IP 3 receptor purified from liver plasma membrane is an (1,4,5)IP 3 activated and (1,3,4,5)IP 4 inhibited calcium permeable ion channel

Abstract

The IP 3 receptor is involved in Ca 2+ mobilization from intracellular stores. Recently, we purified an inositol (1,4,5)-trisphosphate receptor from rat liver plasma membrane (LPM-IP 3R) [Schäfer R. Hell K. Fleischer S. (1993) Purification of an IP 3 receptor from liver plasma membrane. Biophys. J. 66, A146] . The purified LPM-IP 3 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)IP 3] (1 μM) and inhibited by inositol (1,3,4,5)-tetrakisphosphate (IC 50 ∼1 μM) and by heparin (IC 50 ∼20 gm/ml). The channel displays a unitary conductance of 9 pS, and 13 pS in symmetrical 100 mM and 500 mM KCI, respectively, and in symmetrical 250 mM cesium methanesulfonate the slope conductance is 11 pS. Activation by (1,4,5)IP 3 is specific to the cis-side of the chamber, equivalent to the cytoplasmic face. The receptor is a Ca 2+ permeable ion channel based on ion selectivity (Ca 2+ > K + > Na + ⪢ Cl −). The LPM-IP 3 receptor was also permeable to Cs (Cs + ≥ K +), similar to other intracellular Ca 2+ release channels, i.e. the IP 3 receptor from brain and smooth muscle (IP 3R-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 Ca 2+. The open probability of the (1,4,5)IP 3 activated channel activity displays a bell shaped response to cis Ca 2+ ion concentration of our system. The LPM-IP 3 receptor differs from intracellular IP 3R-1 in that the Ca 2+ and ATP concentration required for maximum activation is about 10 times higher as compared with IP 3R-1 from brain cerebellum and smooth muscle. We conclude that the LPM-IP 3 receptor is an (1,4,5)IP 3 activated Ca 2+ permeable ion channel. The implication of our studies is that in liver, (1,4,5)IP 3 regulates Ca 2+ influx via the plasma membrane.