ACUTE ADAPTATION AND RESETTING OF THE BAROREFLEX CONTROL OF VASCULAR-RESISTANCE IN THE CANINE HINDQUARTERS AND MESENTERY

To determine whether acute adaptation and resetting occur in the baror eflex control of regional vascular resistance, experiments were conduc ted in anesthetized and vagotomized dogs. The carotid sinuses were vas cularly isolated to regulate the carotid sinus pressure (CSP) in an op en-loop fashion. The hindquarters (n = 12) and mesenteric (n = 10) bed s were perfused with constant flow and arterial perfusion pressures (H PP and MPP) were used to reflect changes in hindquarters and mesenteri c resistance respectively. We first observed alterations in HPP and MP P during the course of CSP holding (conditioning pressure) at various levels for 15 min. Thereafter, the CSP was lowered to 50 mm Hg and inc reased stepwise to obtain the CSP-HPP and CSP-MPP baroreflex function curves. In experiments in the hindquarters bed, HPP stabilized at an a verage of 104.7 mm Hg during the initial conditioning pressure at 100 mm Hg. When conditioning pressure decreased to 50 mm Hg, the HPP incre ased 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 1 50 mm Hg caused HPP to decrease to 54.8 mm Hg followed by an upward ad aptation to a steady level (80.2 mm Hg) in 5 min. The CSP/HPP curves c onstructed from the CSP step protocol were also affected by conditioni ng pressure. There were significant increases in the threshold and sat uration 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. Althoug h the changes in mesenteric resistance in response to CSP changes were relatively weaker (lower gain), the phenomena of acute adaptation (MP P changes during 15-min conditioning pressure) and resetting (curve sh ift following different conditioning pressures) were still observed. I n addition to the demonstration of adaptation and resetting of baroref lex control on the resistance in these two vascular beds, a graphical analysis is used to indicate that acute adaptation of the baroreflex r esponses is part of the resetting process. It is not necessarily assoc iated with a decrease in sensitivity. Adaptation occurs as the barorec eptors ''recognize'' a new pressure in minutes and results from a shif t of the HPP or MPP to a new level along the newly reset function curv e.