The effect of selenium-deficiency on rat fat-cell glucose oxidation.

When rats are fed a selenium-deficient diet, the glutathione peroxidase activity of epididymal fat-cells decreases to 5-9% of that of control rats fed the same diet supplemented with 0.5 p.p.m. of selenium as sodium selenite. [1-14C]Glucose oxidation in fat-cells from rats fed a selenium-deficient diet is unresponsive to the action of t-butyl hydroperoxide, which stimulates 14CO2 formation from [1-14C]glucose 4-fold in control rats. Insulin enhances [1-14C]glucose oxidation and incorporation into lipids in fat-cells from both groups of rats; however, the response elicited is reduced in fat-cells prepared from selenium-deficient animals. The 'C-1/C-6 ratio' (ratio of glucose C-1 to glucose C-6 oxidized) is enhanced by insulin to a similar degree in fat-cells from both groups of animals. The stimulatory action of Zn2+ and dithiothreitol on [1-14C]glucose oxidation observed in fat-cells from selenium-supplemented rats is greatly reduced in fat-cells from selenium-deficient rats. [1-14C]Glucose oxidation in fat-cells from both groups of animals is highly sensitive to the stimulatory action of adenosine. It is concluded that the enhanced formation and glutathione-linked destruction of H2O2 plays, at the most, only a minor role in the stimulation of the flux of glucose through the pentose phosphate pathway elicited by insulin, although elimination of glutathione peroxidase activity may influence the action of insulin on glucose oxidation. Production and subsequent destruction of H2O2 may play an important role in the stimulatory action of Zn2+ and dithiothreitol on fat-cell [1-14C]glucose oxidation.

The effect of N6-phenylisopropyladenosine on the regulation of fat cell hexose transport, glucose oxidation and fatty acid release by insulin and catecholamines.

N6-Phenylisopropyladenosine was employed in the absence of endogenous adenosine to explore the influence exerted by the R-site over the antagonistic interaction of insulin and catecholamines on several parameters of fat cell metabolism. When no hormones were present, N6-phenylisopropyladenosine had little or no effect; however, the nucleoside potentiated insulin inhibition of catecholamine-stimulated events, such as lipolysis, and, conversely, diminished or blocked catecholamine inhibition of insulin-stimulated processes, such as 2-deoxyglucose uptake, glucose oxidation and esterification, even under conditions where N6-phenylisopropyladenosine, alone, was ineffective in reversing catecholamine actions.

Effect of N6-methyladenosine on fat-cell glucose metabolism. Evidence for two modes of action.

N6-Methyladenosine strongly stimulates [1-14C]glucose oxidation in rat adipocytes [J.E. Souness and V. Chagoya de Sanchez, Fedn Eur. Biochem. Soc. Lett. 125, 249 (1981)]. The effect of the adenosine analogue is largely independent of its action as an R-site agonist. Removal of endogenous adenosine was a prerequisite for the manifestation of the effect of N6-methyladenosine. Nucleoside uptake inhibitors, dipyridamole and nitrobenzylthioinosine, did not block the action of N6-methyladenosine on [1-14C]glucose oxidation. The effect of the adenosine analogue was not greatly influenced by N6-phenylisopropyladenosine, nicotinic acid or theophylline. Although N6-methyladenosine stimulated 3-O-methylglucose uptake into fat cells, it is uncertain whether this was its only effect on glucose metabolism, in view of the comparable enhancement of hexose transport elicited by N6-phenylisopropyladenosine, a much weaker stimulator of glucose oxidation. That hexose transport is not the sole site of action of N6-methyladenosine was supported by the finding that, under conditions which have been proposed to make glucose transport rate limiting, the adenosine analogue only weakly enhanced [1-14C]glucose oxidation. The conversion of glucose carbon 1 to 14CO2 was enhanced by N6-methyladenosine to a greater degree than that of carbon 6, suggesting an increase in pentose phosphate shunt activity. Mechanisms by which this could arise are discussed. Although similarities exist between the effects of insulin and N6-methyladenosine on adipocyte glucose metabolism, the mechanisms by which they stimulate glucose oxidation appear to be distinct, in view of the additivity of their actions on [1-14C]glucose conversion to 14CO2. The results indicate that N6-methyladenosine affects fat-cell glucose metabolism via two different mechanisms: (1) a weak R-site-dependent mode of action related to stimulation of glucose transport and inhibition of lipolysis, and (2) a strong R-site-independent effect of unknown mechanism.

Regulation of phosphate uptake in rat adipocytes by adenosine, catecholamines and thyroid hormones.

Adrenaline and adenosine deaminase decreased the rate of phosphate uptake in isolated rat adipocytes. This effect was inhibited by N6(phenylisopropyl)adenosine. The rate of uptake was decreased by one third if the rats were given triiodothyronine prior to killing. In fat cells from hypothyroid rats, phosphate uptake was not sensitive to adrenaline. Sensitivity was restored by exogenous adenosine deaminase.

Adenosine, thyroid status and regulation of lipolysis.

Adipocytes from hypothyroid rats do not respond to adrenaline with increased glycerol release. Adenosine deaminase largely restores lipolytic sensitivity. This effect is reversed by 2-deoxycoformycin, an inhibitor of the enzyme, and by N6-(phenylisopropyl)adenosine, which is not deaminated. Lipolytic response of normal cells to adrenaline is only 50% inhibited by phenylisopropyladenosine, whereas in cells from hypothyroid rats blockage is total. Inhibition of 50% was seen at 100 and 1 nM concentrations respectively. Insensitivity to adrenaline of hypothyroid-rat adipocytes can, at least partly, be explained by increased sensitivity to adenosine.