Reduction by phentolamine of the hypotensive effect of methionine enkephalin in anaesthetized rabbits

Peripheral sympatho-inhibitory cardiovascular effects of opioid peptides in anaesthetized rabbits

1. Opioid agonists influence isolated cardiovascular tissues from rabbits, as well as the cardiovascular system of pithed rabbits, through presynaptic receptors on postganglionic sympathetic nerve fibres. The present experiments were carried out in order to study effects which result from activation of these receptors in anaesthetized rabbits. 2. In pithed rabbits with electrically stimulated sympathetic outflow, infusion of [D-Ala2-D-Leu5]-enkephalin (DADLE) 10 micrograms kg-1 min-1 and dynorphin-(1-13) (dynorphin) 1 microgram kg-1 min-1 decreased the plasma noradrenaline concentration, mean arterial pressure (MAP) and heart rate. The effects of dynorphin and, less completely, those of DADLE were antagonized by the peripherally selective opioid antagonists N-methyl naloxone bromide (NMN) 1.3 mg kg-1 and N-methyl levallorphan methanesulphonate (NML) 1-3 mg kg-1. 3. In pentobarbitone-anaesthetized rabbits, DADLE 3-30 micrograms kg-1 min-1 and dynorphin 0.3-3 micrograms kg-1 min-1 decreased the plasma noradrenaline concentration and MAP. The highest dose of dynorphin also decreased heart rate, whereas DADLE 10 micrograms kg-1 min-1 caused slight cardioacceleration. The effects of DADLE but not those of dynorphin decreased upon repeated administration. 4. The effects of dynorphin 10 micrograms kg-1 min-1 were abolished or greatly attenuated by NMN 1.3 mg kg-1 and NML 3 mg kg-1. In contrast, the antagonists reduced only slightly the blood pressure-lowering effect of DADLE 10 micrograms kg-1 min-1 and did not reduce significantly the effects of DADLE on the plasma noradrenaline level and heart rate. 5. It was concluded that systemically administered dynorphin produces sympatho-inhibition and an ensuing fall in blood pressure by an action at peripheral receptors, in all probability presynaptic Kappa-receptors on postganglionic sympathetic nerve fibres. The effects of DADLE are more complex and may involve both central and peripheral components.

Systemic methionine-enkephalin evokes cardiostimulatory responses in the human

The cardiovascular effects of bolus doses of methionine-enkephalin (Met5-ENK) (1 to 100 micrograms/kg) were studied in 9 subjects in whom, at cardiac catheterization for evaluation of chest pain, patent coronary arteries were found. Met5-ENK produced a simultaneous increase in blood pressure and heart rate beginning within 20 sec, reaching maximal values between 30 and 40 sec, and then terminating by 60 sec. Heart rate, systolic, diastolic, and mean arterial blood pressures increased significantly (p less than 0.0005); pulse pressure remained unchanged. Positive dose-effect relationships were observed for heart rate (p less than 0.002), systolic, diastolic, and mean arterial blood pressures (p less than 0.05). Naloxone (0.5 mg/kg), given to 4 subjects, prevented the heart rate and blood pressure changes associated with Met5-ENK administration, demonstrating that the cardiovascular changes were mediated by opiate receptors. Subjects also described cutaneous paresthesias which were not prevented by naloxone pretreatment. These data suggest a role for peripheral enkephalins in cardiovascular regulation.

Effects of [Met5]enkephalin on regional blood flow and vascular resistance in rabbits

Methionine enkephalin [( Met5]enkephalin) has different cardiovascular effects, depending on species and routes of peptide administration. In anesthetized animals [Met5]enkephalin decreases blood pressure and heart rate. The site(s) and the mechanism of the hypotensive effects of the peptide are not known. The main purpose of this study was to test the hypothesis that [Met5]enkephalin dilates specifically a certain vascular bed which may account for the hypotensive effect of the peptide. Anesthetized male rabbits were instrumented for the measurement of blood pressure, heart rate, electrocardiogram and renal nerve activity. Four different microspheres (15 microns) were infused into the left ventricle in 10-15 s in either saline or with [Met5]enkephalin (1 mg/kg). Upon completion of the last microsphere injection the animals were killed and 35 tissues samples were taken for the determination of blood flow. Blood flows to many organs such as brain, glandular and cardiac tissues were not altered significantly by [Met5]enkephalin. However, [Met5]enkephalin increased blood flow to skeletal muscular bed. The increase in muscle blood flow was antagonized by naloxone, a specific opioid antagonist. These results suggest that the hypotensive action of [Met5]enkephalin in anesthetized rabbit is in part due to its vasodilatory effect in skeletal muscle.

