Baroreflex adaptation and resetting on the control of arterial pressure

In anesthetized and vagotomized dogs, the carotid sinuses were vascularly isolated to control and adjust the carotid sinus pressure (CSP). In 10 animals, we first observed the changes in systemic arterial pressure (SAP) during CSP holding at different levels. Thereafter, the CSP was lowered to 40 mmHg and increased to 180 mmHg in a ramp fashion to obtain the CSP-SAP baroreflex function curves. The system was initially stabilized at a sinus holding pressure (HP) of 100 mmHg and an average SAP of 107.5 mmHg. During a 15 min HP at 50 mmHg, the SAP elevated to 119.4 mmHg at the beginning and then adapted downward to a stable level of 107.2 mmHg in 5 min. On the other hand, a change of HP to 150 mmHg caused a SAP decrease to 69.0 mmHg and then adapted upward to a quasi-steady level of 87.5 mmHg in 5 min. The CSP-SAP curves constructed by logistic function analysis from the ramp protocol were also significantly affected by the HPs. When the CSP-SAP curve following a HP of 100 mmHg was used as a control, the curve was shifted to the left and to the right following a HP of 50 and 150 mmHg, respectively. There were significant increases in the threshold, midpoint and saturation pressures as the HP was elevated. The degree of acute resetting (delta threshold pressure/delta HP) was calculated to be 33.0% (16.5/50) and 55.4% (27.7/50) for a change of HP from 100 mmHg to 50 mmHg and to 150 mmHg, respectively. However, the resetting was characterized by a parallel shift of the CSP-SAP curves without significant changes in the baroreflex sensitivity or gain. As the acute adaptation and resetting are correlated using a graphic analysis, we hypothesize that the baroreceptors can recognize a new pressure level within minutes. The inherent ability incurs downward and upward adaptation as well as resetting at lower and higher holding pressure, respectively.