The inositol 1,4,5-trisphosphate (IP3) receptor, a Ca2+permeable channel, plays a key role in intracellular Ca2+ signaling. The effects of ATP on the IP3 receptor at the single-channel level were characterized after channel incorporation into planar lipid bilayers. ATP alone was not sufficient to open the IP3-gated channel, but addition of ATP or nonhydrolyzable ATP analogs in the presence of IP3 increased the frequency of channel openings 4.8-fold and increased the average duration of channel openings 2.5-fold; channel conductance was unchanged. High concentrations of ATP (>4 mM) decreased channel activity most probably by competing with IP3 for the IP3-binding site. Allosteric modulation of IP3-induced Ca2+ release by ATP may contribute to the maintenance of cell viability during periods of energy starvation.
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