The purpose of this study was to examine the effect of acute (24 h) and chr
onic (5 wk) hypobaric hypoxic exposure equivalent to a simulated altitude o
f 4,300 m (446 mmHg) on the enzymes of fat metabolism. Heart, liver, and sk
eletal muscle were taken from 32 male Sprague-Dawley rats. Altitude exposur
e did not affect the activity of citrate synthase in any of the tissues, su
ggesting that mitochondrial content was unchanged. Carnitine palmitoyltrans
ferase-I (CPT-I) activity was significantly reduced in the heart by both ac
ute and chronic high altitude exposure compared with controls. A similar re
duction was found for CPT-I activity in extensor digitorum longus after acu
te and chronic exposure compared with control animals. CPT-I activity was n
ot affected by altitude exposure in the soleus muscle or the liver. 3-Hydro
xyacyl-CoA dehydrogenase (beta -HAD) activity was significantly depressed i
n the hearts of chronically exposed animals compared with controls. No diff
erence between acute and control animals was found in the heart for beta -H
AD activity. Liver beta -HAD activity was also significantly decreased in t
he acclimatized as well as in the acute animals compared with the control g
roup. Quadriceps beta -HAD activity was reduced for the chronic animals onl
y compared with controls. These data suggest that acclimatization to high a
ltitude selectively decreases key enzymes in fat utilization and oxidation
in the heart, liver, and select skeletal muscles.