Bimodal delta-opioid receptors regulate vagal bradycardia in canine sinoatrial node

Methionine-enkephalin-arginine-phenylalanine (MEAP) introduced into the interstitium of the canine sinoatrial (SA) node by microdialysis interrupts vagal bradycardia. In contrast, raising endogenous MEAP by occluding the SA node artery improves vagal bradycardia. Both are blocked by the same delta-selective antagonist, naltrindole. We tested the hypothesis that vagal responses to intranodal enkephalin are bimodal and that the polarity of the response is both dose- and opioid receptor subtype dependent. Ultralow doses of MEAP were introduced into the canine SA node by microdialysis. Heart rate frequency responses were constructed by stimulating the right vagus nerve at 1, 2, and 3 Hz. Ultralow MEAP infusions produced a 50-100% increase in bradycardia during vagal stimulation. Maximal improvement was observed at a dose rate of 500 fmol/min with an ED50 near 50 fmol/min. Vagal improvement was returned to control when MEAP was combined with the delta-antagonist naltrindole. The dose of naltrindole (500 fmol/min) was previously determined as ineffective vs. the vagolytic effect of higher dose MEAP. When MEAP was later reintroduced in the same animals at nanomoles per minute, a clear vagolytic response was observed. The delta1-selective antagonist 7-benzylidenenaltrexone (BNTX) reversed the vagal improvement with an ED50 near 1 x 10-21 mol/min, whereas the delta2-antagonist naltriben had no effect through 10-9 mol/min. Finally, the improved vagal bradycardia previously associated with nodal artery occlusion and endogenous MEAP was blocked by the selective delta1-antagonist BNTX. These data support the hypothesis that opioid effects within the SA node are bimodal in character, that low doses are vagotonic, acting on delta1-receptors, and that higher doses are vagolytic, acting on delta2-receptors.     

Vagotonic effects of enkephalin are not mediated by sympatholytic mechanisms

This study examined the hypothesis that vagotonic and sympatholytic effects of cardiac enkephalins are independently mediated by different receptors. A dose-response was constructed by administering the delta-receptor opioid methionine-enkephalin-arginine-phenylalanine (MEAP) by microdialysis into the interstitium of the canine sinoatrial node during vagal and sympathetic stimulation. The right cardiac sympathetic nerves were stimulated as they exited the stellate ganglion at frequencies selected to increase heart rate approximately 35 bpm. The right cervical vagus was stimulated at frequencies selected to produce a two-step decline in heart rate of 25 and 50 bpm. A six-step dose-response was constructed by recording heart rates during nerve stimulation as the dose of MEAP was increased between 0.05 pmol/min and 1.5 nmol/min. Vagal transmission improved during MEAP at 0.5 pmol/min. However, sympathetically mediated tachycardia was unaltered with any dose of MEAP. In Study 2, a similar dose-response was constructed with the kappa-opioid receptor agonist trans(+/-)-3-4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide-HCl (U-50488H) to illustrate an independent sympatholytic effect and to verify its kappa-receptor character. U-50488H gradually suppressed the sympathetic tachycardia, with a significant effect obtained only at the highest dose (1.5 nmol/min). U-50488H had no effect on vagally mediated bradycardia. Surprisingly, the sympatholytic effect was not reversed by withdrawing U-50488H or by the subsequent addition of the kappa-antagonist 17,17'-(dichloropropylmethyl)-6,6',7,7'-6,6'-imino-7,7'-binorphinan-3,4',14,14'-tetroldi-hydrochloride (norBNI). Study 3 was conducted to determine whether the sympatholytic effect of U-50488H could be prevented by norBNI. NorBNI blocked the sympatholytic effect of the U50488H for 90 mins. When norBNI was discontinued afterward and U-50488H was continued alone, a sympatholytic effect emerged within 30 mins. Collectively these observations support the hypothesis that the vagotonic influence of MEAP is not dependent on a sympatholytic influence. Furthermore, the sympatholytic effect is mediated independently by kappa-receptors. The sympatholytic effect of sustained kappa-receptor stimulation appears to evolve gradually into a functional state not easily reversed.     

