Vagal and sinoaortic reflexes in postural control of circulation and renin release

Changes in blood pressure, blood flow towards the head and heart rate during 90 deg head-up tilting for 30 min in anaesthetized male rats

NEW FINDINGS

What is the central question of this study? How does the baroreceptor reflex contribute to systemic blood pressure (BP) control during prolonged 90 deg head-up tilting (HUT) in unconscious rats? What is the main finding and its importance? In intact rats, heart rate (HR) increased after the transition to upright by HUT, and BP was maintained in this posture throughout the 30 min experimental period. After sinoaortic denervation, which results in lack of discharge of the baroreflex afferents, HR and BP decreased gradually during 30 min of 90 deg HUT, suggesting that the baroreceptor reflex is an important factor for maintenance of BP during long-term upright posture. Changes in cardiovascular parameters during prolonged 90 deg head-up tilting (HUT) in animals have not been elucidated in detail. We clarified changes in systemic blood pressure (BP), blood flow towards the head (BF) and heart rate (HR) and the role of the baroreceptor reflex after a transition from the supine posture to 90 deg HUT for 30 min in anaesthetized rats (n = 13). In control rats with the baroreceptor reflex afferents intact, mean BP and BF after the onset of 90 deg HUT decreased significantly by -15.4 ± 5.9 and -26.2 ± 11.5% at 2.9 ± 1.1 s (mean ± SD, n = 12), respectively, compared with control values and then immediately increased and steadied at 30.7 ± 13.1 s (plateau; -2.8 ± 8.5% in BP and -17.5 ± 17.4% in BF compared with control values; BP was maintained during 90 deg HUT). After acute sinoaortic denervation in seven rats, initial reductions in BP and BF after 90 deg HUT were observed at 3.9 ± 1.0 s, similar to the reductions in the nerve-intact rats; the percentage changes from control were -19.2 ± 3.7 and -32.3 ± 8.4%, respectively. These parameters reached a plateau at 22.4 ± 5.8 s at -8.6 ± 7.7 and -29.5 ± 15.0%, respectively, and then BP decreased gradually throughout 90 deg HUT. Heart rate increased slightly after 90 deg HUT in nerve-intact rats, but after sinoaortic denervation this increase disappeared and HR decreased gradually during 90 deg HUT. These results suggest that the baroreceptor reflex contributes to the maintenance of adequate BP during long-term 90 deg HUT.

Additive role of the vestibular end organ and baroreceptors on the regulation of blood pressure in rats

Contribution of the vestibular end organ to regulation of arterial pressure was quantitatively compared with the role of baroreceptors in terms of baroreflex sensitivity and c-Fos protein expression in the rostral ventrolateral medulla (RVLM). Baroreflex sensitivity and c-Fos protein expression in the RVLM were measured in conscious rats that had undergone bilateral labyrinthectomy (BL) and/or baroreceptor unloading. BL attenuated baroreflex sensitivity during intravenous infusion of sodium nitroprusside (SNP), but did not significantly affect the sensitivity following infusion of phenylephrine (PE). Baroreflex sensitivity became positive following sinoaortic denervation (SAD) during infusion of PE and attenuated sensitivity during infusion of SNP. Baroreflex sensitivity also became positive following double ablation (BL+SAD) during infusion of PE, and attenuated sensitivity during infusion of SNP. c-Fos protein expression increased significantly in the RVLM in the sham group after SNP administration. However, the BL, SAD, and SAD+BL groups showed significant decreases in c-Fos protein expression compared with that in the sham group. The SAD group showed more reduced c-Fos protein expression than that in the BL group, and the SAD+BL group showed less expression than that in the SAD group. These results suggest that the vestibular system cooperates with baroreceptors to maintain arterial pressure during hypotension but that baroreceptors regulate arterial pressure during both hypotension and hypertension. Additionally, afferent signals for maintaining blood pressure from the vestibular end organs and the baroreceptors may be integrated in the RVLM.

Interaction between vestibulo-cardiovascular reflex and arterial baroreflex during postural change in rats

To examine a cooperative role for the baroreflex and the vestibular system in controlling arterial pressure (AP) during voluntary postural change, AP was measured in freely moving conscious rats, with or without sinoaortic baroreceptor denervation (SAD) and/or peripheral vestibular lesion (VL). Voluntary rear-up induced a slight decrease in AP (-5.6 ± 0.8 mmHg), which was significantly augmented by SAD (-14.7 ± 1.0 mmHg) and further augmented by a combination of VL and SAD (-21 ± 1.0 mmHg). Thus we hypothesized that the vestibular system sensitizes the baroreflex during postural change. To test this hypothesis, open-loop baroreflex analysis was conducted on anesthetized sham-treated and VL rats. The isolated carotid sinus pressure was increased stepwise from 60 to 180 mmHg while rats were placed horizontal prone or in a 60° head-up tilt (HUT) position. HUT shifted the carotid sinus pressure-sympathetic nerve activity (SNA) relationship (neural arc) to a higher SNA, shifted the SNA-AP relationship (peripheral arc) to a lower AP, and, consequently, moved the operating point to a higher SNA while maintaining AP (from 113 ± 5 to 114 ± 5 mmHg). The HUT-induced neural arc shift was completely abolished in VL rats, whereas the peripheral arc shifted to a lower AP and the operating point moved to a lower AP (from 116 ± 3 to 84 ± 5 mmHg). These results indicate that the vestibular system elicits sympathoexcitation, shifting the baroreflex neural arc to a higher SNA and maintaining AP during HUT.

Sympatho-renal interactions

The renal nerves appear to be involved in the control of cardiovascular homeostasis and volume balance both in physiological and in pathological conditions such as experimental hypertension. Anatomical and electrophysiological evidence suggests that the kidney has a diffuse sensory innervation connected with areas in the brain and spinal cord that are known to regulate cardiovascular functions by both neural and humoral mechanisms. The demonstration of the existence of neural reno-renal reflexes controlling several renal functions indicates that a functional balance between the two kidneys exists and may play an important role in the moment-to-moment control of kidney functions.

The effect of vagotomy on sodium reabsorption and renin release in anaesthetized dogs subjected to 60 degrees head-up tilt

1. Anaesthetized dogs were subjected to two 15 min periods of 60 degrees head-up tilt. Renal perfusion pressure was regulated to minimize changes during tilting. 2. In both intact and vagotomized animals there was a fall in systemic arterial pressure and a rise in heart rate. In both intact and vagotomized animals glomerular filtration rate fell slightly during the first tilt but remained unchanged in the second tilt period. Renal blood flow was unchanged throughout. 3. Both absolute and fractional excretions of sodium were reduced in intact animals subjected to the two consecutive periods of tilt. Following vagotomy these responses were as large as those observed in intact animals. 4. Tilting caused an increase in renin secretion from the kidney in both tilting periods to which the intact animals were subjected. In the vagotomized animals the increase in renin secretion was as large as that observed in intact animals. 5. The results of the present study show that, under the experimental conditions used, vagal afferent activity had no measurable effect on the decreases in sodium excretion or increases in renin secretion mediated by the renal nerves during short periods of 60 degrees head-up tilt.