Effect of propionate on lipogenesis in adipose tissue

The metabolism of propionate in adipose tissue and its effect on lipogenesis was investigated. Fasting induced changes in propionate metabolism of adipose tissue, drastically reducing higher fatty acid synthesis and increasing glyceride-glyerol formation from low concentrations of propionate (0.25 mM). Propionate also promoted lipogenesis from acetate-1-(14)C in tissues of fasted rats, while it inhibited lipogenesis and CO(2) formation from acetate in the fed animal. Treatment with actinomycin D or ethionine abolished both the increased glyceride-glycerol formation from propionate and the promoting effect on lipogenesis from acetate. Synthesis of long-chain fatty acids from propionate-1-(14)C was increased by actinomycin treatment. The change in propionate metabolism induced by fasting is, however, not entirely due to its conversion to glyceride-glycerol, since the latter was almost completely blocked by malonate while part of the promoting effect on fatty acid synthesis persisted.     

Effect of insulin on pyruvate metabolism in epididymal adipose tissue of the rat. Correlation of intracellular pyruvate contents and pyruvate dehydrogenase activity

A method is described to measure the intracellular content of pyruvate and lactate in epididymal adipose tissue of the rat. The intracellular pyruvate concentration was approx. 330mum. Intracellular pyruvate contents and the rates of pyruvate output were increased when NNN'N'-tetramethyl-p-phenylenediamine was added, and decreased in the presence of alanine. Insulin addition caused an increase in intracellular pyruvate contents only at the earlier time-period studied (1.5min as against 20min). Pyruvate dehydrogenase activity was increased in adipose tissue incubated in vitro with insulin. This increase occurred subsequent to the rise in the intracellular pyruvate content induced by insulin addition. The possible physiological implications are discussed.     

Mechanisms regulating adipose tissue pyruvate dehydrogenase

1. Isolated rat epididymal fat-cell mitochondria showed an inverse relationship between ATP content and pyruvate dehydrogenase activity consistent with competitive inhibition of pyruvate dehydrogenase kinase by ADP. At constant ATP concentration pyruvate rapidly activated pyruvate dehydrogenase in fat-cell mitochondria, an observation consistent with inhibition of fat-cell pyruvate dehydrogenase kinase by pyruvate. Pyruvate dehydrogenase in fat-cell mitochondria was also activated by nicotinate (100mum) and by extramitochondrial Na(+) (replacing K(+)) but not by ouabain or insulin. 2. In rat epididymal fat-pads incubated in vitro pyruvate dehydrogenase was activated by addition of insulin in the absence of substrate or in the presence of glucose (10mm) or fructose (10mm). Glucose and fructose activated the dehydrogenase in the absence or in the presence of insulin, and pyruvate also activated in the absence of insulin. It is concluded that extracellular glucose, fructose and pyruvate may activate the dehydrogenase by raising intracellular pyruvate and that insulin may activate the dehydrogenase by some other mechanism. 3. Ouabain (300mum) and medium in which K(+) was replaced by Na(+), activated pyruvate dehydrogenase in epididymal fat-pads. Prostaglandin E(1) (1mug/ml), 5-methylpyrazole-3-carboxylate (10mum) and nicotinate (10mum), which are as effective as insulin as inhibitors of lipolysis and which like insulin lower tissue concentration of cyclic AMP (adenosine 3':5'-cyclic monophosphate), did not activate pyruvate dehydrogenase. Higher concentrations of prostaglandin E(1) (10mug/ml) and nicotinate (100mum) produced some activation of the dehydrogenase. 4. It is concluded that the activation of pyruvate dehydrogenase by insulin is not due to the antilipolytic effect of the hormone and that the action of insulin in lowering adipose-cell concentrations of cyclic AMP does not afford an obvious explanation for the effect of the hormone on pyruvate dehydrogenase. The possibility that the effects of insulin, ouabain and K(+)-free medium may be mediated by Ca(2+) is discussed.     

Effect of rat serum on glycerol metabolism in adipose tissue in vitro

It has been proposed that the lipolytic effect of serum is based on the presence of either lipoproteins or catecholamines. To test these hypotheses, pieces of epididymal fat pads from fed rats were incubated in the presence of albumin and glucose for 120 min. The addition of rat serum (5 mul/vial) enhanced the rates of both glycerol release to the media and [U-14C] glycerol utilization by the tissue. Heparin did not alter these parameters or the response produced by serum. VLDL from rat plasma also enhanced glycerol release and utilization for the formation of CO2 and lipids, and heparin significantly augmented these effects. Neither of the conditions studied affected the percentual distribution of 14C-lipid fractions in the tissues. It is known that in similar conditions to those used in the present study, adrenaline produces a decrease in the utilization of glycerol. Thus our findings do not support the proposed hypothesis explaining the fat-mobilizing action of serum, the mechanism of which remains to be determined.