Influence of ACTH-(1-24) on metabolic acidosis and hypoxemia induced by massive hemorrhage in rats

Melanocortins and the cholinergic anti-inflammatory pathway

Experimental evidence indicates that small concentrations of inflammatory molecules produced by damaged tissues activate afferent signals through ascending vagus nerve fibers, that act as the sensory arm of an "inflammatory reflex". The subsequent activation of vagal efferent fibers, which represent the motor arm of the inflammatory reflex, rapidly leads to acetylcholine release in organs of the reticuloendothelial system. Acetylcholine interacts with α7 subunit-containing nicotinic receptors in tissue macrophages and other immune cells and rapidly inhibits the synthesis/release of tumor necrosis factor-α and other inflammatory cytokines. This neural anti-inflammatory response called "cholinergic anti-inflammatory pathway" is fast and integrated through the central nervous system. Preclinical studies are in progress, with the aim to develop therapeutic agents able to activate the cholinergic anti-inflammatory pathway. Melanocortin peptides bearing the adrenocorticotropin/α-melanocyte-stimulating hormone sequences exert a protective and life-saving effect in animals and humans in conditions of circulatory shock. These neuropeptides are likewise protective in other severe hypoxic conditions, such as prolonged respiratory arrest, myocardial ischemia, renal ischemia and ischemic stroke, as well as in experimental heart transplantation. Moreover, experimental evidence indicates that melanocortins reverse circulatory shock, prevent myocardial ischemia/reperfusion damage and exert neuroprotection against ischemic stroke through activation of the cholinergic anti-inflammatory pathway. This action occurs via stimulation of brain melanocortin MC3/MC4 receptors. Investigations that determine the molecular mechanisms of the cholinergic anti-inflammatory pathway activation could help design of superselective activators of this pathway.

Melanocortins as potential therapeutic agents in severe hypoxic conditions

Melanocortin peptides with the adrenocorticotropin/melanocyte-stimulating hormone (ACTH/MSH) sequences and synthetic analogs have protective and life-saving effects in experimental conditions of circulatory shock, myocardial ischemia, ischemic stroke, traumatic brain injury, respiratory arrest, renal ischemia, intestinal ischemia and testicular ischemia, as well as in experimental heart transplantation. Moreover, melanocortins improve functional recovery and stimulate neurogenesis in experimental models of cerebral ischemia. These beneficial effects of ACTH/MSH-like peptides are mostly mediated by brain melanocortin MC(3)/MC(4) receptors, whose activation triggers protective pathways that counteract the main ischemia/reperfusion-related mechanisms of damage. Induction of signaling pathways and other molecular regulators of neural stem/progenitor cell proliferation, differentiation and integration seems to be the key mechanism of neurogenesis stimulation. Synthesis of stable and highly selective agonists at MC(3) and MC(4) receptors could provide the potential for development of a new class of drugs for a novel approach to management of severe ischemic diseases.

Adrenocorticotropin normalizes the blood levels of nitric oxide in hemorrhage-shocked rats

Anesthetized rats were subjected to volume-controlled hemorrhagic shock by stepwise bleeding. Besides cardiovascular and respiratory functions, nitric oxide (NO)-hemoglobin formation in arterial blood was directly evaluated by means of electron spin resonance spectroscopy. During hemorrhagic shock there was a massive increase in NO-hemoglobin, associated with a fall in mean arterial pressure, pulse pressure, respiratory rate and heart rate, and there was a further increase in NO-hemoglobin 15 min after intravenous (i.v.) treatment with saline. All rats died within 30 min. The reversal of the shock condition induced by the i.v. injection of the adrenocorticotropin (ACTH) fragment 1-24 (160 microg/kg, 5 min after bleeding termination) was associated with a prompt disappearance of NO-hemoglobin. Also S-methylisothiourea (3 mg/kg i.v.), a selective inhibitor of inducible NO synthase, provoked a disappearance of NO-hemoglobin and reversal of the shock condition. The present results provide a direct demonstration that volume-controlled hemorrhagic shock is associated with highly increased blood levels of NO, as indicated by increased NO-hemoglobin, and indicate that ACTH-induced reversal of the shock condition is associated with the normalization of NO blood levels, and a parallel improvement of cardiovascular and respiratory functions. This occurs probably through the inhibition of inducible NO synthase, as suggested by the fact that S-methylisothiourea, a selective inhibitor of this NO synthase isoform, produced the same results.

