Simultaneous inhibition of neprilysin and activation of ACE2 prevented diabetic cardiomyopathy

BACKGROUND

Neprilysin inhibitors (NEPi) are assisting the renin-angiotensin system (RAS) inhibitors in halting diabetic cardiomyopathy (DCM). Away from conventional tactic, a recent report revealed the renoprotective potential of NEPi and angiotensin-converting enzyme (ACE2) activator combination therapy against diabetic nephropathy. However, this combination so far not evaluated against DCM, thus the present investigation aiming the same.

METHODS

Streptozotocin-induced (55 mg/kg, ip) type 1 diabetic (T1D) male Wistar rats were treated with either monotherapy of thiorphan (0.1 mg/kg/day, po) or diminazene aceturate (5 mg/kg/day, po), or their combination therapy, for four weeks. After hemodynamic measurements, all the rats' heart and plasma were collected for biochemistry, ELISA, histopathology, and immunoblotting.

RESULTS

Metabolic perturbations and failing cardiac functions associated with diabetes were markedly attenuated by combination therapy. Besides, unfavourable alterations in RAS and natriuretic peptides system (NPS) were corrected by combination therapy. Interestingly, combination therapy significantly increased plasma and heart cGMP levels compared to T1D and monotherapy receiving rats. Moreover, rats receiving combination therapy exhibited significant inhibition of activated NF-κB, TGF-β and apoptotic signalling, and a notable reduction in cardiac fibrosis when compared to T1D rats. Expressions of posttranslational histone modifications markers; H3K4Me2 and its methyltransferases (SET7/9 and RBBP5) were significantly enhanced in T1D hearts, which were significantly reduced by combination therapy.

CONCLUSIONS

The NEPi and ACE2 activator combination therapy effectively prevented DCM by normalising RAS and NPS activities, increasing cGMP, inhibiting inflammatory, pro-fibrotic and apoptotic signalling, and reversing H3K4Me2 and its methyl transferases expressions.

Inhibition of insulin degrading enzyme by racecadotril in the brain of Wistar rats

Racecadotril is an enkephalinase inhibitor used to treat abdominal discomfort in the clinic. The blood-glucose lowering action of racecadotril has been observed in rats; however, the mechanisms remain obscure. 8-week-old Wistar rats were intravenously injected with racecadotril and the levels of insulin in the brain were measured. Additionally, brain homogenates were co-incubated with racecadotril or thiorphan to evaluate insulin degrading enzyme (IDE) activity. Otherwise, rats were pretreated by intracerebroventricular (i. c. v.) injection of insulin antibody or glibenclamide at a dose sufficient to inhibit K (ATP) channels prior to injection of racecadotril. Moreover, rats were vagotomized to evaluate the role of the cholinergic nerve. Racecadotril significantly decreased the plasma glucose in rats; this action of racecadotril was abolished by i. c. v. pretreatment with insulin antibody or glibenclamide. Also, i. c. v. injection of thiorphan, the active form of racecadotril, lowered blood glucose, but this effect disappeared in the presence of the insulin antibody. In rat brain homogenates, racecadotril and thiorphan inhibited IDE activity and increased the cerebral insulin level. The blood-glucose lowering action of racecadotril or thiorphan was diminished in vagotomized rats. Our results suggest that racecadotril lowers blood glucose mainly through inhibition of IDE activity and increases endogenous insulin in the brain. Subsequently, the increased insulin might activate insulin receptor, which opens the K (ATP) channel and induces peripheral insulin release through the vagal nerve. Thus, we provide the new finding that racecadotril has the ability to inhibit IDE in rat brain.

A nonhuman primate model of Alzheimer's disease generated by intracranial injection of amyloid-β42 and thiorphan

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive impairment and neuropathological changes, including the deposition of amyloid-beta (Aβ) peptide. Aged monkeys have proven to be invaluable in the study of AD, as their brains naturally develop amyloid plaques similar to those in AD brains. However, spontaneous development of AD-like pathologies in aged monkeys is time-consuming, often taking several years. Here, we created an experimentally induced AD model in middle-aged (16-17 years) rhesus monkeys by intracranial injection of Aβ42 and thiorphan, an inhibitor of neprilysin that is responsible for Aβ clearance. The working memory capacity of the monkeys in a delayed-response task was little affected following the delivery of Aβ42 and thiorphan. However, the administration of Aβ42 and thiorphan resulted in a significant intracellular accumulation of Aβ in the neurons of the basal ganglia, the cortex, and the hippocampus, accompanied by neuronal atrophy and loss. Moreover, immunohistochemistry revealed a degeneration of choline acetyltransferase-positive cholinergic neurons and an increase of glial fibrillary acidic protein-positive astrocytes. In conclusion, our data demonstrate a primate model of AD generated by combined infusion of Aβ42 and thiorphan, which duplicates a subset of neuropathological changes in AD brains, thereby having implications in the elucidation of this disease.

Increase of plasma insulin by racecadotril, an inhibitor of enkephalinase, in Wistar rats

Racecadotril is known as an inhibitor of enkephalinase. Increase of plasma insulin by racecadotril has been observed in rats while the mechanism of the action remains obscure. In the present study, intravenous injection of male Wistar rats with racecadotril significantly decreased blood glucose levels. However, this effect of racecadotril was not modified by naloxone at the dose sufficient to block opioid receptors. Thus, the blood glucose-lowering action of racecadotril might be through an endogenous opioid independent mechanism. Otherwise, we found that C-peptide content was also raised by racecadotril in parallel with the increase of insulin in Wistar rats. Thus, the blood glucose-lowering action of racecadotril was related to insulin secretion, but not through the inhibition of plasma insulin degradation. In addition, racecadotril showed no direct effect on insulin secretion in isolated islets or cultured HIT-T15 beta cells. The increase of plasma insulin and blood glucose-lowering action induced by racecadotril were reduced by pretreatment with atropine and enhanced by physotigmine. Direct inhibition of cholinesterase was not observed in brain homogenates treated with racecadotril. Moreover, actions of racecadotril were significantly reduced in rats receiving hemicholinium-3 at a sufficient dose to decrease endogenous acetylcholine. Activation of cholinergic tone is possibly involved in the blood glucose-lowering effect of racecadotril. Our results suggested that racecadotril increased insulin secretion to lower blood glucose mainly via regulation of parasympathetic tone in Wistar rats.

[Changes in the activity of amyloid-degrading metallopeptidases leads to disruption of memory in rats]

In old male Wistar rats (older than 12 months), or adult males (3-4 months) subjected to prenatal hypoxia (7% 02, 3 h, E14), a disruption of short-term memory was observed. The prenatal hypoxia also led to a decrease in the brain cortex expression of metallopeptidases neprilysin (NEP) and endothelin-converting enzyme (ECE-1) which regulate some neuropeptides and are the main beta-amyloid-degrading enzymes. Moreover, a significant decrease (by 2.7 times) in NEP activity in the sensorimotor cortex of old and adult rats subjected to prenatal hypoxia (by 1.7 times) was observed. To confirm possible involvement of these enzymes in memory, the analysis of the effect of microinjections of phosphoramidon (an inhibitor of NEP and ECE-1), and thiorphan (an inhibitor of NEP) into the rat sensorimotor cortex was carried out. In a two-level radial maze test, a disruption of short-term memory was observed 60 and 120 min after i.c. injection ofphosphoramidon (5.9 microg/microl) and 30 and 60 min after i.c. injection of thiorphan (2.5 microg/microl). The involvement of NEP and ECE-1 in short-term memory suggests that a decrease in the level of expression and activity of metallopeptidases involved in metabolism of beta-amyloid peptide (Abeta) and other neuropeptides is one of the main factors in disruption of cognitive functions after prenatal hypoxia or in the process of ageing.

