Single-Channel Kinetics, Inactivation, and Spatial Distribution of Inositol Trisphosphate (IP(3)) Receptors in Xenopus Oocyte Nucleus

Single-channel properties of the Xenopus inositol trisphosphate receptor (IP(3)R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K(+) as the charge carrier (cytoplasmic [IP(3)] = 10 μM, free [Ca(2+)] = 200 nM), the IP(3)R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and ∼300 pS at 60 mV. The channel was frequently observed with high open probability (mean P (o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants τ < 5 ms and ∼20 ms; and at least three closed states with τ ∼1 ms, ∼10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and τ of the longest closed state. A novel “flicker” kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F(1) and F(2). The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F(1) to F(2) transitions. Channel run-down or inactivation (τ ∼ 30 s) was consistently observed in the continuous presence of IP(3) and the absence of change in [Ca(2+)]. Some (∼10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage- reversible inactivation. Mapping of functional channels indicates that the IP(3)R tends to aggregate into microscopic (<1 μm) as well as macroscopic (∼10 μm) clusters. Ca(2+)-independent inactivation of IP(3)R and channel clustering may contribute to complex [Ca(2+)] signals in cells.