Venous pressures in the alert rat: effects of activity and single injections of catecholamines.

Short bursts of activity are accompanied by a sharp rise, then a sharp drop, followed facultatively by a tonic rise of pressure in the portal and mesenteric vein. Caval vein and abdominal pressure show only a monophasic rise during activity. The drop of portal pressure immediately after the activity peak can be shown to be passive, due to a fall in portal flow rate. The tonic rise after activity can be reproduced by single injections of catecholamines, noradrenalin being more effective than adrenalin. Systemic injections as against intramesenteric injections show, on the other hand, that adrenalin more effectively produces the postactivity pressure drop in the V. portae than noradrenalin. Raising venous pressures in the abdominal cavity by 15-40 mm H2O through acute introduction of air into the abdomen produces no change of venous hematocrit values. The results are discussed in connection with transvascular fluid shifts caused by activity and catecholamines. Portal vein pressure fluctuates with the same frequency, but with a phase displacement of 180 degrees, as Mayer waves.