Metabolic response to a specific lipid-depleting factor in parabiotic rats.

Parabiosis has been used as a technique for demonstrating the existence of a humoral factor in the control of body fat. The timing and metabolic basis for specific loss of fat from parabiotic partners of obese rats were examined. One member of a pair received 200% control intake, by stomach tube, for 8, 23, 39, or 57 days. Their partners ate 9.8 +/- 0.1 g/day. Members of ad libitum-fed pairs ate 9.6 +/- 0.1 g/day. All rats received the same diet. After 39 days, body fat in partners of obese rats was 6 +/- 1 g/rat compared with 17 +/- 1 g/rat in members of ad libitum pairs. Body protein was not different. In vitro hepatic fatty acid synthesis (FAS) and esterification (FAE) and inguinal FAS, FAE, and glycerol release suggested that fat loss was due to inhibition of adipose FAE. Partners of overfed rats and members of ad libitum pairs were then compared after 27 days of tube feeding when loss of fat was expected to be most rapid. Hepatic FAS, FAE, and fatty acid oxidation were the same for both groups. Inguinal FAS and FAE were decreased in partners of obese rats. An unidentified "lipid-depleting" agent, originating in obese rats, appears to inhibit adipose FAS and FAE in their partners independently of changes in feeding.