Inositol hexakisphosphate binding sites in rat heart and brain

1 . Inositol 1,4,5‐trisphosphate (Ins(1,4,5)P3) and inositol hexakisphosphate (InsP6) are produced in response to stimulation of cardiac α1‐adrenoceptors. While the role of Ins(1,4,5)P3 and Ins(1,4,5)P3 receptors is well‐defined in many tissues including brain, the functional role of the putative InsP6‐InsP6 receptor system in cardiac function is less clear. Using quantitative autoradiography, this study examined the characteristics and regional localization of [3H]‐InsP6 binding sites in rat heart and compared the affinity of a range of inositol polyphosphates for [3H]‐InsP6 and [3H]‐Ins(1,4,5)P3 binding sites in heart and brain. 2 . [3H]‐InsP6 bound to a single, high affinity site in sections of rat heart (KD ranging from 22 ± 1.9 nM in right atria to 35 ± 2.6 nM in the interventricular septum, n = 7). The maximal number of binding sites (Bmax) ranged from 5.1 ± 0.48 to 12 ± 1.8 pmol mg−1 protein in left atrium and left ventricle, respectively. Inositol phosphates inhibited binding of [3H]‐InsP6 with the order of potency: InsP6 > Ins(1,4,5)PS3 > inositol 1,3,4,5‐tetrakisphosphate ≥ inositol pentakisphosphate > Ins(1,4,5)P3 > > inositol mono‐ and bisphosphates, consistent with the labelling of an InsP6 binding site. 3 . The Ins(1,4,5)P3 analogue, Ins(1,4,5)PS3, originally investigated as a putative selective radioligand for the Ins(1,4,5)P3 receptor, was a potent inhibitor of [3H]‐InsP6 binding in all heart regions (KI = 170–260 nM). The KI of Ins(1,4,5)PS3 for the inhibition of [3H]‐Ins(1,4,5)P3 binding in rat brain (60–220 nM) was similar to that observed for the inhibition of [3H]‐InsP6 binding in heart, suggesting that Ins(1,4,5)PS3 is not a specific ligand for either Ins(1,4,5)P3 or InsP6 receptor binding sites. 4 . Previous studies have detected [3H]‐InsP6 binding in mitochondrial and sarcoplasmic reticulum fractions of heart and links between InsP6 and cardiac mitochondrial Ca2+ regulation have been proposed, suggesting further studies are warranted to determine the functional role(s) of InsP6 and InsP6 receptor binding sites in cardiac tissue.