Decrease of dehydrogenase activity of cerebral glyceraldehyde-3-phosphate dehydrogenase in different animal models of Alzheimer's disease

Recently, a relationship between glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the beta-amyloid precursor protein (betaAPP) in relationship with the pathogenesis of Alzheimer's disease (AD) has been suggested. Therefore, we studied the specific activity of GAPDH in the different animal models of AD: transgenic mice (Tg2576) and rats treated with beta-amyloid, or thiorphan, or lipopolysaccharides (LPS) and interferon gamma (INFgamma). We observed that GAPDH activity was significantly decreased in the brain samples from TG mice. The injection of beta-amyloid, or thiorphan, an inhibitor of neprilysin involved in beta-amyloid catabolism, in rat brains resulted in a pronounced reduction of the enzyme activity. The infusion of LPS and IFNgamma, which can influence the progression of the AD, significantly reduced the enzyme activity.

Thiorphan, a neutral endopeptidase inhibitor used for diarrhoea, is neuroprotective in newborn mice

Excitotoxic damage appears to be a critical factor in the formation of perinatal brain lesions associated with cerebral palsy (CP). When injected into newborn mice, the glutamatergic analogue, ibotenate, produces cortical lesions and white matter cysts that mimic human perinatal brain lesions. Neuropeptides are neuronal activity modulators and could therefore modulate glutamate-induced lesions. However, neuropeptides are rapidly degraded by peptidases. Racecadotril, which is rapidly metabolized to its active metabolite thiorphan, is a neutral endopeptidase (NEP) inhibitor used in clinical practice for diarrhoea with a remarkable safety profile. This study aimed to test the original hypothesis that thiorphan could be neuroprotective against ibotenate-induced lesions in newborn mice. Intraperitoneal administration of thiorphan reduced ibotenate-induced cortical lesions by up to 57% and cortical caspase-3 cleavage by up to 59%. This neuroprotective effect was long-lasting and was still observed when thiorphan was administered 12 h after the insult, showing a remarkable window for therapeutic intervention. Further supporting the neuroprotective effect of pharmacological blockade of NEP, mouse pups with a genetic deletion of NEP displayed a significantly reduced size of the ibotenate-induced cortical grey matter lesion when compared with wild-type animals. Thiorphan effects were mimicked by substance P (SP) and, in a less potent manner, by neurokinin A. Thiorphan effects were inhibited by blockers of NK1 and NK2 receptors. Real-time reverse transcription-polymerase chain reaction, autoradiography and immunohistochemistry confirmed the expression of NK1 and NK2 receptors in the neonatal murine neocortex. These data demonstrate that thiorphan prevents neonatal excitotoxic cortical damage, an effect largely mediated by SP. Thiorphan could represent a promising drug for the prevention of CP, which remains a challenging disease. In a broader context, these results also raise potential implications for the prevention of neurodegenerative diseases involving glutamate-mediated excitotoxic neuronal death.

Inhibition of neprilysin by thiorphan (i.c.v.) causes an accumulation of amyloid β and impairment of learning and memory

An accumulation of amyloid beta peptide (Abeta) due to an imbalance between anabolism and catabolism triggers Alzheimer's disease (AD). Neprilysin is a rate-limiting peptidase, which participates in the catabolism of Abeta in brain. We investigated whether rats continuously infused with thiorphan, a specific inhibitor for neprilysin, into the cerebral ventricle cause cognitive dysfunction, with an accumulation of Abeta in the brain. Thiorphan-infused rats displayed significant cognitive dysfunction in the ability to discriminate in the object recognition test and spatial memory in the water maze test, but not in other hippocampus-dependent learning and memory tasks. Thiorphan infusion also elevated the Abeta40 level in the insoluble fraction of the cerebral cortex, but not that of the hippocampus. There was no significant difference in the nicotine-stimulated release of acetylcholine in the hippocampus between vehicle- and thiorphan-infused rats. These results indicate that continuous infusion of thiorphan into the cerebral ventricle causes cognitive dysfunction by raising the level of Abeta in the cerebral cortex, and suggest that a reduction of neprilysin activity contribute to the deposition of Abeta and development of AD.

Thiorphan-induced neprilysin inhibition raises amyloid beta levels in rabbit cortex and cerebrospinal fluid

Studies on the pathogenesis of Alzheimer's disease (AD) suggest overproduction of amyloid beta (Abeta) may not be the only pathogenic route to AD. Decreased degradation of Abeta is another possible disease mechanism. Neprilysin is a neutral endopeptidase that has been proposed to be the major enzyme responsible for Abeta degradation. Studies have reported correlations between Abeta deposition and neprilysin activity in the human brain. This study shows that intracerebroventricular infusion of thiorphan, a neprilysin inhibitor, raises cortical and cerebrospinal fluid (CSF) Abeta concentrations in rabbits. Rabbits treated with thiorphan for 5 days had levels of CSF and cortical Abeta40 that were 147 and 142% of the control group, respectively. Results for Abeta42 showed a similar trend. The results indicate that age-related decreases of neprilysin could lead to increased brain concentrations of Abeta, plaque formation, and AD.

Racecadotril versus loperamide: antidiarrheal research revisited

Racecadotril is an enkephalinase inhibitor, presented as a purely antisecretory agent with advantages over the opiate-receptor agonist loperamide in the treatment of diarrhea. A critical review of the literature and the models used was performed. Although pretreatment with high doses of racecadotril reduced cholera toxin-induced secretion and although clinical efficacy was demonstrated in young infants--a population characterized by 10-fold higher plasma enkephalin concentrations compared with adults, the analysis calls into question the peripheral antisecretory selectivity and relative clinical efficacy. Conversely, loperamide can be proposed as an antisecretory agent at therapeutic concentrations. Its efficacy is well established in acute and chronic diarrhea. Current experimental and clinical comparative studies of both drugs have problems with regard to the selection of the doses, the validity of models, and/or the trial design. The conclusion is that more research is needed before reliable conclusions can be drawn on the place of racecadotril in diarrhea treatment.

Hormonal, renal, hemodynamic responses to acute neutral endopeptidase inhibition in heart transplant patients

We investigated the hemodynamic, renal, and hormonal responses to neutral endopeptidase (NEP) inhibition during a 6-h, double-blind, randomized, placebo-controlled study in seven chronic, stable heart transplant patients. Baseline characteristics were similar during both experiments, and no significant changes were observed after placebo. NEP inhibition increased circulating endothelin-1 (from 2.01 +/- 0.1 to 2.90 +/- 0.2 pmol/l; P < 0.01), atrial natriuretic peptide (ANP; from 21.5 +/- 2.7 to 29.6 +/- 3.7 pmol/l; P < 0.01), and the ANP second messenger cGMP. Noteworthy, systemic blood pressure did not increase. Renal plasma flow and glomerular filtration rate remained unmodified after NEP inhibition. Filtration fraction (33 +/- 13%), diuresis (196 +/- 62%), and natriuresis (315 +/- 105%) increased significantly in relation to ANP and cGMP. A strong inverse relationship was observed between excreted cGMP and sodium reabsorption (r = -0.71, P < 0.0001). Thus, despite significantly increasing endothelin-1, NEP inhibition did not adversely influence systemic or renal hemodynamics in transplant patients. ANP, possibly through a tubular action, enhances the natriuresis observed after NEP inhibition.

