Renal Nerve Activity Responses Research Articles

Reflexes from pulmonary arterial baroreceptors in dogs: interaction with carotid sinus baroreceptors

In contrast to the reflex vasodilatation occurring in response to stimulation of baroreceptors in the aortic arch, carotid sinuses and coronary arteries, stimulation of receptors in the wall of pulmonary arteries results in reflex systemic vasoconstriction. It is rare for interventions to activate only one reflexogenic region, therefore we investigated how these two types of reflexes interact. In anaesthetized dogs connected to cardiopulmonary bypass, reflexogenic areas of the carotid sinuses, aortic arch and coronary arteries and the pulmonary artery were subjected to independently controlled pressures. Systemic perfusion pressure (SPP) measured in the descending aorta (constant flow) provided an index of systemic vascular resistance. In other experiments, sympathetic efferent neural activity was recorded in fibres dissected from the renal nerve (RSNA). Physiological increases in pulmonary arterial pressure (PAP) induced significant increases in SPP (+39.1 ± 10.4 mmHg) and RSNA (+17.6 ± 2.2 impulses s(−1)) whereas increases in carotid sinus pressure (CSP) induced significant decreases in SPP (−42.6 ± 10.8 mmHg) and RSNA (−42.8 ± 18.2 impulses s(−1)) (P < 0.05 for each comparison; paired t test). To examine possible interactions, PAP was changed at different levels of CSP in both studies. With CSP controlled at 124 ± 2 mmHg, the threshold, 'set point' and saturation pressures of the PAP–SPP relationship were higher than those with CSP at 60 ± 1 mmHg; this rightward shift was associated with a significant decrease in the reflex gain. Similarly, increasing CSP produced a rightward shift of the PAP–RSNA relationship, although the effect on reflex gain was inconsistent. Furthermore, the responses to changes in CSP were influenced by setting PAP at different levels; increasing the level of PAP from 5 ± 1 to 33 ± 3 mmHg significantly increased the set point and threshold pressures of the CSP–SPP relationship; the reflex gain was not affected. These results indicate the existence of interaction between pulmonary arterial and carotid sinus baroreceptor reflexes; physiological and pathological states that alter the stimulus to one may alter the reflex responses from the other.

Contribution of nerve growth factor to upregulation of P2X3 expression in DRG neurons of rats with femoral artery occlusion

Femoral artery occlusion augments the sympathetic nerve and pressor responses to muscle contraction and muscle metabolites injected into the arterial blood supply of the hindlimb muscles in rats. The underlying mechanism by which these reflex responses are enhanced after muscle vascular insufficiency is unclear. Purinergic P2X(3) receptor has been reported to contribute to the metabolic component of the exercise pressor reflex. Thus the purpose of this study was to examine if chronic femoral occlusion would alter the expression of P2X(3) in dorsal root ganglion (DRG) neurons of rats. Also, P2X(3)-mediated sympathetic responsiveness was examined after femoral occlusion. In addition, the role played by nerve growth factor (NGF) in regulating the expression and response of P2X(3) was examined. Western blot analysis showed that 24 h of femoral ligation increased the levels of P2X(3) (optical density: 0.93 ± 0.07 in control and 1.37 ± 0.10 after occlusion; P < 0.05 vs. control). The fluorescence immunohistochemistry further demonstrated that the occlusion elevated P2X(3) expression in DRG neurons (percentage of P2X(3)-positive cells: 33 ± 3% in control and 51 ± 3% in occlusion; P < 0.05 vs. control). Furthermore, the results showed that responses of renal sympathetic nerve activity and blood pressure to stimulation of P2X were greater in occluded rats than responses in control rats by injection of α,β-methylene ATP into the arterial blood supply of the hindlimb muscle. Finally, infusion of NGF in the hindlimb muscles of healthy rats increased P2X(3) (optical density: 0.98 ± 0.12 in control and 1.37 ± 0.16 with NGF; P < 0.05 vs. control). The pressor response to injection of α,β-methylene ATP was increased in the rats with NGF infusion. Likewise, blocking NGF attenuated exaggeration of the reflex response induced by α,β-methylene ATP in occluded rats. The findings of this study suggest that the levels of P2X(3) in primary afferent neurons are upregulated as the blood supply to the hindlimb is deficient under ischemic conditions, leading to augmentation of the muscle reflex. NGF is closely related to increases in P2X(3) receptor expression and response.

