Sympathetic nervous system responses to cold exposure and diet in rat skeletal muscle

Norepinephrine (NE) turnover measured in individual skeletal muscles of the rat assessed sympathetic responses to cold (4°C) and diet. Acute cold exposure increased NE turnover slightly (15-50%) in all muscles examined, including gastrocnemius, tibialis anterior, tensor fascia lata, and soleus; in contrast, cold-accelerated NE turnover in heart by one- to twofold and in interscapular brown adipose tissue (IBAT) by more than threefold. Similar results in skeletal muscle and heart were obtained after 1 wk of cold exposure. Examination of dietary effects on NE turnover, comparing rats fed a sucrose-supplemented diet for 3 days with those fasted for 2 days, indicated that sympathetic activity in skeletal muscle was unresponsive to dietary intake. In heart and IBAT, on the other hand, NE turnover in sucrose-fed rats was twice that obtained in fasted animals. Taken together, the lesser effect of cold and the undetectable response to diet indicate that sympathetic outflow to skeletal muscle is regulated differently than that to heart or IBAT and imply that skeletal muscle is not a major site for sympathetically mediated thermogenic responses to cold exposure or to dietary alteration such as fasting and sucrose feeding.