Resuscitating effect of melanocortin peptides after prolonged respiratory arrest

1. The resuscitating activity of melanocortin peptides (MSH-ACTH peptides) was tested in an experimental model of prolonged respiratory arrest. 2. Anaesthetized, endotracheally intubated rats subjected to a 5 min period of ventilation interruption, invariably died from cardiac arrest within 6-9 min of resumption of ventilation. 3. When resumption of ventilation was associated with the simultaneous intravenous (i.v.) injection of a melanocortin peptide (alpha-MSH or ACTH-(1-24)) (160 microg kg(-1) there was an almost immediate (within 1 min), impressive increase in cardiac output, heart rate, mean arterial pressure (+ 560% of the before-treatment value) and pulse pressure (+356% of the before-treatment value), with full recovery of electroencephalogram after 30-45 min. Blood gases and pH were normalized within 15-60 min after treatment, and all treated animals eventually recovered completely and survived indefinitely (= more than 15 days). 4. The same response was observed in adrenalectomized animals, as well as in animals pretreated with a beta1-adrenoceptor blocking agent (atenolol, 3 mg kg(-1), i.v.), or with an alpha1-adrenoceptor blocking agent (prazosin, 0.1 mg kg(-1), i.v.), or with an adrenergic neurone blocking agent (guanethidine, 10 mg kg(-1), intraperitoneally). 5. An effect quite similar to that produced by melanocortins was obtained with ouabain (0.1 mg kg(-1), i.v.); the antioxidant drug, glutathione (75 mg kg(-1), i.v.) also produced 100% resuscitation, but the effect was slower in onset. On the other hand, adrenaline (0.005 mg kg(-1), i.v.) was able to resuscitate only 1 out of 8 rats and dobutamine (0.02 mg kg(-1), i.v.) resuscitated 4 out of 8 rats; moreover, the effect of both catecholamines was much slower in onset than that of melanocortins and the initial, impressive stimulation of cardiovascular function was absent. 6. These results show that melanocortin peptides have a resuscitating effect in a pre-terminal condition produced in rats by prolonged asphyxia. This effect seems primarily due to the restoration of cardiac function, not mediated by catecholamines. These data also suggest that these peptides may have potential therapeutic value in conditions of transient cardiac hypoxia and re-oxygenation such as occur in coronary artery disease.

Influence of ACTH-(1-24) on free radical levels in the blood of haemorrhage-shocked rats: direct ex vivo detection by electron spin resonance spectrometry

1. The influence of ACTH-(1-24) on the blood levels of highly reactive free radicals in haemorrhagic shock was studied in rats. 2. Volume-controlled haemorrhagic shock was produced in adult rats under general anaesthesia (urethane, 1.25 g kg-1 intraperitoneally) by stepwise bleeding until mean arterial pressure stabilized at 20-23 mmHg. Rats were intravenously (i.v.) treated with either ACTH-(1-24) (160 micrograms kg-1 in a volume of 1 ml kg-1) or equivolume saline. Free radicals were measured in arterial blood by electron spin resonance spectrometry using an ex vivo method that avoids injection of the spin-trapping agent (alpha-phenyl-N-tert-butylnitrone). 3. Blood levels of free radicals were 6490 +/- 273 [arbitrary units (a.u.) ml-1 whole blood, before starting bleeding, and 30762 +/- 2650 after bleeding termination (means +/- s.e. mean of the values obtained in all experimental groups). All rats treated with saline died within 30 min, their blood levels of free radicals being 35450 +/- 5450 a.u. ml-1 blood, 15 min after treatment. Treatment with ACTH-(1-24) produced a rapid and sustained restoration of arterial pressure, pulse pressure, heart rate and respiratory function, with 100% survival at the end of the observation period (2 h); this was associated with an impressive reduction in the blood levels of free radicals, that were 12807 +/- 2995, 10462 +/- 2850, 12294 +/- 4120, and 10360 +/- 2080 a.u. ml-1 blood, 15, 30, 60 and 120 min after ACTH-(1-24) administration, respectively. 4. These results provide a direct demonstration that (i) in haemorrhagic shock there is a rapid and massive production of highly reactive free radicals, and that (ii) the sustained restoration of cardiovascular and respiratory functions induced by the i.v. injection of ACTH-(1-24) is associated with a substantial reduction of free radical blood levels. It is suggested that ACTH-(1-24) prevents the burst of free radical generation during blood mobilisation and subsequent tissue reperfusion, and this may be an important component of its mechanism of action in effectively preventing death for haemorrhagic shock.

