Intravenous morphine reduces plasma endothelin 1 concentration through activation of neutral endopeptidase 24.11 in patients with myocardial infarction

Reduced release of vasoconstrictors from the porcine heart after repeated periods of ischemia

OBJECTIVE

To examine if the decline in post-ischemic hyperemic flow after repeated brief periods of myocardial ischemia is accompanied by augmented cardiac release of the vasoconstrictors endothelin-1 (ET-1) and norepinephrine (NE).

DESIGN

Mid-LAD (left anterior descending coronary artery) was occluded for 10 min with 30 min intervals a total of four times in six anesthetized pigs. Blood from the anterior interventricular coronary vein was drained through a shunt to the right atrium to facilitate blood sampling. Plasma concentrations of ET-1 and NE were repeatedly measured in arterial and coronary venous blood to estimate cardiac vasoconstrictor release.

RESULTS

Plasma concentrations of ET-1 and NE remained unaltered, but cardiac release of both vasoconstrictors rose briefly during reperfusion due to the hyperemia. However, release declined progressively after repeated periods of ischemia and reperfusion and amounted to 53% (NE) and 17% (ET-1) of initial release after the fourth period of ischemia.

CONCLUSION

The decline in post-ischemic hyperemia after repeated brief periods of myocardial ischemia is not accompanied by a progressive accentuation of cardiac ET-1 and NE release.

[Is morphine still the analgesic of choice in acute myocardial infarction?]

Chest pain is the most common symptom of patients who present with ischemic heart disease. Morphine has traditionally been the drug of choice for managing chest pain in acute coronary syndrome (ACS) due to its high analgesic potency, though its physiological effects are poorly understood. Routinely used for managing chest pain, morphine is recommended in the 2002 guidelines of the American College of Cardiology/American Heart Association. This recommendation, however, is not based on a high level of scientific evidence but on expert opinion. Studies have found both for and against the use of morphine in ACS, suggesting that its benefits are perhaps not altogether clear. This review examines the pathophysiological effects of morphine and their cardiac implications, with special attention to a possible negative effect on ACS. We reviewed articles in the MEDLINE database from 1982 to 2006.

Aminopeptidase activity in the postmortem brain of human heroin addicts

Several studies have reported that the chronic administration of opioids induces changes in the biosynthesis of endogenous opioid peptides or their precursors in specific brain regions of the adult central nervous system. However, little is known about the catabolic regulation of opioid peptides and its contribution to neuroadaptative changes underlying drug addiction. In the present study, we have analyzed the activity of two enkephalin-degrading enzymes (puromycin-sensitive aminopeptidase or PSA and aminopeptidase N or APN) and two functionally different, soluble aminopeptidases (aminopeptidase B and aspartyl-aminopeptidase) in postmortem samples of prefrontal cortex and caudate nucleus of eight human heroin addict brains and eight matched-controls. Enzyme activities were fluorimetrically measured using beta-naphthylamide derivatives. An increase in the activity of soluble PSA in the prefrontal cortex of heroin abusers was observed (heroin addict group: 51,452+/-3892 UAP/mg protein versus control group: 42,003+/-2597 UAP/mg protein; P<0.05), while the activity of the other peptidases in both brain regions remained unaltered. This result agrees with previous findings in morphine-tolerant rats, and indicates that soluble PSA may be involved in neurobiological processes which underlie heroin addiction.

Influence of morphine and naloxone on endothelin and its receptors in newborn piglet brain vascular endothelial cells: clinical implications in neonatal care

The present study examines the hypothesis that morphine exposure alters newborn brain vascular endothelial cell production of endothelin (ET)-1, as well as the mRNA expression of its receptors. Newborn piglet vascular endothelial cells were treated with morphine (100 ng/mL media), naloxone (100 ng/mL media), or drug-free media (control) for 6, 24, 48, and 96 h. Media was analyzed for ET-1 and big ET-1 levels and the cells were assessed for ETA and ETB receptor mRNA expression. Morphine exposure progressively increased ET-1 production from 6 to 96 h with concurrent reductions in big ET-1 levels starting at 24 h to almost undetectable levels by 96 h. Whereas ETA receptor mRNA expression increased 2-fold at 6 h and 4-fold at 96 h, ETB receptor mRNA expression remained unchanged. Naloxone exposure caused significant decreases in ET-1 levels, whereas an opposite effect was noted in big ET-1 levels, which increased from 6 through 96 h. Naloxone caused a progressive decrease in ETA receptor mRNA expression at 6 h through 96 h and a 2-fold increase in ETB receptor mRNA expression at 48 and 96 h. Increased ET-1 and its receptors in response to morphine may suggest altered cerebrovascular perfusion and brain metabolism in the immature piglet brain.

Synthesis and degradation of endothelin-1

The endothelin-converting enzyme (ECE) is the main enzyme responsible for the genesis of the potent pressor peptide endothelin-1 (ET-1). It is suggested that the ECE is pivotal in the genesis of ET-1, considering that the knockout of both genes generates the same lethal developments during the embryonic stage. Several isoforms of the ECE have been disclosed, namely ECE-1, ECE-2, and ECE-3. Within each of the first two groups, several sub-isoforms derived through splicing of single genes have also been identified. In this review, the characteristics of each sub-isoform for ECE-1 and 2 will be discussed. It is important to mention that the ECE is, however, not the sole enzyme involved in the genesis of endothelins. Indeed, other moieties, such as chymase and matrix metalloproteinase II, have been suggested to be involved in the production of ET intermediates, such as ET-1 (1-31) and ET-1 (1-32), respectively. Other enzymes, such as the neutral endopeptidase 24-11, is curiously not only involved in the degradation and inactivation of ET-1, but is also responsible for the final production of the peptide via the hydrolysis of ET-1 (1-31). In this review, we will attempt to summarize, through the above-mentioned characteristics, the current wisdom on the role of these different enzymes in the genesis and termination of effect of the most potent pressor peptide reported to date.

Effects of morphine administration and its withdrawal on rat brain aminopeptidase activities

The endogenous opioid neuropeptide system seems to be involved in the neural processes which underlie drug addiction. Several studies have reported that the administration of morphine induces changes in the levels and/or activity of endogenous opioid peptides (enkephalin, dynorphin) and their precursors in specific brain regions of the adult CNS. The aim of this work was to study the effects of chronic morphine exposure and its withdrawal on certain aminopeptidases capable of degrading opioid peptides in brain areas including the amygdala, hypothalamus, hippocampus, striatum and brain cortices. In animals treated with morphine, aminopeptidase N presented higher enzyme activity levels in the striatum, the hypothalamus and the amygdala compared to control animals, although statistically significant differences were observed only in the case of the striatum. In addition, the activity of soluble puromycin-sensitive aminopeptidase (PSA) was found to be higher in the frontal cortex of these rats. In contrast, rats experiencing withdrawal symptoms presented decreased levels of aminopeptidase activity in certain brain areas. Thus, the activity of aminopeptidase N in the hippocampus and soluble puromycin-sensitive aminopeptidase in the frontal cortex were found to be lower in rats experiencing naloxone precipitated withdrawal symptoms, compared to the corresponding controls. Finally, the activity of the three studied aminopeptidases in vitro was unaltered by incubation with morphine, suggesting that the observed effects are not due to a direct action of this opioid upon the aminopeptidases. The results of the present report indicate that aminopeptidases may play an important role in the processes of tolerance and withdrawal associated with morphine administration.