Mechanism of action of isoproterenol on venous return

The systemic vascular effects of isoproterenol infused in a dose of 1 mug-kg-1-min-1 was studied in 10 anesthetized dogs. A right heart bypass preparation allowed the separation of venous return into splanchnic and extrasplanchnic flows. Each channel was drained by gravity into an external reservoir. Venous return was then pumped into the pulmonary artery. During the infusion of isoproterenol, the pump was set at sufficient speed to maintain a constant level of blood in the external reservoir. Venous resistances and compliances of both channels were calculated from transient and steady-state volume shifts that occurred after rapid drops in splanchnic and then extrasplanchnic venous pressures. Isoproterenol affected both arterial and venous systems. Venous return increased from 1.62+/-0.11 to 2.40+/-0.19 liter/min (P less than 0.001) while arterial pressure fell from 97.5+/-3.8 to 70.2+/-5.9 mmHg (P less than 0.01). The compliances of the splanchnic and extrasplanchnic channels did not change significantly from their control values of 0.025+/-0.004 and 0.024+/-0.002 liter/mmHg. The venous resistance of the extrasplanchnic channel also did not change from its control value of 5.0 mmHg-liter-1-min-1; however, the splanchnic venous resistance decreased from 16.3+/-3.2 to 9.4+/-2.8 mmHg-liter-1-min-1 (P less than 0.001). The effective splanchnic back pressure, estimated by measuring the level to which hepatic venous pressure had to be raised to cause a change in portal pressure, decreased from 3.9 to 3.0 mmHg (P less than 0.01).