Exercise training changes autonomic cardiovascular balance in mice.

Experiments were performed to investigate the influence of exercise training on cardiovascular function in mice. Heart rate, arterial pressure, baroreflex sensitivity, and autonomic control of heart rate were measured in conscious, unrestrained male C57/6J sedentary (n = 8) and trained mice (n = 8). The exercise training protocol used a treadmill (1 h/day; 5 days/wk for 4 wk). Baroreflex sensitivity was evaluated by the tachycardic and bradycardic responses induced by sodium nitroprusside and phenylephrine, respectively. Autonomic control of heart rate and intrinsic heart rate were determined by use of methylatropine and propranolol. Resting bradycardia was observed in trained mice compared with sedentary animals [485 +/- 9 vs. 612 +/- 5 beats/min (bpm)], whereas mean arterial pressure was not different between the groups (106 +/- 2 vs. 108 +/- 3 mmHg). Baroreflex-mediated tachycardia was significantly enhanced in the trained group (6.97 +/- 0.97 vs. 1.6 +/- 0.21 bpm/mmHg, trained vs. sedentary), whereas baroreflex-mediated bradycardia was not altered by training. The tachycardia induced by methylatropine was significantly increased in trained animals (139 +/- 12 vs. 40 +/- 9 bpm, trained vs. sedentary), whereas the propranolol effect was significantly reduced in the trained group (49 +/- 11 vs. 97 +/- 11 bpm, trained vs. sedentary). Intrinsic heart rate was similar between groups. In conclusion, dynamic exercise training in mice induced a resting bradycardia and an improvement in baroreflex-mediated tachycardia. These changes are likely related to an increased vagal and decreased sympathetic tone, similar to the exercise response observed in humans.     

Autonomic control in rats with overactivity of tissue renin-angiotensin or kallikrein-kinin system

The renin-angiotensin system (RAS) plays an important role in the regulation of the cardiovascular system and the kallikrein-kinin system (KKS) appears to counteract most of the RAS effects. In this study the vagal and the sympathetic influences on the heart rate and the baroreflex control of the heart rate were evaluated in transgenics rats with human tissue kallikrein gene expression [TGR(hKLK1)], and transgenics rats with tissue renin overexpression [TGR(mREN2)27]. Heart rate was similar in all groups but mean arterial pressure was higher in mREN2 rats than in kallikrein and control rats (149+/-4 vs. 114+/-3 vs. 113+/-3 mm Hg, respectively). The intrinsic heart rate was lower in mREN2 rats than in kallikrein and control rats (324+/-5 vs. 331+/-3 vs. 343+/-7 bpm). The HR response to atropine was similar but the response to propranolol was higher in kallikrein rats than control group (61+/-7 vs. 60+/-9 vs. 38+/-7 bpm, respectively). The vagal tonus was lower in mREN2 than in SD and hKAL rats (18+/-3 vs. 40+/-6 vs. 35+/-6 bpm) whereas the sympathetic tonus was higher in kallikrein rats (118+/-7 vs. 96+/-1 vs. 81+/-9 bpm in the mREN2 and SD rats), respectively. Baroreflex sensitivity to bradycardic responses was attenuated in mREN2 rats (0.37+/-0.05 vs. 1.34+/-0.08 vs. 1.34+/-0,13 bpm/mm Hg) while the tachycardic responses were unchanged. The bradycardic responses to electrical stimulation of the vagal nerve were depressed in both renin and kallikrein rats (129+/-47 vs. 129+/-22 vs. 193+/-25 bpm in control group in response to 32 Hz). In conclusion: 1.The rats with overexpression of renin showed decreased intrinsic heart rate and impairment of vagal function, characterized by decreased vagal tonus, reduced response of HR to electrical stimulation of vagus nerve, and depressed reflex bradycardia provoked by increases of blood pressure. 2. The rats with overexpression of kallikrein showed an increase of sympathetic activity that regulates the heart rate, characterized by increased HR response to propranolol and increased sympathetic tonus, accompanied by decreased bradycardic responses to electrical vagal stimulation.     