Nerve growth factor contributes augmented P2X3 expression of primary afferent neurons by femoral occlusion

Femoral artery occlusion augments the reflex sympathetic nerve and pressor responses to muscle metabolites and to muscle contraction in rats. Also, P2X3 receptors contribute the metabolic component of the exercise pressor reflex. Thus, the purpose of this study was to examine if femoral occlusion alters expression of P2X3 in dorsal root ganglion (DRG) neurons and its responsiveness in rats. In addition, we examined role of nerve growth factor (NGF) in the levels of P2X3 proteins in sensory neurons. Our western blot results showed that 24hrs of femoral ligation increased the levels of P2X3 (optical density: 0.93±0.07 in control vs. 1.37±0.10 after occlusion, P<0.05). The fluorescence immunohistochemistry further demonstrated that the occlusion elevated P2X3 expression in DRG neurons (percentage of P2X3 positive cells: 33±3% in control vs.51±3% in occlusion, P<0.05). Furthermore, our results showed that responses of renal sympathetic nerve activity and blood pressure to stimulation of P2X were greater in occluded rats than in control rats by injection of α,β me-ATP into the arterial blood supply of hindlimb muscle. Finally, infusion of NGF in the hindlimb muscles of healthy rats increased P2X3 (optical density: 0.98±0.12 in control vs. 1.37±0.16 with NGF, P<0.05). Our findings suggest that the levels of P2X3 in primary afferent neurons are increased as the blood supply to the hindlimb is insufficient, and NGF is likely responsible for enhanced P2X3. (Supported by NIH R01 HL-090720, P01HL-096570, and AHA grant 0840130N).

Cardiovascular and Sympathetic Effects of Disrupting Tyrosine 985 of the Leptin Receptor

Leptin acts in the brain to regulate food intake and energy expenditure. Leptin also increases renal sympathetic nerve activity and arterial pressure. The divergent signaling capacities of the leptin receptor (ObRb) mediate the stimulation of various intracellular pathways that are important for leptin control of physiological processes. We evaluated the cardiovascular and sympathetic consequences of disrupting the signal emanating from tyrosine985 of ObRb. For this, we used Lepr(L985) (l/l) mice, which carry a loss of function mutation replacing tyrosine985 of ObRb with leucine. Body weight of l/l mice was not significantly different from wild-type controls. In contrast, radiotelemetry measurements revealed that the l/l mice had higher arterial pressure and heart rate as compared with controls. Ganglionic blockade caused a greater arterial pressure fall in the l/l mice relative to controls. In addition, leptin treatment induced a larger increase in arterial pressure and heart rate in the l/l versus wild-type mice. Finally, we compared the response of renal and brown adipose tissue sympathetic nerve activity to intracerebroventricular injection of leptin (2 μg) between l/l and control mice. Leptin-induced increase in renal sympathetic nerve activity was greater in l/l mice relative to controls. In contrast, the brown adipose tissue sympathetic nerve activity response to leptin was attenuated in the l/l mice relative to controls. These data indicate that selective loss of leptin receptor signaling emanating from tyrosine985 enhances the cardiovascular and renal sympathetic effects of leptin. These findings provide important insight into the molecular mechanisms underlying leptin's effects on the sympathetic cardiovascular function and arterial pressure.

Neuronal nitric oxide synthase within paraventricular nucleus: blood pressure and baroreflex in two‐kidney, one‐clip hypertensive rats

