Renal Sympathetic Nerve Activity Responses Research Articles

Mechanisms mediating renal sympathetic activation to leptin in obesity

Leptin plays a critical role in the control of energy homeostasis. The sympathetic cardiovascular actions of leptin have emerged as a potential link between obesity and hypertension. We previously demonstrated that in mice, modest obesity induced by 10 wk of a high-fat diet is associated with preservation of leptin ability to increase renal sympathetic nerve activity (SNA) despite the resistance to the metabolic effects of leptin. Here, we examined whether selective leptin resistance exists in mice with late-stage diet-induced obesity (DIO) produced by 20 wk of a high-fat diet. The decrease in food intake and body weight induced by intraperitoneal or intracerebroventricular injection of leptin was significantly attenuated in the DIO mice. Regional SNA responses to intravenous leptin were also attenuated in DIO mice. In contrast, intracerebroventricularly administered leptin caused contrasting effects on regional SNA in DIO mice. Renal SNA response to intracerebroventricular leptin was preserved, whereas lumbar and brown adipose tissue SNA responses were attenuated. Intact renal SNA response to leptin combined with the increased cerebrospinal fluid leptin levels in DIO mice represents a potential mechanism for the adverse cardiovascular consequences of obesity. Lastly, we examined the role of phosphoinositol-3 kinase (PI3K) and melanocortin receptors (MCR) in mediating the preserved renal SNA response to leptin in obesity. Presence of PI3K inhibitor (LY294002) or MC3/4R antagonist (SHU9119) significantly attenuated the renal SNA response to leptin in DIO and agouti obese mice. Our results demonstrate the importance of PI3K and melanocortin receptors in the transduction of leptin-induced renal sympathetic activation in obesity.

Exercise training improves peripheral chemoreflex function in heart failure rabbits

An enhancement of peripheral chemoreflex sensitivity contributes to sympathetic hyperactivity in chronic heart failure (CHF) rabbits. The enhanced chemoreflex function in CHF involves augmented carotid body (CB) chemoreceptor activity via upregulation of the angiotensin II (ANG II) type 1 (AT(1))-receptor pathway and downregulation of the neuronal nitric oxide synthase (nNOS)-nitric oxide (NO) pathway in the CB. Here we investigated whether exercise training (EXT) normalizes the enhanced peripheral chemoreflex function in CHF rabbits and possible mechanisms mediating this effect. EXT partially, but not fully, normalized the exaggerated baseline renal sympathetic nerve activity (RSNA) and the response of RSNA to hypoxia in CHF rabbits. EXT also decreased the baseline CB nerve single-fiber discharge (4.9 +/- 0.4 vs. 7.7 +/- 0.4 imp/s at Po(2) = 103 +/- 2.3 Torr) and the response to hypoxia (20.6 +/- 1.1 vs. 36.3 +/- 1.3 imp/s at Po(2) = 41 +/- 2.2 Torr) from CB chemoreceptors in CHF rabbits, which could be reversed by treatment of the CB with ANG II or a nNOS inhibitor. Our results also showed that NO concentration and protein expression of nNOS were increased in the CBs from EXT + CHF rabbits, compared with that in CHF rabbits. On the other hand, elevated ANG II concentration and AT(1)-receptor overexpression of the CBs in CHF state were blunted by EXT. These results indicate that EXT normalizes the CB chemoreflex in CHF by preventing an increase in afferent CB chemoreceptor activity. EXT reverses the alterations in the nNOS-NO and ANG II-AT(1)-receptor pathways in the CB responsible for chemoreceptor sensitization in CHF.