Cardiac enkephalins interrupt vagal bradycardia via delta 2-opioid receptors in sinoatrial node

Local cardiac opioids appear to be important in determining the quality of vagal control of heart rate. Introduction of the endogenous opioid methionine-enkephalin-arginine-phenylalanine (MEAP) into the interstitium of the canine sinoatrial node by microdialysis attenuates vagally mediated bradycardia through a delta-opioid receptor mechanism. The following studies were conducted to test the hypothesis that a delta(2)-opiate receptor subtype mediates the interruption of vagal transmission. Twenty mongrel dogs were anesthetized and instrumented with microdialysis probes inserted into the sinoatrial node. Vagal frequency responses were performed at 1, 2, and 3 Hz during vehicle infusion and during treatment with the native agonist MEAP, the delta(1)-opioids 2-methyl-4aa-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aalpha-octahydroquinolino[2,3,3- g]isoquinoline (TAN-67) and [d-pen(2,5)]-enkephalin (DPDPE), and the delta(2) opioid deltorphin II. The vagolytic effects of intranodal MEAP and deltorphin were then challenged with the delta(1)- and delta(2)-opioid receptor antagonists 7-benzylidenenaltrexone (BNTX) and naltriben, respectively. Although the positive control deltorphin II was clearly vagolytic in each experimental group, TAN-67 and DPDPE were vagolytically ineffective in the same animals. In contrast, TAN-67 improved vagal bradycardia by 30-35%. Naltriben completely reversed the vagolytic effects of MEAP and deltorphin. BNTX was ineffective in this regard but did reverse the vagal improvement observed with TAN-67. These data support the hypothesis that the vagolytic effect of the endogenous opioid MEAP was mediated by delta(2)-opioid receptors located in the sinoatrial node. These data also support the existence of vagotonic delta(1)-opioid receptors also in the sinoatrial node.     

Opposite effects of iv amiodarone on cardiovascular vagal and sympathetic efferent activities in rats

It is unknown whether amiodarone exerts a direct central action on the cardiovascular autonomic nervous system. This study was designed to evaluate the effects of acute amiodarone administration on vagal and sympathetic efferent nerve discharges. Experiments were carried out in 25 decerebrate unanesthetized rats. In one group, vagal activity was recorded from preganglionic fibers isolated from the cervical vagus nerve. In another group, sympathetic recordings were obtained from fibers isolated from the cervical sympathetic trunk in intact conditions or after barodenervation. Recordings were performed before and for 60 min after amiodarone (50 mg/kg iv) administration. In all groups, amiodarone induced bradycardia and hypotension. Vagal activity increased immediately, reaching a significant difference after 20 min (260 +/- 131% from 16.4 +/- 3.3 spikes/s) and was unmodified by the barodenervation. At difference, sympathetic activity after an initial and short-lasting increase (150 +/- 83% from 24.8 +/- 5.7 spikes/s) began to decrease significantly after 20 min (36 +/- 17%) throughout the experiment. The initial increase in sympathetic activity was not observed in barodenervated animals. These changes in vagal and sympathetic activity could play an important role in contributing to the antiarrhythmic action of amiodarone.     

Interactions with the cardiac cholinergic system: effects of disopyramide and its mono-N-dealkylated metabolite.

The cardiac vagolytic effects of disopyramide and its mono-N-dealkylated metabolite (MND), and their interactions with the cardiac cholinergic system, were assessed using in vivo and in vitro experiments. In chloralose anesthetized dogs, disopyramide phosphate (0.25 mg/kg/min) and MND at equimolar dose (0.173 mg/kg/min) reduced vagal bradycardia. As indicated by the ED80, MND exhibits a vagolytic activity 1.5-2 times less potent than disopyramide. Concomitantly, increases in heart rate and mean blood pressure were observed with disopyramide, whereas with MND only a rise in mean blood pressure occurred. In conscious dogs, where vagal tone is fully expressed, disopyramide and MND increased heart rate and, interestingly, prevented any atropine-induced additional tachycardia, though heart rate was relatively low. Binding studies on rat heart membranes yielded Ki values 2-2.5 times higher for MND than for disopyramide, and demonstrated that neither disopyramide nor MND binding modified the cardiac muscarinic receptor sites. Taken together, these results show that disopyramide exhibits a more potent cardiac vagolytic action than MND, very likely linked to a greater ability to bind to cardiac muscarinic receptors. They also show that disopyramide and MND are very potent in preventing atropine-induced "excess tachycardia", very likely by inhibiting the ionic pacemaker current(s) involved in its genesis.     

