Cooperative activation of IP_3 receptors by sequential binding of IP_3 and Ca~(2+) safeguards against spontaneous activity

摘要

Background: Ca~(2+) waves allow effective delivery of intracellular Ca~(2+) signals to cytosolic targets. Propagation of these regenerative Ca~(2+) signals probably results from the activation of intracellular Ca~(2+) channels by the increase in cytosolic [Ca~(2+)] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca~(2+) channels are therefore likely to have evolved in parallel with the mechanism of Ca~(2+)-induced Ca~(2+) release. Results: Maximal rates of ~(45)Ca~(2+) efflux from permeabilised hepatocytes superfused with medium in which the [Ca~(2+)] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP)_3). A minimalinterval of -400 msec between IP_3 addition and the peak rate of Ca~(2+) release as well as an absolute latency of 30 msec before initiation of Ca~(2+) mobilisation indicate that channel opening does not immediately follow binding of IP_3. Although the absolute latency of Ca~(2+) release was unaffected by further increasing the IP_3 concentration, it was reduced by increased [Ca~(2+)]. Conclusions: We propose that the closed conformation of the IP_3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP_3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extremeform of coincidence detection. Binding of IP_3 to each of its four receptor subunits unmasks a site to which Ca~(2+) must bind before the channel can open. As IP_3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate.