In the accompanying article, we compared main functional properties of the three mammalian inositol 1,4,5-trisphosphate receptors (InsP3R) isoforms. In this article we focused on modulation of mammalian InsP 3R isoforms by cytosolic Ca2+. We found that: 1), when recorded in the presence of 2 μM InsP3 and 0.5 mM ATP all three mammalian InsP3R isoforms display bell-shaped Ca2+ dependence in physiological range of Ca2+ concentrations (pCa 8-5); 2), in the same experimental conditions InsP3R3 is most sensitive to modulation by Ca2+ (peak at 107 nM Ca2+), followed by InsP3R2 (peak at 154 nM Ca2+), and then by InsP 3R1 (peak at 257 nM Ca2+); 3), increase in ATP concentration to 5 mM had no significant effect of Ca2+ dependence of InsP3R1 and InsP3R2; 4), increase in ATP concentration to 5 mM converted Ca2+ dependence of InsP3R3 from "narrow" shape to "square" shape; 5), ATP-induced change in the shape of InsP3R3 Ca2+ dependence was mainly due to an >200-fold reduction in the apparent affinity of the Ca2+- inhibitory site; 6), the apparent Ca2+ affinity of the Ca 2+ sensor region (Cas) determined in biochemical experiments is equal to 0.23 μM Ca2+ for RT1-Cas, 0.16 μM Ca2+ for RT2-Cas, and 0.10 μM Ca2+ for RT3-Cas; and 7), Ca2+ sensitivity of InsP3R1 and InsP3R3 isoforms recorded in the presence of 2 μM InsP3 and 0.5 mM ATP or 2 μM InsP 3 and 5 mM ATP can be exchanged by swapping their Cas regions. Obtained results provide novel information about functional properties of mammalian InsP3R isoforms and support the importance of the Ca 2+ sensor region (Cas) in determining the sensitivity of InsP 3R isoforms to modulation by Ca2+.
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