The effect of triiodothyronine treatment on fatty acid synthesis was investigated in liver and adipose tissue of fed and 48‐h fasted rats. In the liver, the thyrotoxic state was accompanied by an increased incorporation of [1‐14C]acetyl‐CoA to fatty acids and a rise in the activity of acetyl‐CoA carboxylase and fatty acid synthetase, both in the fed and fasted rats. The activity of glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase, NADP‐malate dehydro‐genase and ATP citrate lyase was also enhanced by triiodothyronine treatment. Liver malate and citrate content was not changed in fed thyrotoxic rats, but was increased in fasted triiodo‐thyronine‐treated animals. Palmitate oxidation was increased in the liver of the same animals, either fed or fasted. The rise in acetyl‐CoA carboxylase activity due to triiodothyronine administration was apparent when assayed both before and after citrate activation in vitro. Maximal activity of this enzyme in the presence of citrate was reached earlier in the liver of triiodothyronine‐treated rats. In adipose tissue, triiodothyronine treatment induced an increase in the activity of acetyl‐CoA carboxylase, fatty acid synthetase and in glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase. The activity of NADP‐malate dehydrogenase and ATP citrate lyase was slightly but insignificantly increased. Malate and citrate levels in adipose tissue were unchanged by triiodothyronine treatment. It is suggested that the triiodothyronine‐induced simultaneous stimulation of opposing pathways (the synthesis of fatty acids and their oxidation) creates a futile metabolic cycle which contributes to the general energy waste and thermogenesis characteristic of thyrotoxicosis.

Incorporation of [2‐14C]pyruvate into adipose tissue triglycerides was measured in fasted, diabetic and triamcinolone treated rats and related to the activity of enzymes active in the elaboration of glyceride glycerol. Incorporation of pyruvate into glyceride glycerol was increased in fasting and diabetes. The activity of phosphoenolpyruvate carboxykinase increased in parallel with the enhanced glyceroneogenesis, whereas the activity of pyruvate carboxylase changed in the opposite direction. After triamcinolone treatment, the activity of both enzymes was decreased as was the incorporation of [2‐14C]pyruvate into glyceride glycerol. In all experimental animals glutamate‐pyruvate transaminase activity in adipose tissue decreased, whereas the activity of glutamate‐oxaloacetate transaminase increased slightly. These results were interpreted as implying that the enhanced glyceroneogenesis in diabetes and fasting is related to the increase in phosphoenolpyruvate carboxykinase activity, whereas pyruvate carboxylase activity seems associated with changes in fatty acid synthesis. In contrast to adipose tissue, in the liver the activity of both enzymes of the dicarboxylic acid shuttle and of transaminases increased in the situations associated with enhanced gluconeogenesis. This finding is discussed in the light of possible differences in the availability of precursors for gluco‐ and glyceroneogenesis in the respective tissues. The role of glyceroneogenesis is pointed out, as one of the factors maintaining the balance between synthesis and breakdown of triglycerides in adipose tissue, particularly in conditions associated with rapid lipolysis.

