The role of the splanchnic circulation in the regulation of total intravascular volume during alpha adrenergic receptor stimulation

Previous studies have not defined the contribution of the splanchnic circulation to the total intravascular volume change associated with selective alpha adrenergic receptor stimulation. Since the splanchnic circulation is responsible for the total volume changes associated with other types of selective autonomic receptor stimulation, the present study was undertaken to examine the influence of alpha adrenergic receptor stimulation on splanchnic intravascular volume, the hemodynamic mechanism responsible for the splanchnic volume change, and the contribution of the splanchnic volume change to the change in total volume. In 35 anesthetized dogs, blood from the vena cavae was drained into an extracorporeal reservoir and returned to the right atrium at a constant rate so that changes in total intravascular volume could be measured as reciprocal changes in reservoir volume. Phenylephrine infusion (100 μg/min) for 20 min in 28 dogs was associated with a decrease in total volume of 64±17 (SEM) ml (P<0.0001). The response was abolished by either alpha adrenergic blockade or evisceration but was not attenuated by beta adrenergic blockade, sinoaortic baroreceptor denervation, ganglionic blockade, or splenectomy. In 5 animals with separate splanchnic perfusion and drainage, total and splanchnic volumes decreased 59±8 ml (P<0.0001) and 317±20 ml (P<0.0001), respectively, while transhepatic vascular resistance increased 17±4 cm H₂O·min/l (P<0.0001). These responses were abolished after alpha adrenergic blockade. Thus, splanchnic volume decreases with alpha adrenergic receptor stimulation, despite an increase in hepatic resistance to splanchnic, venous outflow. The splanchnic volume decrement is entirely responsible for the total volume decrement.