Effects of Diet on Norepinephrine Turnover in Obese (ob/ob) Mice

Abstract

Effects of energy restriction and of supplemental sucrose or fat on norepinephrine (NE) turnover in brown adipose tissue (BAT) and heart of female obese (ob/ob) and lean mice were examined. Rates of NE turnover in BAT of control female obese mice (8 weeks old) were 39% lower than in control lean mice; no differences were evident in hearts of these mice. Energy intake of 6-week-old ob/ob mice was restricted to the intake of lean mice for 2 weeks. This restriction decreased intake by 41% and reduced rates of NE turnover in heart (-15%), but not in BAT, of obese mice. Next, 7½-week-old lean and obese mice were given access to stock diet and a 30% sucrose solution for 3–4 days. Intake of stock diet decreased by more than 50%. Total energy intake increased in lean mice (+18%), but decreased (-19%) in obese mice. When a 10% sucrose solution was offered intake of stock diet still decreased, but total energy intake was unchanged in lean and obese mice. Oxygen consumption increased in lean mice (+19 to +24%), but not in obese mice, when fed either 30 or 10% sucrose solutions, or a fat-supplemented diet where fat isocalorically replaced the sucrose consumed from 30% sucrose solution. Rates of NE turnover were not significantly increased in lean mice fed the 30% sucrose solution, but were increased in BAT (+34%) and heart (+25%) in lean mice fed the fat-supplemented diet. In obese mice rates of NE turnover were increased in BAT (+40%) and heart (+33 to +47%) when fed either the 30% sucrose solution or the fat-supplemented diet. Thus, obese mice failed to exhibit dietary-induced thermogenesis even though the dietary manipulation increased rates of NE turnover in their BAT. Possibly BAT of obese mice is refractory to NE in short-term feeding trials.