MECHANISMS RESPONSIBLE FOR CHANGES IN ABDOMINAL, VASCULAR VOLUME DURING SYMPATHETIC-NERVE STIMULATION IN ANESTHETIZED DOGS

This study was designed to determine the extent to which the decrease in volume of blood in the abdominal circulation in response to sympath etic stimulation was due to a passive effect of decreasing flow rather than an active constriction of the capacitance vessels. In dogs anaes thetized with alpha-chloralose (100 mg kg(-1) I.V.) the abdominal circ ulation was vascularly isolated and perfused either at constant flow o r at constant pressure, and drained at constant pressure from the infe rior vena cava. Changes in volume were determined by integration of th e differences between inflow and outflow. Supramaximal stimulation of both splanchnic (sympathetic) nerves at I Hz decreased abdominal volum e during constant pressure perfusion (active and passive components) b y 3.04 +/- 0.58 ml kg(-1) and at constant flow (active responses only) by 2.30 +/- 0.49 ml kg(-1) (means +/- S.E.M.). The responses at 8 Hz were respectively 9.52 +/- 0.91 and 5.09 +/- 0.49 ml kg(-1). The propo rtion of the responses calculated to be passive at 1 and 8 Hz was 23 /- 6.3 and 45 +/- 5.1%, respectively. These responses were almost iden tical to those induced by changing inflow by increasing the pump speed . Following ligation of the splenic pedicle, the responses during both constant pressure and constant flow were reduced by similar amounts, indicating that only the active response was affected. After ligation of the splenic pedicle, the proportion of the response calculated to b e passive at 1 and 8 Hz increased to 44 +/- 8.0 and 62 +/- 3.7%, respe ctively. These results indicate the importance of passive volume chang es in affecting abdominal volume, particularly following ligation of t he splenic circulation.