Sympathetic afferent stimulation inhibits central vagal activation induced by intravenous medetomidine in rats

Changes in gap junction proteins Connexin30.2 and Connexin40 expression in the sinoatrial node of rats with dexmedetomidine-induced sinus bradycardia

BACKGROUND

Dexmedetomidine (Dex) is widely used, and its most common side effect is bradycardia. The complete mechanism through which Dex induces bradycardia has not been elucidated. This research investigates the expression of gap junction proteins Connexin30.2 (Cx30.2) and Connexin40 (Cx40) within the sinoatrial node of rats with Dex-induced sinus bradycardia.

METHODS

Eighty rats were randomly assigned to five groups. Saline was administered to rats in Group C. In the other four groups, the rats were administered Dex to induce bradycardia. In groups D₁ and D₂, the rats were administered Dex at a loading dose of 30 μg.kg^-1 and 100 μg.kg^-1 for 10 min, then at 15 μg.kg^-1.h^-1 and 50 μg.kg^-1.h^-1 for 120 min separately. The rats in group D₁A and D₂A were administered Dex in the same way as in group D₁ and D₂; however, immediately after the administration of the loading dose, 0.5 mg atropine was administered intravenously, and then at 0.5 mg.kg^-1.h^-1 for 120 min. The sinoatrial node was acquired after intravenous infusion was completed. Quantitative real-time polymerase chain reaction and western blot analyses were performed to measure mRNA and protein expression of Cx30.2 and Cx40, respectively.

RESULTS

The expression of Cx30.2 increased, whereas the expression of Cx40 decreased within the sinoatrial node of rats with Dex-induced sinus bradycardia. Atropine reversed the effects of Dex on the expression of gap junction proteins.

CONCLUSION

Dex possibly altered the expression of gap junction proteins to slow down cardiac conduction velocity in the sinoatrial node.

Contribution of afferent pathway to vagal nerve stimulation-induced myocardial interstitial acetylcholine release in rats

Vagal nerve stimulation (VNS) has been explored as a potential therapy for chronic heart failure. The contribution of the afferent pathway to myocardial interstitial acetylcholine (ACh) release during VNS has yet to be clarified. In seven anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release during right VNS with the following combinations of stimulation frequency (F in Hz) and voltage readout (V in volts): F0V0 (no stimulation), F5V3, F20V3, F5V10, and F20V10. F5V3 did not affect the ACh level. F20V3, F5V10, and F20V10 increased the ACh level to 2.83 ± 0.47 (P < 0.01), 4.31 ± 1.09 (P < 0.001), and 4.33 ± 0.82 (P < 0.001) nM, respectively, compared with F0V0 (1.76 ± 0.22 nM). After right vagal afferent transection (rVAX), F20V3 and F20V10 increased the ACh level to 2.90 ± 0.53 (P < 0.001) and 3.48 ± 0.63 (P < 0.001) nM, respectively, compared with F0V0 (1.61 ± 0.19 nM), but F5V10 did not (2.11 ± 0.24 nM). The ratio of the ACh levels after rVAX relative to before was significantly <100% in F5V10 (59.4 ± 8.7%) but not in F20V3 (102.0 ± 8.7%). These results suggest that high-frequency and low-voltage stimulation (F20V3) evoked the ACh release mainly via direct activation of the vagal efferent pathway. By contrast, low-frequency and high-voltage stimulation (F5V10) evoked the ACh release in a manner dependent on the vagal afferent pathway.

Blockade of adenosine A1 receptor in nucleus tractus solitarius attenuates baroreflex sensitivity response to dexmedetomidine in rats