The possibility of peripheral cholinolytic action of psychostimulants

The cholinolytic effect of sydnophen discovered in earlier anesthetized cats was confirmed on unanesthetized fish and frogs: the vagal bradycardia induced by electric stimulation of peripheral vagal end was decreased or even abolished by intravenous injection of sydnophen (0.2-20 mg/kg). The amphetamine (0.2-30 mg/kg) also blocked the vagal bradycardia in anesthetized cats and unanesthetized frogs. The maximum vagolytic action of amphetamine appeared later (in 4-8 min after injection) in compared with sydnophen (1-3 min). The small dose of amphetamine (0.2-0.3 mg/kg) in contrast to sydnophen didn't decrease the vagal bradycardia but even increased it. It was suggested that the cholinolytic effect of sydnophen and amphetamine is due to different mechanisms.     

Cardiac sympathetic nerve stimulation does not attenuate dynamic vagal control of heart rate via alpha-adrenergic mechanism

Complex sympathovagal interactions govern heart rate (HR). Activation of the postjunctional beta-adrenergic receptors on the sinus nodal cells augments the HR response to vagal stimulation, whereas exogenous activation of the presynaptic alpha-adrenergic receptors on the vagal nerve terminals attenuates vagal control of HR. Whether the alpha-adrenergic mechanism associated with cardiac postganglionic sympathetic nerve activation plays a significant role in modulation of the dynamic vagal control of HR remains unknown. The right vagal nerve was stimulated in seven anesthetized rabbits that had undergone sinoaortic denervation and vagotomy according to a binary white-noise signal (0-10 Hz) for 10 min; subsequently, the transfer function from vagal stimulation to HR was estimated. The effects of beta-adrenergic blockade with propranolol (1 mg/kg i.v.) and the combined effects of beta-adrenergic blockade and tonic cardiac sympathetic nerve stimulation at 5 Hz were examined. The transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with pure delay. beta-Adrenergic blockade decreased the dynamic gain from 6.0 +/- 0.4 to 3.7 +/- 0.6 beats x min(-1) x Hz(-1) (P < 0.01) with no alteration of the corner frequency or pure delay. Under beta-adrenergic blockade conditions, tonic sympathetic stimulation did not further change the dynamic gain (3.8 +/- 0.5 beats x min(-1) x Hz(-1)). In conclusion, cardiac postganglionic sympathetic nerve stimulation did not affect the dynamic HR response to vagal stimulation via the alpha-adrenergic mechanism.     

Vagal function impairment after exercise training.

The present investigation was undertaken to evaluate the vagal function of trained (T) and sedentary (S) rats by use of different approaches in the same animal. After 13 wk of exercise training (treadmill for 1 h 5 times/wk at 26.8 m/min and 15% grade), T rats had a resting heart rate (HR) slightly but significantly lower than S rats (299 +/- 3 vs. 308 +/- 3 beats/min). T rats had marked reduction of the intrinsic HR (329 +/- 4 vs. 369 +/- 5 beats/min) after blockade by methylatropine and propranolol. They also exhibited depressed vagal and sympathetic tonus. Baroreflex bradycardia (phenylephrine injections) was reduced, bradycardic responses produced by electrical stimulation of the vagus were depressed, and responses to methacholine injection were decreased in T rats. Therefore several evidences of vagal function impairment were observed in T rats. The resting bradycardia after exercise training is more likely to be dependent on alterations of the pacemaker cells, inasmuch as the intrinsic HR was markedly reduced.     

