Differential role of the paraventricular nucleus of the hypothalamus in modulating the sympathoexcitatory component of peripheral and central chemoreflexes

In the present study we investigated the involvement of the hypothalamic paraventricular nucleus (PVN) in the modulation of sympathoexcitatory reflex activated by peripheral and central chemoreceptors. We measured mean arterial blood pressure (MAP), heart rate (HR), renal sympathetic nerve activity (RSNA), and phrenic nerve activity (PNA) before and after blocking neurotransmission within the PVN by bilateral microinjection of 2% lidocaine (100 nl) during specific stimulation of peripheral chemoreceptors by potassium cyanide (KCN, 75 microg/kg iv, bolus dose) or stimulation of central chemoreceptors with hypercapnia (10% CO(2)). Typically stimulation of peripheral chemoreceptors evoked a reflex response characterized by an increase in MAP, RSNA, and PNA and a decrease in HR. Bilateral microinjection of 2% lidocaine into the PVN had no effect on basal sympathetic and cardiorespiratory variables; however, the RSNA and PNA responses evoked by peripheral chemoreceptor stimulation were attenuated (P < 0.05). Bilateral microinjection of bicuculline (50 pmol/50 nl, n = 5) into the PVN augmented the RSNA and PNA response to peripheral chemoreceptor stimulation (P < 0.05). Conversely, the GABA agonist muscimol (0.2 nmol/50 nl, n = 5) injected into the PVN attenuated these reflex responses (P < 0.05). Blocking neurotransmission within the PVN had no effect on the hypercapnia-induced central chemoreflex responses in carotid body denervated animals. These results suggest a selective role of the PVN in processing the sympathoexcitatory and ventilatory component of the peripheral, but not central, chemoreflex.     

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.     

Enhanced angiotensin II-mediated central sympathoexcitation in streptozotocin-induced diabetes: role of superoxide anion

Studies have shown that the superoxide mechanism is involved in angiotensin II (ANG II) signaling in the central nervous system. We hypothesized that ANG II activates sympathetic outflow by stimulation of superoxide anion in the paraventricular nucleus (PVN) of streptozotocin (STZ)-induced diabetic rats. In α-chloralose- and urethane-anesthetized rats, microinjection of ANG II into the PVN (50, 100, and 200 pmol) produced dose-dependent increases in renal sympathetic nerve activity (RSNA), arterial pressure (AP), and heart rate (HR) in control and STZ-induced diabetic rats. There was a potentiation of the increase in RSNA (35.0 ± 5.0 vs. 23.0 ± 4.3%, P < 0.05), AP, and HR due to ANG II type I (AT(1)) receptor activation in diabetic rats compared with control rats. Blocking endogenous AT(1) receptors within the PVN with AT(1) receptor antagonist losartan produced significantly greater decreases in RSNA, AP, and HR in diabetic rats compared with control rats. Concomitantly, there were significant increases in mRNA and protein expression of AT(1) receptor with increased superoxide levels and expression of NAD(P)H oxidase subunits p22(phox), p47(phox), and p67(phox) in the PVN of rats with diabetes. Pretreatment with losartan (10 mg·kg(-1)·day(-1) in drinking water for 3 wk) significantly reduced protein expression of NAD(P)H oxidase subunits (p22(phox) and p47(phox)) in the PVN of diabetic rats. Pretreatment with adenoviral vector-mediated overexpression of human cytoplasmic superoxide dismutase (AdCuZnSOD) within the PVN attenuated the increased central responses to ANG II in diabetes (RSNA: 20.4 ± 0.7 vs. 27.7 ± 2.1%, n = 6, P < 0.05). These data support the concept that superoxide anion contributes to an enhanced ANG II-mediated signaling in the PVN involved with the exaggerated sympathoexcitation in diabetes.     

