Origin of the hypotensive and sympathoinhibitory effect of morphinomimetic agents

Pharmacologic Evidence for the Involvement of Central and Peripheral Opioid Receptors in the Cardioprotective Effects of Fentanyl

BACKGROUND

We investigated the involvement of central and peripheral opioid receptors (OR) in the cardioprotective effects of fentanyl (FENT) in a model of myocardial ischemia/reperfusion injury associated with pharmacologically induced sympathetic overactivity in anesthetized rabbits.

METHODS

Central sympathetic stimulation was achieved through intracerebroventricular injection of l-glutamate in animals submitted to 35 min of coronary occlusion followed by 120 min of reperfusion. Rabbits received naloxone HCl intracerebroventricularly or naloxone methiodide IV, a quaternary compound that does not cross the blood-brain barrier, 5 min before FENT treatment (5 or 50 microg/kg, IV).

RESULTS

Infarct area was reduced only by FENT 50 (from 51% +/- 2% to 24% +/- 2%). This protective effect was abolished by peripheral (42% +/- 4%), but not central, OR blockade (32% +/- 3%). The number of premature ventricular complexes during the ischemic period (54 +/- 3) was reduced by FENT 50 (19 +/- 7), an effect blunted by central (40 +/- 3) but not peripheral (18 +/- 7) blockade of OR. During reperfusion, the number of premature ventricular complexes (134 +/- 50) was reduced to 9 +/- 5 by FENT 50 and was prevented by central (42 +/- 4) as well as peripheral (20 +/- 11) OR blockade. The mortality rate (50%) and incidence of ventricular tachycardia (55%) were completely abolished by FENT 50.

CONCLUSIONS

We conclude that fentanyl's effects for limiting myocardial ischemic injury are mediated via peripheral ORs while opioid's antiarrhythmic actions are mediated via central OR agonism.

Cardioprotective action of fentanyl in a model of central sympathetic overactivity in rabbits: antiarrhythmic and anti-ischemic effects

BACKGROUND

Sympathetic overactivity resulting from perioperative noxious stimuli elicits hyperdynamic cardiovascular responses that may lead to myocardial ischemia and/or ventricular arrhythmia, especially in patients presenting with coronary artery disease. In the present study we investigated the cardioprotective effects of clinically relevant doses of fentanyl in an experimental model of sympathetic overactivity associated with myocardial ischemia in anesthetized rabbits.

METHODS

Central sympathetic stimulation was achieved through intracerebroventricular (i.c.v.) injection of L-glutamate (10 micro mol), with simultaneous inhibition of nitric oxide (NO) synthesis through i.v. administration of N(omega)-nitro-l-arginine methyl ester (L-NAME; 40 mg kg(-1)).

RESULTS

L-glutamate triggered ventricular arrhythmia and electrocardiographic alterations indicative of myocardial ischemia. The intravenous administration of fentanyl (5, 10 or 50 micro g kg(-1)) reduced the incidence of ST-segment shift (70, 20 and 10%, respectively, vs. 66.7% in controls) as well as of T-wave inversion from 58.3% to 30, 20 and 10%, respectively. The total number of ventricular premature complexes per minute fell from 65.2 +/- 16 in the control group to 6.8 +/- 3, 3.5 +/- 2 and 2.6 +/- 1.5, respectively. The occurrence of ventricular tachycardia and bigeminy was completely abolished by fentanyl. Finally, the i.v. administration of fentanyl did not induce significant hemodynamic effects (except for dP/dt(max) in the 50 micro g kg(-1)-dose).

CONCLUSION

Fentanyl elicits significant cardioprotective effects in a model of arrhythmia resulting from the association of central sympathetic overactivity with myocardial ischemia in rabbits, independently from its systemic hemodynamic actions.

Chronic kappa-opioid receptor antagonism delays the rise in blood pressure in spontaneously hypertensive rats

A 4-week subcutaneous treatment with the kappa-opioid receptor antagonists, MR 2266 or MR 1452, delayed the age-dependent increase in systolic blood pressure in young spontaneously hypertensive rats (SHR), but not in normotensive rats. The content of beta-endorphin, [Leu5]enkephalin, and catecholamines in various brain tissues was not affected by MR 2266. Specificity of the kappa-opioid receptor antagonism was tested in SHR by injection of the kappa-opioid agonist, MR 2033, and the mu agonist, morphiceptin, after 28 days of MR 1452 or saline and 2 days after the end of treatment. The increase in mean arterial blood pressure (MAP) after MR 2033 did not differ between the two groups at the 28th day of treatment but was higher 2 days later in the previous MR 1452 group, possibly indicating up-regulation of kappa-opioid receptors. Morphiceptin did not alter MAP during or after MR 1452. It is concluded that kappa-opioid receptors may have a tonic influence on the age-dependent increase of blood pressure in SHR but not in normotensive rats.

Analgesic activity of morphiceptin, beta-casomorphin-4, and deltakephalin in normotensive Wistar-Glaxo and spontaneously hypertensive rats

The effects of intraventricular injection of beta-casomorphin-4, morphiceptin and deltakephalin (DTLET) on hot water tail flick and tail compression responses were investigated in Wistar Albino Glaxo (WAG) and spontaneously hypertensive rats (SHR). The effects of the mu agonist morphiceptin (20 nmol/rat), as assessed by the tail compression test, were significantly greater in SHR rats but did not differ between both strains when measured by tail flick latency. Opioid agonist deltakephalin (2 nmol/rat) in both tests elicited stronger analgesic effects in SHR as compared to WAG and these effects were blocked by naloxone in both tests used. beta-Casomorphin-4 exhibits moderate activity for mu receptors. In the tail flick test peptide (60 nmol/rat) produced an increase in latencies in SHR rats that was significantly greater than was observed in WAG rats. Naloxone pretreatment abolished the analgesic activity of beta-casomorphin-4 solely in the tail compression test in SHR. Analysis of the slopes of the dose-response curves seems to suggest that differences between the activity of these opioid peptides in SHR and WAG rats are based on a difference in the density and affinity of the subpopulation of the opioid receptors in these strains of rats.

Fentanyl-induced airway hyperreactivity in the guinea pig

The bronchopulmonary effects of fentanyl were studied in mechanically ventilated, paralyzed guinea pigs that had been anaesthetized with pentobarbitone sodium. Fentanyl did not alter the resting bronchial tone but enhanced the bronchoconstrictor effects of 5-hydroxytryptamine in a dose-related manner. The enhancement induced by 20 micrograms kg-1 fentanyl was suppressed by pretreatments with 0.5 mg kg-1 naloxone or 5 mg kg-1 propranolol, but did not change after 3 mg kg-1 atropine. The bronchoconstrictor responses to histamine were also enhanced by 20 micrograms kg-1 fentanyl. These results suggest that fentanyl-induced airway hyperreactivity is not mediated by an increase in vagal tone but is due to a reduction in the central sympathetic drive and/or in the levels of circulating catecholamines, which occurs through stimulation of opiate receptors.