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(Received for publication, June 4, 1996, and in revised form, May 15, 1997)
From the Laboratory of Molecular Endocrinology, Howard Hughes
Medical Institute, Harvard Medical School, Massachusetts General
Hospital, Boston, Massachusetts 02114
Maitotoxin (MTX) activates a
Ca2+-dependent non-selective cation
current (ICa-NS) in insulinoma cells whose time course is
identical to non-selective cation currents activated by incretin
hormones such as glucagon-like peptide-1 (GLP-1), which stimulate
glucose-dependent insulin secretion by activating cAMP
signaling pathways. We investigated the mechanism of activation of
ICa-NS in insulinoma cells using specific pharmacological
reagents, and these studies further support an identity between MTX-
and GLP-1-activated currents. ICa-NS is inhibited by
extracellular application of genistein, econazole, and SKF 96365. This
inhibition by genistein suggests that tyrosine phophorylation may play
a role in the activation of ICa-NS. ICa-NS is
not inhibited by incubation of cells in glucose-free solution, by
extracellular tetrodotoxin, nimodipine, or tetraethylammonium, or by
intracellular dialysis with 4-aminopyridine, ATP, ryanodine, or
heparin. ICa-NS is also not significantly inhibited by
staurosporine, which does, however, partially inhibit the MTX-induced
rise of intracellular Ca2+ concentration. These effects of
staurosporine suggest that protein kinase C may not be involved in the
activation of ICa-NS but that it may regulate intracellular
Ca2+ release. Alternatively, ICa-NS may have a
small component that is carried through separate divalent
cation-selective channels that are inhibited by staurosporine.
ICa-NS is neither activated nor inhibited by dialysis with
KF, KF + AlF3 or GTP
Volume 272, Number 29,
Issue of July 18, 1997
pp. 17987-17993
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.
S (guanosine 5
-O-(3-thiotriphosphate)), suggesting that GTP-binding
proteins do not play a major role in the activation of this
current.
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