Interaction between cardiac sympathetic afferent reflex and chemoreflex is mediated by the NTS AT1 receptors in heart failure

Abstract

Several sympathoexcitatory reflexes, such as the cardiac sympathetic afferent reflex (CSAR) and arterial chemoreflex, are significantly augmented and contribute to elevated sympathetic outflow in chronic heart failure (CHF). This study was undertaken to investigate the interaction between the CSAR and the chemoreflex in CHF and to further identify the involvement of angiotensin II type 1 receptors (AT1Rs) in the nucleus of the tractus solitarius (NTS) in this interaction. CHF was induced in rats by coronary ligation. Acute experiments were performed in anesthetized rats. The chemoreflex-induced increase in cardiovascular responses was significantly greater in CHF than in sham-operated rats after either chemical or electrical activation of the CSAR. The inhibition of the CSAR by epicardial lidocaine reduced the chemoreflex-induced effects in CHF rats but not in sham-operated rats. Bilateral NTS injection of the AT1R antagonist losartan (10 and 100 pmol) dose-dependently decreased basal sympathetic nerve activity in CHF but not in sham-operated rats. This procedure also abolished the CSAR-induced enhancement of the chemoreflex. The discharge and chemosensitivity of NTS chemosensitive neurons were significantly increased following the stimulation of the CSAR in sham-operated and CHF rats, whereas CSAR inhibition by epicardial lidocaine significantly attenuated chemosensitivity of NTS neurons in CHF but not in sham-operated rats. Finally, the protein expression of AT1R in the NTS was significantly higher in CHF than in sham-operated rats. These results demonstrate that the enhanced cardiac sympathetic afferent input contributes to an excitatory effect of chemoreflex function in CHF, which is mediated by an NTS-AT1R-dependent mechanism.

Fig. 1.

Changes in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) in response to chemoreflex activation with potassium cyanide (KCN; 1 and 10 μg) injected into right carotid artery in intact sham-operated (baseline MAP, 93.1 ± 3.2 mmHg, n = 9) and chronic heart failure (CHF) (baseline MAP, 89.8 ± 3.6 mmHg, n = 10) rats. *P < 0.05 and **P < 0.01 (2-way ANOVA).

Fig. 2.

Effects of cardiac sympathetic afferent stimulation on the chemoreflex response. A and B: representative recordings of MAP, heart rate (HR), and RSNA in response to carotid arterial injection of KCN (10 μg) during epicardial application of saline (control), capsaicin (Cap, 0.4 μg), or electrical stimulation (Ele Stim) of cardiac sympathetic afferent in CHF rats. ABP, arterial blood pressure. C: changes in MAP and RSNA in response to KCN injection during stimulation of cardiac sympathetic afferent in intact sham-operated (baseline MAP, 93.1 ± 3.2 mmHg, n = 9) and CHF (baseline MAP, 89.8 ± 3.6 mmHg, n = 10) rats. *P < 0.05 vs. control; #P < 0.05 vs. sham (2-way ANOVA).

Fig. 3.

Effects of epicardial application of lidocaine on the KCN-induced change in MAP and RSNA in vagotomized sham-operated and CHF rats. A: representative recordings of MAP, HR, and RSNA in response to carotid artery bolus injection of KCN (10 μg) and after epicardial application of normal saline (control, left) and lidocaine (2% in 20 μl, right) in CHF rats. B: decreased change in MAP (left) and RSNA (right) responses to KCN by lidocaine in sham-operated (baseline MAP, 91.4 ± 3.1 mmHg, n = 6) and CHF (baseline MAP, 87.4 ± 2.7 mmHg, n = 8) rats. *P < 0.05 vs. control (2-way ANOVA).

Fig. 4.

Effects of bilateral injection of losartan into the nucleus of the tractus solitarius (NTS) on basal cardiovascular activity and the KCN-induced change in MAP and RSNA in intact sham-operated and CHF rats. A: representative recordings of MAP, HR, and RSNA in response to NTS injection of losartan (100 pmol) in an intact CHF rat. B: change in MAP (left), HR (middle), and RSNA (right) after losartan (10 and 100 pmol) or vehicle [artificial cerebrospinal fluid (aCSF), 50 nl] injected into the NTS in sham-operated (baseline MAP and HR, 89.6 ± 3.8 mmHg and 342 ± 12.6 beats/min, n = 7) and CHF (baseline MAP and HR, 87.3 ± 3.5 mmHg and 336 ± 14.5 beats/min, n = 12) rats. *P < 0.05 vs. aCSF; #P < 0.05 vs. 10 pmol losartan (2-way ANOVA). C: changes in MAP (left) and RSNA (right) evoked by KCN injection after treatment with NTS injection of losartan (100 pmol) and epicardial application of capsaicin (0.4 μg) in CHF rats (baseline MAP and HR, 88.5 ± 3.3 mmHg and 331 ± 17.2 beats/min, n = 7). *P < 0.05 vs. control; NS, no significance vs. losartan (1-way ANOVA).

Fig. 5.

Histological analysis for microinjection and recording sites in the lower brain stem. A: distributions of the microinjection (○, left) or neuronal recording sites (•, right) plotted on standard coronal sections according to the atlas of Paxinos and Watson (Ref. 27). AP, area postrema; Cu, cuneate nucleus; Gr, gracile nucleus; pyx, pyramidal tract; Sol, nucleus of tract solitary; Sp5, spinal trigemina nucleus; 12, nucleus of hypoglossal nerve. B: the arrow-pointed spots in the raw picture marked the microinjection sites in a 50-μm-thick section of brain stem.

Fig. 6.

Effects of cardiac sympathetic afferent input on the activity of NTS chemosensitive neurons. A and B: representative recordings of neuronal discharge in response to right carotid artery bolus injection of KCN (10 μg) after epicardial application of normal saline (control), capsaicin (A, recording from an intact CHF rat) or lidocaine (B, recording from a vagotomized CHF rat). C and D: percent change of baseline discharge (C) and chemosensitivity (D, control level taken as 100%) of NTS chemosensitive neurons NTS in response to epicardial capsaicin or lidocaine in sham-operated and CHF rats. *P < 0.05 vs. control; #P < 0.05 vs. sham (2-way ANOVA).

Fig. 7.

ANG II type 1 receptor (AT₁R) protein expression in the NTS. Top: representative Western blot showing the NTS AT₁R protein expression in intact sham-operated and CHF rats. Bottom: ratio of AT₁R to GAPDH expression in the NTS. *P < 0.01 vs. sham (unpaired t-test).