Baroreflex control of sympathetic nerve activity after elevations of pressure in conscious rabbits

K. P. Undesser, J. Y. Pan, M. P. Lynn and V. S. Bishop The purpose of this study was to assess the effect of rapid baroreceptor resetting on the baroreflex control of renal sympathetic nerve activity in conscious rabbits. Renal sympathetic nerve activity was recorded and used as an index of the efferent limb of the baroreflex. Heart rate and arterial pressure were also recorded. Arterial pressure was raised with either phenylephrine or angiotensin II to a level that eliminated renal sympathetic nerve activity and was maintained at this level for periods of time ranging from 1 to 60 min. On returning pressure to control levels, renal sympathetic nerve activity remained suppressed for up to 90 min, with the duration of the suppression dependent on the magnitude and duration of the pressure stimulus. During this period of suppressed nerve activity, baroreflex curves were generated. The curves produced at this time were also suppressed as compared with control baroreflex curves. With time, the suppressed baroreflex curves returned to control. Further studies were performed to show that the suppression of renal sympathetic nerve activity was mediated via the prolonged increase in baroreceptor afferent activity during the pressure stimulus and was not due to a central effect of phenylephrine. This study indicates that although baroreceptor afferent activity may reset rapidly, there does not appear to be an augmentation of renal sympathetic nerve activity as would be expected.