Adipocytes isolated from the epididymal fat pads of normal rats specifically bound [125I]human GH [( 125I]hGH). Preincubation of cells with 20 micrograms/ml cycloheximide, an inhibitor of protein synthesis, produced a progressive loss of ability to bind [125I]hGH specifically. Loss of binding sites with time followed first order kinetics and had a half-time of about 45 min regardless of whether GH was present or absent during treatment with cycloheximide. Nonspecific binding of labeled hormone was unchanged by cycloheximide. Similar results were obtained when adipocytes were incubated with 200 micrograms/ml puromycin, another inhibitor of translation, but incubation with 5 micrograms/ml actinomycin D, an inhibitor of transcription, for 2.5 h had no effect on the binding of [125I]hGH by adipocytes. The findings are not attributable to cell death, since oxidation of [U-14C] glucose to 14CO2 and binding of [125I]insulin were unaffected in replicate cell populations exposed to the same treatments. Diminished binding could not be attributed to an effect of cycloheximide to hasten the degradation of receptor-bound hGH. Treatment of adipocytes with 0.1 mg/ml trypsin for 10 min virtually abolished their ability to bind [125I]hGH specifically, but binding capability gradually returned after removal of trypsin and was nearly restored to pretrypsin levels by 2 h. Addition of cycloheximide to the incubation medium after removal of trypsin completely prevented recovery of binding capability. Covalent binding of [125I]hGH to its receptors with disuccinimidyl suberate followed by sodium dodecyl sulfate-gel electrophoresis and autoradiography of proteins isolated from adipocyte membranes revealed three specifically labeled bands corresponding to mol wt of 250-300, 130, and 56 kilodaltons. Treatment of adipocytes with cycloheximide before cross-linking resulted in a proportional reduction in all three labeled bands, suggesting a similar half-life for all three entities. Similarly, all three labeled entities reappeared in parallel as adipocytes recovered from treatment with trypsin. The data strongly suggest that receptors for GH turn over rapidly on the surface of adipocytes and that ongoing protein synthesis is required to maintain binding capacity. The data do not permit distinction between rapid turnover of the receptor proteins themselves and a short-lived protein(s) which might be required to insert the receptors into the membrane.

No abstract

Adipose tissue has long been recognized as a target for the action of growth hormone (GH). Clinical and experimental data provide compelling evidence that GH plays an important, but imprecisely defined, role in the overall regulation of fat metabolism; it influences both the formation and function of fat cells. Over 50 years ago, Lee and Shaffer reported that the treatment of rats with pituitary extracts rich in growth-promoting activity resulted in a relative decrease in carcass fat (1). Similar findings have been obtained repeatedly using highly purified preparations of GH (2).Although the total amount of body fat is determined by many factors, GH deficiency (GHD) in reasonably well-nourished children results in a characteristic mildly obese state (3-5) and chronic GH replacement therapy leads to a decrease in body fat, just as is seen in experimental animals. Prior to replacement therapy, the average size and lipid content of subcutaneous fat cells in GH-deficient children is greater than normal, but the total number of fat cells is lower than normal (6, 7). Although adipose cell number is mainly determined between 30 weeks' gestation and 1 year of age (8), some increase in fat cell number also occurs during later growth in children. Chronic treatment of GH-deficient children for periods of 0.5-2 years decreased the average size and lipid content of adipocytes in subcutaneous fat depots and, paradoxically, accelerated the rate of increase in the total number of fat cells, presumably by hastening the conversion of pre-adipocytes to adipocytes. Green et al. established the importance of GH in the differentiation of mouse fibroblasts to adipocytes and provided a basis for the understanding of the mutually dependent actions of GH and insulin-like growth factor I (IGF-I) on this process (9, 10).The role of GH in the regulation of metabolism in mature fat cells has been established by studies of rat adipose tissue in vitro (1 1 , 12). When GH is first added to adipose tissue or adipocytes that have been deprived of GH for at least 3 hours, it stimulates the inward transport of glucose (13, 14) and also accelerates glucose metabolism by stimulating other reactions in the metabolic pathway (15, 16). At the same time, GH is anti-lipolytic and opposes the lipolytic actions of adrenalin (17). These early, insulin-like effects appear within a few minutes of GH administration, become maximal at about 1 hour and gradually disappear over the next 1-2 hours. At this stage, accelerated lipolysis is readily demonstrable. Once the initial insulin-like effect has dissipated and the basal rate of glucose metabolism has been restored, a second insulin-like response cannot be initiated for many hours, even with extremely high concentrations of GH. During this period of refractoriness to insulin-like stimulation of GH, the tissues remain responsive to insulin and other insulin-like agonists, such as IGF-I (18, 19) and oxytocin (18), which suggests that a specific change has taken place in the GH effector pathway.Locally produced auto...