Release of arachidonic acid by agonist-mediated activation of pla2
initiates ca2+ mobilization in intestinal smooth muscle.
Murthy, Karnam S., John F. Kuemmerle, and Gabriel M. Makhlouf.
Departments of Physiology and Medicine, Medical College of
Virginia, Richmond, Virginia 23298-0711
APStracts 2:0033G, 1995.
Recent studies have shown that Ca2+ mobilization in circular smooth
muscle of the intestine is mediated by IP3-dependent Ca2+ release
while Ca2+ mobilization in longitudinal muscle is initiated by IP3
-independent influx of Ca2+ that acts as a trigger for Ca2+-induced
Ca2+ release. The present study examined whether arachidonic acid
(AA) acts as mediator of the initial Ca2+ influx in longitudinal
muscle. CCK-8 caused transient concentration-dependent increase in AA
release in dispersed longitudinal muscle cells only followed by a
sustained increase in both muscle cell types. The initial increase in
AA release coincided with an increase in [Ca2+]i and muscle
contraction: all three events occurred in the presence of cyclo
-oxygenase and lipoxygenase inhibitors, were abolished by GDP[beta]S
and pertussis toxin (PTx) and were dose-dependently inhibited by the
PLA2 inhibitor, dimethyleicosadienoic acid (DEDA), but were not
affected by the PKC inhibitor, calphostin C, or by neomycin, which
abolished concomitant PI hydrolysis. Exogenous AA caused
concentration-dependent contraction and increase in [Ca2+]i in
longitudinal but not circular muscle cells; contraction and the
increase in [Ca2+]i were abolished by Ca2+ channel blockers.
Depletion of Ca2+ stores with thapsigargin attenuated agonist- and
AA-mediated increase in [Ca2+]i and contraction in longitudinal
muscle cells: the residual [Ca2+]i increase (35%) and contraction
(25%) reflected the component of Ca2+ influx; similar treatment of
circular muscle cells abolished agonist-mediated, IP3-dependent
increase in [Ca2+]i and contraction. We conclude that AA released by
agonist-mediated, G protein-dependent, PTx-sensitive activation of
PLA2 acts as a mediator of Ca2+ influx that triggers Ca2+-induced
Ca2+ release. The process is independent of PI hydrolysis and occurs
exclusively in longitudinal smooth muscle, where Ca2+ release
channels are highly sensitive to Ca2+, ryanodine and cADP ribose, and
insensitive to IP3.
Received 9 December 1994; accepted in final form 16 February
1995.
APS Manuscript Number G480-4.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 March 1995.