[Effect of racecadotril in the management of acute diarrhea in infants and children]

BACKGROUND

The treatment of diarrhoea relies on the maintenance or restoration of hydration with maintenance of an adequate nutritional intake. Racecadotril has been shown to reduce the stools output during acute diarrhoea. The present work was aimed at measuring the number of emergency department visits for acute diarrhoea either the children received racecadotril or not.

METHOD

Racecadotril and rehydration were compared with rehydration alone in children aged three months to three years who had acute diarrhoea and were evaluated in the emergency department (ED). The primary end point was the number of medical exams during the week after starting treatment. Secondary end points were the number of stools during the first 48 hours, the duration of the diarrhoea and the weight on day 7.

RESULTS

One hundred and sixty-six children were alternatively randomized to the treated and the control groups. There was no difference for age, degree of dehydration and length of illness before the first visit between the groups. Whatever type of rehydration (oral or i.v.), the treated group had a significant lower number of stools (p < 0.001) and a faster recovery (p < 10(-9)). The children receiving racecadotril needed less additional ED visits for the same episode (p < 0.05). There was no difference for the weight-gain on day 7.

CONCLUSIONS

This study demonstrates the efficacy of racecadotril as adjuvant therapy to oral and i.v. rehydration in the treatment of acute diarrhoea and a fewer emergency department second visit before recovery.

A multinational comparison of racecadotril and loperamide in the treatment of acute watery diarrhoea in adults

BACKGROUND

Racecadotril (acetorphan) is an orally active, potent inhibitor of enkephalinase, which exerts an antihypersecretory effect without increasing intestinal transit time. The aim of this study was to compare the efficacy, safety and tolerability of racecadotril with those of loperamide by assessing their effects on the resolution of the signs and symptoms of diarrhoea in patients in developing countries who had acute watery diarrhoea of less than 5 days' duration.

METHODS

945 outpatients from 21 centres in 14 countries received racecadotril (100 mg) or loperamide (2 mg) three times daily in a single-blind study. Duration of diarrhoea was the primary measure of efficacy; secondary criteria were overall clinical response, occurrence and duration of abdominal pain and distension, and occurrence of other associated signs and symptoms. Occurrence of constipation and adverse events were the main safety assessments.

RESULTS

Diarrhoea resolved rapidly with both racecadotril and loperamide (55.0 h in both groups), 92% of patients on racecadotril and 93% on loperamide being treatment successes. Racecadotril produced a significantly greater reduction in abdominal pain and distension than loperamide (P = 0.024 and 0.03, respectively). The duration of abdominal distension was significantly shorter with racecadotril (5.4 versus 24.4 h; P = 0.0001), and constipation was also significantly less frequent (16% versus 25%; P = 0.001). One-hundred-and-eighty patients (19%) experienced one or more adverse event during the study: 67 (14.2%) in the racecadotril group and 113 (23.9%) in the loperamide group (P = 0.001).

CONCLUSIONS

Racecadotril resolved the symptoms of acute diarrhoea rapidly and effectively, and produced more rapid resolution of abdominal symptoms and less constipation than loperamide.

Effects of chronic neutral endopeptidase inhibition on the progression of left ventricular dysfunction and remodeling in dogs with moderate heart failure

BACKGROUND

The diuretic actions of endogenously produced atrial natriuretic factor (ANF) may be beneficial in the treatment of chronic heart failure (CHF). Neutral endopeptidase (NEP) is the primary enzyme responsible for the degradation of ANF. The present study investigates the effects of long-term NEP inhibition on the progression of left ventricular (LV) dysfunction and remodeling in dogs with moderate heart failure.

METHODS

LV dysfunction was produced in 12 dogs by multiple sequential intracoronary microembolizations. Embolizations were discontinued when LV ejection fraction (EF) was between 30-40%. Two weeks after the last embolization, dogs were randomized to 3 months of oral therapy with the NEP inhibitor ecadotril (100 mg, once daily, n = 6) or to no therapy at all (control, n = 6).

RESULTS

During the 3 months of follow-up, LV EF in control dogs decreased from 37 +/- 1% to 28 +/- 1% (P < 0.01) and LV end-diastolic volume (EDV) and end-systolic volume (ESV) increased (EDV: 72 +/- 3 vs. 84 +/- 5 ml, P < 0.01); ESV: 45 +/- 1 vs. 60 +/- 4 ml, P < 0.01). In dogs treated with ecadotril, LV EF (34 +/- 1% vs. 37 +/- 2%), EDV (79+/- 5 vs. 78+/- 6 ml) and ESV (52 +/- 3 vs. 49 +/- 4) remained essentially unchanged after 3 months of therapy. Histomorphometric measurements at the termination of the study showed that ecadotril was associated with significantly reduced cardiomyocyte hypertrophy compared to control.

CONCLUSION

Early, long-term NEP inhibition with ecadotril prevents the progression of LV dysfunction and attenuates progressive LV remodeling in dogs with moderate heart failure.

Beta-amyloid deposition in the brains of rats chronically infused with thiorphan or lipopolysaccharide: the role of ascorbic acid in the vehicle

Beta-amyloid deposition and neuroinflammation are two important features of Alzheimer's disease (AD) that may influence its progression. Chronic infusion of lipopolysaccharide (LPS) into the rodent 4th ventricle reproduces many of the neurobiological changes seen in AD. Chronic infusion of ascorbic acid containing thiorphan, an inhibitor of the enzyme neprilysin that catabolizes beta-amyloid, into the hippocampus induces extracellular deposition of beta-amyloid fibrils. We investigated whether the combined presence of chronic neuroinflammation could exacerbate the deposition of beta-amyloid induced by thiorphan. The infusion of any solution containing ascorbic acid alone or with thiorphan or LPS increased the level of intraneuronal beta-amyloid immunoreactivity. Solutions that did not contain ascorbic acid were not associated with increased intraneuronal beta-amyloid immunoreactivity. The role of neprilysin in the deposition of beta-amyloid in AD brains remains undetermined.

Endogenous opioids in dopaminergic cell body regions modulate amphetamine-induced increases in extracellular dopamine levels in the terminal regions

Opioid antagonists attenuate behavioral effects of amphetamine and amphetamine-induced increases in extracellular dopamine levels in nucleus accumbens and striatum of rats but do not alter those effects of cocaine. This study was performed to determine 1) if the effect of opioid antagonists on the dopamine response to amphetamine is mediated in either the terminal or cell body region of the nigrostriatal and mesolimbic pathways, and 2) if the enkephalinase inhibitor thiorphan, which slows degradation of endogenous opioid peptides, increases the dopamine response to amphetamine but not to cocaine. Microdialysis probes were placed either into a dopaminergic terminal region or into both a terminal and cell body region of rats. Naloxone methiodide (1.0 microM), a lipophobic opioid antagonist, was administered into either the terminal or cell body region by reverse dialysis, whereas extracellular dopamine was collected in the terminal region. Increases in extracellular dopamine in nucleus accumbens and striatum caused by amphetamine (0.1-6.4 mg/kg, s.c.) were reduced significantly (28-39%) by naloxone methiodide administered into either substantia nigra or ventral tegmentum but not into terminal regions. Thiorphan (10 microM) administered into substantia nigra increased significantly the dopamine response to amphetamine in the ipsilateral striatum by as much as 42% but did not affect the dopamine response to cocaine (3.0-56 mg/kg, i.p.). These results suggest that amphetamine promotes release of endogenous opioids, which, through actions in the ventral tegmentum and substantia nigra, contribute to amphetamine-induced increases in extracellular dopamine in the nucleus accumbens and striatum.

