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 binding 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 (Kir6.2 deltaC26). There were two nonadrenergic imidazoline binding sites for [(3)H]clonidine with K(d) values of 61 nM and 4.5 microM, respectively. Quinine reduced specific binding incompletely (73%) with K(i) values of 75 nM and 133 microM. 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.