Functional characterization of mammalian inositol 1,4,5-trisphosphate receptor isoforms

Inositol 1,4,5-trisphosphate receptors (InsP₃R) play a key role in intracellular calcium (Ca²⁺) signaling. Three mammalian InsP ₃R isoforms-InsP₃R type 1 (InsP₃R1), InsP ₃R type 2 (InsP₃R2), and InsP₃R type 3 (InsP₃R3) are expressed in mammals, but the functional differences between the three mammalian InsP₃R isoforms are poorly understood. Here we compared single-channel behavior of the recombinant rat InsP ₃R1, InsP₃R2, and InsP₃R3 expressed in Sf9 cells, reconstituted into planar lipid bilayers and recorded with 50 mM Ba ²⁺ as a current carrier. We found that: 1), for all three mammalian InsP₃R isoforms the size of the unitary current is 1.9 pA and single-channel conductance is 74-80 pS; 2), in optimal recording conditions the maximal single-channel open probability for all three mammalian InsP ₃R isoforms is in the range 30-40%; 3), in optimal recording conditions the mean open dwell time for all three mammalian InsP₃R isoforms is 7-8 ms, the mean closed dwell time is ∼10 ms; 4), InsP ₃R2 has the highest apparent affinity for InsP₃ (0.10 μ), followed by InsP₃R1 (0.27 μM), and then by InsP ₃R3 (0.40 μM); 5), InsP₃R1 has a high-affinity (0.13 mM) ATP modulatory site, InsP₃R2 gating is ATP independent, and InsP₃R3 has a low-affinity (2 mM) ATP modulatory site; 6), ATP modulates InsP₃R1 gating in a noncooperative manner (n_Hill = 1.3); 7), ATP modulates InsP₃R3 gating in a highly cooperative manner (n_Hill = 4.1). Obtained results provide novel information about functional properties of mammalian InsP₃R isoforms.