Intracerebroventricular met-enkephalin administration modulates adjuvant arthritis

The purpose of this study was to investigate the anti-inflammatory properties of intracerebroventricular met-enkephalin (met-enk) administration in an animal model of arthritis. Adjuvant arthritis was induced in rats by intradermal inoculation of mycobacterium butyricum and the effects of intraventricular met-enk+thiorphan (enkephalinase inhibitor) were studied. Treatment was initiated either simultaneously with the bacterial inoculation (preventive group) or on post-inoculation day 17 after the appearance of inflammation (treatment group). The degree of inflammation was evaluated by measuring the diameter and the circumference of the ankle joint immediately before the sacrifice (day 31) and by histologic examination of ankle joint sections. The results of this study revealed that combined intraventricular injections of met-enk+thiorphan reduced the arthritic-like inflammation in the preventive group as well as in the treatment group. These findings suggest that centrally applied met-enk+thiorphan may suppress the development adjuvant arthritis as well as the symptoms of manifest arthritis. Thus central met-enk may be involved in both hypothalamic pituitary adrenal axis and immune forms of stress-induced modulation.

Short-term effects of ecadotril in dogs with induced congestive heart failure

OBJECTIVE

To evaluate short-term hemodynamic effects of ecadotril in a model of congestive heart failure in dogs.

ANIMALS

6 conscious adult male dogs.

PROCEDURES

Instruments were placed in dogs to measure left ventricular, aortic, and atrial blood pressures. Heart failure was induced by repeated coronary embolization with latex microspheres. Four times, and in random order, dogs were given vehicle or active drug (3, 10, or 30 mg/kg of body weight) orally. Hemodynamic variables, urine flow, and urinary electrolyte excretion were measured before and 30, 90, and 150 minutes, and 10 and 21 hours after drug administration.

RESULTS

Changes in urine flow, heart rate, mean arterial pressure, or peak positive and negative rate of change in ventricular pressure were not apparent. Urinary sodium excretion significantly increased in response to the low and high doses of ecadotril but not in response to the 10 mg/kg dose. Left ventricular end diastolic pressure (LVEDP) consistently decreased in dose- and time-dependent manner. Maximal group-averaged reductions in LVEDP were 5.2, 8.1, and 10 mm Hg for the low, middle, and high doses, respectively. The magnitude of the decrease in LVEDP was not related to cumulative change in urine flow.

CONCLUSIONS AND CLINICAL RELEVANCE

Orally administered ecadotril reduced left ventricular filling pressures in these dogs by a mechanism that does not require a substantial diuretic effect. Ecadotril may be effective for alleviating clinical signs in dogs with left-sided heart failure and may be particularly beneficial for use in dogs that are refractory to traditional diuretic therapy.

Effects of a neutral endopeptidase inhibitor, BP102, on the development of deoxycorticosterone acetate-salt hypertension in kininogen-deficient Brown Norway Katholiek rats

The nature of all of the peptides critical to the mechanism(s) of the antihypertensive action of neutral endopeptidase (NEP) inhibitors is still unclear, but bradykinin is thought to be one such peptide. This study was designed to assess the effectiveness of an NEP inhibitor in deoxycorticosterone acetate (DOCA)-salt treated kininogen-deficient Brown Norway Katholiek (BN-Ka) rats. Oral administration of BP102 (10-100 mg/kg), an NEP inhibitor, increased urine volume and urinary sodium excretion in a dose-dependent manner in anesthetized Sprague-Dawley rats. DOCA-salt hypertension was induced in both BN-Ka and Brown Norway Kitasato (BN-Ki) rats after left nephrectomy. The development of DOCA-salt hypertension in normal BN-Ki rats was prevented, and that in BN-Ka rats was also significantly reduced, by an 8-day administration of BP102. When BP102 was administered for 5 weeks, the high blood pressure of DOCA-salt treated BN-Ka rats was markedly lowered, and their heart weights were reduced. These results suggest that kinins play no role in the antihypertensive effect of this inhibitor and that other factors may be involved in this effect.

Inhibition of neutral endopeptidase causes vasoconstriction of human resistance vessels in vivo

BACKGROUND

Neutral endopeptidase (NEP) degrades vasoactive peptides, including the natriuretic peptides, angiotensin II, and endothelin-1. Systemic inhibition of NEP does not consistently lower blood pressure, even though it increases natriuretic peptide concentrations and causes natriuresis and diuresis. We therefore investigated the direct effects of local inhibition of NEP on forearm resistance vessel tone.

METHODS AND RESULTS

Four separate studies were performed, each with 90-minute drug infusions. In the first study, 10 healthy subjects received a brachial artery infusion of the NEP inhibitor candoxatrilat (125 nmol/min), which caused a slowly progressive forearm vasoconstriction (12+/-2%; P=0.001). In a second two-phase study, 6 healthy subjects received, 4 hours after enalapril (20 mg) or placebo, an intra-arterial infusion of the NEP inhibitor thiorphan (30 nmol/min). Thiorphan caused similar degrees of local forearm vasoconstriction (P=0.6) after pretreatment with both placebo (13+/-1%, P=0.006) and enalapril (17+/-6%, P=0.05). In a third three-phase study, 8 healthy subjects received intra-arterial thiorphan (30 nmol/min), the endothelin ETA antagonist BQ-123 (100 nmol/min), and both combined. Thiorphan caused local forearm vasoconstriction (13+/-1%, P=0.0001); BQ-123 caused local vasodilatation (33+/-3%, P=0.0001). Combined thiorphan and BQ-123 caused vasodilatation (32+/-1%, P=0.0001) similar to BQ-123 alone (P=0.98). In a fourth study, 6 hypertensive patients (blood pressure >160/100 mm Hg) received intra-arterial thiorphan (30 nmol/min). Thiorphan caused a slowly progressive forearm vasoconstriction (10+/-2%, P=0.0001).

CONCLUSIONS

Inhibition of local NEP causes vasoconstriction in forearm resistance vessels of both healthy volunteers and patients with hypertension. The lack of effect of ACE inhibition on the vasoconstriction produced by thiorphan and its absence during concomitant ETA receptor blockade suggest that it is mediated by endothelin-1 and not angiotensin II. These findings may help to explain the failure of systemic NEP inhibition to lower blood pressure.

Use of antisense oligodeoxynucleotide to delta-opioid receptor mRNA in the study of turnover of delta-opioid receptors in the spinal cord of the mouse

Pretreatment of male ICR mice with an antisense oligodeoxynucleotide to delta-opioid receptor mRNA (DOR AS oligo, 163 pmol) given intrathecally (i.t.) once a day for 1-3 days produced a time-dependent attenuation of antinociception produced by i.t.-challenged [D-Ala2] deltorphin II (6.4 nmol), a delta-opioid receptor agonist. The attenuation of the [D-Ala2]deltorphin II-induced antinociception caused by pretreatment with DOR AS oligo given i.t. daily was blocked by co-pretreatment with naltriben (14.5 nmol), a delta-opioid receptor antagonist, but was markedly enhanced by concomitant pretreatment with thiorphan (19.7 nmol) or bestatin (14.5 nmol), which inhibits the degradation of endogenously released Met-enkephalin. Concomitant pretreatment with antiserum to Met-enkephalin, but not with antiserum to Leu-enkephalin, beta-endorphin or dynorphin A (1-17), and DOR AS oligo given i.t. daily for 3 days prevented the attenuation of i.t.-challenged [D-Ala2]deltorphin II-induced antinociception caused by the DOR AS oligo pretreatment. Our results support the existence of a turnover of delta-opioid receptors in the mouse spinal cord caused by the release of Met-enkephalin.