Responses to GABA-A receptor blockade in the hypothalamic PVN are attenuated by local AT1 receptor antagonism

Blockade of GABA-A receptors in the hypothalamic paraventricular nucleus (PVN) has been repeatedly shown to increase arterial blood pressure (ABP), heart rate (HR), and sympathetic nerve activity (SNA), but the mechanism(s) that underlies this response has not been determined. Here, we tested whether full expression of the response requires activation of local ANG II AT1 receptors. ABP, HR, and renal SNA responses to PVN microinjection of bicuculline methobromide (BIC; 0.1 nmol) were recorded before and after microinjection of vehicle (saline); losartan (or L-158809), to block local AT1 receptors; or PD123319, to block AT2 receptors. After PVN microinjection of vehicle or PD123319 (10 nmol), BIC significantly (P < 0.05) increased mean arterial pressure (MAP), HR, and renal SNA. However, PVN microinjection of 2 and 20 nmol of losartan dose dependently reduced responses to PVN-injected BIC, with the 20-nmol dose nearly abolishing MAP (P < 0.005), HR (P < 0.05), and renal SNA (P < 0.005) responses. Another AT1 receptor antagonist, L-158809 (10 nmol), produced similar effects. Neither losartan nor L-158809 altered baseline parameters. Responses to PVN injection of BIC were unchanged by losartan (20 nmol) given intravenously or into the PVN on the opposite side. MAP, HR, and renal SNA responses to PVN microinjection of l-glutamate (10 nmol) were unaffected by PVN injection of losartan (20 nmol), indicating that effects of losartan were not due to nonspecific depression of neuronal excitability. We conclude that pressor, tachycardic, and renal sympathoexcitatory responses to acute blockade of GABA-A receptors in the PVN depend on activation of local AT1 receptors.     

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.     

Blunted nitric oxide-mediated inhibition of sympathetic nerve activity within the paraventricular nucleus in diabetic rats

Recent evidence suggests that a central mechanism may be contributing to the sympathetic abnormality in diabetes. Nitric oxide (NO) has been known as a neurotransmitter in the central nervous system. The goal of this study was to examine the role of the endogenous NO system of the paraventricular nucleus (PVN) in regulation of renal sympathetic nerve activity (RSNA) in streptozotocin (STZ)-induced diabetic rats. The change in number of NADPH-diaphorase-positive neurons [a marker for neuronal NO synthase (nNOS) activity] in the PVN was measured. Diabetic rats were found to have significantly fewer nNOS positive cells in the PVN than in the control group (120 +/- 11 vs. 149 +/- 13, P < 0.05). Using RT PCR, Western blotting and immunofluorescent staining, it was also found that nNOS mRNA expression and protein level in the PVN were significantly decreased in the diabetic rats. Furthermore, using an in vivo microdialysis technique, we found that there was a lower NO(x) release from the PVN perfusates in rats with diabetes compared with the control rats (142 +/- 33 nM vs. 228 +/- 29 nM, P < 0.05). In alpha-chloralose- and urethane-anesthetized rats, an inhibitor of NO synthase, l-NMMA, microinjected into the PVN produced a dose-dependent increase in RSNA, mean arterial pressure (MAP), and heart rate (HR) in both control and diabetic rats. These responses were significantly attenuated in rats with diabetes compared with control rats (RSNA: 11 +/- 3% vs. 35 +/- 3%, P < 0.05). On the other hand, an NO donor, sodium nitroprusside (SNP), microinjected into the PVN produced a dose-dependent decrease in RSNA, MAP, and HR in the control and diabetic rats. RSNA (17 +/- 3%, vs. 41 +/- 6%, P < 0.05) and MAP in response to SNP were significantly blunted in the diabetic group compared with the control group. In conclusion, these data indicate an altered NO mechanism in the PVN of diabetic rats. This altered mechanism may contribute to the increased renal sympathetic neural activity observed in diabetes.     

Acute inhibition of the hypothalamic paraventricular nucleus decreases renal sympathetic nerve activity and arterial blood pressure in water-deprived rats

