Glucose stimulates and insulin inhibits release of pancreatic TRH in vitro

Objective: Pancreatic TRH is present in insulin-producing B-cells of the islets of Langerhans. There is fragmentary evidence that it may be involved in glucoregulation, The aim of our present study was to analyze how glucose and insulin affect TRH secretion by the pancreatic islets. Design: Isolated pancreatic islets were incubated with different concentrations of glucose, insulin and glucagon, and TRH release was measured. Results: In the present study, 6 and 12 mmol/l D-glucose caused significant TRH release from isolated adult rat pancreatic islets when compared with that in the presence of the same concentrations of biologically ineffective L-glucose. Thirty mmol/l D-glucose was also ineffective, but this was not due to depression of secretion by hyperosmolarity since isosmotic compensation for the high glucose addition did not restore its stimulatory effect, Five mu mol/l dibutyryl cyclic 3',5'-adenosine monophosphate (db-cAMP) increased both basal and glucose-stimulated TRH release, but this effect was not seen with 50 mu mol/l db-cAMP. Stimulation of phosphodiesterase by imidazole resulted in decreased basal but not glucose-stimulated release of TRH. Glucagon (10(-7) mol/l) did not affect either basal or glucose-stimulated release of TRH, while insulin (10(-7) and 10(-6) mol/l) inhibited both, Conclusion: Our present data showing that glucose stimulates and insulin inhibits pancreatic TRH release are compatible with the possibility that this substance may play a role in glucoregulation.