The α₂-adrenergic receptor (α₂-AR) agonist dexmedetomidine increases baroreflex sensitivity (BRS). In the current study, we examined the potential role of adenosine A1 receptor (A1R) within the nucleus tractus solitaries (NTS) in such a response. Briefly, adult male Sprague-Dawley rats were anesthetized and randomly received microinjection of selective A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.1 pmol/1 μl) or saline vehicle into the right NTS. Ten min after the microinjection, dexmedetomidine infusion started at a rate of 30 μg/kg over 15 min followed by infusion at 15 μg·kg^-1·h^-1 for 105 min, or 100 μg/kg over 15 min followed by infusion at 50 μg·kg^-1·h^-1 for 105 min. BRS was examined using a standard phenylephrine method prior to infusion (T₀), 60 min (T₁) and 120 min (T₂) after dexmedetomidine infusion started. Adenosine concentration in plasma and brainstem was measured with high-performance liquid chromatography with vs. without α₂-AR antagonist atipamezole pretreatment (0.5 mg/kg, i.p.). Dexmedetomidine increased BRS at both 30 (T₀: 0.55 ± 0.25 vs. T₁: 2.45 ± 0.37, T₂: 2.26 ± 0.56 ms/mmHg, P < 0.05) and 100 μg/kg (T₀: 0.63 ± 0.24 vs. T₁: 6.21 ± 1.87, T₂: 6.30 ± 2.12 ms/mmHg, P < 0.05). DPCPX pretreatment obliterated BRS response to 100-μg/kg dexmedetomidine. At 100 μg/kg, dexmedetomidine increased adenosine concentration in plasma (0.23 ± 0.11 to 0.45 ± 0.07 μg/ml, P < 0.05) and brainstem (1.46 ± 0.30 to 2.52 ± 0.22 μg/ml, P < 0.05); such effect was blocked by atipamezole pretreatment. Western blot analysis showed α₂-AR up-regulation by 100-μg/kg dexmedetomidine, which can be prevented by DPCPX. Double-labeling with glial fibrillary acidic protein showed α₂-AR up-regulation in astrocytes in the NTS. These results suggest that dexmedetomidine enhances baroreflex sensitivity, possibly by increasing adenosine in NTS and α₂-AR expression in astrocytes.

Mild Hypothermia Is Ineffective to Protect Against Myocardial Injury Induced by Chemical Anoxia or Forced Calcium Overload

Although hypothermia suppresses myocardial ischemia/reperfusion injury, whether it also protects the myocardium against cellular stresses such as chemical anoxia and calcium overload remains unknown. We examined the effect of mild hypothermia (33°C) on myocardial injury during ischemia/reperfusion, local administration of sodium cyanide (chemical anoxia), or local administration of maitotoxin (forced Ca overload) using cardiac microdialysis applied to the feline left ventricle. Baseline myoglobin levels (in ng/mL) were 237 ± 57 and 150 ± 46 under normothermia and hypothermia, respectively (mean ± SE, n = 6 probes each). Coronary artery occlusion increased the myoglobin level to 2600 ± 424 under normothermia, which was suppressed to 1160 ± 149 under hypothermia (P < 0.05). Reperfusion further increased the myoglobin level to 6790 ± 1550 under normothermia, which was also suppressed to 2060 ± 343 under hypothermia (P < 0.05). By contrast, hypothermia did not affect the cyanide-induced myoglobin release (930 ± 130 vs. 912 ± 62, n = 6 probes each) or the maitotoxin-induced myoglobin release (2070 ± 511 vs. 2110 ± 567, n = 6 probes each). In conclusion, mild hypothermia does not make the myocardium resistant to cellular stresses such as chemical anoxia and forced Ca overload.

Central activation of cardiac vagal nerve by α2-adrenergic stimulation is impaired in streptozotocin-induced type 1 diabetic rats

To elucidate the abnormality of cardiac vagal control in streptozotocin-induced type 1 diabetic rats, we measured left ventricular myocardial interstitial acetylcholine (ACh) release in response to α₂-adrenergic stimulation as an index of in vivo cardiac vagal nerve activity. A cardiac microdialysis technique was applied to the rat left ventricle, and the effect of α₂-adrenergic stimulation by intravenous medetomidine (100 μg/kg) on myocardial interstitial ACh levels was examined in anesthetized diabetic rats (4-6 weeks after intraperitoneal streptozotocin) and age-matched control rats (protocol 1). The effect of electrical vagal nerve stimulation on ACh levels was also examined in separate rats (protocol 2). In protocol 1, medetomidine increased the ACh levels in control (from 1.76 ± 0.65 to 3.13 ± 1.41 nM, P < 0.05, n = 7) but not in diabetic rats (from 2.01 ± 0.47 to 1.62 ± 0.34 nM, not significant, n = 7). In protocol 2, electrical vagal nerve stimulation at 20 Hz significantly increased the ACh levels in both control (from 1.49 ± 0.26 to 6.39 ± 1.81 nM, P < 0.001, n = 6) and diabetic rats (from 1.77 ± 0.54 to 6.98 ± 1.38 nM, P < 0.001, n = 6). In conclusion, medetomidine-induced central vagal activation was impaired in diabetic rats, whereas peripheral cardiac vagal control of ACh release was preserved. The impairment of central vagal activation may lead to relative sympathetic predominance and promote cardiovascular complications in diabetes.