Role of neutral endopeptidase in exercise-induced bronchoconstriction in asthmatic subjects

The membrane-bound metalloproteinase, neutral endopeptidase (NEP), is a degrading enzyme of both bronchoconstrictor and bronchodilator peptides within the airways. To examine the role of NEP in exercise-induced bronchoconstriction (EIB) in asthmatic subjects, we used inhaled thiorphan, a NEP inhibitor, as pretreatment to a 6-min standardized exercise challenge. Thirteen clinically stable asthmatic subjects participated in this double-blind, placebo-controlled, crossover study that was performed on 2 days separated by 48 h. Thiorphan was administered by two inhalations of 0.5 ml containing 1.25 mg/ml. Subsequently, exercise was performed on a bicycle ergometer at 40-50% of predicted maximal voluntary ventilation while inhaling dry air (20 degrees C, relative humidity 6%). The airway response to exercise was measured by forced expiratory volume in 1 s (FEV1) every 3 min, up to 30 min postexercise challenge, and was expressed both as the maximal percent fall in FEV1 from baseline and as the area under the time-response curve (AUC) (0-30 min). The acute effects of both pretreatments on baseline FEV1 were not different (P > 0.2), neither was there any difference in maximal percent fall in FEV1 between thiorphan and placebo (P > 0.7). However, compared with placebo, thiorphan reduced the AUC by, on average, 26% [AUC (0-30 min, +/-SE): 213.6 +/- 47.7 (thiorphan) and 288.6 +/- 46.0%fall.h (placebo); P = 0.047]. These data indicate that NEP inhibition by thiorphan reduces EIB during the recovery period. This suggests that bronchodilator NEP substrates, such as vasoactive intestinal polypeptide or atrial natriuretic peptide, modulate EIB in patients with asthma.

The effect of inhaled thiorphan on allergen-induced airway responses in asthmatic subjects

BACKGROUND

Neuropeptides are likely to be implicated in the pathophysiology of allergen-induced airway responses. However, upon release in the airways, neuropeptides are potentially inactivated by neutral endopeptidase (NEP).

OBJECTIVE

We hypothesized that NEP-inhibition by inhaled thiorphan (TH) would increase allergen-induced early (EAR) and late (LAR) asthmatic responses, and allergen-induced airway hyperresponsiveness to histamine in asthmatic subjects in vivo. The dose and dosing intervals of TH were derived from previous pharmacokinetic and dose-finding studies.

METHODS

Nine non-smoking, atopic, asthmatic men with dual asthmatic responses to inhaled house-dust mite extract participated in a double-blind, placebo-controlled, cross-over study. During each study period PC20 histamine was measured 24 h before, and 3 and 24 h post-allergen. TH (1.25 mg/mL, 0.5 mL) or placebo (P) were aerosolized pre-allergen, and three times at 2 h intervals post-allergen (total dose of TH: 2.5 mg). Forced expiratory volume in one second (FEV1) was recorded and expressed as percentage fall from baseline. The EAR (0-3 h) and the LAR (3-8 h) were defined as maximum percentage fall from the pre-allergen baseline and as corresponding areas under the time-response curves (AUC).

RESULTS

As compared with P, TH failed to induce an acute effect on FEV1 at any of the timepoints (P > 0.08). There was no significant difference between P and TH in the EAR and the LAR: neither in terms of maximum percentage fall from baseline (mean +/- SEM: EAR: 22.3 +/- 4.7% (P) and 20.4 +/- 4.1% (TH), P = 0.75; LAR: 25.2 +/- 4.7% (P) and 26.4 +/- 5.8% (TH), P = 0.77) nor in terms of AUC (P = 0.76). Correspondingly, the changes in PC20 histamine were not different between the two treatments (P > 0.40).

CONCLUSION

We conclude that four adequate doses of the inhaled NEP-inhibitor, thiorphan, failed to potentiate allergen-induced airway responses in asthma. These results suggest that either neuropeptides do not play a predominant role in allergen-induced airway responses, or that allergen challenge induces NEP-dysfunction in humans in vivo.

The tachykinin NK2 receptor antagonist SR 48968 inhibits citric acid-induced airway hyperresponsiveness in guinea pigs

Airway hyperresponsiveness is a main feature of asthma, and several lines of evidence suggest that tachykinins might be involved in the pathogenesis of airway hyperresponsiveness in rodents. We conducted a study designed to describe an original model of airway hyperresponsiveness induced by citric acid administered as aerosol to guinea pigs, and to investigate the effects of the nonpeptide neurokinin1 (NK1) and neurokinin2 (NK2)-receptor antagonists, SR 140333 and SR 48968, respectively, on the development of this airway hyperresponsiveness. Animals received thiorphan 1 mg/kg intraperitoneally and 30 min later were exposed to an aerosol of citric acid 0.4 M for 1 h. After 24 h, the animals were anesthetized and ventilated. Airway hyperresponsiveness was evidenced by significant shifts to the left of dose-response curves for intravenous acetylcholine (ACh) without a change in maximum responses to ACh. Exposure to citric acid induced an airway hyperresponsive that was abolished by chronic pretreatment with capsaicin (120 mg/kg, 5 d before citric acid exposure). SR 48968 1 mg/kg intraperitoneally, given once at 30 min before the citric acid exposure, inhibited airway hyperresponsiveness, whereas SR 140333 1 mg/kg or codeine 30 mg/kg given under similar conditions did not. The inhibition of airway hyperresponsiveness by SR 48968 did not result from functional antagonism, since SR 48968 did not affect ACh-induced bronchoconstriction, nor did it result from inhibition of tachykinin, which could have been released under the influence of ACh in hyperresponsive animals, since SR 48968 given after the exposure to aerosolized citric acid failed to inhibit airway hyperresponsiveness. In conclusion, these results show that inhaled citric acid can induce the development of an airway hyperresponsiveness in the guinea pig through a release of tachykinins, and also demonstrate that NK2-receptor stimulation plays a predominant role in the development of airway hyperresponsiveness.

Effect of inhaled thiorphan, a neutral endopeptidase inhibitor, on the bronchodilator response to inhaled atrial natriuretic peptide (ANP)

BACKGROUND

The hormone atrial natriuretic peptide (ANP) causes bronchodilation and partially protects against direct and indirect bronchial challenges. Both in vitro and in vivo studies have found that the protective effect of ANP against bronchoconstriction is enhanced by inhibition of the enzyme neutral endopeptidase (NEP). It was hypothesised that pretreatment with thiorphan, an NEP inhibitor, might enhance the bronchodilator response to inhaled ANP.

METHODS

In a randomised double blind placebo controlled crossover study, six asthmatic patients (one woman) of mean (SD) age 47.3 (3.8) years and forced expiratory volume in one second (FEV1) 1.91 (0.42) 1, 55 (3.8)% predicted, were studied. All were shown at screening to have at least a 25% improvement in FEV1 to inhaled salbutamol. On five study visits the patients received either thiorphan 1 mg (in 2 ml) followed by ANP 5 mg or placebo (saline), or placebo (saline) followed by ANP (5 mg), placebo or salbutamol 5 mg. Spirometric parameters were measured after each inhalation and thereafter for the next two hours.

