Hysteresis in the sympathetic baroreflex: role of baseline nerve activity

Non technical summary Sympathetic activity is altered by the baroreflex in response to changes in blood pressure. Thus, the sympathetic baroreflex is essential in preventing dangerous changes in blood pressure. The sympathetic baroreflex is more responsive to falling blood pressure than rising blood pressure in young men and women. This changes with age where older men and women become more sensitive to rising blood pressure. Our findings indicate that in young and older men and older women, the sensitivity of the sympathetic baroreflex depends on the level of sympathetic activity at rest. Therefore, individuals with high sympathetic activity respond better to rising than falling blood pressure and vice versa for people with low resting sympathetic activity. This might explain why older men and women cannot tolerate large decreases in blood pressure, as they have high sympathetic activity. In young women, factors other than sympathetic activity may be more important in determining baroreflex sensitivity. Sympathetic baroreflex sensitivity (BRS) is greater during decreasing compared to increasing diastolic blood pressure (DBP) in young men and women. In older men and women there is no difference in sympathetic BRS to increasing and decreasing DBP. We investigated whether the sensitivity of the central nervous system to increasing and decreasing DBP is dependent upon baseline muscle sympathetic nerve activity (MSNA). We hypothesised that the difference in sympathetic BRS between falling and rising segments of DBP would be positively related to baseline MSNA in 30 young men, 21 young women, 14 older men and 14 postmenopausal women. MSNA was measured using peroneal microneurography and BRS was measured using the spontaneous baroreflex threshold technique. On average, sympathetic BRS was greater during decreasing compared to increasing DBP in young men (P < 0.05) and women (P < 0.05). In older men and women, mean sympathetic BRS was similar in response to increasing and decreasing DBP. The difference (delta) between the falling and rising BRS correlated with baseline MSNA in young (r= 0.58, P < 0.05) and older men (r= 0.66, P < 0.05) and postmenopausal women (r= 0.74, P < 0.05). Thus, all men, and older women, with higher BRS to falling DBP had lower baseline MSNA. This relationship was not observed in young women (r= 0.14, P > 0.05). In summary, baseline MSNA plays a role in determining sympathetic BRS to falling and rising DBP in young and older men and po