The opioid/anti-opioid balance in shock: a new target for therapy in resuscitation

Teleologically, pain is of paramount importance for survival and induces the organism to cope in an active way with aggressions from a basically hostile environment. While the activation of endogenous analgesic (opioid) systems typically occurs in conditions of surrender (pre-terminal conditions, sustained tortures, etc.), the activation of endogenous anti-analgesic systems triggers mechanisms of active or passive defence (such as camouflage) aimed at survival. The distinctive features of the main anti-analgesic systems (melanocortinergic, cholecystokininergic, thyroliberinergic) and the dramatic results obtained in experimental pre-terminal conditions (hemorrhagic shock, prolonged respiratory arrest) with the administration of their neuropeptide transmitters (ACTH and several ACTH-fragments, including alpha-MSH, CCK peptides and thyrotropin-releasing hormone) are here reviewed. The study of the mechanisms underlying the resuscitating effects of these neuropeptides has led to the discovery of the (often extremely potent) resuscitating effect of other drugs (protoveratrines, nicotine, centrally-acting cholinergic agents, ganglion-stimulating drugs). It is particularly remarkable that in pre-terminal conditions these neuropeptides and drugs have highly impressive effects on cardiocirculatory parameters at doses that are almost or actually inactive under normal conditions, and that their resuscitating effect is obtained without the need for any other supportive treatment and at dose-levels well below toxic ranges. Finally, in hemorrhage-shocked animals, the treatment with anti-analgesic neuropeptides shortly after bleeding considerably extends the time-limit for an effective and definitively curing blood reinfusion. This would be of self-evident importance in clinical practice, because an extremely simple, non-toxic first-aid treatment in the field, shortly after a massive hemorrhage, could resuscitate the patient for a period sufficient to effectively set up the most appropriate in-hospital treatment.

Dopamine D1 receptors are involved in the ACTH-induced reversal of hemorrhagic shock

In an experimental model of volume-controlled hemorrhagic shock causing the death of all rats within 30 min, the intravenous (i.v.) bolus injection of the adrenocorticotropic hormone fragment 1-24 (ACTH-(1-24)) (160 micrograms/kg) induced a prompt and sustained improvement of cardiovascular and respiratory function, with 100% survival 2 h after treatment. Pretreatment with either haloperidol, 300 micrograms/kg i.v. (antagonist at dopamine D1 and D2 receptors), or (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepin-7-ol hemimaleate (SCH 23390), 50 micrograms/kg intraperitoneally (selective antagonist at dopamine D1 receptors), significantly inhibited the effect of ACTH-(1-24). A complete inhibition was produced by intracerebroventricular pretreatment with SCH 23390 (0.1 micrograms/rat). On the other hand, both i.v. and i.c.v. pretreatment with l-sulpiride (selective antagonist at dopamine D2 receptors) (25 mg/kg and 80 micrograms/rat, respectively) had only minor effects. These data suggest that the mechanism of the ACTH-induced reversal of hemorrhagic shock involves the activation of dopamine D1 receptors in the brain.

Comparison of the effects of ACTH-(1-24), methylprednisolone, aprotinin, and norepinephrine in a model of hemorrhagic shock in rats

Rats bled to a severe condition of volume-controlled hemorrhagic shock were randomly assigned to one of the following treatments: (1) saline, 1 ml/kg i.v.; (2) saline, 0.2 ml/kg per min i.v. for 10 min; (3) ACTH-(1-24), 160 micrograms/kg i.v.; 4) methylprednisolone, 40 mg/kg i.v.; (5) methylprednisolone, 80 mg/kg i.v.; (6) aprotinin, 10,000 KIU/kg i.v.; (7) norepinephrine, 5 micrograms/kg per min i.v. for 10 min; (8) norepinephrine, 10 micrograms/kg per min i.v. for 10 min. All rats treated with saline or with either of the two doses of methylprednisolone, and half of the rats treated with aprotinin, died within the subsequent 2 h. On the other hand, rats treated with norepinephrine, at either dose, or with ACTH-(1-24) were all still alive 2 h later, a similar improvement in cardiovascular and respiratory parameters being obtained with the two treatments. The effect of ACTH on mean arterial pressure was however more sustained throughout the observation period. These results further support the potential usefulness of ACTH-(1-24) as first-aid treatment in cases of severe blood losses.