AJP - Gastrointestinal and Liver Physiology, Vol 249, Issue 5 572-G579, Copyright © 1985 by American Physiological Society
Control mechanism of spontaneous in vitro contractions of the opossum sphincter of Oddi
J. F. Helm, W. J. Dodds, J. Christensen and S. K. Sarna
We evaluated the control mechanism of peristaltic contractions in the
opossum sphincter of Oddi (SO) by means of an in vitro preparation. At each
of four sites spaced uniformly along the sphincter segment, a force
transducer recorded contractions while a monopolar electrode recorded
myoelectric activity. Spontaneous myoelectric and contractile activity
occurred in 15 of the 20 intact SO specimens studied. Electrical recordings
showed characteristic control waves and response activity. Each control
wave was invariably accompanied by a phasic contraction, irrespective of
whether or not response activity was superimposed on the control wave. The
predominant motor activity of the SO was antegrade peristalsis. Retrograde
peristalsis occurred when antegrade peristalsis failed to traverse the
entire sphincter. Spontaneous SO phasic contractions were not antagonized
by tetrodotoxin. Muscle rings sectioned from the SO exhibited spontaneous
phasic contractions with a proximal-to-distal gradient of intrinsic
contraction frequencies. We conclude that a) the frequency of SO phasic
contractions is determined by control wave frequency, b) spontaneous SO
peristalsis is myogenic in origin and may be modeled by a linear array of
bidirectionally coupled relaxation oscillators, c) the predominance of
antegrade peristalsis may be explained by a high-frequency oscillator in
the proximal SO that drives the slower, more distal oscillators, d)
retrograde peristalsis is initiated by an ectopic oscillator in the distal
SO when antegrade contractions fail to propagate the entire length of the
SO, and e) ectopic SO contractions can propagate retrograde when the more
proximal oscillators are not in their absolutely refractory state.
