Pertussis Toxin–Sensitive G Protein Mediation of PGE₂ Inhibition of cAMP Metabolism and Phasic Glucose-Induced Insulin Secretion in HIT Cells

Although prostaglandin E₂ (PGE₂) is known to inhibit glucose-induced insulin secretion, it is uncertain whether PGE₂ actions on the β-cell are direct, whether they are equipotent for both phases of hormone secretion, and whether the same mechanism of action prevails throughout. Study of the HIT cell, a clonai line of pancreatic β-cells, provides answers to these questions because perifusion with glucose and 3-isobutyl-1-methylxanthine stimulates biphasic insulin secretion. Perifusion with PGE₂ decreased both the first and second phases of glucose-induced insulin release to 47 ± 4% of controls. Pretreatment with pertussis toxin partly prevented PGE₂ inhibition to 80 ± 4% of controls for first phase and 79 ± 4% of controls for second phase. To evaluate whether the partial prevention of PGE₂ inhibition seen with pertussis toxin pretreatment was caused by G₁ heterotrimer association between the preincubation period and the end of perifusion, PGE₂ actions were also examined during continuous treatment with pertussis toxin. Under these conditions, PGE₂ inhibition of both phases was totally prevented. However, no difference was observed in membrane protein ADP ribosylation when cells were examined by sodium dodecyl sulfate–polyacrylamide gel electrophoresis after pretreatment or continuous treatment with pertussis toxin. Cyclic AMP (cAMP) accumulation was inhibited by PGE₂ (from 3263 ± 153 to 1549 ± 158 fmol/10⁶ cells) but less so after pretreatment with pertussis toxin (correlation between insulin release and cAMP accumulation during perifusion; n = 18, r = .85, P < .001). Thus, PGE₂ equally inhibits both phases of glucose-induced insulin secretion and cAMP generation through a pertussis toxin–sensitive G protein–mediated direct effect on the pancreatic β-cell.