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Biophys J, July 2000, p. 271-278, Vol. 79, No. 1
and
*Department of Physiology, University of Connecticut Health Center,
Farmington, Connecticut 06030, Departments of
Pharmacology and Cellular and Molecular Physiology, Yale
University, New Haven, Connecticut 06520 USA
Many hormones and neurotransmitters raise intracellular
calcium (Ca2+) by generating InsP3 and
activating the inositol 1,4,5-trisphosphate receptor
(InsP3R). Multiple isoforms with distinct InsP3
binding properties (Cardy et al., 1997) have been identified (Morgan et al., 1996). The type III InsP3R lacks
Ca2+-dependent inhibition, a property that makes it ideal
for signal initiation (Hagar et al., 1998). Regulation of the type III
InsP3R by InsP3 and ATP was explored in detail
using planar lipid bilayers. In comparison to the type I
InsP3R, the type III InsP3R required a higher
concentration of InsP3 to reach maximal channel activity (EC50 of 3.2 µM versus 0.5 µM for the types III and I
InsP3R, respectively). However, the type III
InsP3R did reach a 2.5-fold higher level of activity.
Although activation by InsP3 was isoform-specific, regulation by ATP was similar for both isoforms. In the presence of 2 µM InsP3, low ATP concentrations (<6 mM) increased the
open probability and mean open time. High ATP concentrations (>6 mM) decreased channel activity. These results illustrate the complex nature
of type III InsP3R regulation. Enhanced channel activity in
the presence of high InsP3 may be important during periods of prolonged stimulation, whereas allosteric modulation by ATP may help
to modulate intracellular Ca2+ signaling.
Biophys J, July 2000, p. 271-278, Vol. 79, No. 1
© 2000 by the Biophysical Society 0006-3495/00/07/271/08 $2.00
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