Identification and functional reconstitution of the type 2 inositol 1,4,5-trisphosphate receptor from ventricular cardiac myocytes

The inositol 1,4,5-trisphosphate receptor (InsP(3)R) is an intracellular Ca2+ release channel that mediates the rise in cytoplasmic calcium in response to receptor-activated production of InsP(3). The InsP(3)R-mediated signaling pathway appears to be ubiquitous and is involved in many cellular processes including cell division, smooth muscle contraction, and neuronal signaling. Different regions of the heart also express InsP(3) receptors, We report here that acutely dissociated ventricular myocytes from ferret and rat hearts express significant levels of InsP(3)R as indicated by immunoblotting with a receptor consensus antibody, InsP(3) binding experiments (K-D = 23.6 nM and B-max = 0.46 pmol/mg) suggest the myocytes contain the high affinity type 2 InsP(3) receptor. Exhaustive mRNA screening by polymerase chain reaction, RNase protection, and subsequent DNA sequencing positively identify the InsP(3)R as type 2, The type 2 receptor from ferret heart was then incorporated into planar lipid bilayers and formed Ca2+-selective, InsP(3)-activated, heparin-blocked ion channels. We conclude that the predominant InsP(3) receptor isoform expressed in cardiac myocytes is type 2 and that it forms a functional InsP(3)-gated Ca2+ channel when reconstituted in planar lipid bilayers.