Angiotensin in the brain suppresses epinephrine-induced cardiac arrhythmias through CNS opioid mechanisms

Endogenous kappa opioids mediate the action of brain angiotensin II to increase blood pressure

The objectives of this study were to determine whether endogenous opioids are operative in modulating the CNS action of angiotensin II (ang II) on blood pressure and to determine whether this is mediated by endogenous mu or kappa opioid receptor agonists. The study design was: unanesthetized Wistar rats, 300-400g, previously instrumented with a cannula in the lateral cerebral ventricle and a catheter in the femoral artery, had ang II, 0.5microg, injected into the lateral cerebral ventricle (ICV). Groups were allocated to receive naloxone, a mu opioid receptor antagonist or MR 2266 a selective kappa opioid receptor antagonist prior to ang II. In other experiments in unanesthetized rats, baroreceptor reflex function was assessed by intravenous injection of phenylephrine or nitroprusside and the interaction of endogenous opioids and ang II ascertained with use of the mu or kappa opioid receptor antagonist .

RESULTS

Ang II significantly (p<0.05) increased systolic and diastolic blood pressure. The kappa opioid antagonist, MR 2266, 25microg/kg ICV, significantly (p<0.05) reduced and MR 2266, 50microg/kg ICV, completely prevented the increase in blood pressure produced by ang II. In contrast, the mu opioid receptor antagonist, naloxone, 50microg/kg, ICV, did not significantly attenuate the blood pressure responses to ang II. Ang II induced alteration in baroreceptor function. The effect of ang II on baroreceptor function was significantly antagonized by the kappa opioid receptor antagonist MR 2266. In conclusion, these data indicate that: (a) endogenous opioids modulate the pressor response to intracerebral ang II, (b) this effect is mediated mainly through endogenous kappa opioid agonists and kappa rather than mu opioid receptors, (c) alteration of baroreceptor sensitivity by ang II is modulated by endogenous kappa opioids.

Role of epinephrine in acute stress

This article presents the likely pathway of stimuli generated by the recognition of high-intensity stressors to ultimately produce a fight-or-flight response. A key element is the recognition that psychological stressors that do not directly alter the internal environment represent the most important etiology of a fight-or-flight response. Adrenomedullary secretion is a critical component of that response; impromptu stimulation of the adrenal medulla can produce plasma epinephrine concentrations greater than 10,000 pg/mL. When these plasma levels reach the hypothalamus to act on the CNS, the result is facilitation of the decision making, and decision execution processes (fight-or-flight), and perhaps further sympathetic stimulation and vasopressin release. Subjects with underlying cardiovascular and/or metabolic pathology may be particularly susceptible to potentially lethal reactions to this neuroendocrine response. Additionally, since this biological reaction may be triggered by sudden changes in the social environment, the coordinated actions of epinephrine, sympathetic stimulation and vasopressin must be directed at not only optimizing the chances for survival, but also at attaining maximal preservation of the individual environmental and social domains.

Magnesium deficiency in adult rats promotes the induction of ventricular tachycardia by the administration of epinephrine

The effects of magnesium deficiency on epinephrine-induced ventricular tachyarrhythmia were investigated in adult rats. Forty-two adult Wistar rats were fed a magnesium-deficient diet while 30 rats were fed a standard diet for 20 days. The plasma magnesium concentration was lower in the magnesium-deficient rats (0.22+/-0.01 mmol/l) than in the control rats (0.76+/-0.03 mmol/l, P < 0.001). Using a telemetry system, electrocardiograms and arterial blood pressure were recorded on a polygraph in an unrestrained condition. Epinephrine was infused intravenously starting at 5 microg/kg per minute. The QT interval was prolonged to 50+/-1 ms in the magnesium-deficient rats compared with 44+/-1 ms in the control rats (P < 0.001). Before the administration of epinephrine, no ventricular tachyarrhythmias or seizures were found in either the control or the magnesium-deficient rats. The incidence of epinephrine-induced sustained ventricular tachycardia (VT) was higher in the magnesium-deficient rats (86%) than in the control rats (43%, P < 0.01). However, this VT did not result in sudden death. Seizures always preceded death in both the magnesium-deficient and control rats while the arrhythmias observed immediately before death were mainly bradyarrhythmias. The present study in an adult rat magnesium-deficient model revealed that magnesium deficiency enhances the susceptibility to epinephrine-induced ventricular tachyarrhythmias.

Morphine and morphiceptin increase the threshold for epinephrine-induced cardiac arrhythmias in the rat through brain mu opioid receptors

1. To determine whether morphine modulates the development of cardiac arrhythmias through mu opioid receptors by an action within the central nervous system (CNS). Catecholamine-induced ventricular arrhythmias were produced, in the rat, by continuous infusion of epinephrine at incremental doses until the development of fatal arrhythmias, usually ventricular fibrillation. 2. Morphine, 0.1 mg/kg i.v., significantly suppressed (P < 0.05) the development of epinephrine-induced arrhythmias compared with the control group. This was opposed by the mu opioid antagonist naloxone (1 or 2 mg/kg) in a dose-dependent manner. 3. To determine whether these effects were operative in the brain, rats received an injection of either morphine 50 micrograms/kg or its diluent (control) into the lateral cerebral ventricle intracerebroventricularly (i.c.v.). Morphine significantly increased (P < 0.05) the threshold for the development of arrhythmias. 4. To further explore whether this effect was operative at the mu opioid receptor, a more specific mu opioid receptor agonist morphiceptin (50 micrograms/kg) was administered i.c.v. and produced a significant increase (P < 0.05) in the threshold for cardiac arrhythmias compared with controls. 5. The action of morphine was further established to be operating through mu opioid receptors from experiments with the i.c.v. administration of naloxone (+) and naloxone (-) followed by morphine showing that the action of morphine in the brain was prevented by the opioid antagonist naloxone but not by its stereo-isomer that is not a mu opioid receptor antagonist. 6. These data suggest a role for morphine to modulate cardiac arrhythmias, specifically to increase arrhythmia threshold, through an action within the CNS at mu opioid receptors.