The renin-angiotensin system is activated in the early phase of two-kidney, one-clip (2K-1C) hypertension. The paraventricular nucleus (PVN) integrates inputs regulating sympathetic outflow. The PVN receives inputs from plasma angiotensin II via projections from circumventricular organs and from renal afferent nerves transmitted via the nucleus tractus solitarii. Nitric oxide within the PVN may exert a sympathoinhibitory effect. These studies tested whether decreasing endogenous nitric oxide by introducing dominant negative (DN) constructs for neuronal nitric oxide synthase (nNOS) into PVN chronically augments hypertension and/or modulates baroreflex function. Male 6-week-old Sprague-Dawley rats underwent sham surgery or right renal artery clipping and placement of radiotelemetry transmitters. One week later, the PVN was injected bilaterally with 250 nl artificial cerebrospinal fluid containing 250 ng microl(-1) of RSV beta-galactosidase (beta-Gal), cytomegalovirus (CMV) wild-type (WT nNOS), or respiratory syncytial virus (RSV) haeme domain or RSV haemeRedF (DN nNOS). Haemodynamics were monitored for 5 weeks. Then left renal nerve electrodes were placed, and 2 days later the rats underwent baroreflex testing in the conscious state. The rise in mean arterial pressure (MAP) was significantly potentiated in the DN nNOS 2K-1C group beyond 15 days after PVN injection. By day 35, MAP in the 2K-1C groups was 152 +/- 6.3 (beta-Gal), 155.1 +/- 6.6 (WT nNOS) and 179 +/- 5.4 mmHg (DN nNOS; P < 0.01 versus all other groups). Sham-clipped rats remained normotensive. All groups displayed progressive bradycardia over time that was attenuated in the DN nNOS 2K-1C group. Baroreflex curves shifted to higher pressures, and baroreflex sensitivity of heart rate was diminished to a similar extent in all groups of 2K-1C rats. The baroreflex response of renal sympathetic nerve activity was preserved. The PVN tissue from DN nNOS rats had decreased dimerization of nNOS and generation of total nitric oxide. These findings indicate that chronic interference of nNOS dimerization required for generation of nitric oxide within the PVN potentiates the increase of blood pressure by modulating the sympathoexcitation that accompanies renovascular hypertension.

Comparison of the pharmacological and sequence methods for assessing baroreflex control blood pressure via heart rate and renal sympathetic nerve activity

In many cardiovascular diseases, the sensitivity of the baroreflex is impaired. To determine baroreflex sensitivity in the laboratory, we have traditionally infused vasoactive drugs to alter blood pressure and recorded the responses of renal sympathetic nerve activity and heart rate. However, the pharmacological method is not always practical and thus there is a shift to obtaining baroreflex response curves from naturally occurring fluctuations in arterial pressure. The aim of this project was to establish the practicalities of using spontaneous fluctuations in arterial pressure to determine the sensitivity of the baroreflex in conscious unrestrained animals and make comparisons to traditional pharmacological methods. Gains of the baroreflex control of heart rate were comparable between the two methods but spontaneous baroreflex gains for the baroreflex control for sympathetic nerve activity were greater than those obtained from pharmacological methods. It must also be noted that the sequence methods only provides gain measurements and no information on the operating range of the reflex. Supported by the Health Research Council of New Zealand and University of Auckland Research Committee.

ASIC3 in dorsal root ganglion neurons contributes to femoral occlusion enhanced sympathetic responsiveness

Acid sensing ion channel subtype 3 (ASIC3) is responsive to acid in the extracellular medium. Prior studies suggest that muscle metabolite lactic acid and ASIC play a role in muscle afferent mediated reflex sympathetic response. Also, femoral occlusion increases the reflex sympathetic nerve response. Thus, we tested the hypothesis that enhanced ASIC3 expression and its responsiveness in primary afferent neurons innervating muscles contribute to the exaggerated reflex sympathetic responses after femoral occlusion. The femoral occlusion was induced by the femoral artery ligation in rats. First, we employed Western blot to examine expression of ASIC3 in dorsal root ganglion (DRG). The data showed that 24 hrs of femoral occlusion led to upregulation of ASIC3 expression in DRG (optical density: 0.93 ± 0.08 in control vs. 1.47 ± 0.04 after occlusion, P&lt;0.05), e.g. 1.58‐fold increase. Second, experiments using whole cell patch‐clamp demonstrated that current amplitude of DRG neurons innervating muscle was larger in response to a lower pH in femoral occlusion. Third, responses of renal sympathetic nerve activity and blood pressure to acid injected into the arterial blood supply of hindlimb muscles were also enhanced after the ligation. This study suggests that ASIC3 has an important contribution to augmented sympathetic responsiveness after femoral occlusion. (Supported by R01 HL078866 and R01 HL090720).