Activation of 5-HT(1A) receptors attenuates tachycardia induced by restraint stress in rats

To better understand the central mechanisms that mediate increases in heart rate (HR) during psychological stress, we examined the effects of systemic and intramedullary (raphe region) administration of the serotonin-1A (5-HT(1A)) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) on cardiac changes elicited by restraint in hooded Wistar rats with preimplanted ECG telemetric transmitters. 8-OH-DPAT reduced basal HR from 356 +/- 12 to 284 +/- 12 beats/min, predominantly via a nonadrenergic, noncholinergic mechanism. Restraint stress caused tachycardia (an initial transient increase from 318 +/- 3 to 492 +/- 21 beats/min with a sustained component of 379 +/- 12 beats/min). beta-Adrenoreceptor blockade with atenolol suppressed the sustained component, whereas muscarinic blockade with methylscopolamine (50 microg/kg) abolished the initial transient increase, indicating that sympathetic activation and vagal withdrawal were responsible for the tachycardia. Systemic administration of 8-OH-DPAT (10, 30, and 100 microg/kg) attenuated stress-induced tachycardia in a dose-dependent manner, and this effect was suppressed by the 5-HT(1A) antagonist WAY-100635 (100 microg/kg). Given alone, the antagonist had no effect. Systemically injected 8-OH-DPAT (100 microg/kg) attenuated the sympathetically mediated sustained component (from +85 +/- 19 to +32 +/- 9 beats/min) and the vagally mediated transient (from +62 +/- 5 to +25 +/- 3 beats/min). Activation of 5-HT(1A) receptors in the medullary raphe by microinjection of 8-OH-DPAT mimicked the antitachycardic effect of the systemically administered drug but did not affect basal HR. We conclude that tachycardia induced by restraint stress is due to a sustained increase in cardiac sympathetic activity associated with a transient vagal withdrawal. Activation of central 5-HT(1A) receptors attenuates this tachycardia by suppressing autonomic effects. At least some of the relevant receptors are located in the medullary raphe-parapyramidal area.     

Sympathetic and parasympathetic component of bradycardia triggered by stimulation of NTS P2X receptors

We have previously shown that activation of P2X purinoceptors in the subpostremal nucleus tractus solitarius (NTS) produces a rapid bradycardia and hypotension. This bradycardia could occur via sympathetic withdrawal, parasympathetic activation, or a combination of both mechanisms. Thus we investigated the relative roles of parasympathetic activation and sympathetic withdrawal in mediating this bradycardia in chloralose-urethane anesthetized male Sprague-Dawley rats. Microinjections of the selective P2X purinoceptor agonist alpha,beta-methylene ATP (25 pmol/50 nl and 100 pmol/50 nl) were made into the subpostremal NTS in control animals, after atenolol (2 mg/kg i.v.), a beta1-selective antagonist, and after atropine methyl bromide (2 mg/kg i.v.), a muscarinic receptor antagonist. The bradycardia observed with activation of P2X receptors at the low dose of the agonist is mediated almost entirely by sympathetic withdrawal. After beta1-adrenergic blockade, the bradycardia was reduced to just -5.1 +/- 0.5 versus -28.8 +/- 5.1 beats/min in intact animals. Muscarinic blockade did not produce any significant change in the bradycardic response at the low dose. At the high dose, both beta1-adrenergic blockade and muscarinic blockade attenuated the bradycardia similarly, -37.4 +/- 6.4 and -40.6 +/- 3.7 beats/min, respectively, compared with -88.0 +/- 11 beats/min in control animals. Double blockade of both beta1-adrenergic and muscarinic receptors virtually abolished the response (-2.5 +/- 0.8 beats/min). We conclude that the relative contributions of parasympathetic activation and sympathetic withdrawal are dependent on the extent of P2X receptor activation.     

