The effect on insulin release of efaroxan, an alpha 2-adrenoceptor antagonist and a highly potent drug at imidazoline I1 receptors, and the effects of seven other imidazoline compounds selective for the imidazoline I1 or I2 receptors, were studied in the rat insulinoma cell line RIN-5AH. The cells released insulin in response to glucose (0.3-10 mM), and efaroxan (100 microM) potentiated glucose-induced insulin release. (-)-Adrenaline completely displaced the binding of [125I]p-iodoclonidine to membranes of RIN-5AH cells, indicating that these cells do not express imidazoline I1 receptors. Cirazoline and idazoxan (100 microM), both highly potent drugs at imidazoline I2 receptors, and the guanidines guanoxan and amiloride (200 microM), also promoted insulin release from RIN-5AH cells. Irreversible blockade of imidazoline I2 receptors with 10 microM clorgyline did not prevent the stimulatory effects of cirazoline or idazoxan; however, these compounds completely reversed the inhibition by diazoxide (250 microM), an opener of ATP-dependent K+ channels (K+ATP channels), of glucose-induced insulin release. These data indicate that the imidazoline/guanidine compounds promote insulin release from RIN-5AH cells, by interacting with a novel binding site related to K+ATP channels that does not represent any of the known imidazoline I1 or I2 receptors.