Long-term responses of renal sympathetic nerve activity and arterial pressure to chronic sino-aortic denervation in rats

Long-term responses of renal sympathetic nerve activity and arterial pressure to chronic sino-aortic denervation in rats

Elevated mitochondrial superoxide contributes to enhanced chemoreflex in heart failure rabbits

Peripheral chemoreflex sensitivity is enhanced in both clinical and experimental chronic heart failure (CHF). Here we investigated the role of manganese superoxide dismutase (MnSOD), the SOD isoform specially targeted to mitochondria, and mitochondrial superoxide levels in the enhanced chemoreceptor activity and function of the carotid body (CB) in CHF rabbits. CHF suppressed MnSOD protein expression and elevated mitochondrial superoxide levels in CB compared with that in sham CB. Adenovirus (Ad) MnSOD (1 x 10(8) plaque-forming units/ml) gene transfer selectively to the CBs normalized mitochondrial superoxide levels in glomus cells from CHF CB. In addition, Ad MnSOD reduced the elevation of superoxide level in CB tissue from CHF rabbits. Ad MnSOD significantly increased MnSOD expression in CHF CBs and normalized the baseline renal sympathetic nerve activity and the response of renal sympathetic nerve activity to hypoxia in CHF rabbits. Ad MnSOD decreased baseline single-fiber discharge from CB chemoreceptors compared with Ad Empty (6.3 + or - 1.5 vs. 12.7 + or - 1.4 imp/s at approximately 100-Torr Po(2), P < 0.05) and in response to hypoxia (20.5 + or - 1.8 vs. 32.6 + or - 1.4 imp/s at approximately 40-Torr Po(2), P < 0.05) in CHF rabbits. Compared with Ad Empty, Ad MnSOD reversed the blunted K(+) currents in CB glomus cells from CHF rabbits (385 + or - 11 vs. 551 + or - 20 pA/pF at +70 mV, P < 0.05). The results suggest that decreased MnSOD in the CB and elevated mitochondrial superoxide levels contribute to the enhanced CB chemoreceptor activity and peripheral chemoreflex function in CHF rabbits.

Nitric oxide in rostral ventrolateral medulla regulates cardiac-sympathetic reflexes: role of synthase isoforms

Our previous studies have shown that nitric oxide (NO) synthase (NOS)-containing neurons in the rostral ventrolateral medulla (rVLM) are activated during cardiac sympathoexcitatory reflexes (Refs. 12 and 13). However, the precise function of NO in the rVLM in regulation of these reflexes has not been defined. Three isoforms of NOS, including neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS), are located in the rVLM. We explored the role of NO, derived from different NOS isoforms in the rVLM, in processing cardiac-sympathetic reflexes using whole animal reflex and electrophysiological approaches. We found that, in anesthetized cats, increased mean arterial blood pressure and renal sympathetic nerve activity elicited by epicardial application of bradykinin (BK; 1-10 microg/ml, 50 microl) were significantly attenuated following unilateral rVLM microinjection of the nonselective NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (50 nmol/50 nl), or a specific nNOS inhibitor, 7-nitroindazole (7-NI; 5-10 pmol/50 nl; both P < 0.05). In contrast, the responses of mean arterial blood pressure and renal sympathetic nerve activity to cardiac BK stimulation were unchanged by unilateral rVLM microinjection of N(omega)-nitro-D-arginine methyl ester (inactive isomer of N(omega)-nitro-L-arginine methyl ester, 50 nmol/50 nl), 3-6% methanol (7-NI vehicle), N(6)-(1-iminoethyl)-L-lysine (250 pmol/50 nl; iNOS inhibitor), or N(5)-(1-iminoethyl)-L-ornithine (250 nmol/50 nl; eNOS inhibitor). Furthermore, in separate cats, we noted that iontophoresis of 7-NI (0.1 mM) reduced the increased discharge of cardiovascular sympathoexcitatory rVLM neurons in response to cardiac stimulation with BK (P < 0.05). These neurons were characterized by their responses to inputs from baroreceptors, and their cardiac rhythmicity was determined through frequency and time domain analyses, correlating their discharge to arterial blood pressure and cardiac sympathetic efferent nerve activity. These data suggest that NO, specifically nNOS, mediates sympathetic cardiac-cardiovascular responses through its action in the rVLM.