Circulating [Met]enkephalin and catecholamine responses to acute hypotension and hypertension in anaesthetized greyhounds

The effects of either hypotension induced by sodium nitroprusside or hexamethonium or hypertension produced by angiotensin II or noradrenaline on the circulating levels of methionine enkephalin ([Met]enkephalin)-like immunoreactivity (MLI), adrenaline and noradrenaline in anaesthetized greyhounds were examined. Nitroprusside infusions (200 and 400 micrograms min-1) induced a fall in blood pressure accompanied by significant rises in plasma MLI and catecholamine concentrations. Concomitant administration of a high dose of naloxone did not alter the fall in blood pressure produced by nitroprusside but was associated with greater rises in circulating MLI and catecholamines when compared to nitroprusside alone, suggesting that [Met]enkephalin is not involved in the hypotensive action of nitroprusside. Intravenous hexamethonium (2.5 mg kg-1) provoked a fall in blood pressure which was not associated with any changes in plasma MLI. However, it produced a fall in plasma noradrenaline and a rise in plasma adrenaline. Thus it appears that neural mechanisms are required, at least in part, for the release of MLI. Angiotensin II (1.25 micrograms kg-1 min-1) and noradrenaline (8 micrograms kg-1 min-1) infusions produced an elevation in blood pressure without altering the circulating MLI levels. Study of the molecular forms of circulating MLI, before and during hypotension, revealed that the large molecular weight enkephalin-containing peptides with approximate molecular sizes of 18kD and 8kD were the predominant forms both in the basal and stimulated states. It is concluded that circulating [Met]enkephalin is not involved in the tonic control of blood pressure but it may modulate catecholamine release following hypotension as part of the stress response.

Opioid peptides decrease noradrenaline release and blood pressure in the rabbit at peripheral receptors

Effects of dynorphin-(1-13), Leu5-enkephalin, D-Ala2,D-Leu5-enkephalin (DADLE), and for comparison bremazocine, on plasma noradrenaline concentration and mean arterial pressure (MAP) were studied in pithed rabbits. In the first series of experiments, the sympathetic outflow was stimulated electrically via the pithing rod at 2 Hz twice for 3 min each (S1, S2). Drugs were administered before S2. Bremazocine 10 micrograms/kg + 2 micrograms/kg/h and 100 micrograms/kg + 20 micrograms/kg/h, dynorphin 1 and 3 micrograms/kg/min, Leu5-enkephalin 100 micrograms/kg/min and DADLE 10 and 30 micrograms/kg/min all diminished the electrically-evoked increase in plasma noradrenaline and MAP. The effects were antagonized by naloxone. In the second series, an infusion of noradrenaline (2 micrograms/kg/min) was given twice for 3 min each (N1, N2). Drugs were administered before N2. Bremazocine 100 micrograms/kg + 20 micrograms/kg/h slightly enhanced the pressor effect of exogenous noradrenaline, whereas dynorphin 3 micrograms/kg/min, Leu5-enkephalin 100 micrograms/kg/min and DADLE 30 micrograms/kg/min caused no significant change. In the third series, the sympathetic outflow was stimulated continuously at 2 Hz, and the interaction of dynorphin and DADLE was studied. Dynorphin 1 microgram/kg/min and DADLE 10 micrograms/kg/min initially decreased MAP to a similar extent. The effect of DADLE faded with time. When, during continuous infusion of DADLE 10 micrograms/kg/min, and after return of MAP to the pre-DADLE level, dynorphin 1 microgram/kg/min or DADLE 10 micrograms/kg/min was infused additionally, the effect of dynorphin was unchanged, whereas that of DADLE was almost abolished. We conclude that the opioid peptides as well as bremazocine decrease action potential-evoked release of noradrenaline and, secondarily, blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.