Prolonged Activation of the Baroreflex Produces Sustained Hypotension

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Abstract
The role of baroreflexes in long-term control of arterial pressure is unresolved. To determine whether chronic activation of the baroreflex produces sustained hypotension, we developed a method for prolonged activation of the carotid baroreflex in conscious dogs. This was achieved by chronically implanting electrodes around both carotid sinuses and using an externally adjustable pulse generator to electrically activate the carotid baroreflex. Control values for mean arterial pressure (MAP) and heart rate were 93±3 mm Hg and 64±4 bpm, respectively. After control measurements, the carotid baroreflex was activated bilaterally for 7 days at a level that produced a prompt and substantial reduction in MAP, and for day 1 MAP was reduced to 75±4 mm Hg. Moreover, this hypotensive response was sustained throughout the entire 7 days of baroreflex activation (day 7, MAP=72±5 mm Hg). During prolonged baroreflex activation, heart rate decreased in parallel with MAP, although the changes were not as pronounced (day 7, heart rate=51±3 bpm). Prolonged baroreflex activation was also associated with ≈35% reduction in plasma norepinephrine concentration (control=87±15 pg/mL). After baroreflex activation, hemodynamic measures and plasma levels of norepinephrine returned to control levels. Interestingly, despite the pronounced fall in MAP, plasma renin activity did not increase during prolonged baroreflex activation. These data indicate that prolonged baroreflex activation can lead to substantial reductions in MAP by suppressing the sympathetic nervous system. Furthermore, sustained sympathoinhibitory effects on renin secretion may play an important role in mediating the long-term hypotensive response.