Single-channel inositol 1,4,5-trisphosphate receptor currents revealed by patch clamp of isolated Xenopus oocyte nuclei.

Patch clamp of the outer nuclear membrane of isolated Xenopus oocyte nucleus was used to measure the single-channel properties of the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R). The observed channel was activated by IP3, inhibited by heparin, and Ca(2+)-selective, with ion permeabilities PCa:PK:PCl = 8:1:0.05. In symmetric KCl buffer, the channel was ohmic (113 picosiemens in 140 mM KCl) at low channel currents but rectified at higher positive currents. The nuclear IP3R exhibited three conductance substates: a main substate occurring approximately 90% of channel open time, a double substate with twice the main substate conductance and a third substate with half the main substate conductance, which was observed rarely. Channel open probability fluctuated over time and among nuclei. Mean open channel durations of the main and double substates were approximately 5 and 1 ms, respectively. Many channels exhibited periods of closure lasting seconds, and most inactivated permanently within 5 min of IP3 stimulation. These results provide the first characterization of the single-channel properties of the IP3R in its native membrane environment and demonstrate that patch clamp electrophysiology of intact nuclei can be used to directly record currents through the IP3R.