Central cardiovascular effects of morphinomimetic peptides in dogs.

Morphinomimetic peptides were injected into the cisterna magna of chloralosed dogs. Methionine enkepalin (500 microgram/kg i.c.) was found ineffective but beta-endorphin (50 microgram/kg i.c.) induced an initial and transient increase in blood pressure and heart rate followed by a delayed hypotension and bradycardia. The synthetic pentapeptides, [d-ala2]met-enkephalin (500 microgram/kg i.c.), [d-ala2]met-enkephalinamide 500 microgram/kg i.c.) also induced a marked hypotensive, bradycardic and sympatho-inhibitory effect. High doses of naloxone (100 microgram/kg i.v.) were required to antagonize these effects transiently.