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 inosi tol 1,4,5-trisphosphate receptor (IP3R) were studied by patch clamp of isolated nuclei from Xenopus oocytes. In 140 mM K+ the IP3R channel k inetics and presence of conductance substates were similar over a rang e (0-9.5 mM) of free Mg2+. In 0 mM Mg2+ the channel current-voltage (I -V) relation was linear with conductance of similar to 320 pS. Conduct ance varied slowly and continuously over a wide range (SD approximate to 60 pS) and sometimes fluctuated during single openings. The presenc e of Mg2+ on either or both sides of the channel reduced the current ( blocking constant similar to 0.6 mM in symmetrical Mg2+), as well as t he range of conductances observed, and made the I-V relation nonlinear (slope conductance similar to 120 pS near 0 mV and similar to 360 pS at +/-70 mV in symmetrical 2.5 mM Mg2+). Ba2+ exhibited similar effect s on channel conductance. Mg2+ and Ba2+ permeated the channel with a r atio 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 relatio n and reduce current by a mechanism involving permeation block of the IP3R due to strong binding to site(s) in the conduction pathway. Furth ermore, stabilization of conductance by divalent cations reveals a nov el interaction between the cations and the IP3R.