Amino acid and glucose uptake by rat brown adipose tissue. Effect of cold-exposure and acclimation

Abstract

The net uptake/release of glucose, lactate and amino acids from the bloodstream by the interscapular brown adipose tissue of control, cold-exposed and cold-acclimated rats was estimated by measurement of arteriovenous differences in their concentrations. In the control animals amino acids contributed little to the overall energetic needs of the tissue; glucose uptake was more than compensated by lactate efflux. Cold-exposure resulted in an enhancement of amino acid utilization and of glucose uptake, with high lactate efflux. There was a net glycine-and proline efflux-that partly compensated the positive nitrogen balance of the tissue; amino acids accounted for about one-third of the energy supplied by glucose to the tissue. Cold-acclimation resulted in a very high increase in glucose uptake, with a parallel decrease in lactate efflux and amino acid consumption. Branched-chain amino acids, however, were more actively utilized. This was related with a much higher alanine efflux, in addition to that of glycine and proline. It is suggested that most of the glucose used during cold-exposure is returned to the bloodstream as lactate under conditions of active lipid utilization, amino acids contributing their skeletons largely in anaplerotic pathways. On the other hand, cold-acclimation resulted in an important enhancement of glucose utilization, with lowered amino acid oxidation. Amino acids are thus used as metabolic substrates by the brown adipose tissue of rats under conditions of relatively scarce substrate availability, but mainly as anaplerotic substrates, in parallel to glucose. Cold-acclimation results in a shift of the main substrates used in thermogenesis from lipid to glucose, with a much lower need for amino acids.

Comparison of Uptake of Multiple Clinical Radiotracers into Brown Adipose Tissue Under Cold-Stimulated and Nonstimulated Conditions

“…BAT has glycolytic capabilities, as well as enzymes involved in amino acid metabolism (25). Uptake and release of several amino acids in BAT was evaluated by Lopez-Soriano et al, who studied arteriovenous differences in vivo (13). Our results indicate that increased 3 H-L-methionine metabolism in BAT is also measurable with radiolabeled tracers and may potentially be visualized as high uptake in 3 H-L-methionine PET studies.…”

“…We chose this condition (4°C for 4 h) according to previous reports to ensure that the BAT of all animals was stimulated adequately and reproducibly (1,11,13). To avoid the effect of anesthesia on BAT activity, no anesthetic agent was used in any procedure throughout the study except for euthanasia.…”

Cold but not sympathomimetics activates human brown adipose tissue in vivo

“…The cold-associated increase in C-peptide without changes in glucose reflects a state of relative insulin resistance that may have been caused by elevated norepinephrine, as previously described (24). The elevated lactic acid levels were likely a result of glycolysis and amino acid metabolism in the BAT itself (18,25) and possibly also from subclinical, low-grade muscle contraction. Our data support a model in which mild cold exposure activates a specific, sympathetic response to only certain organs, principally BAT and the peripheral vasculature, but not other organs, such as the liver, heart, or adrenal glands.…”

Short‐term oscillations of aortic core temperature and thermogenic organ blood flow in the rat

“…The figures found in the literature for blood flows across portions of this tissue are very large only under intense adrenergic stimulation (Foster, Depocas & Frydman, 1980;Lopez-Soriano et al 1988).…”

“…3f64 J g-1 0C-1 or 3-43 J ml-'1C-1 (Medlowitz, 1948), since the density of whole blood is about 1 06 g ml-'. The blood flow across non-stimulated interscapular brown adipose tissue (about 0 21 g for a 200 g rat) is in the range of 0-16 ml min-m (Ma et al 1986;Lopez-Soriano et al 1988;Foster & Ma, 1989), i.e. 0 0026 ml s-.…”