Specificity of nonadrenergic imidazoline binding sites in insulin-secreting cells and relation to the block of ATP-sensitive K+ channels

To characterize the specificity of nonadrenergic imidazoline hinding sites of insulin-secreting HIT cells, competitive binding of insulinotropic imidazolines and quinine was measured and compared with the effect of these compounds on native K-ATP channels and with a heterologously expressed variant of the pore-forming subunit (Kir6.2DeltaC26). There were two nonadrenergic imidazoline binding sites for [H-3]clonidine with K-d values of 61 nM and 4.5 muM, respectively. Quinine reduced specific binding incompletely (73%) with K-i values of 75 nM and 133 muM. Clonidine, N-allyl-clonidine (alinidine), and quinine inhibited native K-ATP channels as well as Kir6.2DeltaC26 channels. Coexpression of Kir6.2DeltaC26 and SUR1 (the regulatory subunit of K-ATP) did not increase the potency of quinine. There are nonadrenergic imidazoline binding sites in insulin-secreting HIT cells which also recognize quinine. One of these sites is Kir6.2, the pore-forming subunit of the K-ATP channel.