Reflex effects of stimulation of carotid sinus and aortic baroreceptors on hindlimb vascular resistance in dogs

Dampney, Roger AL, Michael G. Taylor, and Elspeth M. McLachian. "Reflex effects of stimulation of carotid sinus and aortic baroreceptors on hindlimb vascular resistance in dogs." Circulation research 29.2 (1971): 119-127. Before this study, published in 1971, the baroreflex had been established as a regulator of cardiovascular circulation via sensory receptors known as the carotid sinus and aortic baroreceptors. As their names describe, these two sets of sensory receptors are located on blood vessels in the carotid bodies on either side of the neck and at the level of the aortic arch. Each set of receptors are sensitive to the stretching of the vascular wall, and we now know that when stimulated they will relay information to the NTS which plays a role in modulating sympathetic outflow from the RVLM via GABAergic inhibition from the CVLM. At the time the baroreceptors located in the carotid bodies had been characterize much more extensively than the receptors located at the aortic arch. The goal then of this study was to define reflex changes following variations of aortic stimulation as well as compare the sensitivity of the aortic receptors to the already defined carotid receptors. To do this, two experimental setups were used. In both set ups canine hindlimbs were perfused at the level of the abdominal aorta directly below the bifurcation of the renal arteries. Blood was perfused into the hindlimbs at a constant rate using a pulsitile pump to mimic a heartbeat, and arterial pressure in the hindlimbs was measured from a branch of one of the femoral arteries. In the first set of experiments the carotid receptors were isolated and the vessels were perfused at a constant pressure to ensure no sensory information was being relayed to the brain. In this way, they were able to ensure that the reflex changes being observed were strictly from the aortic receptors. In the second experiment the opposite was done and the vagi were cut diminishing sensory information from the aortic receptors. After stimulation of the aortic receptors at different frequencies, what was found is that at a specific frequency of stimulation, compared to the carotid receptors, the aortic receptors did not produce as large of a reflex change seen at the level of the hindlimb vasculature. For example the maximal gain observed from stimulation of the aortic receptors was 1.1, where as the maximal gain observed from stimulation of the carotid receptors was 2.2. These observations allowed for the conclusion that aortic baroreceptor have lower sensitivity to changes in blood pressure than the carotid baroreceptors, in a way that greater changes in pressure must occur for the aortic receptors to induces changes in vascular resistance at the level of the hindlegs. ~JI