Dynamic and static conditioning pressures evoke equivalent rapid resetting in rat aortic baroreceptors

A recent study suggested that exposure of carotid sinus baroreceptors to pulsatile pressures for a period of minutes can decrease the static threshold pressure at which they begin to discharge, that is, the exposure can sensitize baroreceptors. Another study found that the rapid resetting of carotid sinus baroreceptors to elevated conditioning pressures was reduced or eliminated by pulsatile conditioning. In the present study, we tested the responses of aortic baroreceptors in an in vitro preparation to a range of static and dynamic conditioning pressures lasting 15 minutes. Slow test ramps of increasing pressure were used to assess static discharge properties (threshold and gain). To be accepted for analysis (n = 12), each baroreceptor had to successfully complete static and dynamic test sequences for at least three different conditioning mean arterial pressure levels. We measured aortic diameter simultaneously with pressure and baroreceptor discharge. Generally, we found no significant difference between the static pressure threshold measured before and after dynamic conditioning. The static pressure threshold was linearly related to the mean level of the conditioning pressure, and no differences in the slopes of these relations (a measure of the ability of a baroreceptor to rapidly reset) were found between static and dynamic conditioning. After dynamic conditioning, discharge rates returned to near control levels in all cases within a few seconds of the return to static conditioning. Two baroreceptors exposed to the vasodilator nitroprusside throughout the experiment showed similar results. Diameter measurements indicated no role of vessel diameter changes during dynamic or static conditioning. In conclusion, we found no evidence of a long-lasting sensitization of aortic baroreceptors by dynamic pressure inputs. The results of this study of aortic baroreceptors during static and dynamic conditions together with previous reports in conscious or intact animals suggest that the mean level of pressure is the primary determinant of the magnitude of the shift in the baroreceptor discharge curve during rapid resetting.