Femoral artery occlusion augments TRPV1-mediated sympathetic responsiveness

Muscle metabolic by-products stimulate thin fiber muscle afferent nerves and evoke reflex increases in blood pressure and sympathetic nerve activity. Previous studies reported that chemically sensitive transient receptor potential vanilloid type 1 (TRPV1) channels present on sensory muscle afferent neurons have an important impact on sympathetically mediated cardiovascular responses. The reflex-mediated reduction in blood flow to skeletal muscle leads to limited exercise capacity in patients with peripheral arterial occlusive disease. Thus, in this study, we tested the hypothesis that the expression of enhanced TRPV1 receptor and its responsiveness in primary afferent neurons innervating muscles initiate exaggerated reflex sympathetic responses after vascular insufficiency to the muscle. Muscle vascular insufficiency was induced by the femoral artery ligation in rats for 24 h. Our data show that 1) the ligation surgery leads to the upregulation of TRPV1 expression in the dorsal root ganglion; 2) the magnitude of the dorsal root ganglion neuron TRPV1 response induced by capsaicin is greater in vascular insufficiency (4.0 +/- 0.31 nA, P < 0.05 vs. sham-operated control) than that in sham-operated control (2.9 +/- 0.23 nA); and 3) renal sympathetic nerve activity and mean arterial pressure responses to capsaicin (0.5 microg/kg body wt) are also enhanced by vascular insufficiency (54 +/- 11%, 9 +/- 2 mmHg in sham-operated controls vs. 98 +/- 13%, 33 +/- 5 mmHg after vascular insufficiency, P < 0.05). In conclusion, sympathetic nerve responses to the activation of metabolite-sensitive TRPV1 receptors are augmented in rats with the femoral artery occlusion compared with sham-operated control animals, due to alterations in the expression of TRPV1 receptor and its responsiveness in sensory neurons.

Hypothalamic Corticotrophin‐Releasing Factor and Norepinephrine Mediate Sympathetic and Cardiovascular Responses to Acute Intracarotid Injection of Tumour Necrosis Factor‐α in the Rat

Systemic administration of tumour necrosis factor (TNF)-alpha induces the release of norepinephrine in the paraventricular nucleus (PVN) of hypothalamus and an increase in expression of corticotrophin-releasing factor (CRF) and CRF type 1 receptors. We explored the hypothesis that CRF and norepinephrine in PVN mediate the cardiovascular and sympathetic responses to acute systemic administration of TNF-alpha. In anaesthetised rats, the increases in arterial pressure and heart rate induced by intracarotid artery injection of TNF-alpha were attenuated by intracerebroventricular (ICV) injection of either the alpha 1-adrenergic antagonist prazosin or the CRF antagonist alpha-helical CRF. Prazosin blocked the TNF-alpha-induced increase in renal sympathetic nerve activity (RSNA), whereas alpha-helical CRF substantially reduced the RSNA response. Conversely, CRF and the alpha 1-adrenergic agonist phenylephrine, administered ICV, both elicited increases in PVN neuronal activity, RSNA, arterial pressure and heart rate. Microinjection of CRF and phenylephrine directly into PVN evoked smaller responses. These results are consistent with the hypothesis that norepinephrine and CRF in the PVN mediate the cardiovascular and sympathetic responses to acute systemic administration of TNF-alpha.

Central endothelin: effects on vasopressin and the arterial baroreflex in doxorubicin heart failure ratsThis article is one of a selection of papers published in the special issue (part 1 of 2) on Forefronts in Endothelin.

Endothelin 1 (ET-1) is increased in heart failure, both in plasma and within the central nervous system. Centrally, ET-1 induces sympathetic hyperactivity and arginine vasopressin (AVP) secretion. Both sympathetic activity and AVP secretion are regulated by the arterial baroreflex, which is typically impaired in heart failure. We hypothesized that central blockade of ETA receptors (ETAR) alters the baroreflex response of heart rate, renal sympathetic nerve activity (RSNA), and plasma AVP levels in a cardiomyopathic model of heart failure. Female Sprague-Dawley rats received weekly intraperitoneal injections of doxorubicin 2.5 mg x kg(-1) (doxorubicin heart failure, doxo-HF) or saline vehicle (control). After 8 weeks, they were instrumented, conditioned to the study environment, and then studied in the awake, non-restrained state. Baseline mean arterial pressure (MAP), RSNA, and plasma osmolality were similar in both groups, but heart rate (p<0.02), left ventricular pressure (p<0.001), and plasma AVP (p<0.01) were higher in the doxo-HF group. ET-1 dose dependently increased MAP, but the rise was significantly attenuated in doxo-HF rats at all doses. Baseline baroreflex control of heart rate and RSNA was similar in both groups. ETAR blockade with 4 nmol BQ123 i.c.v. significantly decreased both the upper plateau (p<0.05) and the range (p<0.05) of the baroreflex response of both heart rate and RSNA in doxo-HF but not in control rats. Despite higher basal plasma levels of AVP, ET-1 evoked a rise in plasma AVP of 13.6+/-3.2 pg x mL(-1) in doxo-HF compared with 0.4+/-0.4 pg x mL(-1) in control rats (p<0.001). To account for the blunted pressor response to ET-1 in the doxo-HF rats, gain of AVP release was calculated as DeltaAVP/DeltaMAP and was also found to be significantly greater in the doxo-HF rats (p<0.001). BQ123 prevented the rise in AVP and restored the gain in doxo-HF rats to that seen in controls. Thus, central ETAR contribute to the sympathoexcitation and AVP responses observed in heart failure due to doxorubicin cardiomyopathy.