Neuropeptide Y and peptide YY mediate nonadrenergic vasoconstriction and modulate sympathetic responses in rats

The modulation of cardiovascular sympathetic responses by neuropeptide Y (NPY) and peptide YY (PYY) was assessed in vivo, in pithed rats. Both peptides (0.02-2 nmol/kg) caused similar dose-dependent pressor responses, resistant to adrenergic blockade but antagonized by the calcium channel blocker, nifedipine. Only NPY, at the lowest dose, slightly accelerated heart rate (by 10 +/- 4 beats/min). At the pressor dose (0.6 nmol/kg) but not subpressor dose (0.2 nmol/kg), the increase in blood pressure induced by stimulation of the sympathetic outflow (ST: 0.3 Hz, 50 V, 1 min) was attenuated by PYY (by 40%), whereas ST-evoked tachycardia was reduced by NPY (by 35%). Neither NPY- nor PYY-pretreatment affected ST-induced increments in plasma norepinephrine (NE) and epinephrine concentrations. In addition, regional hemodynamic effects of NPY were studied in conscious rats instrumented with Doppler flow probes. The hypertension caused by NPY was attended by reflex bradycardia and marked rise in peripheral vascular resistance in renal (+ 233 +/- 59%), superior mesenteric (+ 183 +/- 65%) and hindquarter (+ 65 +/- 10%) circulation. The pattern of hemodynamic responses of NPY was similar to that of NE but, unlike the latter, persisted after adrenergic blockade.     

Evaluation of baroreflex control of heart rate in renovascular hypertensive mice

The objective of the present study was to evaluate the baroreflex and the autonomic control of heart rate (HR) in renovascular hypertensive mice. Experiments were carried out in conscious C57BL/6 (n = 16) mice 28 days after a 2-kidney 1-clip procedure (2K1C mice) or a sham operation (sham mice). Baroreflex sensitivity was evaluated by measuring changes in heart rate (HR) in response to increases or decreases in mean arterial pressure (MAP) induced by phenylephrine or sodium nitroprusside. Cardiac autonomic tone was determined by use of atropine and atenolol. Basal HR and MAP were significantly higher in 2K1C mice than in sham mice. The reflex tachycardia induced by decreases in MAP was greatly attenuated in 2K1C mice compared with sham mice. Consequently, the baroreflex sensitivity was greatly decreased (2.2 +/- 0.4 vs. 4.4 +/- 0.3 beats x min(-1) x mmHg(-1)) in hypertensive mice compared with sham mice. The reflex bradycardia induced by increases in MAP and the baroreflex sensitivity were similar in both groups. Evaluation of autonomic control of HR showed an increased sympathetic tone and a tendency to a decreased vagal tone in 2K1C mice compared with that in sham mice. 2K1C hypertension in mice is accompanied by resting tachycardia, increased predominance of the cardiac sympathetic tone over the cardiac vagal tone, and impairment of baroreflex sensitivity.     

Postvagal potentiation of the chronotropic effect of norepinephrine

Following termination of vagal stimulation, heart rate increases above control (postvagal tachycardia). This phenomenon has been attributed to vagally mediated release of norepinephrine in the sinus node region, although other contributory factors may be important. The possibility that, during the postvagal period, the chronotropic efficacy of norepinephrine is enhanced was investigated. Mongrel dogs (N = 6) were pretreated with reserpine in order to minimize postvagal tachycardia and hence allow reliable detection of enhanced responsiveness to norepinephrine. The dogs were then anesthetized with chloralose, autonomically decentralized, and instrumented to record electrocardiogram, aortic blood pressure, and electrograms from right atrium and right ventricle. Thirty-, forty-, or sixty-second infusions of norepinephrine were administered via the sinus node artery. The mean cycle length decrease produced by norepinephrine alone was 95 msec (which corresponds to a heart rate increase of + 19.6 bpm). After a 30-sec period of vagal stimulation, norepinephrine infusions produced a cycle length decrease of 139 msec (+32.5 bpm). These results are significant at the P less than 0.05 level. It is concluded that norepinephrine infusions produce a significantly greater magnitude of tachycardia when administered postvagally. It is proposed that this postvagal potentiation of the chronotropic effect of norepinephrine may contribute to postvagal tachycardia. Indeed, there may be a synergistic relationship between this phenomenon and vagally mediated release of norepinephrine in the mediation of postvagal tachycardia.     