Desipramine increases cardiac parasympathetic activity via α2-adrenergic mechanism in rats

Desipramine (DMI) is a blocker of neuronal norepinephrine (NE) uptake transporter. Although intravenous DMI has been shown to cause centrally-mediated sympathoinhibition and peripheral NE accumulation, its parasympathetic effect remains to be elucidated. We hypothesized that intravenous DMI activates the cardiac vagal nerve via an α₂-adrenergic mechanism. Using a cardiac microdialysis technique, changes in myocardial interstitial acetylcholine (ACh) levels in the left ventricular free wall in response to intravenous DMI (1mg·kg^-1) were examined in anesthetized rats. In rats with intact vagi (n=7), intravenous DMI increased ACh from 1.67±0.43 to 2.48±0.66nM (P<0.01). In rats with vagotomy (n=5), DMI did not significantly change ACh (from 0.92±0.16 to 0.85±0.23nM). In rats with intact vagi pretreated with intravenous yohimbine (2mg·kg^-1), DMI did not significantly change ACh (from 1.25±0.23 to 1.13±0.15nM). In conclusion, while DMI is generally considered to be an agent that predominantly affects sympathetic neurotransmission, it can activate the cardiac vagal nerve via α₂-adrenergic stimulation in experimental settings in vivo.

Dexmedetomidine preconditioning may attenuate myocardial ischemia/reperfusion injury by down-regulating the HMGB1-TLR4-MyD88-NF-кB signaling pathway

Aims

To investigate whether dexmedetomidine (DEX) preconditioning could alleviate the inflammation caused by myocardial ischemia/reperfusion (I/R) injury by reducing HMGB1-TLR4-MyD88-NF-кB signaling.

Methods

Seventy rats were randomly assigned into five groups: sham group, myocardial I/R group (I/R), DEX+I/R group (DEX), DEX+yohimbine+I/R group (DEX/YOH), and yohimbine+I/R group (YOH). Animals were subjected to 30 min of ischemia induced by occluding the left anterior descending artery followed by 120 min of reperfusion. Myocardial infarct size and histological scores were evaluated. The levels of IL-6 and TNF-α in serum and myocardium were quantified by enzyme-linked immunosorbent assay, and expression of HMGB1, TLR4, MyD88, IκB and NF-κB in the myocardial I/R area were determined with Western blot and immunocytochemistry.

Results

Myocardial infarct sizes, histological scores, levels of circulating and myocardial IL-6 and TNF-α, the expression of HMGB1, TLR4, MyD88 and NF-κB, and the degradation of IκB were significantly increased in the I/R group compared with the sham group (P<0.01). DEX preconditioning significantly reduced the myocardial infarct size and histological scores (P<0.01 vs. I/R group). Similarly, the serum and myocardial levels of IL-6 and TNF-α, the expression of HMGB1, TLR4, MyD88 and NF-κB, and the degradation of IκB were significantly reduced in the DEX group (P<0.01 vs. I/R group). These effects were partly reversed by yohimbine, a selective α₂-adrenergic receptor antagonist, while yohimbine alone had no significant effect on any of the above indicators.

Conclusion

DEX preconditioning reduces myocardial I/R injury in part by attenuating inflammation, which may be attributed to the downregulation of the HMGB1-TLR4-MyD88-NF-кB signaling pathway mediated by the α₂-adrenergic receptor activation.

Chemical sympathectomy attenuates inflammation, glycocalyx shedding and coagulation disorders in rats with acute traumatic coagulopathy

Acute traumatic coagulopathy (ATC) may trigger sympathoadrenal activation associated with endothelial damage and coagulation disturbances. Overexcitation of sympathetic nerve in this state would disrupt sympathetic-vagal balance, leading to autonomic nervous system dysfunction. The aim of this study was to evaluate the autonomic function in ATC and its influence on inflammation, endothelial and coagulation activation. Male Sprague-Dawley rats were randomly assigned to sham, ATC control (ATCC) and ATC with sympathectomy by 6-hydroxydopamine (ATCS) group. Sham animals underwent the same procedure without trauma and bleeding. Following trauma and hemorrhage, rats underwent heart rate variability (HRV) test, which predicts autonomic dysfunction through the analysis of variation in individual R-R intervals. Then, rats were euthanized at baseline, and at 0, 1 and 2 h after shock and blood gas, conventional coagulation test and markers of inflammation, coagulation, fibrinolysis, endothelial damage and catecholamine were measured. HRV showed an attenuation of total power and high frequency, along with a rise of low frequency and low frequency : high frequency ratio in the ATC rats, which both were reversed by sympathectomy in the ATCS group. Additionally, sympathetic denervation significantly suppressed the increase of proinflammatory cytokines, tumor necrosis factor-α and the fibrinolysis markers including tissue-type plasminogen activator and plasmin-antiplasmin complex. Serum catecholamine, soluble thrombomodulin and syndecan-1 were also effectively inhibited by sympathectomy. These data indicated that autonomic dysfunction in ATC involves both sympathetic activation and parasympathetic inhibition. Moreover, sympathectomy yielded anti-inflammatory, antifibrinolysis and endothelial protective effects in rats with ATC. The role of autonomic neuropathy in ATC should be explored further.

