Stimulation of metabolic activity of adipose tissue from fasted rats by prolonged incubation in vitro: I. requirement for glucose and insulin

Abstract

1. 1. Rat epididymal adipose tissue may be incubated in Eagle's tissue-culture medium for periods as long as 36 h without evidence of serious metabolic deterioration. Tissue transfer to new medium was carried out every 4.5 or 6 h. Sterile media were used, although without strictly sterile transfer technique; penicillin and streptomycin were present throughout. 2. 2. Continued viability of the tissue was evidenced by persistent glucose uptake, oxidation of glucose to CO 2, and fatty acid as well as protein synthesis from glucose, together with intact sensitivity to insulin throughout. However, from 24 h on there was a tendency to increased production of lactate by the tissue, together with a slight decrease in glucose oxidation, suggesting increasing occurrence within the tissue of anaerobic glycolysis. 3. 3. The grossly depressed metabolic activity of adipose tissue obtained from animals fasted 96 h was restored toward normal by preincubation of the tissue for periods ranging from 4.5 to 24 h, provided that the preincubation medium contained glucose and insulin or human serum. Increased lipogenesis, together with increased ability to oxidize glucose to CO 2 and to incorporate glucose carbon into protein, reached a plateau after approx. 9 h of preincubation. 4. 4. Although tissue-culture medium was used in most of these studies, the presence of essential amino acids and co-factors required for tissue cell growth in vitro was not required for the occurrence of the preincubation effect upon lipogenesis and glucose oxidation in adipose tissue. 5. 5. Lipogenesis from pyruvate, and pyruvate oxidation to CO 2 a were not affected by preincubation of epididymal adipose tissue from severely fasted rats. The presence of insulin was without effect upon pyruvate metabolism either before or after pre-incubation with glucose. However, preincubation without glucose in the presence of pyruvate with or without insulin, restored the lipogenetic activity of adipose tissue from glucose in a manner similar to that observed when preincuuation was carried out in the presence of glucose and insulin. 6. 6. Several enzymic activities concerned with glucose and fatty acid synthesis were measured in the homogenate of adipose tissue from severely fasted rats, both immediately after killing the animal and following preincubation with glucose and insulin for 9 h. In no instance was an increase in any of the enzymic activities measured noted; in most instances a very slight decrease, of the order of magnitude of 5%, was seen. 7. 7. The most likely but still hypothetical explanation for the increased lipo-genetic activity of adipose tissue of fasted rats after preincubation is the accumulation (or re-accumulation) within the tissue of metabolic products or metabolites which in themselves stimulate lipogenesis. These metabolites would require the presence of insulin to arise in sufficient quantity from the metabolism of glucose, but would arise from the metabolism of pyruvate in adequate amounts even in the absence of insulin.