Baroreflex Control of Heart Rate and Cardiac Hypertrophy in Angiotensin II–Induced Hypertension in Rabbits

Abstract

Abstract The cardiac hypertrophy observed in hypertension is thought to be responsible for the accompanying deficiency in the baroreflex control of heart rate. In this study, we assessed the baroreflex relationship between heart rate and arterial pressure in a group of seven rabbits during a normotensive period, during the early phase of angiotensin II (Ang II)–induced hypertension (1 week) (50 ng/kg per minute IV via osmotic minipumps), after 7 weeks of continuous hypertension, then 2 days after Ang II was stopped, and finally 7 days after Ang II. Left ventricles were weighed for measurement of left ventricular weight–body weight ratio. One week of intravenous Ang II infusion produced hypertension (mean arterial pressure from 80±2 up to 115±8 mm Hg), with significantly increased heart rate and hematocrit. The heart rate–arterial pressure baroreflex curve was shifted to the right, with a significant 45% reduction in the gain of the reflex (−6.4±1.5 to −3.5±0.2 beats per minute/mm Hg). After 7 weeks of Ang II, arterial pressure was still elevated (112±4 mm Hg) and the gain of the baroreflex curve still somewhat attenuated, although it was no longer markedly different from normotensive levels (gain, −5.09±0.95, 20% reduction from normotensive level). Two days after the Ang II infusion was stopped, arterial pressure had returned to normotensive levels, although hematocrit and heart rate remained elevated. At this time, the baroreflex curve was similar to prehypertensive control levels, with no further changes when measured again 7 days after Ang II. Cardiac hypertrophy was present when measured at 7 days after angiotensin (left ventricular weight–body weight ratio: 1.78±0.05 versus 1.35±0.04 g/kg, hypertensive versus normotensive, P <.05). Thus, although Ang II infusion produced an initial deficit in the baroreflex control of heart rate, this effect became less as the hypertension continued. Furthermore, although cardiac hypertrophy developed, its presence did not appear to be sufficient to produce a decrease in barosensitivity independent of raised arterial pressure.