Contribution of the arterial baroreflex to the differential responses of renal and lumbar sympathetic nerve activity and heart rate to freezing behavior in conscious rats

The present study examined the potential contribution of the arterial baroreflex to the differential responses of renal (RSNA) and lumbar sympathetic nerve activity (LSNA) and heart rate (HR) observed during freezing behavior, which was reported previously (FASEB J. 21:6 A882, 2007). Male Wistar rats were chronically instrumented with electrodes for measurements of RSNA, LSNA, and electrocardiogram, and with arterial and venous catheters. At least three days after the surgery, rats were exposed to white noise with 90dB over 10 min following a 60 min control period. The baroreflex curve for RSNA, LSNA and HR was determined by changing systemic arterial pressure using rapid intravenous infusions of vasoactive drugs and then fitted to an inverse sigmoid function curve. During the freezing behavior, the baroreflex curve for RSNA was shifted upward with a significant (P&lt;0.05) increase in the response range of 115% compared with control level; while the baroreflex curve for LSNA remained unchanged during freezing behavior. The baroreflex curve for HR was shifted leftward with a significant (P&lt;0.05) decrease in the midpoint pressure of 11mmHg relative to the control level. These data suggest that the differential responses of RSNA, LSNA and HR observed during freezing behavior may be explained, in part, by differential shifts in baroreflex curves for RSNA, LSNA and HR in rats.

Stimulation of NTS A1adenosine receptors differentially resets baroreflex control of regional sympathetic outputs

Previously we showed that pressor and differential regional sympathoexcitatory responses (adrenal > renal >/= lumbar) evoked by stimulation of A(1) adenosine receptors located in the nucleus of the solitary tract (NTS) were attenuated/abolished by baroreceptor denervation or blockade of glutamatergic transmission in the NTS, suggesting A(1) receptor-elicited inhibition of glutamatergic transmission in baroreflex pathways. Therefore we tested the hypothesis that stimulation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex responses of preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activity. In urethane-chloralose-anesthetized male Sprague-Dawley rats (n = 65) we compared baroreflex-response curves (iv nitroprusside and phenylephrine) evoked before and after bilateral microinjections into the NTS of A(1) adenosine receptor agonist (N(6)-cyclopentyladenosine, CPA; 0.033-330 pmol/50 nl). CPA evoked typical dose-dependent pressor and differential sympathoexcitatory responses and similarly shifted baroreflex curves for pre-ASNA, RSNA, and LSNA toward higher mean arterial pressure (MAP) in a dose-dependent manner; the maximal shifts were 52.6 +/- 2.8, 48.0 +/- 3.6, and 56.8 +/- 6.7 mmHg for pre-ASNA, RSNA, and LSNA, respectively. These shifts were not a result of simple baroreceptor resetting because they were two to three times greater than respective increases in baseline MAP evoked by CPA. Baroreflex curves for pre-ASNA were additionally shifted upward: the maximal increases of upper and lower plateaus were 41.8 +/- 16.4% and 45.3 +/- 8.7%, respectively. Maximal gain (%/mmHg) measured before vs. after CPA increased for pre-ASNA (3.0 +/- 0.6 vs. 4.9 +/- 1.3), decreased for RSNA (4.1 +/- 0.6 vs. 2.3 +/- 0.3), and remained unaltered for LSNA (2.1 +/- 0.2 vs. 2.0 +/- 0.1). Vehicle control did not alter the baroreflex curves. We conclude that the activation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex control of regional sympathetic outputs.

Reactive oxygen species in the paraventricular nucleus mediate the cardiac sympathetic afferent reflex in chronic heart failure rats