The present study was performed to determine whether sympathetic outflow and arterial blood pressure in water-deprived rats are dependent on the ongoing neuronal activity of the hypothalamic paraventricular nucleus (PVN). Renal sympathetic nerve activity (RSNA), mean arterial blood pressure (MAP), and heart rate were recorded in urethane-alpha-chloralose-anesthetized rats that were deprived of water but not food for 48 h before experiments. Acute inhibition of the PVN by bilateral microinjection of the GABA(A) agonist muscimol (100 pmol/side) significantly decreased RSNA in water-deprived rats (-26.7 +/- 4.7%, n = 7) but was without effect in control rats (1.3 +/- 6.3%, n = 7). Similarly, injection of muscimol produced a greater decrease in MAP in water-deprived rats than in control rats (-46 +/- 3 vs. -16 +/- 3 mmHg, respectively), although baseline MAP was not different between groups (105 +/- 4 vs. 107 +/- 4 mmHg, respectively). Neither bilateral microinjection of isotonic saline vehicle (100 nl/side) into the PVN nor muscimol (100 pmol/side) outside the PVN altered RSNA or MAP in either group. In addition, ganglionic blockade with hexamethonium (30 mg/kg i.v.) significantly decreased MAP in both groups; however, the decrease in MAP was significantly greater in water-deprived rats than in control rats (62 +/- 2 vs. 48 +/- 2 mmHg, respectively). Collectively, these findings suggest that sympathetic outflow contributes more to the maintenance of blood pressure in the water-deprived rat, and this depends, at least partly, on the ongoing activity of PVN neurons.     

Hydrogen Sulfide in Paraventricular Nucleus Enhances Sympathetic Activity and Cardiac Sympathetic Afferent Reflex in Chronic Heart Failure Rats

Background

Intracerebroventricular infusion of NaHS, a hydrogen sulfide (H₂S) donor, increased mean arterial pressure (MAP). This study was designed to determine the roles of H₂S in the paraventricular nucleus (PVN) in modulating sympathetic activity and cardiac sympathetic afferent reflex (CSAR) in chronic heart failure (CHF).

Methodology/Principal Findings

CHF was induced by left descending coronary artery ligation in rats. Renal sympathetic nerve activity (RSNA) and MAP were recorded under anesthesia. CSAR was evaluated by the RSNA and MAP responses to epicardial application of capsaicin. PVN microinjection of low doses of a H₂S donor, GYY4137 (0.01 and 0.1 nmol), had no significant effects on RSNA, MAP and CSAR. High doses of GYY4137 (1, 2 and 4 nmol) increased baseline RSNA, MAP and heart rate (HR), and enhanced CSAR. The effects were greater in CHF rats than sham-operated rats. A cystathionine-β-synthase (CBS) inhibitor, hydroxylamine (HA) in PVN had no significant effect on the RSNA, MAP and CSAR. CBS activity and H₂S level in the PVN were decreased in CHF rats. No significant difference in CBS level in PVN was found between sham-operated rats and CHF rats. Stimulation of cardiac sympathetic afferents with capsaicin decreased CBS activity and H₂S level in the PVN in both sham-operated rats and CHF rats.

Conclusions

Exogenous H₂S in PVN increases RSNA, MAP and HR, and enhances CSAR. The effects are greater in CHF rats than those in sham-operated rats. Endogenous H₂S in PVN is not responsible for the sympathetic activation and enhanced CSAR in CHF rats.     

Inhibition of sympathetic responses at birth in sheep by lesion of the paraventricular nucleus

Birth is characterized by a surge in sympathetic outflow, heart rate (HR), mean arterial blood pressure (MABP) and circulating catecholamines. The paraventricular nucleus (PVN) of the hypothalamus is an important central regulatory site of sympathetic activity, but its role in the regulation of sympathoexcitation at birth is unknown. To test the hypothesis that the PVN regulates sympathetic activity at birth, experiments were performed in chronically instrumented near-term (137- to 142-day gestation, term 145 days) sheep before and after delivery by cesarean section. Stereotaxic guided electrolytic lesioning of the PVN (n = 6) or sham lesioning (n = 6) was performed 48 h before study. At 30 min after birth, renal sympathetic nerve activity (RSNA) increased 128 +/- 26% above fetal values in the sham-lesioned animals (P < 0.05). In contrast, at a similar time point, RSNA decreased to 52 +/- 12% of the fetal value in the PVN-lesioned animals. Lesioning of the PVN did not affect the usual postnatal increases in MABP and epinephrine levels although HR failed to rise above fetal values. ANG II but not arginine vasopressin or norepinephrine levels increased in PVN-lesioned animals after birth, whereas all three hormones increased (P < 0.05) in sham-lesioned animals. Fetal and newborn HR baroreflex responses were similar in both groups. However, the usual postnatal attenuation of baroreflex-mediated inhibition of RSNA was blunted in the PVN-lesioned group. The results of this study demonstrate that ablation of the PVN abolishes sympathoexcitation with birth at near-term gestation. The PVN may play a critical role in physiological adaptation at birth.     

