AJP - Heart and Circulatory Physiology, Vol 250, Issue 5 796-H805, Copyright © 1986 by American Physiological Society
Effect of endorphins on heart rate and blood pressure in adult dogs
G. G. Haddad, H. J. Jeng and T. L. Lai
To investigate the role of opioids in regulating cardiovascular function,
we administered delta-opioid receptor agonists D-Ala-D-Leu enkephalin
(DADLE) and D-Ala-Met enkephalinamide (DAME), and mu-opioid receptor
agonist, a morphiceptin analogue (MA), intracisternally in 13
unanesthetized, chronically instrumented adult dogs in 2 doses (25 and 125
micrograms/kg). After an initial transient drop, the R-R interval increased
(peak approximately 25-60 min) postadministration of opioids. The time
course and the magnitude of the change in R-R interval depended on the
agonist: delta-agonists induced a more prolonged and marked change in R-R
interval than mu-agonists at both doses. Mean arterial blood pressure (MAP)
increased initially but dropped toward or even below base line 30 min after
opioids administration. Atropine, given intravenously or intra-arterially
at peak action of agonist in relatively low doses (0.02 mg/kg), induced an
AV block followed by a marked decrease in R-R interval. There was also an
increase in MAP after atropine. Naloxone, given intracisternally, reversed
both delta- and mu-opioid effects but did not induce changes in the R-R
interval without prior administration of opioids. We conclude that in
unanesthetized adult dogs 1) both mu- and delta-receptor opioid agonists
prolong the R-R interval, and this depends on the type of receptor
stimulated; 2) opioids induce slowing in heart rate, possibly by increasing
parasympathetic activity to the heart; 3) enkephalin and morphiceptin
analogues induce a biphasic response in MAP; and 4) endorphins do not
modulate cardiovascular function tonically; we speculate that they can
alter the R-R interval and MAP in the presence of stimuli.
