Different Modes of Action of the Imidazoline Compound RX871024 in Pancreatic β‐Cells: Blocking of K⁺ Channels, Mobilization of Ca²⁺ from Endoplasmic Reticulum, and Interaction with Exocytotic Machinery^a

Abstract

The imidazoline compound RX871024 glucose-dependently potentiates the release of insulin in pancreatic islets and beta-cell lines. This activity of the compound is not related to its action by stimulating alpha 2-adrenoceptors and I1- and I2-imidazoline receptors. There are at least three modes of action of RX871024 in beta-cells: (1) RX871024 blocks the ATP-dependent, Ca(2+)-activated, and delayed rectifier K+ channel activity; (2) RX871024 causes mobilization of Ca2+ 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 Ca2+ 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.

Imidazoline compounds protect against interleukin 1beta-induced beta-cell apoptosis.

“…In addition, we have demonstrated that long-term exposure to the imidazoline compound RX871024 is not toxic to ␤-cells. These data, together with the observations published previously (3)(4)(5)(6)(7)42), suggest that imidazoline compounds should be explored as potential therapeutic agents for the treatment of both type 1 and type 2 diabetes. …”

“…Previously, we showed that imidazolines stimulate insulin release both by blocking ATP-dependent K + channels, with a subsequent increase in cytoplasmic free Ca 2+ concentration ([Ca 2+ ] i ), and by directly affecting the exocytotic machinery (3,7). In addition, imidazoline compounds induce release of Ca 2+ from nonmitochondrial thapsigargin-sensitive intracellular stores (8).…”

Two Generations of Insulinotropic Imidazoline Compounds

“…Second, measurements of protein phosphorylation in permeabilized insulin-secreting HIT T15 cells show that RX871024 induced protein phosphorylation (24). Finally, the increase in phosphorylation induced by the imidazoline was blocked by the protein kinase inhibitors RpBrcAMPS, H-89, and staurosporine (24). Thus, the imidazoline RX871024 induced insulin secretion not only by depolarizing the membrane and increasing [Ca 2ϩ ] i , but also by stimulating PKA-and PKC-dependent protein phosphorylation in pancreatic ␤-cells.…”

“…First, inhibitors of protein kinase A (PKA) (H-89 and Rp-BrcAMPS) or protein kinase C (PKC) (calphostin C and staurosporine) completely abolished imidazoline-induced increases in insulin secretion, whereas glucose-and KClinduced stimulation of insulin secretion were unaffected (18). Second, measurements of protein phosphorylation in permeabilized insulin-secreting HIT T15 cells show that RX871024 induced protein phosphorylation (24). Finally, the increase in phosphorylation induced by the imidazoline was blocked by the protein kinase inhibitors RpBrcAMPS, H-89, and staurosporine (24).…”

“…Activity of protein kinase A and C and the insulinotropic effect of RX871024. The effects of the imidazoline RX871024 on insulin secretion were also studied under conditions in which [Ca 2ϩ ] i was kept constant (18,24). Islets were either depolarized with high concentrations of KCl and glucose or electropermeabilized.…”