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 stores. Recently, we purified an inositol (1,4,5)-trisphosphate receptor from rat liver plasma membrane (LPM-IP(3)R) [Schafer R. Hell K. Fleischer S. (1993) Purification of an IP3 receptor from liver plasma membrane. Biophys. J. 66, A146]. The purified LPM-IP3 receptor was incorporated into vesicle derived planar bilayers and its channel properties characterized. The receptor displayed ion channel activity that was activated by inositol (1,4,5)-trisphosphate [(1,4,5)IP3] (1 mu M) and inhibited 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 methanesulfonate 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. The 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) and 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 bell shaped response to cis Ca2+ ion concentration of our system. The LPM-IP3 receptor differs from intracellular IP(3)R-1 in that the Ca2+ and ATP concentration required for maximum activation is about 10 times higher as compared with IP(3)R-1 from brain cerebellum and smooth muscle. We conclude that the LPM-IP3 receptor is an (1,4,5)IP3 activated Ca2+ permeable ion channel. The implication of our studies is that in liver, (1,4,5)IP3 regulates Ca2+ influx via the plasma membrane.