David J. Serdar

Prolonged Activation of the Baroreflex Produces Sustained Hypotension

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.

Chronic Baroreceptor Activation Enhances Survival in Dogs With Pacing-Induced Heart Failure

Much of the current pharmacological therapy for chronic heart failure targets neurohormonal activation. In spite of recent advances in drug therapy, the mortality rate for chronic heart failure remains high. Activation of the carotid baroreceptor (BR) reduces sympathetic outflow and augments vagal tone. We investigated the effect of chronic activation of the carotid BR on hemodynamic and neurohormonal parameters and on mortality in dogs with chronic heart failure. Fifteen dogs were instrumented to record hemodynamics. Electrodes were applied around the carotid sinuses to allow for activation of the BR. After 2 weeks of pacing (250 bpm), electrical carotid BR activation was initiated in 7 dogs and continued for the remainder of the study. The start of BR activation was used as a time reference point for the remaining 8 control dogs that did not receive BR activation. Survival was significantly greater for dogs undergoing carotid BR activation compared with control dogs (68.1±7.4 versus 37.3±3.2 days, respectively; P<0.01), although arterial pressure, resting heart rate, and left ventricular pressure were not different over time in BR-activated versus control dogs. Plasma norepinephrine was lower in dogs receiving BR activation therapy 31 days after the start of BR activation (401.9±151.5 versus 1121.9±389.1 pg/mL in dogs not receiving activation therapy; P<0.05). Plasma angiotensin II increased less in dogs receiving activation therapy (plasma angiotensin II increased by 157.4±58.6 pg/mL in control dogs versus 10.1±14.0 pg/mL in dogs receiving activation therapy; P<0.02). We conclude that chronic activation of the carotid BR improves survival and suppresses neurohormonal activation in chronic heart failure.

Influence of Prolonged Baroreflex Activation on Arterial Pressure in Angiotensin Hypertension

Despite recent evidence indicating sustained activation of the baroreflex during chronic infusion of angiotensin II (Ang II), sinoaortic denervation does not exacerbate the severity of the hypertension. Therefore, to determine whether Ang II hypertension is relatively resistant to the blood pressure-lowering effects of the baroreflex, the carotid baroreflex was electrically activated bilaterally for 7 days in 5 dogs both in the presence and absence of a continuous infusion of Ang II (5 ng/kg per minute) producing high physiological plasma levels of the peptide. Under control conditions, basal values for mean arterial pressure (MAP) and plasma norepinephrine concentration (NE) were 93±1 mm Hg and 99±25 pg/mL, respectively. By day 7 of baroreflex activation, MAP and NE were reduced to 72±4 mm Hg (−21±3 mm Hg) and 56±15 pg/mL, respectively, but PRA was unchanged (control=0.41±0.06 ng ANG I/mL per hour). All values returned to basal levels by the end of a 7-day recovery period. After 7 days of Ang II infusion, MAP increased from 93±3 to 129±3 mm Hg, whereas NE fell from 117±15 to 86±23 pg/mL. During the next 7 days of baroreflex activation/Ang II infusion, further reductions in NE were not statistically significant, and on the final day of baroreflex activation, the reduction in MAP was only 5±1 mm Hg, compared with 21±3 mm Hg in the control normotensive state. These findings indicate that long-term baroreflex-mediated reductions in arterial pressure are markedly diminished, but not totally eliminated, in the presence of hypertension produced by chronic infusion of Ang II.