Synthesis and characterization of a novel tritiated KATP channel opener with a benzopyran structure.

The synthesis of a tritiated benzopyran-type opener of the ATP-dependent K+ channel (KATP channel), [3H]-PKF217 - 744 (3S,4R)-N-[3,4-dihydro-2,2-dimethyl-3-hydroxy-6-(2-methyl-4-pyridinyl)-2H-1-benzopyran-4-yl]-3-[2,6-3H]pyridinecarboxamide with a specific activity of 50 Ci mmol(-1) is described. Binding of the ligand was studied in membranes from human embryonic kidney cells transfected with the sulphonylurea receptor isoforms, SUR2B and SUR2A, respectively. PKF217 - 744 was confirmed as being a KATP channel opener by its ability to open the Kir6.1/SUR2B channel, the recombinant form of the vascular KATP channel, and to inhibit binding of the pinacidil analogue, [3H]-P1075, to SUR2B (Ki=26 nM). The kinetics of [3H]-PKF217 - 744 binding to SUR2B was described by rate constants of association and dissociation of 6.9x10(6) M(-1) min(-1) and 0.09 min(-1), respectively. Binding of [3H]-PKF217 - 744 to SUR2B/2A was activated by MgATP (EC50 approximately 3 microM) and inhibited (SUR2B) or enhanced (SUR2A) by MGADP: Binding of [3H]-PKF217 - 744 to SUR2B was inhibited by representatives of the different structural classes of openers and sulphonylureas. Ki values were identical with those obtained using the opener [3H]-P1075 as the radioligand. Glibenclamide accelerated dissociation of the SUR2B-[3H]-PKF217 - 744 complex. The data show that the affinity of [3H]-PKF217 - 744 binding to SUR2B is approximately 6 times lower than that of [3H]-P1075. This is due to a surprisingly slow association rate of the benzopyran-type ligand, suggesting a complex mechanism of opener binding to SUR. The other pharmacological properties of the two opener radioligands are identical.