Regulation of cardiac sympathetic afferent reflex by GABAA and GABAB receptors in paraventricular nucleus in rats

The aim of the present study was to determine the role of GABA(A) and GABA(B) receptors in paraventricular nucleus (PVN) in regulating cardiac sympathetic afferent reflex (CSAR). Under urethane (800 mg/kg) and alpha-chloralose (40 mg/kg) anesthesia, renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were recorded in sinoaortic-denervated and cervical-vagotomized rats. CSAR was evaluated based in the response of RSNA to epicardial application of capsaicin (0.3 nmol) or bradykinin (1 nmol). Bilateral PVN microinjection of the GABA(A) receptor agonist isoguvacine (10 nmol) attenuated CSAR, while the GABA(B) receptor agonist baclofen (1 nmol) abolished CSAR. Both isoguvacine and baclofen greatly decreased baseline RSNA and MAP. The GABA(A) receptor antagonist gabazine (0.1 nmol) had no significant effect on CSAR, but the GABA(B) receptor antagonist CGP-35348 (10 nmol) enhanced CSAR. Gabazine caused greater increases in baseline RSNA and MAP than CGP-35348. Vigabatrin (10 nmol), a selective GABA-transaminase inhibitor which increases endogenous GABA level, abolished CSAR, and decreased baseline RSNA, MAP and HR. The effects of vigabatrin were antagonized by combined gabazine (0.1 nmol) and CGP-35348 (10 nmol). The results indicate that activation of either GABA(A) or GABA(B) receptors in the PVN inhibits CSAR, while blockage of GABA(B) receptors in the PVN enhances CSAR. Endogenous GABA in the PVN could have an important role in regulating CSAR.

Impairment of vestibular-mediated cardiovascular response and motor coordination in rats born and reared under hypergravity

It is well known that environmental stimulation is important for the proper development of sensory function. The vestibular system senses gravitational acceleration and then alters cardiovascular and motor functions through reflex pathways. The development of vestibular-mediated cardiovascular and motor functions may depend on the gravitational environment present at birth and during subsequent growth. To examine this hypothesis, arterial pressure (AP) and renal sympathetic nerve activity (RSNA) were monitored during horizontal linear acceleration and performance in a motor coordination task in rats born and reared in 1-G or 2-G environments. Linear acceleration of +/-1 G increased AP and RSNA. These responses were attenuated in rats with a vestibular lesion, suggesting that the vestibular system mediated AP and RSNA responses. These responses were also attenuated in rats born in a 2-G environment. AP and RSNA responses were partially restored in these rats when the hypergravity load was removed, and the rats were maintained in a 1-G environment for 1 wk. The AP response to compressed air, which is mediated independently of the vestibular system, did not change in the 2-G environment. Motor coordination was also impaired in the 2-G environment and remained impaired even after 1 wk of unloading. These results indicate that hypergravity impaired both the vestibulo-cardiovascular reflex and motor coordination. The vestibulo-cardiovascular reflex was only impaired temporarily and partially recovered following 1 wk of unloading. In contrast, motor coordination did not return to normal in response to unloading.