Effects of fetal behavioral states on renal sympathetic nerve activity and arterial pressure of unanesthetized fetal sheep

Fetal behavior, renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) were studied 1-3 days after surgery in seven fetal sheep (aged 127-136 days). Five behavioral states were defined from chart recordings of electrocortical (electrocorticographic; ECoG) activity and eye, limb, and breathing movements. Most records were of high-voltage ECoG (HV) or low-voltage (LV) ECoG with breathing (LVB); 6.7 +/- 1.7% were LV ECoG with no breathing (LV0). RSNA was lower in LV0 (P < 0.001) and greater in LVB than in HV (P < 0.05). MAP was lower in both LV states than in HV and when the fetuses went from LV to HV (P < 0.001 to P < 0.03). HR was highest in HV (P < 0.001). In HV and LVB and when the fetus went from LV to HV, MAP and HR were inversely related (P = 0.012-0.003). In LVB and from LV to HV there were direct relationships between MAP and RSNA (P = 0.0014, P = 0.08), and when the fetus went from LV to HV there was also an inverse relationship between HR and RSNA (P = 0.02). Thus fetal RSNA, MAP, and HR are affected by behavioral state as is fetal cardiovascular control. The increase in RSNA during fetal breathing showed that there was an altered level of fetal RSNA associated with fetal breathing activity.     

最后区注射腺苷对大鼠血压、心率和肾交感神经放电的影响

在 5 3只麻醉Sprague Dawley大鼠观察了最后区内微量注射腺苷 (1ng/ 6 0nl)对平均动脉压 (MAP)、心率(HR)和肾交感神经放电 (RSNA)的影响。实验结果如下 :(1)最后区内微量注射Ado后 ,MAP、HR和RSNA分别由13 76± 0 46kPa、35 6 2 8± 4 2 5bpm和 10 0± 0 %下降至 11 2 3± 0 49kPa (P <0 0 0 1)、336 91± 5 2 3bpm (P <0 0 1)和70 95± 5 19% (P <0 0 0 1) ;(2 )静脉注射非选择性腺苷受体拮抗剂 8 苯茶碱 (8 phenyltheophylline,15 0 μg/kg ,0 2ml)和选择性腺苷A1受体拮抗剂 (8 cyclopentyl 1,3 dipropylxanthine,5 0 0 μg /kg ,0 2ml)后 ,腺苷的上述抑制效应可被完全阻断 ;(3)静脉注射ATP敏感性钾通道阻断剂格列苯脲 (5mg/kg ,0 2ml)后 ,腺苷的上述效应也被消除。以上结果提示 ,最后区微量注射腺苷对血压、心率和肾交感神经放电有抑制作用 ,此作用与A1受体介导的ATP敏感性钾通道开放有关。     

Small-conductance, Ca(2+) -activated K+ channel 2 is the key functional component of SK channels in mouse urinary bladder

Small-conductance Ca(2+)-activated K(+) (SK) channels play an important role in regulating the frequency and in shaping urinary bladder smooth muscle (UBSM) action potentials, thereby modulating contractility. Here we investigated a role for the SK2 member of the SK family (SK1-3) utilizing: 1) mice expressing beta-galactosidase (beta-gal) under the direction of the SK2 promoter (SK2 beta-gal mice) to localize SK2 expression and 2) mice lacking SK2 gene expression (SK2(-/-) mice) to assess SK2 function. In SK2 beta-gal mice, UBSM staining was observed, but staining was undetected in the urothelium. Consistent with this, urothelial SK2 mRNA was determined to be 4% of that in UBSM. Spontaneous phasic contractions in wild-type (SK2(+/+)) UBSM strips were potentiated (259% of control) by the selective SK channel blocker apamin (EC(50) = 0.16 nM), whereas phasic contractions of SK2(-/-) strips were unaffected. Nerve-mediated contractions of SK2(+/+) UBSM strips were also increased by apamin, an effect absent in SK2(-/-) strips. Apamin increased the sensitivity of SK2(+/+) UBSM strips to electrical field stimulation, since pretreatment with apamin decreased the frequency required to reach a 50% maximal contraction (vehicle, 21 +/- 4 Hz, n = 6; apamin, 12 +/- 2 Hz, n = 7; P < 0.05). In contrast, the sensitivity of SK2(-/-) UBSM strips was unaffected by apamin. Here we provide novel insight into the molecular basis of SK channels in the urinary bladder, demonstrating that the SK2 gene is expressed in the bladder and that it is essential for the ability of SK channels to regulate UBSM contractility.     

