Ryanodine-induced calcium release from hepatic microsomes and
permeabilized hepatocytes.
Lilly, Leslie B., and John L. Gollan.
Gastroenterology Division, Department of Medicine, Brigham and
Women's Hospital, and Harvard Digestive Diseases Center, Harvard
Medical School, Boston, MA.
APStracts 2:0026G, 1995.
Inositol 1,4,5-trisphosphate (InsP3) is a second messenger which
releases Ca++ from hepatocyte microsomes. The toxic alkaloid,
ryanodine, modulates Ca++ release via a receptor (RyR) identified in
a variety of cell systems, but its regulation and functional
significance in the liver are undefined. Similarly, the role in
hepatocyte Ca++ regulation of cyclic ADP-ribose (cADPR), which is the
putative endogenous ligand for the RyR in other cell systems, has not
been defined. We have investigated calcium regulation in hepatocytes
and, in particular, the effects of ryanodine, cADPR and other
putative modulators on Ca++ release, utilizing microsomes and
permeabilized cells, and compared these to InsP3-induced calcium
release. Ryanodine at concentrations 3 50 [mu]M released 20% of
microsomal Ca++ and, in contrast to InsP3, no potentiation was
observed with GTP and PEG. InsP3-induced Ca++ release was
demonstrable following maximal ryanodine-induced Ca++ release,
suggesting that distinct Ca++ stores are involved. cADPR (5 [mu]M) did
not induce Ca++ release, either alone or in combination with
calmodulin or hepatic cytosol, nor did it influence ryanodine-induced
release, in either microsomes or permeabilized hepatocytes (in which
ryanodine released 25% of the sequestered Ca++). Ryanodine-induced
Ca++ release in microsomes was not influenced by 20 mM caffeine,
which itself did not mobilize Ca++, but was prevented by 500 [mu]M
tetracaine, which was shown to induce Ca++ release. We conclude that
ryanodine is capable of mobilizing Ca++ in the hepatocyte from
microsomal stores which are distinct from those regulable by InsP3,
but that cADPR has no such effect. These data suggest that cADPR does
not serve as the endogenous ligand for the RyR in liver cells, or
that the site of action of ryanodine in hepatocyte microsomes is
distinct from that in other cell types.
Received 30 November 1994; accepted in final form 29 January
1995.
APS Manuscript Number G470-4.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 February 1995.