20‐HETE increases renal sympathetic nerve activity via activation of chemically and mechanically sensitive muscle afferents

Arachidonic acid and its metabolites produced via cyclooxygenase (COX) and lipoxygenase pathways have been reported to contribute to the cardiovascular reflexes evoked by stimulating thin fibre muscle afferents during muscle contraction. 20-Hydroxyeicosatetraenoic acid (20-HETE), a primarily metabolized product of arachidonic acid by cytochrome P450 enzymes, can be accumulated in contracting muscles. Thus, the purpose of this study was to determine the role of 20-HETE in modulating the reflex sympathetic responses to activation of chemically and mechanically sensitive muscle afferents. The renal sympathetic nerve activity (RSNA) and cardiovascular responses were examined after injections of 20-HETE into the arterial blood supply of the hindlimb muscles of decerebrated rats. This induced a dose-dependent increases in RSNA and mean arterial pressure (MAP). We also tested the hypothesis that 20-HETE would sensitize muscle afferents and, thereby, augment the RSNA and blood pressure response to muscle stretch. The results show that arterial infusion of 20-HETE significantly enhanced the RSNA and MAP responses to muscle stretch. In contrast, N-hydroxy-N'-(4-butyl-2-methylphenyl)formamidine, a potent inhibitor of 20-HETE production, attenuated the reflex muscle responses. Furthermore, the sensitizing effect of 20-HETE on the muscle reflex was significantly attenuated after blocking COX activity with indomethacin. Our data suggest that 20-HETE plays a role in modulating muscle afferent-mediated sympathetic responses, probably through engagement of a COX-dependent mechanism.

Melanocortin receptors mediate renal sympathetic activation to leptin in obesity

Leptin acts in the brain to decrease appetite and body weight and increase sympathetic nerve activity (SNA). Obese mice are resistant to the metabolic actions of leptin but have intact renal SNA responses to leptin. The mechanisms that mediate renal SNA response to leptin in obesity remain unknown. We have previously shown the importance of the melanocortin receptors (MCR) in mediating renal SNA response to leptin. Here, we examined the role of MCR in mediating renal sympathoactivation to leptin in obesity by testing the effect of MC3/4R blockade with SHU9119 (30 pM) on the renal SNA response to intracerebroventricular (ICV) leptin in diet‐induced obese (DIO) and agouti obese mice. DIO in C57BL/6J mice was obtained by exposure to 45% high fat diet for 20 weeks. The rise in renal SNA induced by ICV leptin (5 μg) was of the same magnitude in lean (129±10%, n=15), DIO (117±15%, n=8) and agouti obese (115±16%, n=6) mice. ICV pre‐treatment with SHU9119 inhibited (P&lt;0.05) the increase in renal SNA induced by ICV leptin in lean (23±8%, n=10), DIO (9±11%, n=10) and agouti obese (51±14%, n=5) mice. In lean mice, ICV MTII (agonist of MC3/4R, 2 μg) increased renal SNA by 140±41%, n=4) and only 18±9% in presence of SHU9119 (n=5, P=0.003) demonstrating that blockade of renal SNA response by SHU9119 is due to inhibition of MC3/4R. Our data demonstrate the importance of the MCR as key mediators of renal sympathetic activation to leptin in obesity.

High dietary sodium intake sensitizes the pressor and renal sympathetic nerve activity responses to peripheral and spinal vasopressin infusion

Vasopressinergic (AVP) activation of spinal and vascular V1 receptors may contribute to the sensitized blood pressure (BP) and renal sympathoexcitatory (RSNA) responses to ANGII and ANGIII ICV infusion after high Na+ (HNa+) intake. This hypothesis was tested by comparing the BP and RSNA responses to intravenous (IV) or intrathecal (IT) AVP infusion in male Wistar rats fed a normal Na+ (NNa+, 0.3%) or HNa+ (3.0%) diet for six weeks. At 11 weeks of age, rats were anaesthetized (1ml IP chloralose/urethane) and instrumented with arterial and IV cannulae. The tip of the IT cannula was placed at spinal vertebra T13. RSNA was recorded from the left kidney. ΔBP (mmHg) or ΔRSNA (%baseline) were measured after IV AVP (1, 3, 5 or 10ng/0.2ml; NNa+ n=6, HNa+=7) or IT AVP (1, 5, 10, 20μM; 10μL/2min; NNa+ =7, HNa+ = 5), respectively. Data are presented as mean ± SE. P&lt;0.05 indicated significance. Basal BP (mmHg, NNa+ 89±4, HNa+ 95±4) and RSNA (μV/s, NNa+ 15.1±4.6 vs HNa+10.4±3.0) were similar between groups. ΔBP was greater to all doses of IV AVP in HNa+ (P&lt;0.001). ΔRSNA was greater in HNa+ after 1μM (NNa+:1.0±2.0% vs HNa+: 8.2±2.0%, P&lt;0.05) and 5μM (NNa+: 3.2± 1.2% vs HNa+: 10.2±1.9%, P&lt;0.05) IT AVP. These results show that HNa+ intake sensitizes both AVP induced peripheral vasoconstriction and spinal V1 mediated RSNA, which together may contribute to the development of hypertension. Funding: HRB RP/2004/17.