Characterization of [3H]naltrindole binding to delta opioid receptors in rat brain.

[3H]Naltrindole binding characteristics were determined using homogenized rat brain tissue. Saturation binding studies at 25 degrees C measured an equilibrium dissociation constant (Kd) value of 37.0 +/- 3.0 pM and a receptor density (Bmax) value of 63.4 +/- 2.0 fmol/mg protein. Association binding studies showed that equilibrium was reached within 90 min at a radioligand concentration of 30 pM. Naltrindole, as well as the ligands selective for delta (delta) opioid receptors, such as pCI-DPDPE and Deltorphin II inhibited [3H]naltrindole binding with nanomolar IC50 values. Ligands selective for mu (mu) and kappa (kappa) opioid receptors were only effective in inhibiting [3H]naltrindole binding at micromolar concentrations. From these data, we conclude that [3H]naltrindole is a high affinity, selective radioligand for delta opioid receptors.     

Aging and baroreflex control of RSNA and heart rate in rats

Aging is associated with altered autonomic control of cardiovascular function, but baroreflex function in animal models of aging remains controversial. In this study, pressor and depressor agent-induced reflex bradycardia and tachycardia were attenuated in conscious old (24 mo) rats [57 and 59% of responses in young (10 wk) Wistar rats, respectively]. The intrinsic heart rate (HR, 339 +/- 5 vs. 410 +/- 10 beats/min) was reduced in aged animals, but no intergroup differences in resting mean arterial blood pressure (MAP, 112 +/- 3 vs. 113 +/- 5 mmHg) or HR (344 +/- 9 vs. 347 +/- 9 beats/min) existed between old and young rats, respectively. The aged group also exhibited a depressed (49%) parasympathetic contribution to the resting HR value (vagal effect) but preserved sympathetic function after intravenous methylatropine and propranolol. An implantable electrode revealed tonic renal sympathetic nerve activity (RSNA) was similar between groups. However, old rats showed impaired baroreflex control of HR and RSNA after intravenous nitroprusside (-0.63 +/- 0. 18 vs. -1.84 +/- 0.4 bars x cycle(-1) x mmHg(-1) x s(-1)). Therefore, aging in rats is associated with 1) preserved baseline MAP, HR, and RSNA, 2) impaired baroreflex control of HR and RSNA, and 3) altered autonomic control of resting HR.     

Modulation of cardiopulmonary depressor reflex in nucleus ambiguus by electroacupuncture: roles of opioids and γ-aminobutyric acid

Stimulation of cardiopulmonary receptors with phenylbiguanide (PBG) elicits depressor cardiovascular reflex responses, including decreases in blood pressure and heart rate mediated in part by the brain stem parasympathetic cardiac neurons in the nucleus ambiguus (NAmb). The present study examined NAmb neurotransmitter mechanisms underlying the influence of electroacupuncture (EA) on the PBG-induced hypotension and bradycardia. We hypothesized that somatic stimulation during EA modulates PBG responses through opioid and γ-aminobutyric acid (GABA) modulation in the NAmb. Anesthetized and ventilated cats were studied during repeated stimulation with PBG or cardiac vagal afferents while low-frequency EA (2 Hz) was applied at P5-6 acupoints overlying the median nerve for 30 min and NAmb neuronal activity, heart rate, and blood pressure were recorded. Microinjection of kainic acid into the NAmb attenuated the PBG-induced bradycardia from -60 ± 11 to -36 ± 11 beats/min. Likewise, EA reduced the PBG-induced depressor and bradycardia reflex by 52 and 61%, respectively. Cardiac vagal afferent evoked preganglionic cellular activity in the NAmb was reduced by EA for about 60 min. Blockade of opioid or GABA(A) receptors using naloxone and gabazine reversed the EA-related modulation of the evoked cardiac vagal activity by 73 and 53%, respectively. Similarly, naloxone and gabazine reversed EA modulation of the negative chronotropic responses from -11 ± 5 to -23 ± 6 and -13 ± 4 to -24 ± 3 beats/min, respectively. Thus EA at P5-6 decreases PBG evoked hypotension and bradycardia as well as the NAmb PBG-sensitive preganglionic cardiac vagal outflow through opioid and GABA neurotransmitter systems.     

