On possible mechanisms in an increased haeat production in adipose tissues of the rat.

Abstract

The role of fat breakdown and resynthesis cycle in an augmented oxygen consumption was examined in epididymal white and interscapular brown adipose tissues of both non-adapted control and cold-adapted rats. In vitro addition of norepinephrine or theophylline separately or in combination was used as a stimulant.It was found that in epididymal white adipose tissue of both control and cold-adapted groups increments in oxygen consumption were well correlated with the amount of FFA reesterified. Slopes of both regression lines were essentially identical, while the ordinate intercept of the cold-adapted group was about twofold greater than that of the control. From the slope it was calculated that high-energy phosphate bonds required for each molecule of fatty acid esterified would be 2.6-P, a value comparable to theoretically obtained 2.3-P on the assumption of P: O ratio to be three. Since the amount of oxygen consumption in the basal state (without an added hormone) of the control group increased in parallel with that of FFA esterified after cold exposure, these increments would well be explained as due to relative increase in f atfree dry mass caused by a partial depletion of lipid content. It appears therefore to be reasonable to conclude that white adipose tissue depends basically for an increased heat production on an accelerated turnover of the lipolysis and reesterification cycle and that this basic property is not affected by cold adaptation.Similar analysis on the data obtained from brown adipose tissue of the control group showed again an excellent correlation between oxygen consumption and FFA reesterification. But the slope of the regression line yielded a P: O ratio of 0.234. This very low value, though presumably due partly to the underestimation of esterification because of the neglect of glycerolkinase activity, may still be suggestive of a possible involvement of uncoupling of oxidative phosphorylation in thermogenic systems in brown adipose tissue. Furthermore, when animals were chronically exposed to cold, this correlation was entirely lost. This appears mainly due to the apparently low rate of esterification calculated in spite of the high rate of oxygen consumption. Although the influence of glycerolkinase under these conditions remains to be clarified, these results may suggest that brown adipose tissue, in addition to its liability to uncouple phosphorylation from oxidation, may show an enhanced turnover of fat breakdown and resynthesis cycle under stimulation by the catecholamine in the cold-adapted state.