Effect of cold-acclimation on oxygen uptake and glucose production of perfused duckling liver

Thermogenic mechanisms and their hormonal regulation

Increased heat generation from biological processes is inherent to homeothermy. Homeothermic species produce more heat from sustaining a more active metabolism as well as from reducing fuel efficiency. This article reviews the mechanisms used by homeothermic species to generate more heat and their regulation largely by thyroid hormone (TH) and the sympathetic nervous system (SNS). Thermogenic mechanisms antecede homeothermy, but in homeothermic species they are activated and regulated. Some of these mechanisms increase ATP utilization (same amount of heat per ATP), whereas others increase the heat resulting from aerobic ATP synthesis (more heat per ATP). Among the former, ATP utilization in the maintenance of ionic gradient through membranes seems quantitatively more important, particularly in birds. Regulated reduction of the proton-motive force to produce heat, originally believed specific to brown adipose tissue, is indeed an ancient thermogenic mechanism. A regulated proton leak has been described in the mitochondria of several tissues, but its precise mechanism remains undefined. This leak is more active in homeothermic species and is regulated by TH, explaining a significant fraction of its thermogenic effect. Homeothermic species generate additional heat, in a facultative manner, when obligatory thermogenesis and heat-saving mechanisms become limiting. Facultative thermogenesis is activated by the SNS but is modulated by TH. The type II iodothyronine deiodinase plays a critical role in modulating the amount of the active TH, T(3), in BAT, thereby modulating the responses to SNS. Other hormones affect thermogenesis in an indirect or permissive manner, providing fuel and modulating thermogenesis depending on food availability, but they do not seem to have a primary role in temperature homeostasis. Thermogenesis has a very high energy cost. Cold adaptation and food availability may have been conflicting selection pressures accounting for the variability of thermogenesis in humans.

The role of the liver in lipid metabolism during cold acclimation in non-hibernator rodents

Cold exposure increases the demand for energy substrates. Cold acclimation of rats led to a 3-fold increase in fatty acid (FA) beta-oxidation (P<0.01) for ex vivo livers perfused at 37 degrees C. This increase was preserved following perfusion at 25 degrees C (P<0.001). In vitro measurement of absolute rates of hepatic beta-oxidation revealed no significant difference following cold acclimation, implying changes in fatty acid flux through beta-oxidation rather than increased oxidation capacity. Total FA uptake was increased one-third following perfusion at 25 degrees C (P<0.001) and cold acclimation (P<0.05) and cold acclimation led to diversion of tissue FA from storage to beta-oxidation (P<0.01). In separate experiments, in vivo hepatic lipogenesis rates for saponifiable lipids doubled (P<0.01) and cholesterol synthesis increased one-third (P<0.001). Taken together these data suggest the oxidation and synthesis of lipids occur simultaneously in hepatic tissue possibly to increase prevailing tissue FA concentrations and to generate heat through increased metabolic flux rates.

Increased lipogenesis in isolated hepatocytes from cold-acclimated ducklings

Thermogenic endurance and development of metabolic cold adaptation in birds may critically depend on their ability to synthesize and use fatty acids (FA) as fuel substrates. Hepatic lipogenesis and the capacity to oxidize FA in thermogenic tissues were measured in cold-acclimated (CA) ducklings (Cairina moschata) showing original mechanisms of metabolic cold adaptation in the absence of brown adipose tissue, the specialized thermogenic tissue of rodents. The rate of FA synthesis from [U-(14)C]glucose and from [1-(14)C]acetate, measured in incubated hepatocytes isolated from 5-wk-old thermoneutral (TN; 25 degrees C) or CA (4 degrees C) fed ducklings, was higher than in other species. Hepatic de novo lipogenesis was further increased by cold acclimation with both glucose (+194%) and acetate (+111%) as precursor. Insulin slightly increased (+11-14%) hepatic lipogenesis from both precursors in CA ducklings, whereas glucagon was clearly inhibitory (-29 to -51%). Enhanced de novo lipogenesis was associated with higher (+171%) hepatocyte activity of glucose oxidation and larger capacity (+50 to +100%) of key lipogenic enzymes. The potential for FA oxidation was higher in liver (+61%) and skeletal muscle (+29 to +81%) homogenates from CA than from TN ducklings, suggesting that the higher hepatic lipogenesis may fuel oxidation in thermogenic tissues. Present data underline the high capacity to synthesize lipids from glucose in species like muscovy ducks susceptible to hepatic steatosis. Lipogenic capacity can be further increased in the cold and may represent an important step in the metabolic adaptation to cold of growing ducklings.