Seven-day pyridostigmine administration and thermoregulation during rest and exercise in dry heat

Seven men participated in a double-blind study of effects of multiple-dose oral pyridostigmine bromide (PB) on physiological responses to 4-h heat stress tests (HST's) in a hot dry environment, 42 degrees C, 20% relative humidity. Subjects underwent 2 7-d series of tests, separated by 72 h, taking 30 mg PB every 8 h in one series, and placebo in the other. Each HST began right after the 0800 dose of PB or placebo. Subjects drank ad libitum during each HST, and performed two 55-min treadmill walks at about 40% VO2max during the last 2 h. Inhibition of red cell cholinesterase at the start of exercise averaged 30.0% in subjects taking PB, and did not differ significantly among HST's with PB. PB increased sweating and evaporative water loss by about 4%, and lowered chest skin temperature during exercise by 0.7 degrees C; but it had no significant effect on rectal temperature, other skin temperatures, O2 uptake, or fluid balance. PB alone had no significant effect on heart rate (HR), but had a significant interaction with day: although PB had essentially no effect on HR in the 1st HST, its effect increased progressively so that HR during exercise in the 4th HST was 8 beats.min-1 lower with PB. Multiple-dose PB had only slight effects on responses to moderate exercise-heat stress beyond those described after single-dose PB, and we found no adverse effects of multiple-dose PB administration.