Hierarchical recruitment of the sympathetic and parasympathetic limbs of the baroreflex in normotensive and spontaneously hypertensive rats

The arterial baroreflex acts to buffer acute changes in blood pressure by reciprocal modulation of sympathetic and parasympathetic activity that controls the heart and vasculature. We have examined the baroreflex pressure-function curves for changes in heart rate and non-cardiac sympathetic nerve activity (SNA, thoracic chain T8-12) in artificially perfused in situ rat preparations. We found that the non-cardiac SNA baroreflex is active over a lower range of pressures than the cardiac baroreflex (threshold 66 +/- 1 mmHg versus 82 +/- 5 mmHg and mid-point 77 +/- 3 versus 87 +/- 4 mmHg, respectively, P < 0.05, n = 6). This can manifest as a complete dissociation of the baroreflex limbs at low pressures. This difference between the cardiac and non-cardiac SNA baroreflex is also seen in end-organ sympathetic outflows (adrenal and renal nerves). Recordings of the cardiac vagal (parasympathetic) and the inferior cardiac (sympathetic) nerves identify the cardiac parasympathetic baroreflex component as being active over a higher range of pressures. This difference in the operating range of the baroreflex-function curves is exaggerated in the spontaneously hypertensive rat where the cardiac component has selectively reset by 20-25 mmHg to a higher pressure range (threshold of 104 +/- 4 mmHg and mid-point 113 +/- 4, n = 6). The difference in the pressure-function curves for the cardiac versus the vascular baroreflex indicates that there is a hierarchical recruitment of the output limbs of the baroreflex with a sympathetic predominance at lower arterial pressures.