SYMPATHETIC DRIVE TO LIVER AND NONHEPATIC SPLANCHNIC TISSUE DURING HEAVY EXERCISE

The contribution of sympathetic drive and vascular catecholamine deliv ery to the splanchnic bed during heavy exercise was studied in dogs th at underwent a laparotomy during which flow probes were implanted onto the portal vein and hepatic artery and catheters were inserted into t he carotid artery, portal vein, and hepatic vein. At least 16 days aft er surgery, dogs completed a 20-min heavy exercise protocol (mean work rate of 5.7 +/- 1 miles/h, 20 +/- 2% grade). Arterial epinephrine (Ep i) and norepinephrine (NE) increased by similar to 500 and similar to 900 pg/ml, respectively, after 20 min of heavy exercise. Because Epi i s not released from the splanchnic bed and because Epi fractional extr action (FX) = NE FX, NE uptake by splanchnic tissue can be calculated despite simultaneous release of NE. Basal nonhepatic splanchnic (NHS) FX increased 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 from a 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 vein Epi levels by similar to 60% , the release of NE from NHS tissue maintained portal vein NE; at leve ls similar to those in arterial blood. NHS NE spillover increased from a basal rate of 5.7 +/- 1.4 to 11.7 +/- 2.8 ng kg(-1) min(-1) at 20 m in of exercise. Hepatic NE spillover increased from a basal rate of 5. 0 +/- 1.2 ng kg(-1) min(-1) to a peak of 14.2 +/- 2.8 ng kg(-1) min(-1 ) at 15 min of exercise. These results show that I) approximately two- and threefold increases in NHS and hepatic NE spillover occur during heavy exercise, demonstrating that sympathetic drive to these tissues contributes to the increase in circulating NE; 2) the high catecholami ne FX by the NHS tissues results in an Epi level at the liver that is considerably lower than that in the arterial blood; and 3) circulating NE delivery to the liver is sustained despite high catecholamine FX d ue to simultaneous NHS NE release.