Hypoxic tachycardia in hypoxia-acclimated rats

Acute hypoxia (9.5% O2) increased the heart rate (HR) in hypoxia (12% O2 for 2 months)-acclimated (HX) rats, while it decreased HR in unacclimated (CT) rats. After treatment with propranolol, acute hypoxia decreased HR to the same extent in CT and HX rats. After treatment with atropine, acute hypoxia increased HR in HX rats but not in CT rats. The hypoxia-induced tachycardia in HX rats may be mainly due to an enhanced sympatho-adrenal activity.