Gene transfer of Cu‐Zn superoxide dismutase to the carotid body reverses enhanced chemoreceptor function in heart failure rabbits

Peripheral chemoreflex sensitivity is potentiated in both clinical and experimental chronic heart failure (CHF). NADPH oxidase‐derived superoxide anion mediates angiotensin II‐enhanced carotid body (CB) chemoreceptor sensitivity in CHF rabbits, and tempol, the superoxide dismutase (SOD) mimetic, inhibits this Ang II‐ and CHF‐enhanced superoxide anion effect (Cardiovasc Res. 2007, 75:546–54). Here we investigated the effect of adenoviral Cu‐Zn SOD gene transfer in the CB on chemoreceptor activity and function in CHF rabbits. Ad Cu‐Zn SOD (1×108 pfu/ml) gene transfer to the CBs significantly reduced the baseline renal sympathetic nerve activity (RSNA) and the response of RSNA to hypoxia in the CHF rabbits. Baseline single‐fiber discharge from CB chemoreceptors during normoxia and the response to hypoxia were enhanced in CHF vs. sham rabbits. Ad Cu‐Zn SOD decreased the baseline discharge (2.9 ± 1.1 vs. 8.1 ± 0.8 imp/s at 104 ± 2.4 mm Hg PO2) and the response to hypoxia (16.4 ± 2.3 vs. 38.7 ± 2.3 imp/s at 41 ± 2.6 mm Hg PO2, p&lt;0.05) in CHF rabbits. In addition, Ad Cu‐Zn SOD normalized the blunted IK in CB glomus cells from CHF rabbits (1020 ± 142 vs. 2156 ± 230 pA at +70 mV, p&lt;0.05). Ad SOD did not affect CB afferent discharge and IK of CB glomus cells in sham rabbits. These results suggest that Cu‐Zn SOD activity in the CB may be involved in the enhanced activity of the CB chemoreceptors and peripheral chemoreflex function in CHF rabbits.

Cyclooxygenase products contribute to enhanced muscle reflex in heart failure

In heart failure (HF), reflex sympathetic nerve responses to muscle contraction are exaggerated. The present experiment was designed to test the role played by cyclooxygenase products, which are produced within contracting skeletal muscle, in the enhanced muscle reflex of HF. HF was induced by myocardial infarction (MI) after the coronary artery ligation in rats. Echocardiography was performed to determine fractional shortening (FS), an index of the left ventricular function. In decerebrate rats, we examined renal sympathetic nerve activity (RSNA) during 1‐min intermittent (1‐ to 4‐s stimulation‐to‐relaxation) contraction of left triceps surae muscles. RSNA responded synchronously as tension was developed, and the response was significantly (P&lt;0.05) enhanced in MI rats [+28±2 arbitrary unit (a.u.); n=12] with FS &lt;30% compared to control animals (+16±3 a.u.; n=13) with FS &gt; 40% at the same tension development. After intravenous administration of the cyclooxygenase inhibitor indomethacin (5 mg kg−1), the enhanced RSNA response to contraction was significantly reduced in the MI rats by 44% (+15±5 a.u.), but no significant change was found in the control animals (+12±2 a.u.). These data suggest that cyclooxygenase products within active muscle, such as prostaglandins, play a role in the enhanced muscle reflex in HF. Supported by NIH R01 R01 HL075533 &amp; R01 HL078866 (Li) and R01 HL060800 (Sinoway).

