Comparative effects of idazoxan, efaroxan, and BU 224 on insulin secretion in the rabbit: Not only interaction with pancreatic imidazoline I2 binding sites

The nature of the binding site(s) involved in the insulin secretory activity of imidazoline compounds remains unclear. An imidazoline I2 binding site (I2BS) has been neglected since the classic I2 ligand, idazoxan, does not release insulin. Using the rabbit as an appropriate model for the study of this type of binding sites, we have tried to re-evaluate the effects of idazoxan, the selective I2 compound BU 224, and efaroxan on insulin secretion. Mimicking efaroxan, idazoxan and BU 224 potentiated insulin release from perifused islets in the presence of 8 mM glucose. In static incubation, insulin secretion induced by idazoxan and BU 224 exhibited both dose and glucose dependencies. ATP-sensitive K (KATP) channel blockade, though at a different site from the SUR1 receptor, with subsequent Ca entry, mediates the insulin releasing effect of the three ligands. However, additional MAO independent intracellular steps in stimulussecretion coupling linked to PKA and PKC activation are only involved in the effect of BU 224. Therefore, both an I2 related binding site at the channel level shared by the three ligands and a putative I3-intracellularly located binding site stimulated by BU 224 would be mediating insulin release by these compounds. In vivo experiments reassess the abilities of idazoxan and BU 224 to enhance glucose-induced insulin secretion and to elicit a modest blood glucose lowering response.