Renal responses to acute reflex activation of renal sympathetic nerve activity and renal denervation in secondary hypertension

We tested whether the responsiveness of the kidney to basal renal sympathetic nerve activity (RSNA) or hypoxia-induced reflex increases in RSNA, is enhanced in angiotensin-dependent hypertension in rabbits. Mean arterial pressure, measured in conscious rabbits, was similarly increased (+16 +/- 3 mmHg) 4 wk after clipping the left (n = 6) or right (n = 5) renal artery or commencing a subcutaneous ANG II infusion (n = 9) but was not increased after sham surgery (n = 10). Under pentobarbital sodium anesthesia, reflex increases in RSNA (51 +/- 7%) and whole body norepinephrine spillover (90 +/- 17%), and the reductions in glomerular filtration rate (-27 +/- 5%), urine flow (-43 +/- 7%), sodium excretion (-40 +/- 7%), and renal cortical perfusion (-7 +/- 3%) produced by hypoxia were similar in normotensive and hypertensive groups. Hypoxia-induced increases in renal norepinephrine spillover tended to be less in hypertensive (1.1 +/- 0.5 ng/min) than normotensive (3.7 +/- 1.2 ng/min) rabbits, but basal overflow of endogenous and exogenous dihydroxyphenolglycol was greater. Renal plasma renin activity (PRA) overflow increased less in hypertensive (22 +/- 29 ng/min) than normotensive rabbits (253 +/- 88 ng/min) during hypoxia. Acute renal denervation did not alter renal hemodynamics or excretory function but reduced renal PRA overflow. Renal vascular and excretory responses to reflex increases in RSNA induced by hypoxia are relatively normal in angiotensin-dependent hypertension, possibly due to the combined effects of reduced neural norepinephrine release and increased postjunctional reactivity. In contrast, neurally mediated renin release is attenuated. These findings do not support the hypothesis that enhanced neural control of renal function contributes to maintenance of hypertension associated with activation of the renin-angiotensin system.     

Dopamine and norepinephrine in the alimentary tract changes after chemical sympathectomy and surgical vagotomy

The aim of this study was to examine the distribution of dopamine and norepinephrine in the proximal alimentary tract of the rat and to assess the contributions of sympathetic and vagal fibers to the tissue concentrations of both catecholamines. Tissues were extracted in perchloric acid and the catecholamines were separated by high pressure liquid chromatography and detected electrochemically. In untreated rats (controls) both catecholamines were concentrated in the gastric muscle but norepinephrine levels were 6-8 times higher (corpus, dopamine 35 +/- 7 ng . g-1, norepinephrine 265 +/- 50 ng . g-1, mean +/- SE, n = 6). In the mucosa norepinephrine concentrations were 10-12 times higher (corpus, dopamine 12 +/- 3 ng . g-1, norepinephrine 140 +/- 26 ng . g-1). Chemical sympathectomy (6 hydroxydopamine, 100 mg . kg-1 ip 3 days) significantly reduced dopamine concentrations in muscle and norepinephrine in muscle, mucosa, pylorus and duodenum. In all tissues the effects on norepinephrine were greater. Surgical vagotomy significantly reduced dopamine concentrations in the gastric muscle, but not the mucosa. Norepinephrine concentrations in the stomach of vagotomized rats were significantly reduced only in the pylorus. Differences in the relative concentrations of dopamine and norepinephrine in gastric tissues of the normal rat and differences in the effects of sympathectomy and vagotomy suggest that dopamine and norepinephrine exist, to an extent, in separate populations of cells and that dopamine is not merely a precursor of norepinephrine. Gastric mucosal dopamine, which was mainly unaffected by either treatment, may exist in APUD cells.