Exercise training normalizes enhanced peripheral chemoreflex function in chronic heart failure rabbits

Our previous studies have shown that the enhanced sensitivity of the peripheral chemoreflex contributes to the sympathetic hyperactivity in chronic heart failure (CHF) rabbits and an augmented carotid body (CB) chemoreceptor activity is involved in the enhancement of peripheral chemoreflex function. Furthermore, we have also found that upregulation of angiotensin (Ang) II‐AT1 receptor pathway and downregulation of NOS‐NO pathway in the CB contribute to the enhanced CB chemoreceptor sensitivity in the CHF state. Here we investigated whether exercise training (EXT) normalizes the enhanced peripheral chemoreflex function in CHF rabbits and possible mechanism mediating this effect. EXT partially but not fully normalized the exaggerated baseline renal sympathetic nerve activity (RSNA) and the response of RSNA to hypoxia in CHF rabbits. EXT also decreased the baseline carotid body nerve single‐fiber discharge (4.9 ± 0.4 vs 7.7 ± 0.4 imp/s at PO2 =103 ± 2.3 mm Hg) and the response to hypoxia (20.6 ± 1.1 vs 36.3 ± 1.3 imp/s at PO2 =41 ± 2.2 mm Hg) from CB chemoreceptors in CHF rabbits. Western and immunofluorescence data showed that protein expression of nNOS was increased in the CBs from EXT + CHF rabbits, compared with that in CHF rabbits. On the other hand, the AT1 receptor overexpression of the CB in CHF state was also blunted by EXT. These results indicate that EXT improves the enhanced chemoreflex in CHF, which is due to EXT‐induced upregulation of nNOS‐NO pathway and downexpression of Ang II‐AT1 receptor pathway in the CBs.

Organum vasculosum laminae terminalis contributes to increased sympathetic nerve activity induced by central hyperosmolality

The contribution of the organum vasculosum laminae terminalis (OVLT) in mediating central hyperosmolality-induced increases of sympathetic nerve activity (SNA) and arterial blood pressure (ABP) was assessed in anesthetized rats. Solutions of graded NaCl concentration (150, 375, and 750 mM) were injected (150 mul) into the forebrain vascular supply via an internal carotid artery (ICA). Time-control experiments (n = 6) established that ICA NaCl injections produced short-latency, transient increases of renal SNA (RSNA) and mean ABP (MAP) (P < 0.05-0.001). Responses were graded, highly reproducible, and unaltered by systemic blockade of vasopressin V1 receptors (n = 4). In subsequent studies, stimulus-triggered averaging of RSNA was used to accurately locate the OVLT. Involvement of OVLT in responses to ICA NaCl was assessed by recording RSNA and MAP responses before and 15 min after electrolytic lesion of the OVLT (n = 6). Before lesion, NaCl injections increased RSNA and MAP (P < 0.05-0.001), similar to time control experiments. After lesion, RSNA responses were significantly reduced (P < 0.05-0.001), but MAP responses were unaltered. To exclude a role for fibers of passage, the inhibitory GABA-A receptor agonist muscimol was microinjected into the OVLT (50 pmol in 50 nl) (n = 6). Before muscimol, hypertonic NaCl increased RSNA, lumbar SNA (LSNA), and MAP (P < 0.05-0.001). After muscimol, both RSNA and LSNA were significantly reduced in response to 375 and 750 mM NaCl (P < 0.05). MAP responses were again unaffected. Injections of vehicle (saline) into OVLT (n = 6) and muscimol lateral to OVLT (n = 5) each failed to alter responses to ICA NaCl. We conclude that OVLT neurons contribute to sympathoexcitation by central hyperosmolality.