Sympathetic nerve activity and whole body heat stress in humans

We and others have shown that moderate passive whole body heating (i.e., increased internal temperature ～0.7°C) increases muscle (MSNA) and skin sympathetic nerve activity (SSNA). It is unknown, however, if MSNA and/or SSNA continue to increase with more severe passive whole body heating or whether these responses plateau following moderate heating. The aim of this investigation was to test the hypothesis that MSNA and SSNA continue to increase from a moderate to a more severe heat stress. Thirteen subjects, dressed in a water-perfused suit, underwent at least one passive heat stress that increased internal temperature ～1.3°C, while either MSNA (n = 8) or SSNA (n = 8) was continuously recorded. Heat stress significantly increased mean skin temperature (Δ～5°C, P < 0.001), internal temperature (Δ～1.3°C, P < 0.001), mean body temperature (Δ～2.0°C, P < 0.001), heart rate (Δ～40 beats/min, P < 0.001), and cutaneous vascular conductance [Δ～1.1 arbitrary units (AU)/mmHg, P < 0.001]. Mean arterial blood pressure was well maintained (P = 0.52). Relative to baseline, MSNA increased midway through heat stress (Δ core temperature 0.63 ± 0.01°C) when expressed as burst frequency (26 ± 14 to 45 ± 16 bursts/min, P = 0.001), burst incidence (39 ± 13 to 48 ± 14 bursts/100 cardiac cyles, P = 0.03), or total activity (317 ± 170 to 489 ± 150 units/min, P = 0.02) and continued to increase until the end of heat stress (burst frequency: 61 ± 15 bursts/min, P = 0.01; burst incidence: 56 ± 11 bursts/100 cardiac cyles, P = 0.04; total activity: 648 ± 158 units/min, P = 0.01) relative to the mid-heating stage. Similarly, SSNA (total activity) increased midway through the heat stress (normothermia; 1,486 ± 472 to mid heat stress 6,467 ± 5,256 units/min, P = 0.03) and continued to increase until the end of heat stress (11,217 ± 6,684 units/min, P = 0.002 vs. mid-heat stress). These results indicate that both MSNA and SSNA continue to increase as internal temperature is elevated above previously reported values.     

Glucocorticoids reduce responses to AMPA receptor activation and blockade in nucleus tractus solitarius

We tested the hypothesis that glucocorticoids attenuate changes in arterial pressure and renal sympathetic nerve activity (RSNA) in response to activation and blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors within the nucleus of the solitary tract (NTS). Experiments were performed in Inactin-anesthetized male Sprague-Dawley rats treated for 7 +/- 1 days with a subcutaneous corticosterone (Cort) pellet or in control rats. Baseline mean arterial pressure (MAP) was significantly higher in Cort-treated rats (109 +/- 2 mmHg, n = 39) than in control rats (101 +/- 1 mmHg, n = 48, P < 0.05). In control rats, microinjection of AMPA (0.03, 0.1, and 0.3 pmol/100 nl) into the NTS significantly decreased MAP at all doses and decreased RSNA at 0.1 and 0.3 pmol/100 nl. Responses to AMPA in Cort-treated rats were attenuated at all doses of AMPA (P < 0.05). Responses to the AMPA-kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were also significantly reduced in Cort-treated rats relative to control rats. Blockade of glucocorticoid type II receptors with mifepristone significantly enhanced responses to CNQX in both control and Cort rats. We conclude that glucocorticoids attenuate MAP and RSNA responses to activation and blockade of AMPA receptors in the NTS.     

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.     

