Circulating angiotensin II attenuates the sympathetic baroreflex by reducing the barosensitivity of medullary cardiovascular neurones in the rat

Abstract

Chronic intravenous angiotensin II (Ang II) has been widely used to establish centrally mediated hypertension in experimental animals, and disruption of Ang II activity is a frontline treatment for hypertensive disease. However, the acute central actions of circulating Ang II are poorly understood. We examined the effects of intravenous pressor doses of Ang II on autonomic activity in anaesthetized rats under neuromuscular blockade, and compared baroinhibition evoked by Ang II pressor ramps to equipressor responses evoked by phenylephrine (PE). Baroinhibition of splanchnic sympathetic nerve activity was attenuated during Ang II trials compared with PE, and rats remained sensitive to electrical stimulation of the aortic depressor nerve at higher arterial pressures during Ang II trials. This was not due to a direct effect of Ang II on aortic nerve baroreceptors. In a separate series of experiments, we provide direct evidence that bulbospinal barosensitive neurones in the rostral ventrolateral medulla are differentially sensitive to pressure ramps evoked by Ang II or PE vasoconstriction. Nineteen out of 41 units were equally sensitive to increased arterial pressure evoked by Ang II or PE. In 17 of 41 units, barosensitivity was attenuated during Ang II trials, and in five of 41 cases units that had previously been barosensitive increased their firing rate during Ang II trials. These results show, for the first time, that circulating Ang II acutely modulates central cardiovascular control mechanisms. We suggest that this results from activation by Ang II of a central pathway originating at the circumventricular organs.

Figure 2. Effects of Ang II on baroreceptor encoding of blood pressure

A, experimental record illustrating simultaneous recording of multiunit ADN activity (top trace), ECG (middle trace) and AP (lower trace). B, R-wave-triggered averages constructed over 5 min of rectified ADN (rADN; top trace) and AP (lower trace) before (continuous lines) and after intravenous Ang II (dotted lines). C, Ang II had no effect on the encoding of AP by the ADN (P > 0.05). D, experimental record showing effect of AP responses to Ang II and PE on barosensitive ADN activity (top trace; rectified and smoothed, 500 ms time constant) and SNA (middle trace; rectified and smoothed, 500 ms time constant). Note that although PE and Ang II evoke similar changes in AP and ADN activity, baroinhibition of SNA evoked by Ang II is attenuated compared with that evoked by PE. Responses to electrical stimulation of the central ADN stump are more pronounced during Ang II trials. Stimulus artefacts are removed from ADN recording for clarity. E, there was no effect of Ang II on the encoding of AP by the ADN during PE and Ang II trials (individual experiments in grey, average in black, P > 0.05).

Figure 1. Effects of PE and Ang II on sympathetic nerve activity

A, experimental record illustrating the effects of intravenous PE (5 μg) and Ang II (35 pmol) on heart rate (HR; top trace), rectified splanchnic sympathetic nerve activity (SNA; middle trace) and arterial blood pressure (AP; lower trace) in an anaesthetized vagotomized rat. Marker channel indicates timing of tetanic stimulation of the aortic depressor nerve (ADN; 100 Hz, 1 s). B, SNA was inhibited by PE- and Ang II-evoked baroreceptor loading, but SNA baroinhibition was consistently attenuated by Ang II (individual experiments in grey, pooled average in black; P < 0.05). C, the magnitude of ADN-evoked baroreflex AP reduction was inversely proportional to baseline AP in control (PE) trials; during equipressor Ang II trials, there was a diminished effect of AP on the magnitude of responses to ADN stimulation (P < 0.05).

Figure 3. Properties of RVLM units

Representative examples from the same neurone demonstrating the functional properties of RVLM units included in the present study (same cell as in Fig. 3A). For each unit, R-wave-triggered histograms of firing were plotted with AP (grey overlay) to show pulse modulation (A; bin width 10 ms). B, peristimulus time histogram (200 sweeps, 0.5 Hz, bin width 10 ms) shows unit inhibition by intermittent stimulation of the ADN. C, most units were modulated by central respiratory drive; phrenic neurogram (PNA; grey trace)-triggered histogram demonstrates moderate inspiratory depression. D, some units were tested for bulbospinal projections. Stimulation of the T2 spinal cord at the depth of the intermediolateral cell column (IML; arrow) resulted in a constant latency antidromic action potential that was annihilated by spontaneous orthodromic spikes occurring within the critical period (dotted lines).

Figure 4. Effects of intravenous Ang II on RVLM unit barosensitivity

Examples show RVLM unit firing frequency (top trace; bin width 2 s), raw extracellular recordings (middle trace) and AP (lower trace) during PE and Ang II trials. Data from each example are broken into 2 s bins, plotted against AP, and fitted to a linear regression. In 19 of 41 units, barosensitivity was unaffected by Ang II (A). In 17 of 41 units, barosensitivity was attenuated following Ang II (B). In 5 of 41 units, cells that were previously inhibited during PE trials increased their firing rates during Ang II trials (C).