Sympathetic drive to liver and nonhepatic splanchnic tissue during heavy exercise

Coker, Robert H., Mahesh G. Krishna, D. Brooks Lacy, Eric J. Allen, and David H. Wasserman. Sympathetic drive to liver andnonhepatic splanchnic tissue during heavy exercise. J. Appl. Physiol. 82(4): 1244-1249, 1997.Thecontribution of sympathetic drive and vascular catecholamine deliveryto the splanchnic bed during heavy exercise was studied in dogs thatunderwent a laparotomy during which flow probes were implanted onto theportal vein and hepatic artery and catheters were inserted into thecarotid artery, portal vein, and hepatic vein. At least 16 days aftersurgery, dogs completed a 20-min heavy exercise protocol (mean workrate of 5.7 ± 1 miles/h, 20 ± 2% grade). Arterial epinephrine(Epi) and norepinephrine (NE) increased by ~500 and ~900 pg/ml,respectively, after 20 min of heavy exercise. Because Epi is notreleased from the splanchnic bed and because Epi fractional extraction(FX) = NE FX, NE uptake by splanchnic tissue can be calculated despite simultaneous release of NE. Basal nonhepatic splanchnic (NHS) FXincreased from a basal rate of 0.52 ± 0.09 to a peak of 0.64 ± 0.05 at 10 min of exercise. Hepatic Epi FX increased froma basal rate of 0.68 ± 0.10 to 0.81 ± 0.09 at 20 min of exercise. Even though NHS extraction of Epi reduced portal veinEpi levels by ~60%, the release of NE from NHS tissue maintainedportal vein NE at levels similar to those in arterial blood. NHS NEspillover increased from a basal rate of 5.7 ± 1.4 to 11.7 ± 2.8 ng · kg¹ · min¹at 20 min of exercise. Hepatic NE spillover increased from a basal rateof 5.0 ± 1.2 ng · kg¹ · min¹to a peak of 14.2 ± 2.8 ng · kg¹ · min¹at 15 min of exercise. These results show that1) approximately two- and threefoldincreases in NHS and hepatic NE spillover occur during heavy exercise,demonstrating that sympathetic drive to these tissues contributes tothe increase in circulating NE; 2) the high catecholamine FX by the NHS tissues results in an Epi level atthe liver that is considerably lower than that in the arterial blood;and 3) circulating NE delivery tothe liver is sustained despite high catecholamine FX due tosimultaneous NHS NE release.