Rapid activation and partial inactivation of inositol trisphosphate receptors by inositol trisphosphate

During superfusion of permeabilized hepatocytes, submaximal concentrations of inositol 1,4,5-trisphosphate (InsP(3)) evoked quantal Ca2+ mobilization: a rapid acceleration in the rate of Ca-45(2+) release abruptly followed by a biphasic decline to the basal rate before the InsP(3)-sensitive stores had fully emptied. During the fast component of the decay, the Ca2+ permeability of the stores fell rapidly by 40% (t(1/2) = 250 ms) to a state indistinguishable from that evoked by preincubation with InsP(3) under conditions that prevented Ca2+ mobilization. This change was accompanied by a decrease in the InsP(3) dissociation rate: the response declined more quickly when InsP(3) was removed during the initial stages of a response than later. We suggest that InsP(3) directly causes its receptor to rapidly switch (t(1/2) = 250 ms) between a low-affinity (K-d similar to 1 mu M) active, and a higher-affinity (K-d similar to 100 nM) less active, conformation, and that this transition underlies the fast component of the decaying phase of Ca2+ release. Ca2+ continues to leak through the unchanging less active state of the receptor until those stores that responded initially are completely empty, accounting for the slow phase of the response. The requirements for activation of InsP(3) receptors are more stringent (InsP(3) and then Ca2+ binding) than those for partial inactivation (InsP(3) binding); rapid inactivation is therefore likely to determine whether the cytosolic [Ca2+] reaches the threshold for regenerative Ca2+ signals.