Decrease in liver norepinephrine in response to exercise and hypoglycemia.

The existence of the sympathetic innervation of the liver has been known for many years, but the role of this system in regulation of liver metabolism is unclear. The purpose of these experiments was to identify physiological conditions for activation of liver sympathetics. Liver norepinephrine (NE) was measured in normal resting rats and in rats exposed to swimming, treadmill running, fasting, and insulin-induced hypoglycemia. Liver NE decreased significantly in response to swimming (-71% of control), treadmill running (-53% of control), and hypoglycemia (-24% of control). Rats that are endurance trained by daily bouts of treadmill running for 3 mo show no decrease in liver NE in response to a 60-min run on the treadmill, whereas nontrained rats show a 50% decrease in liver NE with the same amount of exercise. We conclude that the liver sympathetics are activated in response to swimming, treadmill exercise and hypoglycemia, and that endurance training causes a reduction in the degree of exercise-induced activation of these neurons.