Effects of divalent cations on single-channel conduction properties of Xenopus IP3 receptor.
The effects of Mg2+ and Ba2+ on single-channel properties of the inositol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel kinetics and presence of conductance substates were similar over a range (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I-V) relation was linear with conductance of approximately 320 pS. Conductance varied slowly and continuously over a wide range (SD approximately 60 pS) and sometimes fluctuated during single openings. The presence of Mg2+ on either or both sides of the channel reduced the current (blocking constant approximately 0.6 mM in symmetrical Mg2+), as well as the range of conductances observed, and made the I-V relation nonlinear (slope conductance approximately 120 pS near 0 mV and approximately 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effects on channel conductance. Mg2+ and Ba2+ permeated the channel with a ratio of permeability of Ba2+ to Mg2+ to K+ of 3.5:2.6:1. These results indicate that divalent cations induce nonlinearity in the I-V relation and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furthermore, stabilization of conductance by divalent cations reveals a novel interaction between the cations and the IP3R.
Department of Physiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6100, USA.
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D000818:Animals; D001464:Barium / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002413:Cations, Divalent / Q000494:pharmacology*; D004553:Electric Conductivity; D005260:Female; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009865:Oocytes / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D011930:Reaction Time; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects / Q000502:physiology*; D014982:Xenopus laevis
D015220:Calcium Channels; D002413:Cations, Divalent; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D001464:Barium; D015544:Inositol 1,4,5-Trisphosphate; D008274:Magnesium; D011188:Potassium