Baroreflex control of sympathetic nerve activity and blood pressure variability.

1. The simultaneous recording of blood pressure (BP) and renal sympathetic nerve activity (RSNA) in conscious sinoaortic baroreceptor denervated rats has revealed that the sympathetic component of the baroreceptor reflex both limits the amplitude of slow BP fluctuations and generates a faster BP oscillation (approximately 0.4 Hz in rats), the so-called Mayer wave. 2. Using BP and RSNA time series collected in conscious baroreceptor denervated rats and parameters of the transfer function relating RSNA to BP, it has been possible to predict BP and RSNA variabilities actually observed in baroreceptor-intact rats. The most accurate simulation was obtained when the baroreflex gain was set at 20-30% of a critical value leading to the production of self-sustained oscillations of BP and RSNA at Mayer wave's frequency. 3. Recent studies performed on conscious rats have indicated that the gain of the RSNA-BP baroreflex function curve is altered during sleep-wake cycle, grooming, exercise and exposure to environmental stress. These observations raise the possibility that the sympathetic baroreflex sensitivity might be continuously modulated as part of normal behavioural responses. 4. To examine this hypothesis, a method has been developed to obtain a continuous index of sympathetic baroreflex sensitivity. The method is based on the calculation of the gain of the transfer function relating RSNA oscillations to the BP pulse at heart rate frequency. This new spontaneous index correlates with the baroreflex gain measured by the vasoactive drug injection technique and is inversely related to overall indices of BP variability. In addition, it shows large, spontaneous variations over time.