Effects of a cold environment on energy-related enzyme activities in the postnatal rat.

Liver glucose-6-phosphatase and lipase-esterase, liver and muscle glycogen phosphorylase, and brown fat lipase-esterase activity changes were studied during the postnatal development of rats born and growing up in temperatures of +5 and 20 degrees C. Liver glucose-6-phosphatase activity was highest at the age of 4 days in both environments. In the age groups 20-67 days glucose-6-phosphatase activity was higher in animals living in a cold environment than in those reared at room temperature. At birth, glycogen phosphorylase activity was high in the liver but very low in the muscle. No difference was found between the two temperatures. The lipase-esterase activity in the liver was very low at birth, rising to adult level by the age of 30 days, while in the brown fat the activity was already high at the time of birth and clearly higher in rats born in a cold environment than in those born at room temperature. At the time of birth the relative and absolute weight of brown fat were also clearly higher in rats born at +5 degrees C than in those born +20 degrees C.