Phentolamine inhibits exocytosis of glucagon by G i2 protein-dependent activation of calcineurin in rat pancreatic α-cells

Abstract

Capacitance measurements were used to investigate the molecular mechanisms by which imidazoline compounds inhibit glucagon release in rat pancreatic α-cells. The imidazoline compound phentolamine reversibly decreased depolarization-evoked exocytosis >80% without affecting the whole-cell Ca 2+ current. During intracellular application through the recording pipette, phentolamine produced a concentration-dependent decrease in the rate of exocytosis (IC 50 =9.7 μM). Another imidazoline compound, RX871024, exhibited similar effects on exocytosis (IC 50 = 13 μM). These actions were dependent on activation of pertussis toxin-sensitive G i2 proteins but were not associated with stimulation of ATP-sensitive K + channels or adenylate cyclase activity. The inhibitory effect of phentolamine on exocytosis resulted from activation of the protein phosphatase calcineurin and was abollshed by cyclosporin A and deltamethrin. Exocytosis was not affected by intracellular application of specific α 2, I 1, and I 2 ligands. Phentolamine reduced glucagon release (IC 50 =1.2 μM) from intact islets by 40%, an effect abolished by pertussis toxin, cyclosporin A, and deltamethrin. These data suggest that imidazoline compounds inhibit glucagon secretion via G i2-dependent activation of calcineurin in the pancreatic α-cell. The imidazoline binding site is likely to be localized intracellularly and probably closely associated with the secretory granules.