Effect of concomitant fasting and cold exposure on sympathoadrenal activity in rats

Fasting decreases and cold exposure increases sympathetic nervous system activity. The present studies examine the effect of fasting and cold together on sympathoadrenal function. At 4°C animals fasted for 2 days excreted 29% less norepinephrine (NE) than fed animals, averaging 458 ± 32 ng NE/mg creatinine and 646 ± 68, respectively (P <0.005), but 122% more epinephrine (E) 77.9 ± 11.7 ng E/mg creatinine and 35.1 ± 6.7, respectively (P <.01). Fasting for 2 days reduced cardiac NE turnover, a direct measure of sympathetic neuronal activity, 33% in animals at 22°C from 28.2 ± 3.6 ng Ne.heart ^-1. h^-1 to 18.9 ± 4.8 (95% confidence intervals) and 25% in animals acutely exposed to 4°C from 60.7 ± 8.0 to 45.6 ± 9.5. Similar reductions in urinary NE excretion and cardiac NE turnover were observed in adrenal-demedullated rats. Thus fasting at 4°C lowers sympathetic activity and enhances adrenal medullary secretion. This pattern of decreased sympathetic and increased adrenal medullary activity, previously seen with fasting hypoglycemia, suggests an important role for the adrenal medulla in internal homeostasis at times when the sympathetic nervous system is suppressed.