The opioid agonist ethylketocyclazocine accentuates epinephrine-induced cardiac arrhythmias in the rat through an action in the brain

The purpose of this study was to determine whether the opioid receptor agonist ethylketocyclazocine (EKC) modulates the development of cardiac arrhythmias by an action within the central nervous system. Catecholamine-induced ventricular arrhythmias were produced in the rat by continuous infusion of epinephrine, at incremental doses, until the development of fatal arrhythmias that were usually ventricular fibrillation. EKC, 1 mg/kg, intravenously (IV) significantly (p < 0.05) accentuated the manifestations of or reduced the threshold for epinephrine-induced arrhythmias. The effect of EKC was prevented by the kappa opioid antagonist MR 2266 in a dose-dependent manner. To determine whether the central nervous system is a site of this action of EKC, rats received injection of either EKC or the diluent (control) into the lateral cerebral ventricle (ICV). EKC, 100 and 200 micrograms/kg ICV, significantly (p < 0.05) altered the dose-effect relationship between epinephrine and arrhythmias so that EKC accentuated the development of cardiac arrhythmias. These data indicate that EKC, through an action in the brain, modulates cardiac arrhythmias and suggests a role for opioid receptor agonists, such as EKC, in cardiac arrhythmias and perhaps sudden cardiac death in man.

Morphine and the Endogenous Opioid Dynorphin in the Brain Attenuate Digoxin-Induced Arrhythmias in Guinea Pigs

The effects of the opioid receptor agonists morphine and dynorphin on digoxin-induced arrhythmias were examined in guinea pigs that had received intravenous digoxin (50 mu/kg bolus plus 500 mu/kg/hr intravenously). Animals received either morphine (50 or 100 micrograms/kg) or dynorphin A(1-13) (50 or 100 micrograms/kg) or saline (the diluent) into the lateral cerebral ventricle (intracerebroventricularly) prior to digoxin. Morphine and dynorphin produced significant (P < 0.05) dose-dependent increases in the threshold of digoxin-induced arrhythmias. The mean digoxin dose at the development of fatal arrhythmias was 775 +/- 42 micrograms/kg in the control group but was significantly higher namely 958 +/- 45 micrograms/kg after 100 micrograms/kg of morphine ICV, and 984 +/- 47 micrograms/kg after 100 micrograms/kg of dynorphin A (1-13) intracerebroventricularly. In the absence of digoxin, the highest doses of each of these opioids did not produce arrhythmias. Changes in blood pressure and heart rate were unlikely explanations for the observed actions of these opioids as morphine accentuated the increase in blood pressure that accompanied digoxin while dynorphin was associated with a lower blood pressure with digoxin, despite similar effects on arrhythmias. In the control group, fatal digoxin-induced arrhythmias were ventricular tachyarrhythmias in two-thirds of cases and complete heart block in the rest. Morphine and dynorphin reduced the development of ventricular tachyarrhythmias. The role of the cholinergic system was explored, with morphine, utilizing atropine sulfate which crosses the blood brain barrier and atropine methylnitrate which does not enter the CNS. Atropine sulfate but not atropine methylnitrate reversed the effects of morphine on digoxin-induced arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynorphin A (1-13) in the brain suppresses epinephrine-induced ventricular premature complexes and ventricular tachyarrhythmias

The objectives of this study were to test the hypothesis that dynorphin in the central nervous system modulates epinephrine-induced cardiac arrhythmias and that central cholinergic mechanisms are operative in this action of dynorphin. Cardiac arrhythmias were produced by continuous intravenous infusion of epinephrine, in Wistar rats, previously instrumented with catheters in the lateral cerebral ventricle, femoral vein and femoral artery. Epinephrine produced ventricular premature complexes and later the development of fatal ventricular fibrillation. Dynorphin A (1-13), 5 or 20 micrograms (3 or 12 nM) administered into the lateral cerebral ventricle (ICV), significantly (P less than 0.05) increased the threshold for development of cardiac arrhythmias. Dynorphin A (1-13), 20 micrograms, increased the epinephrine dose at the occurrence of ventricular premature beats to 171 +/- 8 (mean +/- 1 S.E.M.) compared to 120 +/- 5 micrograms epinephrine/kg in the control group and increased the dose at the onset of fatal arrhythmias to 186 +/- 8 compared to 141 +/- 10 micrograms epinephrine/kg in the control group. The action of dynorphin was significantly (P less than 0.05) antagonized by the kappa opioid antagonist MR2266. Atropine sulfate, administered ICV or intravenously, produced a dose dependent antagonism of this action of dynorphin A (1-13). This was not due to the peripheral effects of atropine, as atropine methylnitrate, which does not cross the blood brain barrier, did not oppose the effects of dynorphin A (1-13). These data indicate (i) dynorphin A (1-13) increases the threshold for or suppresses the manifestations of epinephrine-induced ventricular arrhythmias, (ii) dynorphin's action on cardiac arrhythmias is mediated through central cholinergic rather than peripheral parasympathetic mechanisms (iii) dynorphin may play a role as an endogenous opioid within the brain that modulates cardiac arrhythmias in circumstances of elevated circulating epinephrine concentration.