Lability of arterial pressure after baroreceptor denervation is not pressure dependent

The mechanisms of increased arterial pressure lability after sinoaortic deafferentation remain unknown. We have shown previously in rats with chronic sinoaortic deafferentation (7-14 days after sinoaortic deafferentation) that ganglionic blockade significantly reduced mean arterial pressure and arterial pressure lability. The present study investigated the possibility that lability is related to the level of arterial pressure. Rats were instrumented chronically and heart rate and mean arterial pressure were sampled every 5 seconds in the conscious, freely moving state. Graded sustained increases in pressure (+10 to +82 mm Hg) produced by constant infusion of angiotensin II, phenylephrine, or vasopressin did not affect lability (standard deviation of 30-minute sampling period); whereas, graded hypotension (-10 to -70 mm Hg) produced by infusions of adenosine, nitroprusside, or nisoldipine appeared to reduce lability. Analysis of covariance and orthogonal polynomial curve fitting demonstrated a significant correlation between the decrease in mean arterial pressure and the decrease in lability produced by nisoldipine but not by adenosine or nitroprusside. Lability does not appear to be solely dependent on the level of arterial pressure because lability was reduced by adenosine when pressure was maintained at control levels by simultaneous infusion of phenylephrine. We conclude that 1) arterial pressure lability is not influenced by elevation of arterial pressure but can be reduced when pressure is lowered by certain vasodilators and 2) pressure alone does not appear to be the major determinant of lability because it can be attenuated by vascular smooth muscle relaxants even when pressure is maintained.