Captopril potentiates the vasodepressor action of Met-enkephalin in the anaesthetized rat

Use of Naloxone in Angiotensin-Converting Enzyme Inhibitor Overdose: A Case Report

BACKGROUND

Angiotensin-converting enzyme (ACE) inhibitor overdose is an uncommonly presenting toxicologic emergency. Management is primarily supportive care, but a small body of evidence exists to support naloxone for management of hypotension.

CASE REPORT

We present a case of accidental ACE inhibitor overdose. The patient took approximately 300 mg lisinopril over 48 h and presented for evaluation of syncope. He was hypotensive and unresponsive to fluids. We administered naloxone with immediate and sustained resolution in hypotension. The mechanism of action is briefly discussed. WHY SHOULD AN EMERGENCY MEDICINE PHYSICIAN BE AWARE OF THIS?: Naloxone is a rapid, low-risk, low-cost, and effective intervention for hypotension due to ACE inhibitor toxicity. It is supported by basic science research and clinical experience.

Interplay between RAS and opioids: opening the Pandora of complexities

Angiotensin and endogenous opioids are important bioactive neuropeptides, which are widely distributed in the brain and peripheral regions to produce diverse biological and neurobiological activities. An endogenous opioid system includes proopiomelanocortin-derived enkephalin, dynorphin and endorphin that act on their specific receptors such as delta (δ), kappa (κ) and mu (μ) receptors. Research evidence demonstrates significant positive as well as negative interactions between renin angiotensin system (RAS) and endogenous opioids in the brain and periphery. The diverse actions of Ang II are possibly mediated indirectly through endogenous opioids, while opioids are also shown to activate RAS components suggesting the up-regulation of each system in concern with each other. On the contrary, there are reports suggesting a negative correlation between RAS and opioid system. Research evidence also supports the notion that Ang II acts as anti-opioid peptide to decrease the actions of opioids. Moreover, opioids-induced decline in angiotensin release and functioning has also been reported. Co-administration of ACE inhibitors with opioids exhibits significant interactions possibly due to decreased metabolism of opioids leading to potentiation of their actions. The present review describes the complexities of positive and negative interactions between RAS and opioids along with possible mechanisms responsible for these interactions.

Angiotensin-converting enzyme inhibitors increase vasoconstrictor requirements after cardiopulmonary bypass

Preoperative use of angiotensin-converting enzyme (ACE) inhibitors is common and has been associated with hypotension at separation from cardiopulmonary bypass (CPB). This study prospectively examined the influence of chronic preoperative ACE inhibitor use and other perioperative factors on the incidence of vasoconstrictor therapy required to maintain systolic blood pressure at more than 85 mm Hg despite a normal cardiac output after CPB in 4301 adults undergoing elective coronary artery and/or valve surgery. Hypothermic, nonpulsatile CPB and either opioid or ketamine-benzodiazepine anesthesia were common features of the operations. At least two vasoconstrictor infusions (phenylephrine, norepinephrine, or dopamine) were required for low perfusion pressure despite adequate cardiac output after CPB in 7.7% of 519 ACE-inhibited patients and 4.0% of 3782 patients not receiving ACE inhibitors (P = 0.0001). In the first 4 h after arrival in the intensive care unit, the need for vasoconstrictor infusions to treat hypotension with adequate cardiac output did not differ, although more ACE-inhibited patients (6.4%) exhibited low values of systemic vascular resistance (< 600 dyne.s.cm-5) than patients not receiving ACE inhibitors (2.8%; P = 0.0002). Logistic regression analysis identified preoperative ACE inhibitor use, congestive heart failure, poor left ventricular function, duration of CPB, reoperative surgery, age, and opioid anesthesia as independent risk factors for requiring > or = 2 vasoconstrictor infusions after CPB. No other preoperative drug therapy significantly altered this outcome.

