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 alpha 1-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 > or = 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 (K1 = 170-260 nM). The K1 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.
Rowley
K G
KG
University of Melbourne, Department of Medicine, Austin and Repatriation Medical Centre, Heidelberg, Victoria, Australia.
Gundlach
A L
AL
Cincotta
M
M
Louis
W J
WJ
D000818:Animals; D001345:Autoradiography; D001665:Binding Sites / Q000187:drug effects; D001923:Brain Chemistry / Q000502:physiology*; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D008931:Mitochondria, Muscle / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D009124:Muscle Proteins / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D010833:Phytic Acid / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism
D009124:Muscle Proteins; D010833:Phytic Acid; D015544:Inositol 1,4,5-Trisphosphate