Influence of baroreceptor reflexes on the capacitance vessel's response to acetylstrophanthidin

The importance of the reflexes in the effects of acetylstrophanthidin on capacitance vessels was evaluated in the present study using chloralose-anesthetized dogs with intact and denervated baroreceptors. In each animal left ventricular bypass was instituted by draining all pulmonary venous blood into a reservoir from which it was pumped into a T-tube inserted into the descending thoracic aorta. Shifts of blood into or out of the reservoir reflected alterations in systematic vascular capacity. Femoral artery and central venous pressures as well as reservoir level, were continuously monitored. Acetylstrophanthidin (12.5 and 25.0 μg/kg) was administered by rapid i.v. injection. In 6 animals with intact baroreceptors, both doses of acetylstrophanthidin were followed by an increase in intravascular blood volume (average 86 and 228 ml, respectively). The small dose had no effect on arterial blood pressure while the large dose produced a significant rise in the pressure. Acetylstrophanthidin administration in 6 denervated dogs was not associated with a significant change in intravascular volume, but both doses produced significant increases in arterial pressure. These results indicate that the increase in systematic vascular capacity observed in animals with intact baroreceptors was due to venodilation resulting from a withdrawal of sympathetic activity to the systemic veins rather than from a direct action of acetylstrophanthidin on these vessels.