+SAL (241 +/- 9 vs 187 +/- 16 min, mean +/- SE) and for cPY+AT over aPY+AT (76 +/- 9 vs 57 +/- 2 min). The improvement in thermoregulation resulted from increased salivary water for evaporative cooling indicated by % weight loss (corrected for fecal loss) during heat stress: cPY+SAL over aPY+SAL (8.4 +/- 0.3 vs 6.6 +/- 0.5%). This increased heat tolerance resulting from subchronic anticholinesterase administration resembles changes seen with heat acclimation.