IP3 RECEPTOR PURIFIED FROM LIVER PLASMA-MEMBRANE IS AN (1,4,5)IP3 ACTIVATED AND (1,3,4,5)IP4 INHIBITED CALCIUM-PERMEABLE ION-CHANNEL

The IP3 receptor is involved in Ca2+ mobilization from intracellular s tores. Recently, we purified an inositol (1,4,5)-trisphosphate recepto r from rat liver plasma membrane (LPM-IP(3)R) [Schafer R. Hell K. Flei scher S. (1993) Purification of an IP3 receptor from liver plasma memb rane. Biophys. J. 66, A146]. The purified LPM-IP3 receptor was incorpo rated into vesicle derived planar bilayers and its channel properties characterized. The receptor displayed ion channel activity that was ac tivated by inositol (1,4,5)-trisphosphate [(1,4,5)IP3] (1 mu M) and in hibited by inositol (1,3,4,5)-tetrakisphosphate (IC50 similar to 1 mu M) and by heparin (IC50 similar to 20 mu g/ml). The channel displays a unitary conductance of 9 pS, and 13 pS in symmetrical 100 mM and 500 mM KCl, respectively, and in symmetrical 250 mM cesium methanesulfonat e the slope conductance is 11 pS. Activation by (1,4,5)IP3 is specific to the cis-side of the chamber, equivalent to the cytoplasmic face. T he receptor is a Ca2+ permeable ion channel based on ion selectivity ( Ca2+ > K+ > Na+ >> Cl-). The LPM-IP3 receptor was also permeable to Cs (Cs+ greater than or equal to K+), similar to other intracellular Ca2 + release channels, i.e. the IP3 receptor from brain and smooth muscle (IP(3)R-1) and the ryanodine receptor from skeletal muscle (RyR-1) an d heart (RyR-2). Channel activity is not voltage dependent (+/- 100 mV applied voltage). The channel is activated by ATP and Ca2+. The open probability of the (1,4,5)IP3 activated channel activity displays a be ll shaped response to cis Ca2+ ion concentration of our system. The LP M-IP3 receptor differs from intracellular IP(3)R-1 in that the Ca2+ an d ATP concentration required for maximum activation is about 10 times higher as compared with IP(3)R-1 from brain cerebellum and smooth musc le. We conclude that the LPM-IP3 receptor is an (1,4,5)IP3 activated C a2+ permeable ion channel. The implication of our studies is that in l iver, (1,4,5)IP3 regulates Ca2+ influx via the plasma membrane.