RESULTS

ANP alone caused a bronchodilator response up to 15 minutes when compared with placebo or thiorphan alone with a mean (SE) change in FEV1 of 16.8 (8.1)% and 16.1 (6.8)% at 10 and 15 minutes from baseline, respectively. Prior inhalation of thiorphan prolonged the duration of the bronchodilator effect of ANP up to 60 minutes with a mean (SE) change in FEV1 of 23.1 (3.4)% at 60 minutes. There was no difference in the maximum degree of bronchodilation following the administration of ANP alone compared with the combination of thiorphan and ANP. The degree and duration of the bronchodilator response produced by ANP, or the combination of the NEP inhibitor and ANP, were less than that produced by salbutamol.

CONCLUSIONS

These results confirm that, at least in part, the bronchodilator response to inhaled ANP is modulated by NEP. Analogues of ANP which are stable to NEP may have greater bronchodilator activity than ANP in the treatment of asthma.

Analgesia produced by immobilization stress and an enkephalinase inhibitor in amphibians

The role of endogenous opioids in modulating pain transmission in amphibians was examined by two methods known to activate endogenous opioids in mammals. Analgesia was assessed using the acetic acid test in the Northern grass frog, Rana pipiens. One or 2 h of immobilization produced a significant analgesia lasting for at least 90 min. Systemic, but not spinal, administration of naloxone before immobilization prevented the analgesic effects seen in saline-pretreated controls. Spinal administration of the enkephalinase inhibitor, thiorphan, but not bestatin (both at 100 nmol/frog), produced significant analgesia. The analgesic effect of thiorphan was blocked by coadministration of intraspinal naloxone. These data are the first to suggest a role for endogenous opioid modulation of noxious stimuli in lower vertebrates by examination of stress-induced analgesia and the action of agents that inhibit enkephalin degradation.

Interaction of a neutral endopeptidase inhibitor with an ANP-C receptor ligand in anesthetized dogs

Inhibition of important degradative pathways of atrial natriuretic peptide (ANP) in vivo could be a valuable therapeutic tool for regulating endogenous levels of ANP. The aim was to investigate the in vivo effects of both blockade of atrial natriuretic peptide clearance receptor and inhibition of neutral endopeptidase 24.11, an enzyme shown to be involved in ANP breakdown. Therefore, we infused a specific neutral endopeptidase inhibitor ((S)-thiorphan) and an ANP-C receptor ligand (AP 811) alone or in combination into anaesthetized beagle dogs. Compared with vehicle controls, coadministration of (S)-thiorphan and AP 811 (100 micrograms/kg/min and 10 micrograms/kg/min, resp.) had greater effects on endocrine and renal parameters than administration of either substance alone. Coadministration of both compounds increased urinary excretion of volume and sodium, cGMP and ANP. We found also increased plasma cGMP, plasma ANP and decreased plasma renin activity. No effects were observed with respect to blood pressure, left ventricular pressure or heart rate during the infusion period of 2 h. We conclude from these investigations, that blocking both degrading pathways of ANP with the ANP-C receptor ligand AP 811 and the neutral endopeptidase inhibitor (S)-thiorphan is more effective than inhibition of either system alone. Such a combination might therefore be a useful therapeutic tool in cardiovascular diseases.

Effect of inhaled atrial natriuretic peptide and a neutral endopeptidase inhibitor on histamine-induced bronchoconstriction

The reduced ability of inhaled compared with intravenous atrial natriuretic peptide (ANP) to modify bronchial reactivity and tone may be due to degradation of the peptide by neutral endopeptidase (NEP) within the airways. To test this hypothesis, we have examined the effect of thiorphan, an NEP inhibitor, on the protection afforded by inhaled ANP against histamine-induced bronchoconstriction in 10 mildly asthmatic patients. Pretreatment with ANP alone attenuated the bronchoconstrictor response to histamine with a mean (SEM) maximum percent fall in FEV1 after histamine of 15.9 (2.9) (p < 0.05) compared with 24 (2.9) after placebo and 24 (4) after pretreatment with thiorphan alone. Prior inhalation of thiorphan greatly enhanced the ANP effect: the mean maximum percent fall after this combination was 5.1 (2.3) (p < 0.01, compared with ANP alone). Our results suggest that airway NEP is important in modulating the effect of inhaled ANP. It may be possible to exploit the guanylyl cyclase pathway, by which ANP acts, in the treatment of asthma by the administration of ANP analogues stable to neutral endopeptidase.

Enkephalinase-blocking agent thiorphan affects cell growth and differentiation in long term culture of mouse bone marrow

Enkephalinase-blocking agent thiorphan was added to long-term cultures of mouse bone marrow cells at the time of culture initiation (time 0) or 2 weeks thereafter, when the stromal layer appears. Cellularity, cell morphology (in cytospin smears) and the yield of granulocyte-macrophage progenitor cells (GM-CFC assay in agar) were recorded. Low concentrations of thiorphan accelerated recovery of the cultures after an initial drop of the cell count. Expansion and maturation of the granulocytic lineage was promoted, with parallel decline of the GM-CFC yield. Thiorphan probably interfered with the activity of enkephalinase (endopeptidase 24.11) in the cultures. That enzyme is the CD10 surface marker (CALLA) of lymphoid, myeloid and stromal elements.

Neutral endopeptidase inhibitor thiorphan increases airway narrowing to inhaled sodium metabisulfite in normal subjects

Possible mechanisms involved in inhaled sodium metabisulfite (MBS)-induced bronchoconstriction include cholinergic reflex and release of tachykinins from sensory nerve endings. Tachykinins are potent bronchoconstrictors cleaved and inactivated by neutral endopeptidase (NEP) in the airways. To investigate the role of tachykinins in airway response to MBS, we assessed the effect of NEP-inhibitor thiorphan on airway response to MBS in nine nonatopic, nonasthmatic subjects. Two inhalational challenges with doubling doses of MBS (0.03 to 16 mumol) were performed 3 d apart. Ten minutes before MBS challenge, subjects randomly inhaled either thiorphan (1.25 mg) or placebo according to a double-blind cross-over design. Airflow at 30% of vital capacity (V30p) from partial expiratory flow-volume curves was measured at baseline, 10 min after thiorphan or placebo, and 2 min after each MBS dose. The dose of MBS causing 40% fall in V30p (PD40V30p) was calculated. Neither thiorphan nor placebo affected baseline airway caliber. Thiorphan caused a leftward shift of the dose-response curve to MBS. After placebo a measurable PD40V30p was obtained in four of nine subjects. In these subjects PD40V30p fell significantly after thiorphan inhalation. Four of five subjects who did not exhibit PD40V30p after placebo showed measurable PD40V30p after thiorphan. Percent fall in V30p caused by highest dose of MBS was significantly greater after thiorphan compared with placebo (55.9 +/- 4.6% versus 30.8 +/- 5.6%; mean +/- SE; p < 0.001). Results of this study demonstrate that the NEP-inhibitor thiorphan increases MBS-induced bronchoconstriction in normal subjects, suggesting that tachykinins are involved in airway responses to inhaled MBS.

