Alterations of mitochondrial phospholipids in the rat brown adipose tissue after chronic treatment with cold or thyroxine.

The discovery of the hypertrophy of the rat brown adipose tissue which is induced by prolonged cold exposure [l] has been followed by many ultrastructural and biochemical studies upon this tissue which presents the peculiarity to contain a great number of mitochondrial [2]. Chronic cold exposure increases the mitochondrial diameter [3], the number of cristae [4] or the concentration of inner membrane [S] , correlatively to an increase of oxidative enzyme activities [6] . These modifications are associated with an increase of the phospholipid percentage [3,7,8] and important alterations of the phospholipid fatty acid pattern [7,8]. Chronic thyroxine treatment induces also a hypertrophy of the rat brown adipose tissue [9] but the phospholipid percentage is unchanged, while there is an increase of the total phospholipid content; some modifications of the phospholipid fatty acid pattern are also observed but they are less important than after cold treatment and in an opposite direction P',81. Since a great part of the phospholipids of brown fat belongs to the mitochondrial fraction we have studied the mitochondrial phospholipids of brown adipose tissue from rats treated chronically with cold or thyroxine. The most striking effect of cold exposure was a large increase of the mitochondrial phospholipid content of the tissue which was accompanied by a decrease of the phosphatidylcholine: phosphatidylethanolamine ratio (PC:PE), the cardiolipin percentage being unchanged; moreover the fatty acid composition of the different phospholipids was greatly modified