Membrane composition, particularly of mitochondria, could be a critical factor 
by determining the propagation of reactions involved in mitochondrial function 
during periods of high oxidative stress such as rapid growth and aging. 
Considering that phospholipids not only contribute to the structural and 
physical properties of biological membranes, but also participate actively in 
cell signaling and apoptosis, changes affecting either class or fatty acid 
compositions could affect phospholipid properties and, thus, alter mitochondrial 
function and cell viability. In the present study, heart and brain mitochondrial 
membrane phospholipid compositions were analyzed in rainbow trout during the 
four first years of life, a period characterized by rapid growth and a sustained 
high metabolic rate. Specifically, farmed fish of three ages (1-, 2- and 
4-years) were studied, and phospholipid class compositions of heart and brain 
mitochondria, and fatty acid compositions of individual phospholipid classes 
were determined. Rainbow trout heart and brain mitochondria showed different 
phospholipid compositions (class and fatty acid), likely related to 
tissue-specific functions. Furthermore, changes in phospholipid class and fatty 
acid compositions with age were also tissue-dependent. Heart mitochondria had 
lower proportions of cardiolipin (CL), phosphatidylserine (PS) and 
phosphatidylinositol, and higher levels of phosphatidylcholine (PC) and 
phosphatidylethanolamine (PE) with age. Heart mitochondrial membranes became 
more unsaturated with age, with a significative increase of peroxidation index 
in CL, PS and sphingomyelin (SM). Therefore, heart mitochondria became more 
susceptible to oxidative damage with age. In contrast, brain mitochondrial PC 
and PS content decreased in 4-year-old animals while there was an increase in 
the proportion of SM. The three main phospholipid classes in brain (PC, PE and 
PS) showed decreased n-3 polyunsaturated fatty acids, docosahexaenoic acid and 
peroxidation index, which indicate a different response of brain mitochondrial 
lipids to rapid growth and maturation.