AJP - Gastrointestinal and Liver Physiology, Vol 252, Issue 3 333-G338, Copyright © 1987 by American Physiological Society
Effect of morphine on electrophysiological properties of circular and longitudinal muscles
R. J. Gilbert, S. K. Sarna and D. R. Harder
We have measured the effects of morphine on the intracellular
electrophysiological properties of smooth muscle cells from the circular
and longitudinal muscle layers of the canine jejunum. Morphine
hyperpolarized the circular muscle membrane by approximately 12 mV and
increased the electrical control activity (ECA) amplitude and dV/dt.
Morphine had no significant effect on the electrical properties of the
longitudinal muscle cells. The morphine-induced hyperpolarization of the
circular muscle membrane was blocked by tetrodotoxin (TTX) and naloxone,
but not by atropine and hexamethonium, propranolol, or phentolamine.
Morphine significantly increased the slope of the resting membrane
potential vs. the log of the potassium concentration in bathing medium from
38 to 50 mV/decade. The sodium permeability to potassium permeability
ratio, calculated from the Goldman constant field equation, was reduced by
morphine from 0.13 to 0.07 at mM of K+. The above results suggest that when
measured by intracellular techniques, morphine hyperpolarizes the circular
muscle membrane by release of a nonadrenergic, noncholinergic
neurotransmitter. The mechanism of this hyperpolarization is consistent
with an increase in potassium conductance.
