In vivo regulation of rainbow trout lipolysis by catecholamines.

Lipolysis provides fatty acids that support key life processes by functioning as membrane components, oxidative fuels and metabolic signals. It is commonly measured as the rate of appearance of glycerol (Ra glycerol). Its in vivo regulation by catecholamines has been thoroughly investigated in mammals, but little information is available for ectotherms. Therefore, the goals of this study were, first, to characterize the effects of the catecholamines norepinephrine (NE) and epinephrine (Epi) on the lipolytic rate of intact rainbow trout (Oncorhynchus mykiss) and, second, to determine whether the plasma glycerol concentration is a reliable index of Ra glycerol. Our results show that baseline Ra glycerol (4.6+/-0.4 micromol kg(-1) min(-1)) is inhibited by NE (-56%), instead of being stimulated, as in mammals, whereas Epi has the same activating effect in both groups of vertebrates (+167%). NE-induced inhibition of fish lipolysis might play a particularly important role during aquatic hypoxia, when survival often depends on regulated metabolic depression. The plasma glycerol concentration is a poor predictor of Ra glycerol, and it should not be used as an index of lipolysis. Trout maintain a particularly high baseline lipolytic rate because only 13% of the fatty acids provided are sufficient to support total energy expenditure, whereas the remaining fatty acids must undergo reesterification (87%).