Neuronal and Endothelial Nitric Oxide Synthases in the Paraventricular Nucleus Modulate Sympathetic Overdrive in Insulin-Resistant Rats

A central mechanism participates in sympathetic overdrive during insulin resistance (IR). Nitric oxide synthase (NOS) and nitric oxide (NO) modulate sympathetic nerve activity (SNA) in the paraventricular nucleus (PVN), which influences the autonomic regulation of cardiovascular responses. The aim of this study was to explore whether the NO system in the PVN is involved in the modulation of SNA in fructose-induced IR rats. Control rats received ordinary drinking water, whereas IR rats received 12.5% fructose-containing drinking water for 12 wks to induce IR. Basal SNA was assessed based on the changes in renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) in response to chemicals administered to the PVN. We found an increased plasma norepinephrine level but significantly reduced NO content and neuronal NOS (nNOS) and endothelial NOS (eNOS) protein expression levels in the PVN of IR rats compared to Control rats. No difference in inducible NOS (iNOS) protein expression was observed between the two groups. In anesthetized rats, the microinjection of sodium nitroprusside (SNP), an NO donor, or Nω-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of NOS, into the PVN significantly decreased and increased basal SNA, respectively, in both normal and IR rats, but these responses to SNP and L-NAME in IR rats were smaller than those in normal rats. The administration of selective inhibitors of nNOS or eNOS, but not iNOS, to the PVN significantly increased basal SNA in both groups, but these responses were also smaller in IR rats. Moreover, IR rats exhibited reduced nNOS and eNOS activity in the PVN. In conclusion, these data indicate that the decreased protein expression and activity levels of nNOS and eNOS in the PVN lead to a reduction in the NO content in the PVN, thereby contributing to a subsequent enhancement in sympathoexcitation during IR.     

Protein kinase CK2 contributes to diminished small conductance Ca2+‐activated K+ channel activity of hypothalamic pre‐sympathetic neurons in hypertension

Small conductance calcium‐activated K⁺ (SK) channels regulate neuronal excitability. However, little is known about changes in SK channel activity of pre‐sympathetic neurons in the hypothalamic paraventricular nucleus (PVN) in essential hypertension. SK channels, calmodulin, and casein kinase II (CK2) form a molecular complex. Because CK2 is up‐regulated in the PVN in spontaneously hypertensive rats (SHRs), we hypothesized that CK2 increases calmodulin phosphorylation and contributes to diminished SK channel activity in PVN pre‐sympathetic neurons in SHRs. Perforated whole‐cell recordings were performed on retrogradely labeled spinally projecting PVN neurons in Wistar‐Kyoto (WKY) rats and SHRs. Blocking SK channels with apamin significantly increased the firing rate of PVN neurons in WKY rats but not in SHRs. CK2 inhibition restored the stimulatory effect of apamin on the firing activity of PVN neurons in SHRs. Furthermore, apamin‐sensitive SK currents and depolarization‐induced medium after‐hyperpolarization potentials of PVN neurons were significantly larger in WKY rats than in SHRs. CK2 inhibition significantly increased the SK channel current and medium after‐depolarization potential of PVN neurons in SHRs. In addition, CK2‐mediated calmodulin phosphorylation level in the PVN was significantly higher in SHRs than in WKY rats. Although SK3 was detected in the PVN, its expression level did not differ significantly between SHRs and WKY rats. Our findings suggest that CK2‐mediated calmodulin phosphorylation is increased and contributes to diminished SK channel function of PVN pre‐sympathetic neurons in SHRs. This information advances our understanding of the mechanisms underlying hyperactivity of PVN pre‐sympathetic neurons and increased sympathetic vasomotor tone in hypertension.

     

Skeletal muscle reflex-mediated changes in sympathetic nerve activity are abnormal in spontaneously hypertensive rats

In hypertension, the blood pressure response to exercise is exaggerated. We demonstrated previously that this heightened pressor response to physical activity is mediated by an overactive skeletal muscle exercise pressor reflex (EPR), with important contributions from its metaboreflex and mechanoreflex components. However, the mechanisms driving the abnormal blood pressure response to EPR activation are largely unknown. Recent evidence in humans suggests that the muscle metaboreflex partially mediates the enhanced EPR-induced pressor response via abnormally large changes in sympathetic nerve activity (SNA). Whether the muscle mechanoreflex induces similarly exaggerated alterations in SNA in hypertension remains unknown, as does the role of the mechanoreceptors mediating muscle reflex activity. To address these issues, the EPR was selectively activated by electrically inducing hindlimb muscle contraction in decerebrate normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Stimulation of the EPR evoked significantly larger increases in mean arterial pressure (MAP) and renal SNA (RSNA) in SHR compared with WKY (ΔRSNA from baseline: 140 ± 11 vs. 48 ± 8%). The mechanoreflex was stimulated by stretching hindlimb muscle which likewise elicited significantly greater elevations in MAP and RSNA in SHR than WKY (ΔRSNA from baseline: 105 ± 11 vs. 35 ± 7%). Blockade of mechanoreceptors in muscle with gadolinium significantly attenuated the MAP and RSNA responses to contraction and stretch in SHR. These data suggest that 1) the exaggerated pressor response to activation of the EPR and muscle mechanoreflex in hypertension is mediated by abnormally large reflex-induced augmentations in SNA and 2) this accentuated sympathetic responsiveness is evoked, in part, by stimulation of muscle mechanoreceptors.     