Naloxone reversal of hypotension due to captopril overdose

The hemodynamic effects of captopril and other angiotensin-converting enzyme inhibitors may be mediated by the endogenous opioid system. The opioid antagonist naloxone has been shown to block or reverse the hypotensive actions of captopril. We report a case of an intentional captopril overdose, manifested by marked hypotension, that resolved promptly with the administration of naloxone. To our knowledge, this is the first reported case of captopril-induced hypotension treated with naloxone. Our experience demonstrates a possible role of naloxone in the reversal of hypotension resulting from captopril.

Naloxone does not antagonize the antihypertensive effect of chronic captopril therapy in hypertensive patients

It has been reported that naloxone, an opiate receptor antagonist, blunts the hypotensive effect of captopril in normotensives. However, our previous data did not show any interaction between captopril given acutely and naloxone (0.1 mg/kg) in hypertensives. To test whether a greater naloxone dose could interfere with the hemodynamic effect of chronically administered captopril, 12 male hypertensives were studied: Six of them had been under captopril treatment (50 mg tid) for at least 1 month, whereas the others had been drug free for the same time. Both groups randomly received a saline or naloxone (0.2 mg/kg) infusion for 1 hour, and blood pressure, heart rate, PRA, plasma aldosterone, adrenaline, and noradrenaline were measured at regular intervals before, during, and after naloxone infusion. In drug-free hypertensives, naloxone tended to reduce blood pressure slightly and did not modify heart rate, PRA, plasma aldosterone, adrenaline, or noradrenaline. In captopril-treated hypertensives, naloxone did not blunt the hypotensive effect of captopril, but rather enhanced it, without changing the heart rate, adrenaline, and noradrenaline. Moreover, naloxone increased the renin-stimulating action and did not modify the aldosterone-inhibiting effect of captopril. Our results show that the hemodynamic action of captopril given chronically is not influenced by opioid receptor blockade and therefore that the antihypertensive effect of this drug seems to be unrelated to the activation of the opioidergic system.

Physiologic role of the peripheral enkephalinergic system in regulating cardiovascular homeostasis: evidence of interactions with the renin-angiotensin and kallikrein-kinin systems

On isolated heart preparation, it was found that Leu5-Enkephalin (Leu5-ENK) did not influence the cardiac function. On the other hand, Leu5-ENK induced a specific dose-related inhibition, in the cardiac perfusate, of the activities of kininase II (KII) and angiotensin converting enzyme (ACE) (but not of kininase I-KI). Instead no detectable alterations of the above enzymatic activities with the used concentrations of Leu5-ENK were observed in vitro. This opioid also increased specifically the effects induced by some of the autacoids, related to both renin-angiotensin and kallikrein-kinin systems, on the KII and ACE activities. A specific correlation between these Leu5-ENK-induced modifications and the functional responses of the heart to the same autacoids was observed. Naloxone (NAL) and more significantly ICI 174864 (ICI) opposed or reversed the inhibitory effect of the used opioid whereas they had neither inhibitory nor synergic effect on both KII and ACE activity by themselves. The possible physiologic role of the enkephalins in regulating cardiovascular function by acting peripherally on some humoral systems through modulatory mechanism was discussed.

Feeding and drinking behaviour following angiotensin converting enzyme blockade: role of injectant pH

Angiotensin converting enzyme (ACE) is a circulating dipeptidase which has a broad specificity and is known to metabolise a range of circulating peptides. While a number of circulating peptides are believed to modulate food intake, it is not known if ACE plays a role in the control of feeding behaviour and therefore in this study we have examined the effect of the potent, specific ACE antagonists captopril (SQ 14225) and enalapril (MK421) on food and water intake following food deprivation and 2-deoxyglucose treatment in the rat. One hour captopril (50 mg/kg, IP) pretreatment significantly reduced the food intake of 24 hr food deprived rats. Because captopril solutions have a low pH (2.0), the effect of buffered captopril on food intake following 24 hr food deprivation was also examined. Buffered captopril also significantly reduced the food intake of 24 hr food deprived rats, but not to the same extent as unbuffered captopril. Naloxone pretreatment (1 mg/kg, IP) did not antagonize the effect of captopril on food intake indicating that the anorexic action of captopril was not due to alterations in opiate peptide levels. Buffered captopril did not reduce the food intake of food-replete rats receiving 2-deoxyglucose (300 mg/kg, IP) or alter blood gases or pH. However, an equimolar buffered dose of the structurally different ACE inhibitor enalapril failed to significantly alter food deprivation-induced food intake, suggesting that this action of captopril in reducing food intake was unrelated to ACE blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensins II and III prevent captopril-induced renin release in the rat