The aim of this study was to determine whether reactive oxygen species (ROS) in the paraventricular nucleus (PVN) mediate both the cardiac sympathetic afferent reflex (CSAR) and angiotensin II-induced CSAR enhancement in chronic heart failure (CHF) rats. CSAR was evaluated from the responses of renal sympathetic nerve activity (RSNA) to epicardial application of bradykinin. In both CHF and sham-operated rats, PVN microinjection of the superoxide anion scavengers tempol or tiron almost abolished the CSAR, but the superoxide dismutase inhibitor DETC potentiated the CSAR. PVN pretreatment with tempol or tiron abolished, whereas DETC augmented, the angiotensin II-induced CSAR enhancement. In CHF rats, superoxide anion and malondialdehyde (MDA) levels in the PVN were increased, but were normalized by the AT(1) receptor antagonist losartan. PVN microinjection of tempol decreased superoxide anion and MDA levels, but epicardial application of bradykinin or PVN microinjection of angiotensin II increased superoxide anion and MDA to higher levels in CHF rats than in sham-operated rats. These results indicate that ROS in the PVN mediates the CSAR and the effect of angiotensin II in the PVN on the CSAR in both CHF and sham-operated rats. Increased ROS in the PVN are involved in the enhanced CSAR in CHF.

Endogenous hydrogen peroxide in paraventricular nucleus mediating cardiac sympathetic afferent reflex and regulating sympathetic activity

We previously reported that reactive oxygen species (ROS) in paraventricular nucleus (PVN) mediated cardiac sympathetic afferent reflex (CSAR). The present study investigated the role of endogenous hydrogen peroxide (H(2)O(2)), a ROS, in the PVN in mediating the CSAR and regulating sympathetic activity. The CSAR was evaluated by the response of renal sympathetic nerve activity (RSNA) to epicardial application of bradykinin (BK) in rats. Bilateral microinjection of polyethylene glycol-catalase (PEG-CAT, an analogue of endogenous catalase) or polyethylene glycol-superoxide dismutase (PEG-SOD, an analogue of endogenous superoxide dismutase) into the PVN abolished the CSAR, decreased baseline RSNA and mean arterial pressure (MAP). Moreover, pretreatment with PEG-CAT or PEG-SOD blocked the enhanced CSAR and RSNA responses induced by exogenous angiotensin II (Ang II) in the PVN. Aminotriazole (ATZ, a catalase inhibitor) alone potentiated the CSAR, increased RSNA and MAP, but failed to augment the Ang II-induced CSAR enhancement responses. Pretreated with PEG-SOD, ATZ still increased baseline RSNA and MAP but inhibited the CSAR and Ang II-induced CSAR and RSNA enhancement responses. These results suggested that endogenous H(2)O(2) in the PVN mediated both the CSAR and Ang II-induced CSAR enhancement responses. H(2)O(2) in the PVN were involved in regulating sympathetic activity and arterial pressure.

Differential response of renal and lumbar sympathetic nerve activity during freezing behaviour in conscious rats

Stressful or threatening sensory stimulation results in decreased motor activity in some species, a response referred to as “freezing” behaviour. The present study was designed to examine sympathetic and hemodynamic responses during freezing behaviour induced by 90 dB noise exposure in conscious rats. Wistar male rats were instrumented chronically with electrodes for measurements of renal (RSNA) and lumbar (LSNA) sympathetic nerve activity, electroencephalogram, electromyogram, and electrocardiogram and catheters for measurements of systemic arterial and central venous pressure. Rats were exposed to 90 dB white-noise for 10 min. 90 dB noise exposure resulted in an immediate and significant increase in RSNA while LSNA did not change significantly. There was a significant difference in the response between RSNA and LSNA during the 90 dB noise exposure. Sino-aortic denervation (SAD) attenuated the magnitude of the increase in RSNA while it has no influence on the changes in LSNA observed in SAD rats. These data suggest that the arterial baroreceptor reflex plays a significant role in the differential control of RSNA and LSNA during freezing behaviour in conscious rats.

Activation of Endothelin-A Receptors Contributes to Angiotensin-Induced Suppression of Renal Sensory Nerve Activation

