EFFECT OF CAROTID-SINUS BARORECEPTOR REFLEX ON HEPATIC AND SPLENIC VASCULAR CAPACITANCE IN VAGOTOMIZED DOGS

Mechanisms of how baroreflex activation changes splanchnic vascular vo lumes were studied in eight vagotomized dogs, anesthetized by chloralo se/urethan. Hepatic and splenic vascular volume changes were determine d from organ dimensions by sonomicrometry. Pulsatile carotid sinus pre ssure (CSP) in isolated and separately perfused carotid sinuses was ch anged among 200, 120, and 40 mmHg. Lowering CSP from 120 to 40 mmHg si gnificantly decreased both hepatic and splenic vascular volume (at sim ilar portal pressure) by 1.9 +/- 0.5 and 1.8 +/- 0.6 ml/kg body wt, re spectively. Increasing CSP from 120 to 200 mmHg tended to increase reg ional vascular volumes (P = NS). The combined volume change of liver a nd spleen between CSP 40 and 200 mmHg was 4.2 +/- 0.6 ml/kg body wt (P < 0.001). Pressure-volume (dimension) curves at high, low, and baseli ne CSP were determined to separate active and passive mechanisms of va scular volume changes. Changes in CSP did not change regional vascular compliance. Low CSP significantly decreased unstressed liver and unst ressed splenic volume by 3.3 +/- 0.9 and 1.9 +/- 0.5 ml/kg body wt, re spectively. These results indicate that liver and spleen both contribu te to blood volume mobilization by vasoconstriction during low CSP and that the carotid sinus baroreceptor reflex modulates hepatic and sple nic vascular capacitance by changing unstressed volume rather than by changing vascular compliance.