Journal of Applied Physiology, Vol 58, Issue 3 853-858, Copyright © 1985 by American Physiological Society
Modulation of cholinergic neurotransmission in airways by enkephalin
J. A. Russell and E. J. Simons
We compared the effects of methionine enkephalin and leucine enkephalin on
contractions of isolated canine tracheal smooth muscle strips induced by
field electrical stimulation (ES) and exogenous acetylcholine
(approximately 10(-5) M). Methionine and leucine enkephalin (10(-8) to
10(-5) M), when added at the peak of airway contractions induced by ES at 1
Hz, depressed the contractions in a concentration-dependent manner by a
maximum of 95 and 99%, respectively. Acetylcholine-induced contractions of
similar magnitude were depressed only 4% by methionine enkephalin and 12%
by leucine enkephalin. Frequency-response curves (0.5-20 Hz) were also
obtained before and after incubation of tracheal strips with 10(-5) M
methionine and leucine enkephalin. Enkephalin depressed contractions
induced by stimulation at 0.5 and 1 Hz by an average of 98 and 95%,
respectively. The inhibitory effect of enkephalin progressively decreased
at successively higher stimulus frequencies until at 20 Hz there was no
significant difference between airway contractions obtained in the presence
and absence of enkephalin. Naloxone (3 X 10(-5) M) antagonized the
inhibitory effects of both enkephalins. We conclude that methionine and
leucine enkephalins inhibit the release of acetylcholine from the
postganglionic parasympathetic neurons that innervate airway smooth muscle.
