SINGLE-CHANNEL KINETICS, INACTIVATION, AND SPATIAL-DISTRIBUTION OF INOSITOL TRISPHOSPHATE (IP3) RECEPTORS IN XENOPUS OOCYTE NUCLEUS

Citation
Dod. Mak et Jk. Foskett, SINGLE-CHANNEL KINETICS, INACTIVATION, AND SPATIAL-DISTRIBUTION OF INOSITOL TRISPHOSPHATE (IP3) RECEPTORS IN XENOPUS OOCYTE NUCLEUS, The Journal of general physiology, 109(5), 1997, pp. 571-587
Citations number
81
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physiology
ISSN journal
0022-1295
Volume
109
Issue
5
Year of publication
1997
Pages
571 - 587
Database
ISI
SICI code
0022-1295(1997)109:5<571:SKIASO>2.0.ZU;2-0
Abstract
Single-channel properties of the Xenopus inositol trisphosphate recept or (IP3R) ion channel were examined by patch clamp electrophysiology o f the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K as the charge carrier (cytoplasmic [IP3]=10 mu M, free [Ca2+]=200 nM) , the IP3R exhibited four and possibly five conductance states. The co nductance of the most-frequently observed state M was 113 pS around 0 mV and similar to 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P-0=0.4 at 20 mV). Dwell time distri bution analysis revealed at least two kinetic states of M with time co nstants tau <5 ms and similar to 20 ms; and at least three closed stat es with tau similar to 1 ms, similar to 10 ms, and >1 s. Higher cytopl asmic potential increased the relative frequency and tau of the longes t closed state. A novel ''flicker'' kinetic mode was observed, in whic h 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 cor responding to 1.33 electron charges moving across the entire electric field during F-1 to F-2 transitions. Channel run-down or inactivation (tau similar to 30 s) was consistently observed in the continuous pres ence of IP3 and the absence of change in [Ca2+]. Some (similar to 10%) channel disappearances could be reversed by an increase in voltage be fore 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 mech anism generates flicker activity and voltage reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggreg ate into microscopic (<1 mu m) as well as macroscopic (similar to 10 m u m) clusters. Ca2+-independent inactivation of IP3R and channel clust ering may contribute to complex [Ca2+] signals in cells.