Hepatomesenteric release and removal of norepinephrine in swine.

Release and removal of norepinephrine (NE) by hepatomesenteric organs in anesthetized swine were examined using measurements of NE in arterial, portal, and hepatic venous plasma. NE spillover from the liver and mesenteric organs increased during splanchnic nerve stimulation, validating these measurements as indexes of sympathetic outflow. Administration of the neuronal uptake-blocking drug desipramine reduced mesenteric NE extraction more than hepatic extraction, suggesting that neuronal uptake was more important for NE removal in mesenteric organs than in the liver. Circulating NE was removed by the liver more efficiently than by mesenteric organs, whereas mesenteric NE spillover (2.46 nmol/min) exceeded liver NE spillover (0.74 nmol/min). Hepatomesenteric NE spillover represented 53% of total body spillover; NE clearance was 42% of total body clearance. Because of efficient hepatic extraction of NE released by mesenteric organs, the sum of mesenteric and hepatic NE spillovers (3.20 pmol/min) exceeded net hepatomesenteric spillover estimated using arterial and hepatic venous measurements alone (1.96 pmol/min). Thus valid assessment of the substantial amounts of NE released by hepatomesenteric organs requires separate examination of mesenteric and hepatic spillovers.