Different Modes of Action of the Imidazoline Compound RX871024 in Pancreatic β‐Cells: Blocking of K + Channels, Mobilization of Ca 2+ from Endoplasmic Reticulum, and Interaction with Exocytotic Machinery a

The imidazoline compound RX871024 glucose‐dependently potentiates the release of insulin in pancreatic islets and β‐cell lines. This activity of the compound is not related to its action by stimulating α 2 ‐adrenoceptors and I 1 ‐ and I 2 ‐imidazoline receptors. There are at least three modes of action of RX871024 in β‐cells: (1) RX871024 blocks the ATP‐dependent, Ca + ‐activated, and delayed rectifier K + channel activity; (2) RX871024 causes mobilization of Ca 2+ from thapsigargin‐sensitive intracellular stores, the effect probably controlled by cytochrome P450; and (3) the stimulatory activity of RX871024 on insulin release involves interaction of the compound with the exocytotic machinery, unrelated to the changes in membrane potential and cytoplasmic‐free Ca 2+ concentration, whereas protein phosphorylation plays an important role in this process. The maximal insulinotropic effect of RX871024 is much higher than that of the sulfonylurea glibenclamide. RX871024 stimulates insulin release and normalizes blood glucose levels in rats in vivo without affecting blood pressure and heart rate.