Increased sympathetic activity in rat white adipose tissue during prolonged fasting

The effect of prolonged fasting on sympathetic activity was examined in rat white adipose tissue (WAT) and, for comparison purposes, in interscapular brown adipose tissue (IBAT). Preliminary experiments showed that 6-hydroxydopamine or tyramine administration to fed animals produced similar reductions in norepinephrine (NE) content of WAT and IBAT. Fasting for 48 h did not affect tissue NE content significantly, but induced a threefold increase in [3H]NE uptake by retroperitoneal and epididymal adipose tissue, contrasting with a 50% reduction in IBAT. Measured with DL-alpha-methyl-p-tyrosine, NE fractional rates of turnover were faster and calculated turnover rates were three times higher in retroperitoneal and epididymal tissue from fasted rats than in tissues from fed controls. In experiments with [3H]NE, although fractional rates did not change significantly, calculated NE turnover also increased in retroperitoneal and epididymal tissue after food deprivation. In contrast, in IBAT, NE turnover either did not change (measured with DL-alpha-methyl-p-tyrosine) or, in the experiments with [3H]NE, decreased significantly after fasting. These and other data suggest that a centrally controlled selective activation of WAT sympathetic fibers contributes to fasting lipolysis.