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 n ucleus was used to measure the single-channel properties of the inosit ol 1,4,5-trisphosphate (IP3) receptor (IP(3)R). The observed channel w as activated by IP3, inhibited by heparin, and Ca2+-selective, with io n permeabilities P-Ca:P-K:P-Cl = 8:1:0.05. In symmetric KCI buffer, th e channel was ohmic (113 picosiemens in 140 mM KCl) at low channel cur rents but rectified at higher positive currents. The nuclear IP(3)R ex hibited three conductance substates: a main substate occurring similar to 90% of channel open time, a double substate with twice the main su bstate conductance and a third substate with half the main substate co nductance, which was observed rarely. Channel open probability fluctua ted over time and among nuclei. Mean open channel durations of the mai n and double substates were similar to 5 and 1 ms, respectively. Many channels exhibited periods of closure lasting seconds, and most inacti vated permanently within 5 min of IP3 stimulation. These results provi de the first characterization of the single-channel properties of the IP(3)R in its native membrane environment and demonstrate that patch c lamp electrophysiology of intact nuclei can be used to directly record currents through the IP(3)R.