1. The effects of exogenous angiotensins II and III (50 pmol/min i.v.) on plasma renin release following captopril injection (5 mg/kg, i.v.) were studied in anaesthetized Sprague-Dawley rats, to determine whether angiotensin II blockade is the major mechanism by which captopril induces renin release. 2. Captopril produced a 12-fold increase in plasma renin concentration compared with saline-injected controls. This was completely reversed by pre-infusion of angiotensin II or III. 3. The fall in blood pressure following captopril treatment was also abolished by angiotensins II and III pre-infusion. Noradrenaline pre-infusion (200-800 ng/min, i.v.) also prevented the captopril-induced hypotension but did not alter the rise in plasma renin. 4. Ureteric ligation did not significantly reduce captopril-induced renin release suggesting that acute changes in sodium excretion or delivery of electrolyte to the macula densa were not involved in renin release. 5. These findings suggest that captopril induces renin release by inhibiting angiotensin II feedback control of renin secretion and that angiotensin III may also modulate renin release.

Effect of angiotensin-converting enzyme inhibitors captopril and enalapril on cAMP content of specific brain areas in spontaneously hypertensive rats

1. The influence of two angiotensin-converting enzyme inhibitors, captopril and enalapril, on the cAMP content of microdissected brain areas was examined in spontaneously hypertensive rats. Both drugs depleted systolic arterial blood pressure significantly. 2. Captopril and enalapril increased the level of cAMP in catecholaminergic cell groups in the lower brain-stem. Captopril was more effective in the substantia nigra, while enalapril treatment resulted in high cAMP levels in the ventrolateral medulla oblongata (A1 catecholaminergic cell group). 3. Both drugs, especially captopril, depleted cAMP content in the cingulate cortex. 4. No changes in cAMP levels were measured in the primary baroreceptor centre (nucleus of the solitary tract) following either captopril or enalapril treatment.

The broad substrate specificity of human angiotensin I converting enzyme

Angiotensin I converting enzyme (kininase II; ACE) has been described as a peptidyldipeptidase or dipeptidyl carboxypeptidase (EC 3.4.15.1) of the pulmonary endothelial cells, which liberates angiotensin II or inactivates kinins. However, ACE has a much wider distribution and substrate specifity; it is concentrated in human epithelial cells (e.g. brush border of the kidney, placenta, intestine and choroid plexus), neuroepithelial cells (subfornical organ, pallidonigral dendrites, median eminence) and male genital tract (testes, prostate, epididymides, seminal plasma). Its substrates include enkaphalins, the C-terminal extended proenkephalins and a protected chemotactic tripeptide. Recent, mostly in vitro studies with purified ACE, indicate that ACE also cleaves peptides by other than peptidyldipeptidase action. Homogeneous human ACE inactivated substance P in spite of its blocked C-terminus (Met11-NH2) primarily by releasing the C-terminal tripeptide. A blocked C-terminal tripeptide, Arg-Pro-Gly-NH2 was also released from the luteinizing hormone releasing hormone (LHRH). Although ACE shares many properties with carboxypeptidases, it surprisingly cleaves the N-terminal tripeptide greater than Glu1-His2-Trp3 from LHRH. Because human ACE hydrolyzes a variety of peptide hormones, actions of its inhibitors may go well beyond blocking the conversion of angiotensin I.

