Studies of some effects of cold exposure on the function and structure of mitochondria of rats.

Abstract

The structure and function of mitochondria of cold-acclimated rats (+2°C) in comparison with those of control rats (+23°C) have been studied in relation to (A) uncoupling of oxidative phosphorylation, (B) protein synthesis, and (C) mitochondrial ultra structure, in an attempt to find the cause of extra heat production (non-shivering thermogenesis) which occurs during cold exposure. The dry mass, number and total protein content of mitochondria per gram of original fresh liver tissue was found to be reduced in cold-acclimated rats. Cold exposure also resulted in a decrease of the protein contents of the soluble and insoluble fractions of mitochondria. The reduction of dry mass per gram of original fresh liver tissue and the reduction of soluble and insoluble protein contents per gram of original fresh liver tissue seems to be due to the reduction of number of mitochondria since the dry mass per unit number of mitochondria was found to be more or less the same. Cold treatment also resulted in a decrease of the P:O ratio, owing to a higher uptake of oxygen and lower uptake of inorganic phosphate. Study on incorporation of uniformly labeled C14-amino acids into mitochondria) protein indicated that cold exposure induced a reduced accumulation of radioactivity in mitochondria) protein both in vitro and in vivo, this effect being more pronounced in the soluble protein fraction. Studies with the electron microscope revealed that a great number of mitochondria of cold-acclimated rats were larger than those of control rats, with more ruffled outer membrane and pale matrix. This enlargement of mitochondria of cold-acclimated rats could be due to swelling in vivo. The glycogen content of the liver cells was found to be higher in cold-acclimated rats than in control rats. It is concluded that cold exposure exerts its effect on mitochondrial protein synthesis, particularly the soluble fraction. This impaired protein synthesis with enlargement of mitochondria of cold-acclimated rats could be the reason for the swelling and the changes in the properties of the membrane, leading to the uncoupling of oxidative phosphorylation.