Activation of renal mechanosensory nerves is enhanced by a high-sodium diet and suppressed by a low-sodium diet. Angiotensin (Ang) II and endothelin (ET)-1 each contributes to the impaired responsiveness of renal mechanosensory nerves in a low-sodium diet. We examined whether stimulation of ETA receptors (Rs) contributes to Ang II-induced suppression of the responsiveness of renal mechanosensory nerves. In anesthetized rats fed a low-sodium diet, renal pelvic administration of the Ang type I receptor (AT1-R) antagonist losartan enhanced the afferent renal nerve activity (ARNA) response to increasing renal pelvic pressure 7.5 mm Hg from 7+/-2% to 15+/-2% and the prostaglandin (PG) E(2)-mediated substance P release from 0+/-1 to 8+/-1 pg/min. Adding the ETA-R antagonist BQ123 to the renal pelvic perfusate containing losartan did not produce any further enhancement of the ARNA response or PGE(2)-mediated release of substance P (17+/-3% and 8+/-1 pg/min). Likewise, renal pelvic administration of BQ123 and BQ123+losartan resulted in similar enhancements of the ARNA responses to increased renal pelvic pressure and PGE(2)-mediated substance P release. In high-sodium-diet rats, pelvic administration of Ang II reduced the ARNA response to increased renal pelvic pressure from 27+/-4% to 8+/-3% and the PGE(2)-mediated substance P release from 9+/-0 to 1+/-1 pg/min. Adding BQ123 to the renal pelvic perfusate containing Ang II restored the increases in ARNA and the PGE(2)-mediated substance P release toward control (27+/-6% and 7+/-1 pg/min). In conclusion, stimulation of ETA-R plays an important contributory role to the Ang II-mediated suppression of the activation of renal mechanosensory nerves in conditions of low-sodium diet.

Differential effects of endothelin on activation of renal mechanosensory nerves: stimulatory in high-sodium diet and inhibitory in low-sodium diet

Activation of renal mechanosensory nerves is enhanced by high and suppressed by low sodium dietary intake. Afferent renal denervation results in salt-sensitive hypertension, suggesting that activation of the afferent renal nerves contributes to water and sodium balance. Another model of salt-sensitive hypertension is the endothelin B receptor (ETBR)-deficient rat. ET and its receptors are present in sensory nerves. Therefore, we examined whether ET receptor blockade altered the responsiveness of the renal sensory nerves. In anesthetized rats fed high-sodium diet, renal pelvic administration of the ETBR antagonist BQ-788 reduced the afferent renal nerve activity (ARNA) response to increasing renal pelvic pressure 7.5 mmHg from 26+/-3 to 9+/-3% and the PGE2-mediated renal pelvic release of substance P from 9+/-1 to 3+/-1 pg/min. Conversely, in rats fed low-sodium diet, renal pelvic administration of the ETAR antagonist BQ-123 enhanced the ARNA response to increased renal pelvic pressure from 9+/-2 to 23+/-6% and the PGE2-mediated renal pelvic release of substance P from 0+/-0 to 6+/-1 pg/min. Adding the ETAR antagonist to ETBR-blocked renal pelvises restored the responsiveness of renal sensory nerves in rats fed a high-sodium diet. Adding the ETBR antagonist to ETAR-blocked pelvises suppressed the responsiveness of the renal sensory nerves in rats fed a low-sodium diet. In conclusion, activation of ETBR and ETAR contributes to the enhanced and suppressed responsiveness of renal sensory nerves in conditions of high- and low-sodium dietary intake, respectively. Impaired renorenal reflexes may contribute to the salt-sensitive hypertension in the ETBR-deficient rat.

Responses of Blood Pressure and Renal Sympathetic Nerve Activity to Colorectal Distension in Anesthetized Rats

We investigated the effects of visceral stimulation by colorectal distension (CRD) on mean arterial blood pressure (MAP) and renal sympathetic nerve activity (RSNA), the latter being an index of vasoconstrictor activity, in anesthetized central nervous system (CNS)-intact and C2 spinalized rats. The CRD stimulation was induced by the distention of a balloon inserted into the colorectum. In CNS-intact rats, there were significant reductions in MAP and RSNA in response to intraballoon pressures of 60 and 80 mmHg, but not to 20 and 40 mmHg. However, spinalized rats demonstrated significant increases in MAP in response to intraballoon pressures of 60 and 80 mmHg and increases in RSNA in response to intraballoon pressures of 40, 60, and 80 mmHg. These results suggest that noxious visceral stimulation at lower spinal levels reduces MAP by inhibiting sympathetic output in CNS-intact anesthetized rats. On the other hand, noxious visceral stimulation results in an increase in sympathetic-induced MAP in spinalized anesthetized rats.