Effect of an inhaled neutral endopeptidase inhibitor, thiorphan, on airway responsiveness to leukotriene D4 in normal and asthmatic subjects

Cysteinyl leukotrienes are potent inflammatory mediators that are considered to play a role in the pathophysiology of asthma. It can be postulated that leukotrienes exert their bronchoconstricting effects, in part, through secondary release of endogenous neuropeptides. We examined the effect of inhaled thiorphan, an inhibitor of a neuropeptide degrading enzyme, on the concentration-response curve to leukotriene D4 (LTD4) in a two-period, double-blind, cross-over and placebo-controlled study, in 16 nonasthmatic and 12 asthmatic subjects. Thiorphan or placebo were aerosolized and administered in two 0.5 ml doses of 1.25 mg.ml-1 each, 10 min prior to LTD4 inhalation. The airway response was measured by forced expiratory volume in one second (FEV1) and partial expiratory flow-volume curves (expiratory flow at 40% of forced vital capacity; V40p), and expressed as % fall from baseline. Complete concentration-response curves to inhaled LTD4 were recorded and characterized by their position (provocative concentration producing a 20% fall in FEV1 and a 40% fall in V40p; PC20FEV1 and PC40 V40p) and, in the nonasthmatics, also by the maximal-response plateau (MFEV1, MV40p). Post-pretreatment baseline values of FEV1 and V40p were not different between thiorphan and placebo pretreatment. In both groups of subjects, there was no significant difference in lnPC40V40p or lnPC20FEV1 to LTD4 between the two pretreatments mean difference +/- SD (in doubling concentrations): 0.12 +/- 0.73 and -0.19 +/- 1.23, respectively, in asthmatics; and 0.17 +/- 0.95 and -0.99 +/- 1.95, respectively, in nonasthmatics. The maximal-response plateau could not be obtained in the majority of the asthmatic subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesic potency of S-acetylthiorphan after intravenous administration to mice

As hydrolysis in serum of acetorphan to acetylthiorphan (N-[(R,S)-3-acetylmercapto-2-benzylpropanoyl]glycine) has been evidenced, both the neutral endopeptidase inhibition in vitro by acetylthiorphan and analgesic potency of acetylthiorphan after intravenous administration to mice in two analgesic models, the hot-plate and the tail-flick tests, were compared with those of thiorphan and acetorphan. Acetylthiorphan showed a decreased degree of neutral endopeptidase inhibition (IC50 = 316 +/- 38 nM) compared to thiorphan (IC50 = 1.8 +/- 0.2 nM). After intravenous administration followed by the hot-plate jump latency test, acetylthiorphan elicited a degree of analgesia equivalent to that with acetorphan but longer lasting. Like acetorphan and thiorphan, acetylthiorphan was devoid of analgesic activity in the tail-flick test. The results indicated that S-acetylation of the thiol function in acetylthiorphan ensures sufficient lipophilicity to permit crossing of the blood-brain barrier and that acetylthiorphan acts via a prodrug mechanism.

The effects of orally active enkephalinase inhibitors on morphine withdrawal syndrome

Considerable evidence has accumulated to suggest that intracerebroventricular administration of enkephalinase inhibitors, which do not penetrate the blood-brain barrier, significantly attenuates opioid withdrawal syndrome. Therefore, the aim of this study was to examine the effect of intraperitoneal (i.p.) administration of orally active enkephalinase inhibitors, acetorphan (2.5-20 mg kg-1) and SCH 34826 (15-120 mg kg-1). These drugs significantly decreased the severity of the naloxone precipitated withdrawal syndrome in morphine dependent rats and mice. It therefore appears that these orally active enkephalinase inhibitors are promising tools in studying modulation of opioid dependence phenomena.

Different effects of peptidase inhibitors on dermophin- and on [D-Arg2] dermorphin-induced antinociceptive activity

The antinociceptive effects produced by the intracerebroventricular (i.c.v.) injection of dermorphin and [D-Arg2]dermorphin were compared in conscious mice, using the combined administration of peptidase inhibitors. Nociception was assessed using a tail pressure assay. Dermorphin-induced antinociception was not potentiated by simultaneous administration of amastatin or captopril as judged from the ED50 values. Co-administration of dermorphin and amastatin gave a longer duration than with dermorphin alone, whereas there was no significant effect on duration with captopril. The antinociceptive activity of dermorphin was significantly enhanced when the heptapeptide was injected simultaneously with both peptidase inhibitors. This result indicates that the heptapeptide sequence is required for the full expression of intrinsic opioid activity of dermorphin. In contrast, co-administration of amastatin brought about a significant enhancement of the antinociceptive activity induced by i.c.v. administration of [D-Arg2]dermorphin, whereas the effect of [D-Arg2]dermorphin was markedly decreased by the concurrent administration of captopril or thiorphan. The potency of captopril was much greater than that of thiorphan. The present results suggest that [D-Arg2]dermorphin may be transformed metabolically to a peptide which has potent antinociceptive activity.

Chronic inhibition of enkephalinase induces changes in the antinociceptive and locomotor effects of the enkephalinase inhibitor acetorphan in rats

The enkephalinase inhibitor thiorphan was infused intracerebroventricularly in rats during 14 days (25 micrograms/5 microliters/hr), inducing an average inhibition of cerebral enkephalinase of about 65%. Animals were tested during the infusion for their response to acetorphan, a parenterally active derivative of thiorphan. When administered intravenously on day 8 of the infusion, acetorphan (5 mg/kg) significantly increased locomotion in chronic saline-infused rats but not in animals receiving thiorphan. Furthermore, when injected at the same dose on day 10, acetorphan did not modify the latency to jump, in the hot plate test, in thiorphan-treated rats, whereas it elicited a significant analgesia in chronic saline-treated controls. These data show that the effects induced by the administration of an enkephalinase inhibitor were diminished after a period of chronic inhibition of the enzyme, suggesting the development of tolerance.

Effect of sinorphan, an enkephalinase inhibitor, on plasma atrial natriuretic factor and sodium urinary excretion in cirrhotic patients with ascites

We examined the acute effects of sinorphan, an inhibitor of enkephalinase, on plasma atrial natriuretic factor (ANF) and urinary sodium excretion in cirrhotic patients with ascites. A single oral dose of sinorphan (100 or 30 mg in 11 and 5 patients, respectively) was administered against placebo according to a double blind cross-over protocol. Basal plasma ANF levels varied over a large range between 2.6-79 pmol/L. Sinorphan, at a dose of 100 mg, inhibited 70% of plasma enkephalinase activity 60 min after ingestion and elicited simultaneously an increase in plasma ANF and cGMP levels 1.8 and 1.5 times basal values, respectively. There was a transient increase in sodium urinary output without a change in creatinine clearance over the initial 2-h period following drug administration. An increase in urinary cGMP was also observed on a longer period of 6 h. Plasma aldosterone decreased significantly, but the lowest concentration was reached 1 h later than the peak of plasma ANF. Mean blood pressure and PRA were unmodified. The effects of 30 mg sinorphan on plasma ANF, cGMP, and aldosterone were also significant, but less marked than those of the higher dose. Therefore, enkephalinase inhibition transiently increases sodium urinary excretion in cirrhotic patients with ascites via a mechanism that is likely to imply reduction of ANF catabolism. These results suggest that ANF could play a role in the control of sodium homeostasis in liver cirrhosis with ascites.