心房钠尿多肽对麻醉大鼠血压和肾神经传出放电的影响

本实验观察了心房肽Ⅱ(Atriopeptin Ⅱ,APⅡ)对麻醉大鼠血压(AP)、心率(HR)和肾交感神经传出放电(RSNA)的影响,并与硝普钠对 AP 和 RSNA 的影响作比较。结果如下:(1)缓冲神经完整和迷走神经完整条件下(n=12)静脉注射 APⅡ(50μg/kg)后,动脉收缩压(SAP)降低23.0±1.66 mmHg(Μ±SE,p<0.001),HR 减慢9±3.5b/min(p<0.05),RSNA 降低4.89±2.95%(P>0.05)。迷走神经切断后,静脉注射 APⅡ引起的~⊿SAP 虽有所减小,但与切断迷走神经前的反应比较,无统计学意义,HR 减慢不再出现,而 RSNA 则有所增加;(2)缓冲神经切断和迷走神经完整条件下(n=7),静脉注射 APⅡ时 SAP 降低27.4±3.25mmHg(P<0.001),HR 减慢13±3.1b/min(P<0.01),RSNA 降低11.67±1.95%(P<0.001)。切断迷走神经后,静脉注射 APⅡ引起的 SAP 降低程度有明显減小(P<0.01),HR减慢不再出现,RSNA 则反而增加(3)无论在迷走神经完整还是切断条件下,静脉注射硝普钠(n=6) SAP 均明显降低,同时伴有 RSNA 的反射性增加。以上结果表明:APⅡ的降压效应,部分是通过迷走神经传入纤维;在切断缓冲神经条件下,APⅡ可经由迷走神经传入纤维的激活而反射地抑制 RSNA。     

Influence of acute alcohol ingestion on sympathetic neural responses to orthostatic stress in humans

Acute alcohol consumption is reported to decrease mean arterial pressure (MAP) during orthostatic challenge, a response that may contribute to alcohol-mediated syncope. Muscle sympathetic nerve activity (MSNA) increases during orthostatic stress to help maintain MAP, yet the effects of alcohol on MSNA responses during orthostatic stress have not been determined. We hypothesized that alcohol ingestion would blunt arterial blood pressure and MSNA responses to lower body negative pressure (LBNP). MAP, MSNA, and heart rate (HR) were recorded during progressive LBNP (-5, -10, -15, -20, -30, and -40 mmHg; 3 min/stage) in 30 subjects (age 24 ± 1 yr). After an initial progressive LBNP (pretreatment), subjects consumed either alcohol (0.8 g ethanol/kg body mass; n = 15) or placebo (n = 15), and progressive LBNP was repeated (posttreatment). Alcohol increased resting HR (59 ± 2 to 65 ± 2 beats/min, P < 0.05), MSNA (13 ± 3 to 19 ± 4 bursts/min, P < 0.05), and MSNA burst latency (1,313 ± 16 to 1,350 ± 17 ms, P < 0.05) compared with placebo (group × treatment interactions, P < 0.05). During progressive LBNP, a pronounced decrease in MAP was observed after alcohol but not placebo (group × time × treatment, P < 0.05). In contrast, MSNA and HR increased during all LBNP protocols, but there were no differences between trials or groups. However, alcohol altered MSNA burst latency response to progressive LBNP. In conclusion, the lack of MSNA adjustment to a larger drop in arterial blood pressure during progressive LBNP, coupled with altered sympathetic burst latency responses, suggests that alcohol blunts MSNA responses to orthostatic stress.