AUTONOMIC REGULATION OF HEART-RATE RESPONSE TO EXERCISE IN TIBETAN AND HAN RESIDENTS OF LHASA (3,658-M)

To test the hypothesis that native high-altitude residents have less b eta-sympathetic and more parasympathetic tone than newcomers, we compa red the effects of beta-sympathetic and parasympathetic blockade in 10 Tibetan and 9 Han acclimatized male residents of Lhasa, Tibet Autonom ous Region, China (elevation 3,658 m). Each subject was studied during cycle ergometer exercise at 70, 132, and 191 W after placebo (normal saline), beta-sympathetic (propranolol, 0.2 mg/kg iv), or parasympathe tic (atropine, 0.04 mg/kg iv) blockade in random order on different da ys. At rest, the fall in resting heart rate with propranolol and the r ise with atropine were equal in Tibetan and Han subjects. During exerc ise, the fall in heart rate with propranolol relative to placebo value s was greater in the Han than in the Tibetan group, whereas the rise i n heart rate with atropine was greater in the Tibetans. Propranolol or atropine administration did not change minute ventilation per unit O2 consumption in either group. At the highest level of exercise on the placebo day, the Tibetans achieved a higher work load and level of O2 consumption than the Han subjects. Propranolol or atropine reduced O2 consumption and work load similarly in the two groups at the highest e xercise level. The results supported our hypothesis that native Tibeta n residents of high altitude exhibit more parasympathetic and less bet a-sympathetic tone during exercise. Neither relatively greater parasym pathetic nor less sympathetic activation appeared implicated in the gr eater exercise capacity of Tibetans compared with that of acclimatized newcomer residents of high altitude.