Atriopeptin regulation and renal function in conscious spontaneously hypertensive rats

We examined the interaction of a non-guanylate cyclase-linked atriopeptin (AP) binding site ligand, SC-46542 (des[Phe106,Gly107,Ala115,Gln116]AP-(103-126], and an endopeptidase 24.11 inhibitor, thiorphan, on mean arterial pressure, urinary sodium excretion, urinary cyclic guanosine monophosphate (cGMP) excretion, plasma cGMP concentration, and plasma AP immunoreactivity (ir) in conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Compared to vehicle control rats, coadministration of SC-46542 and thiorphan increased urinary sodium excretion in SHR from 2.1 +/- 0.3 to 11.6 +/- 0.7 microEq/min/100 g body weight and in WKY from 1.6 +/- 0.4 to 4.4 +/- 0.4 microEq/min/100 g body weight, and increased urinary cGMP excretion in SHR from 2.7 +/- 0.5 to 79.0 +/- 17.5 pmol/min/100 g body weight and in WKY from 7.0 +/- 3.0 to 72.4 +/- 10.6 pmol/min/100 g body weight. The change in urinary sodium excretion was greater in SHR than WKY. The coadministration of SC-46542 and thiorphan had greater effects on urinary sodium excretion and urinary cGMP excretion than administration of either compound alone. Coadministration of thiorphan and SC-46542 had no effect on glomerular filtration rate or plasma cGMP concentration, suggesting that the urinary cGMP excretion response was nephrogenous. Compared to vehicle control rats, plasma APir was increased during coadministration of SC-46542 and thiorphan in both SHR (998 +/- 76 v 5.10 +/- 116 pg/mL) and WKY (775 +/- 36 v 414 +/- 36 pg/mL).(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselective protection of exogenous and endogenous atrial natriuretic factor by enkephalinase inhibitors in mice and humans

We compared the relative potencies of sinorphan and retorphan, the S- and R-enantiomers of acetorphan a potent inhibitor of enkephalinase (EC 3.4.34.11), to inhibit membrane metalloendopeptidase in vivo and to protect exogenous and endogenous ANF after oral administration. In mice, sinorphan was 2-3 fold as potent as retorphan in inhibiting the specific in vivo binding of [3H]acetorphan to kidney enkephalinase. The same potency ratio was found for the enhancement of trichloroacetic acid-precipitated radioactivity in kidneys of mice that had received 125I-ANF, which is used as a test for the protection of the hormone against inactivation in vivo. In nine healthy human volunteers who had received a low oral dosage of sinorphan or retorphan in a double-blind, placebo-controlled, randomized trial, sinorphan was also 2-3 fold more potent than retorphan in inhibiting plasma enkephalinase activity. These effects were accompanied by a related rise in plasma ANF immunoreactivity, which also reflected the difference in the effectiveness of the two compounds. Sinorphan was also more potent than retorphan in enhancing urinary cyclic GMP excretion and sodium excretion in five of these subjects. These data indicate that, in humans as in rodents, enkephalinase plays a crucial role in the inactivation of ANF, its partial inhibition in vivo being accompanied by a significant protection of the exogenous or endogenous hormone as well as by typical ANF-like responses. Thus orally administered sinorphan appears to be a promising compound for therapeutic use in cardiovascular and renal diseases in which ANF has been postulated to exert beneficial effects.

Diuretic and natriuretic responses in rats treated with enkephalinase inhibitors

Rat atrial natriuretic factor (125I-rANF, 99-128) is hydrolysed by pure enkephalinase (EC 3.4.24.11) in vitro at a rate similar to that of 125I-hANF. Trichloroacetic precipitated radioactivity was significantly elevated in the kidneys of rats pretreated with acetorphan, an enkephalinase inhibitor, and receiving 125I-rANF, indicating that the exogenous hormone was protected against degradation. A single oral administration of acetorphan elicited diuretic and natriuretic effects in conscious normotensive rats and natriuretic effects in spontaneously hypertensive rats, effects which were not accompanied by significant changes in kaliuresis. The diuretic and natriuretic effects were still observed in conscious normotensive rats after three days of repeated administration of the drug. In conscious or anesthetized rats in which volume expansion was elicited by hydroelectrolytic loads, the initial rate of urinary elimination of water and sodium was nearly doubled by treatment with enkephalinase inhibitors. This effect was prevented by coadministration of an ANF antiserum, which suggests that the effect was mediated by endogenous ANF. These various observations suggest that enkephalinase inhibitors protect endogenous ANF from degradation and thereby enhance the typical renal effects of the hormone.

Intrathecal administration of thiorphan, bestatin, desipramine and fluoxetine differentially potentiate the antinociceptive effects induced by beta-endorphin and morphine, administered intracerebroventricularly

The effects of the intrathecal injection of thiorphan (an inhibitor of enkephalinase inhibitor), bestatin (an inhibitor of aminopeptidase), desipramine (an inhibitor of the uptake of noradrenaline) and fluoxetine (an inhibitor of the uptake of serotonin) on the antinociception induced by beta-endorphin and morphine, administered intracerebroventricularly, were studied in male ICR mice. Antinociceptive effects were assessed by the tail-flick and hot-plate tests. Thiorphan (16 micrograms) and bestatin (16 micrograms), injected intrathecally, potentiated inhibition of the tail-flick response, induced by beta-endorphin but not by morphine administered intracerebroventricularly, whereas desipramine (6 micrograms) and fluoxetine (6 micrograms), injected intrathecally potentiated inhibition of the tail-flick response induced by morphine, but not by beta-endorphin, administered intracerebroventricularly. Thiorphan, bestatin, desipramine or fluoxetine, given intrathecally, did not antagonize inhibition of the hot-plate response, induced by beta-endorphin or morphine administered intracerebroventricularly. The results indicate that inhibition of the tail-flick response, induced by beta-endorphin administered intracerebroventricularly, is mediated by the opioid system, but not by noradrenergic and serotonergic systems in the spinal cord. On the other hand, the inhibition of the tail-flick response, induced by morphine given intracerebroventricularly, is mediated by noradrenergic and serotonergic systems, but not by the opioid system in the spinal cord. The lack of effect of enzyme inhibitors and inhibitors of the uptake of biogenic amines intrathecally on beta-endorphin- and morphine-induced inhibition of the hot-plate response, is due to the supraspinal nature of the nociceptive hot-plate response. The present results further support the hypothesis, proposed previously, that intracerebroventricularly injected beta-endorphin and morphine elicit antinociception by activating different descending inhibitory systems.

Role of endogenous enkephalins in locomotion and nociception studied with peptidase inhibitors in two inbred strains of mice (C57BL/6J and DBA/2J)

Acetorphan, a parenterally active enkephalinase inhibitor, induced dose-dependently a naloxone-reversible analgesia on the hot-plate jump test in DBA/2J (DBA2) mice but was devoid of effects in C57BL/6J (C57) mice. By contrast, acetorphan increased locomotion in both strains; however, the DBA2 strain was much more sensitive than C57 mice to the locomotor stimulant effect. The increased locomotion was antagonized by naloxone in both strains. These data suggest that endogenous enkephalins modulate nociception and locomotion in the two inbred strains differently.

Augmented analgesic effects of enkephalinase inhibitors combined with transcranial electrostimulation

The analgesic effects of very low current transcranial electrostimulation are naloxone-reversible and thus presumably mediated by endogenous opioid activity. The present experiments indicate that blocking enkephalinase activity by i.c.v. thiorphan or i.p. acetorphan results in an increased analgesic effect of electrostimulation as measured by the 50 degrees C wet tail flick test. In the case of each drug, rats receiving both drug and electrostimulation displayed significantly more analgesia than rats receiving electrostimulation and injection vehicle alone, rats receiving drug and sham stimulation or rats receiving vehicle and sham stimulation.