SELECTIVE MOBILIZATION OF ADIPOSE-TISSUE FATTY-ACIDS DURING ENERGY DEPLETION IN THE RAT

This study extends our previous work (Raclot, T., and R. Groscolas. 19 93. J. Lipid Res. 34: 1515-1526) which demonstrated that in the fed st ate fatty acids are selectively released from white adipocytes in vitr o. It aims at determining whether such selectivity operates in vivo du ring energy depletion and has physiological relevance. This question w as examined in rats by simultaneously measuring, after 1, 7, or 10 day s of fasting, the fatty acid content of retroperitoneal adipose tissue (RP), and the composition of fatty acids released by isolated RP adip ocytes. A preliminary dietary manipulation (fish oil feeding) allowed us to study the mobilization of a wide spectrum of fatty acids. Fastin g resulted in a relative depletion of adipose tissue in fatty acids su ch as alpha-linolenic, arachidonic, and eicosapentaenoic, and in a rel ative enrichment in all very long chain saturated and monounsaturated fatty acids. After a 56% depletion of total fatty acids, 20% (22:1n-11 ) to 90% (20:5n-3) of the initial mass of individual fatty acids was l ost. The in vivo relative mobilization of fatty acids (% in lost fatty acids / % in RP triacylglycerols) ranged from 0.31 to 2.54. For a giv en chain length it increased with unsaturation whereas for a given deg ree of unsaturation it decreased with chain length. The in vitro relat ive mobilization of fatty acids (% in released fatty acids/% in rip tr iacylglycerols) was similarly dependent on their molecular structure a nd, to a significant extent, directly related to in vivo mobilization. It is concluded that during fasting-induced energy depletion, the net in vivo mobilization of fatty acids from adipose tissue is selective. The selectivity of mobilization i) is based on the molecular structur e of fatty acids, ii) is fully accounted for by their selective releas e from adipocytes, iii) leads to a profound remodelling of the composi tion of adipose tissue fatty acids, and iv) does not seem directed tow ards a preferential retention or sparing of particular fatty acids.