Single-channel kinetics, inactivation, and spatial distribution of inositol trisphosphate (IP3) receptors in Xenopus oocyte nucleus.
Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) 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 [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 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 tau < 5 ms and approximately 20 ms; and at least three closed states with tau approximately 1 ms, approximately 10 ms, and >1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. 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 F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 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 IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.
Mak
D O
DO
Department of Physiology, University of Pennsylvania, Philadelphia 19104-6100, USA. dmak@mail.med.upenn.edu
Foskett
J K
JK
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure; D004594:Electrophysiology; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology; D009865:Oocytes / Q000378:metabolism* / Q000648:ultrastructure; D018408:Patch-Clamp Techniques; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014981:Xenopus
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D015544:Inositol 1,4,5-Trisphosphate