Acute angiotensin-converting enzyme inhibition evokes bradykinin-induced sympathetic activation in diabetic rats

We have previously shown that acute intravenous injection of the angiotensin-converting enzyme (ACE) inhibitor enalapril in diabetic rats evokes a baroreflex-independent sympathoexcitatory effect that does not occur with angiotensin receptor blockade alone. As ACE inhibition also blocks bradykinin degradation, we sought to determine whether bradykinin mediated this effect. Experiments were performed in conscious male Sprague-Dawley rats, chronically instrumented to measure mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), 2 wk after streptozotocin (55 mg/kg iv, diabetic, n = 11) or citrate vehicle (normal, n = 10). Enalapril (2.5 mg/kg iv) decreased MAP in normal rats (-15 +/- 3 mmHg), while a smaller response (-4 +/- 1 mmHg) occurred in diabetic rats. Despite these different depressor responses to enalapril, HR (+44 +/- 8 vs. +26 +/- 7 bpm) and RSNA (+90 +/- 21 vs +71 +/- 8% baseline) increased similarly between the groups (P > or = 0.22 for both). Pretreatment with the bradykinin B2 receptor antagonist Hoe 140 (10 microg/kg bolus followed by 0.8.mug(-1)kg.min(-1) infusion) attenuated the decrease in MAP observed with enalapril in normal rats but had no effect in diabetic rats. Moreover, the normal group had smaller HR and RSNA responses (HR: +13 +/- 8 bpm; RSNA: +32 +/- 13% baseline) that were abolished in the diabetic group (HR: -4 +/- 5 bpm; RSNA: -5 +/- 9% baseline; P < 0.05 vs. preenalapril values). Additionally, bradykinin (20 microg/kg iv) evoked a larger, more prolonged sympathoexcitatory effect in diabetic compared with normal rats that was further potentiated after treatment with enalapril. We conclude that enhanced bradykinin signaling mediates the baroreflex-independent sympathoexcitatory effect of enalapril in diabetic rats.

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.

Gene transfer of neuronal nitric oxide synthase to the paraventricular nucleus improves enhanced NMDA NR1 receptor function in rats with chronic heart failure

Our previous studies have shown that the decreased nitric oxide (NO) and increased glutamatergic mechanisms’ effects on sympathetic regulation within the paraventricular nucleus (PVN) may contribute to the elevated sympathoexcitation during chronic heart failure (CHF). In the present study, we investigated the effects of neuronal NO synthase (nNOS) gene transfer on NMDA NR1 receptor function in the rats with coronary ligation model of CHF. Adenovirus vectors encoding either nNOS (AdnNOS) or β-galactosidase (AdβGal, as control) were transfected into the PVN in vivo. Three days after application of AdnNOS, the nNOS mRNA expression and protein level were significantly increased within the PVN in rats with CHF (compared to AdβGal-CHF group, P<0.05). The renal sympathetic nerve activity (RSNA), blood pressure (BP) and heart rate (HR) responses to microinjection of NMDA into the PVN was significantly potentiated (RSNA: 42±2%, P<0.05) in the rats with CHF compared with sham rats. AdnNOS significantly decreased the enhanced RSNA, BP and HR responses to NMDA in the rats with CHF (RSNA: 35±4%, compared to AdβGal-CHF group, P<0.05). These results indicate that the effects of endogenous nNOS on the glutamatergic mechanism within the PVN may play an important role in the altered balance and tone of sympathetic outflow in the CHF condition.

Role of PI3 kinase in mediating renal sympathoactivation to leptin in obesity

Leptin acts in the central nervous system to decrease appetite and body weight and increase sympathetic nerve activity (SNA). Diet-induced obesity in mice is associated with resistance to the metabolic actions of leptin but preservation of its renal SNA and arterial pressure effects, leading to hypertension. The molecular basis of this selectivity in leptin resistance remains unknown. We have previously shown that leptin activation of renal SNA is phosphoinositol-3 kinase (PI3K)-dependent. Here, we tested the hypothesis that in obesity, renal sympathoactivation to leptin is mediated by PI3K. We compared the effect of intracerebroventricular (ICV) leptin on renal SNA in presence or absence of PI3K inhibitor, LY294002. Multifiber recording of SNA was performed under anesthesia (α-chloralose). Obesity was induced in C57BL/6J mice by exposure to a high fat diet (45%) for 20 weeks. After 20 weeks, mice on high-fat diet weighed ~20% more than those fed the normal diet. Weight of fat pads and plasma leptin were also significantly higher in the obese mice. The rise in renal SNA induced by ICV administration of leptin (5 μg) was of the same magnitude in both lean and obese mice. As expected, in lean mice ICV pre-treatment with LY294001 (0.1 μg) inhibited the increase in renal SNA induced by ICV leptin (Figure). Pre-treatment with LY294002 also blocked the increase in renal SNA induced by leptin in the obese mice (Figure). Our data confirm the pivotal role of PI3K in mediating the effects of leptin on renal SNA. Our findings also demonstrate the importance of PI3K as a key link between leptin receptor and renal SNA responses in obesity and provide a molecular substrate for selectivity in leptin resistance. Effect of ICV leptin (5 μg) on renal SNA in lean and obese mice in presence or absence of PI3K inhibitor, LY294002 (0.1 μg). ∗ P<0.05 vs. vehicle-leptin group, n=7–8 mice/group.