Captopril attenuates diuretic and natriuretic actions of furosemide but not atrial natriuretic peptide

Atrial Natriuretic Peptide (ANP) exerts a potent diuretic and natriuretic action in rat, dog and man. To test whether these actions of ANP are due to antagonism of the antinatriuretic actions of endogenous angiotensin II, an examination was made of the renal actions of ANP in anesthetized rats with and without captopril pretreatment. Captopril treatment did not alter the diuretic and natriuretic action of a single, intravenous bolus injection of ANP (1000 ng of the 8-33 ANP). Captopril treatment did significantly attenuate the diuretic and natriuretic actions of Furosemide (20 mg/kg ip). This attenuation was not reversed by concomitant, exogenous angiotensin II infusion. These results suggest that while ANP exerts its renal actions independently of the circulating renin-angiotensin system the diuretic and natriuretic action of Furosemide is modulated by a substrate of angiotensin converting enzyme.

Captopril does not potentiate hypotension and algesia by substance P

Captopril (1-5 mg/kg, i.v.) did not affect the vasodepressor responses to substance P (1-30 micrograms/kg, i.v.) in anaesthetized rats. Substance P (100 micrograms/kg, s.c.) produced significant algesia in mice; this was not potentiated by the smaller doses of captopril (1-2 mg/kg, i.p.), but was instead antagonized by the high dose (5 mg/kg, i.p.). It is concluded that captopril does not have any influence on substance P degradation in vivo since the pharmacological actions of the undecapeptide are not enhanced after captopril treatment.

Attenuation of pressor responses to sympathetic stimuli in the rat by enalapril

Intravenous administration to pithed Wistar rats of the angiotensin converting enzyme inhibitor enalapril (0.1-1.0 mg/kg) lowered the diastolic blood pressure and reduced pressor responses occurring during electrical stimulation (1-30 Hz) of the spinal sympathetic outflow. These doses of enalapril given intravenously also attenuated pressor responses to intravenous injection of the muscarinic ganglion stimulant McNeil-A-343 (50, 100, 150 micrograms/kg) and noradrenaline (0.1-5.0 micrograms/kg). Enalapril (1.0 mg/kg, i.v.) reduced pressor responses to the nicotinic ganglion stimulant 1,1-dimethyl-4-phenyl-piperazinium (300 micrograms/kg, i.v.). These results confirmed that the actions of enalapril resemble those of captopril in the pithed rat, by causing reductions in both blood pressure and pressor responses to sympathetic stimuli.

Captopril and opiate antagonism in essential hypertension

Six patients maintained on 50-100 mg captopril, for 2-25 months, were administered captopril 50 mg orally, together with either naloxone or 0.9% saline vehicle (placebo) given intravenously, in a double-blind crossover study. Naloxone did not appear to modify the circulatory effects of captopril in these patients, in contrast to earlier findings after acute captopril administration in normotensives. The results do not support an important endogenous opioid role in the chronic antihypertensive effect of captopril, but provide evidence that different mechanisms may contribute to the early short term falls in blood pressure, compared to the later long term effects.

Naloxone reverses the effects of enalapril and enalaprilic acid on the pressor responses to afferent vagal stimulation

The effects of intracisternal injection of enalapril (MK 421) or its bioactive form enalaprilic acid (MK 422) (0.075 mg/kg) on the pressor responses elicited by afferent stimulation of the vagus were investigated in the urethane-anaesthetized dog. The angiotensin converting enzyme inhibitors (ACEI)-induced decrease in the systolo-diastolic pressor responses to vagal stimulation was reversed by naloxone (0.1 mg/kg iv). These data support evidence for an additional central site of action of These data support evidence for an additional central site of action of ACEI and suggest the involvement of central opioid mechanisms in the hypotensive properties of ACEI.

Disappearance of enkephalins in the isolated perfused rat lung

Enkephalin disappearance during a single passage through the isolated, Krebs'-perfused rat lung was examined by superfusion bioassay. The rat colon was used to quantitate enkephalin disappearance since it proved to be sensitive to physiologic concentrations (10(-11) M) of met5-enkephalin or an analog D-ala2-D-leu5-enkephalin. The rat stomach strip was used to assess the release of prostaglandins from the pulmonary vasculature. The rat lung rapidly degraded the enkephalins but released no prostaglandins in the dose-range of 0.1 - 50 ng. Captopril at doses which blocked conversion of angiotensin I to II inhibited the degradation of enkephalins across the lung.