1. IntroductionTo challenge the theory that the arterial baroreflex system minimizes the fluctuations of MAP but does not determine the levels of MAP [1,2], we compared histograms of MAP during movements between intact (n = 8) and sinoaortic-denervated (SAD, n = 6) rabbits. We hypothesized that if the baroreflex system only minimizes the variability of MAP, then physical movements should cause MAP to increase to a higher level in SAD rabbits than in intact rabbits.2. MethodsThe rabbits were chronically instrumented with arterial and venous catheters, a reference hydrostatic pressure (RP) catheter, and electrodes for electromyogram (EMG) recording of the nuchal muscles. MAP was measured in a rabbit cage for 24 h and collected at a speed of 10 samples/min by a computer. True mean arterial pressure was obtained from the calculation MAP minus RP. The 24-h data of MAP obtained were divided into three groups, based on EMG and RP: MAP during the animals' resting state (Rest), those during the moving state and the others (Others). The Rest phase had a minimum EMG level and lower RP, while the Move phase had an active EMG and higher RP. No significant difference was found in any of EMG level or RP of the Rest or Move phase between intact and SAD rabbits.3. ResultsIn intact rabbits, MAP remained low with a minimum EMG and lower RP during the Rest phase, while it apparently remained high with an active EMG and higher RP during the Move phase. On the other hand, in SAD rabbits, MAP was fluctuating and not synchronized with EMG activity nor RP level. The MAP data were converted into histograms. A typical histogram of a intact or SAD rabbit is shown in Fig. 1. The peak of the MAP histogram increased from 69 ± 2 to 85 ± 2 mmHg (n = 8) by moving in intact rabbits. In SAD rabbits, however, the peak did not change (from 67 ± 2 to 67 ± 3 mmHg, n = 6).The average of 24-h MAP increased from 70 ± 2 to 84 ± 2 mmHg by moving in intact rabbits. In SAD rabbits, the average of 24-h MAP did not increase (from 73 ± 2 to 74 ± 2 mmHG, n = 6). The correlation between MAP and muscular activity was examined using 24-h data (n = 14 400) of MAP and EMG. The average correlation coefficient was 0.596 in intact rabbits, which indicated a significant linear correlation. It was 0.058 in SAD rabbits, which was not significant.4. DiscussionThese results indicate that physical movements did not cause the MAP to increase to a higher level in SAD rabbits than in intact rabbits. Rather, SAD abolished the movement-induced resetting of the MAP level. Therefore, the baroreflex system not only minimizes the variability of MAP but also functions to increase MAP during movements. Its mechanism remains unknown. The baroreflex system is unlikely to be able to reset MAP directly, because the mode or average of 24-h MAP is not influenced by SAD. The baroreflex system, however, has an unique action in realizing other functions. For example, the afferent inputs from the baroreceptors realizes the vasopressin-induced neurohumoral interaction at the area postrema [3]. The baroreflex system may, therefore, be required to follow through the central commands and reset MAP levels higher during movements, via a hypothetical mechanism as shown in Fig. 2.