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 (InsP₃) evoked quantal Ca²⁺ mobilization: a rapid acceleration in the rate of ⁴⁵Ca²⁺ release abruptly followed by a biphasic decline to the basal rate before the InsP₃-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 InsP₃ under conditions that prevented Ca²⁺ mobilization. This change was accompanied by a decrease in the InsP₃ dissociation rate: the response declined more quickly when InsP₃ was removed during the initial stages of a response than later. We suggest that InsP₃ directly causes its receptor to rapidly switch (t(1/2) = 250 ms) between a low-affinity (K(d) ~ 1 μM) active, and a higher-affinity (K(d) ~ 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 InsP₃ receptors are more stringent (InsP₃ and then Ca²⁺ binding) than those for partial inactivation (InsP₃ binding); rapid inactivation is therefore likely to determine whether the cytosolic [Ca²⁺] reaches the threshold for regenerative Ca²⁺ signals.