Ventricular reflex interactions during transmural myocardial ischemia

The role of left ventricular receptors with sympathetic afferent fibers in the reflex response to myocardial ischemia is controversial, particularly in the canine model. Previous experiments have shown that reflex excitatory responses mediated by left ventricular sympathetic afferents can be detected in sinoaortic denervated and vagotomized dogs during transmural myocardial ischemia. The purpose of these experiments was to determine if reflex excitatory responses occur in dogs with intact left ventricular vagal afferents. Experiments were performed in 27 chloralose-anesthetized dogs with sinoaortic denervation. Responses of efferent renal sympathetic nerve activity, arterial, and left atrial pressures to transmural and non-transmural inferoposterior myocardial ischemia were measured before and after interruption of left ventricular sympathetic afferents by stellectomy. The adequacy of sympathetic deafferentation was assessed by measurement of renal nerve responses to epicardial bradykinin. Prior to stellectomy, excitatory responses were observed in 10 animals and inhibitory responses in 9 animals. The remaining animals had no responses and were excluded from analysis. In the excitatory group, reflex increases in renal nerve activity during both transmural and non-transmural inferoposterior ischemia were abolished by stellectomy and not replaced by inhibitory responses. In the inhibitory group, non-transmural inferoposterior ischemia elicited greater reflex decreases in renal nerve activity when left ventricular sympathetic afferents were intact. After stellectomy, transmural ischemia elicited greater reflex inhibition of renal nerve activity. Renal nerve responses to epicardial bradykinin were abolished by stellectomy. These results indicate that reflex excitatory responses mediated by left ventricular receptors with sympathetic afferent fibers can be elicited in dogs with intact vagal afferents. These excitatory responses are most apparent during transmural myocardial ischemia. In dogs with inhibitory responses to coronary occlusion, activation of sympathetic afferents during transmural ischemia appears to attenuate reflex inhibitory responses mediated by left ventricular vagal afferents.

Activation of EP4 receptors contributes to prostaglandin E2-mediated stimulation of renal sensory nerves

Induction of cyclooxygenase-2 (COX-2) in the renal pelvic wall increases prostaglandin E(2) (PGE(2)) leading to stimulation of cAMP production, which results in substance P (SP) release and activation of renal mechanosensory nerves. The subtype of PGE receptors involved, EP2 and/or EP4, was studied by immunohistochemistry and renal pelvic administration of agonists and antagonists of EP2 and EP4 receptors. EP4 receptor-like immunoreactivity (LI) was colocalized with calcitonin gene-related peptide (CGRP)-LI in dorsal root ganglia (DRGs) at Th(9)-L(1) and in nerve terminals in the renal pelvic wall. Th(9)-L(1) DRG neurons also contained EP3 receptor-LI and COX-2-LI, each of which was colocalized with CGRP-LI in some neurons. No renal pelvic nerves contained EP3 receptor-LI and only very few nerves COX-2-LI. The EP1/EP2 receptor antagonist AH-6809 (20 microM) had no effect on SP release produced by PGE(2) (0.14 microM) from an isolated rat renal pelvic wall preparation. However, the EP4 receptor antagonist L-161,982 (10 microM) blocked the SP release produced by the EP2/EP4 receptor agonist butaprost (10 microM) 12 +/- 2 vs. 2 +/- 1 and PGE(2), 9 +/- 1 vs. 1 +/- 0 pg/min. The SP release by butaprost and PGE(2) was similarly blocked by the EP4 receptor antagonist AH-23848 (30 microM). In anesthetized rats, the afferent renal nerve activity (ARNA) responses to butaprost 700 +/- 100 and PGE(2).780 +/- 100%.s (area under the curve of ARNA vs. time) were unaffected by renal pelvic perfusion with AH-6809. However, 1 microM L-161,982 and 10 microM AH-23848 blocked the ARNA responses to butaprost by 94 +/- 5 and 78 +/- 10%, respectively, and to PGE(2) by 74 +/- 16 and 74 +/- 11%, respectively. L-161,982 also blocked the ARNA response to increasing renal pelvic pressure 10 mmHg, 85 +/- 5%. In conclusion, PGE(2) increases renal pelvic release of SP and ARNA by activating EP4 receptors on renal sensory nerve fibers.