Acute adaptation and resetting of the baroreflex control of vascular resistance in the canine hindquarters and mesentery

Abstract

To determine whether acute adaptation and resetting occur in the baroreflex control of regional vascular resistance, experiments were conducted in anesthetized and vagotomized dogs. The carotid sinuses were vascularly isolated to regulate the carotid sinus pressure (CSP) in an open-loop fashion. The hindquarters (n= 12) and mesenteric (n=10) beds were perfused with constant flow and arterial perfusion pressures (HPP and MPP) were used to reflect changes in hindquarters and mesenteric resistance respectively. We first observed alterations in HPP and MPP during the course of CSP holding (conditioning pressure) at various levels for 15 min. Thereafter, the CSP was lowered to 50 mm Hg and increased stepwise to obtain the CSP-HPP and CSP-MPP baroreflex function curves. In experiments in the hindquarters bed, HPP stabilized at an average of 104.7 mm Hg during the initial conditioning pressure at 100 mm Hg. When conditioning pressure decreased to 50 mm Hg, the HPP increased to 125.5 mm Hg and then gradually declined to a steady level (115.6 mm Hg) in 5 min. An increase in conditioning pressure from 100 to 150 mm Hg caused HPP to decrease to 54.8 mm Hg followed by an upward adaptation to a steady level (80.2 mm Hg) in 5 min. The CSP/HPP curves constructed from the CSP step protocol were also affected by conditioning pressure. There were significant increases in the threshold and saturation pressures as conditioning pressure was elevated. However, the resetting was characterized by a parallel shift of the CSP/HPP curves without significant changes in baroreflex gain or sensitivity. Although the changes in mesenteric resistance in response to CSP changes were relatively weaker (lower gain), the phenomena of acute adaptation (MPP changes during 15-min conditioning pressure) and resetting (curve shift following different conditioning pressures) were still observed. In addition to the demonstration of adaptation and resetting of baroreflex control on the resistance in these two vascular beds, a graphical analysis is used to indicate that acute adaptation of the baroreflex responses is part of the resetting process. It is not necessarily associated with a decrease in sensitivity. Adaptation occurs as the baroreceptors “recognize” a new pressure in minutes and results from a shift of the HPP or MPP to a new level along the newly reset function curve.