Superoxide anions in the paraventricular nucleus mediate cardiac sympathetic afferent reflex in insulin resistance rats

AIM

Cardiac sympathetic afferent reflex (CSAR) participates in sympathetic over-excitation. Superoxide anions and angiotensin II (Ang II) mechanisms are associated with sympathetic outflow and CSAR in the paraventricular nucleus (PVN). This study was designed to investigate whether PVN superoxide anions mediate CSAR and Ang II-induced CSAR enhancement response in fructose-induced insulin resistance (IR) rats.

METHODS

CSAR was evaluated with the changes of renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) responses to the epicardial application of capsaicin (CAP) in anaesthetized rats.

RESULTS

Compared with Control rats, IR rats showed that CSAR, PVN NAD(P)H oxidase activity, superoxide anions, malondialdehyde (MDA), Ang II and AT1 receptor levels were significantly increased, whereas PVN superoxide dismutase (SOD) and catalase (CAT) activities were decreased. In Control and IR rats, PVN microinjection of superoxide anions scavengers tempol, tiron and PEG-SOD (an analogue of endogenous superoxide dismutase) or inhibition of PVN NAD(P)H oxidase with apocynin caused significant reduction of CSAR, respectively, but DETC (a superoxide dismutase inhibitor) strengthened the CSAR. PVN pre-treatment with tempol abolished, whereas DETC potentiated, Ang II-induced CSAR enhancement response. Moreover, PVN pre-treatment with tempol or losartan prevented superoxide anions increase caused by Ang II in IR rats.

CONCLUSION

PVN superoxide anions mediate CSAR and Ang II-induced CSAR response in IR rats. In IR state, increased NAD(P)H oxidase activity and decreased SOD and CAT activities in the PVN promote superoxide anions increase to involve in CSAR enhancement. Ang II may increase NAD(P)H oxidase activity via AT1 receptor to induce superoxide anion production.

Myocardial interstitial serotonin and its major metabolite, 5-hydroxyindole acetic acid levels determined by microdialysis technique in rat heart

AIMS

The aim of this study was to elucidate myocardial interstitial serotonin (5-HT) kinetics in the heart, including 5-HT reuptake and enzymatic degradation to 5-hydroxyindole acetic acid (5-HIAA) via monoamine oxidase (MAO).

MAIN METHODS

Using microdialysis technique in anesthetized rats, we simultaneously monitored myocardial interstitial levels of 5-HT and its major metabolite, 5-HIAA, in the left ventricle and examined the effects of local administration of a MAO inhibitor, pargyline, or a 5-HT uptake inhibitor, fluoxetine.

KEY FINDINGS

Pargyline increased dialysate 5-HT concentration from 1.8±0.3 at baseline to 3.9±0.5nM but decreased dialysate 5-HIAA concentration from 20.7±1.0 at baseline to 15.8±1.4nM at 60-80min of administration. Fluoxetine increased dialysate 5-HT concentration from 1.9±0.4 at baseline to 6.5±0.9nM at 60-80min of administration, but did not change dialysate 5-HIAA concentration. Local administration of ADP (100mM) increased dialysate 5-HT and 5-HIAA concentrations. Pargyline did not affect ADP-induced increase in dialysate 5-HT concentration but suppressed ADP-induced increase in dialysate 5-HIAA concentration during 60min of ADP administration. Fluoxetine increased dialysate 5-HT concentration at 40-60min of ADP administration, but did not affect ADP-induced increase in dialysate 5-HIAA concentration.

SIGNIFICANCE

Simultaneous monitoring of myocardial interstitial 5-HT and 5-HIAA levels provides valuable information on 5-HT kinetics including reuptake and enzymatic degradation by MAO, which play a role in the regulation of myocardial interstitial 5-HT levels at baseline and when 5-HT levels are elevated.