The energy state of the rat liver has been studied at various times of
long-term cold acclimation (5 degrees C). On day 10 of cold exposure
the energy state of the liver is characterized by low levels of ATP an
d adenine nucleotide pool and by low phosphorylation potential and ''e
nergy charge'' (state 2). In mitochondria the respiratory control and
transmembrane potential decrease, and ion conduction of inner membrane
increases. On days 10-15 of cold exposure, when the body temperature
is lowered, the liver presumably becomes actively involved in thermal
stabilization. Cold exposure for 35-42 days increases the rate of elec
tron transport in metabolic state 3, respiratory control, and energiza
tion of liver mitochondria, thus providing ATP content and total adeni
ne nucleotide pool in the tissue at the control level. However, this d
oes not seem to compensate completely for the energy consumption since
phosphorylation potential and ''energy charge'' of the liver are decr
eased.