Azelnidipine Attenuates Cardiovascular and Sympathetic Responses to Air-Jet Stress in Genetically Hypertensive Rats

Azelnidipine is a new dihydropyridine calcium channel blocker that causes minimal stimulation of the sympathetic nervous system despite its significant depressor effect. In the present study, we examined the effects of oral or intravenous administration of azelnidipine on cardiovascular and renal sympathetic nerve activity (RSNA) responses to air-jet stress in conscious, unrestrained stroke-prone spontaneously hypertensive rats. Oral administration of high-dose azelnidipine (10 mg/kg per day) or nicardipine (150 mg/kg per day) for 10 days caused a significant and comparable decrease in blood pressure, but low-dose azelnidipine (3 mg/kg per day) did not. Air-jet stress increased mean arterial pressure (MAP), heart rate (HR) and RSNA. High-dose azelnidipine significantly attenuated the increases in MAP, HR and RSNA in response to air-jet stress while nicardipine did not. Low-dose azelnidipine significantly attenuated the pressor response with a trend of decrease in RSNA. Intravenous injection of azelnidipine induced a slowly developing depressor effect. To obtain a similar time course of decrease in MAP by azelnidipine, nicardipine was continuously infused at adjusted doses. Both drugs increased HR and RSNA significantly, while the change in RSNA was smaller in the azelnidipine group. In addition, intravenous administration of azelnidipine attenuated the responses of MAP, HR, and RSNA to air-jet stress; by comparison, the inhibitory actions of nicardipine were weak. In conclusion, oral or intravenous administration of azelnidipine inhibited cardiovascular and sympathetic responses to air-jet stress. This action of azelnidipine may be mediated at least in part by the inhibition of the sympathetic nervous system.

Cardiac sympathetic afferent stimulation augments the arterial chemoreceptor reflex in anesthetized rats

Chronic heart failure (CHF) is well known to be associated with both an enhanced chemoreceptor reflex and an augmented cardiac "sympathetic afferent reflex" (CSAR). The augmentation of the CSAR may play an important role in the enhanced chemoreceptor reflex in the CHF state because the same central areas are involved in the sympathetic outputs of both reflexes. We determined whether chemical and electrical stimulation of the CSAR augments chemoreceptor reflex function in normal rats. Under anesthesia, renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were recorded. The chemoreceptor reflex was tested by unilateral intra-carotid artery bolus injection of potassium cyanide (KCN) and nicotine. We found that 1) left ventricular epicardial application of capsaicin increased the pressor responses and the RSNA responses to chemoreflex activation induced by both KCN and nicotine; 2) when the central end of the left cardiac sympathetic nerve was electrically stimulated, both the pressor and the RSNA responses to chemoreflex activation induced by KCN were increased; 3) pretreatment with intracerebroventricular injection of losartan (500 nmol) completely prevented the enhanced chemoreceptor reflex induced by electrical stimulation of the cardiac sympathetic nerve; and 4) bilateral microinjection of losartan (250 pmol) into the nucleus tractus solitarii (NTS) completely abolished the enhanced chemoreceptor reflex by epicardial application of capsaicin. These results suggest that both the chemical and electrical stimulation of the CSAR augments chemoreceptor reflex and that central ANG II, specially located in the NTS, plays a major role in these reflex interactions.