High concentration of thyrotropin-releasing hormone in pancreatic islets.

The concentration of thyrotropin-releasing hormone (TRH, thyroliberin) in rat islets of Langerhans is 30-fold higher than in whole rat pancreas, indicating that the islets are the main source of pancreatic TRH. The TRH extracted from islets is indistinguishable from synthetic TRH in its immunological and biological properties and in its inactivation by human serum. The physiologic function of islet TRH is unknown. However, because TRH is antagonistic to somatostatin in other systems, and somatostatin also is concentrated in islets in high concentrations, it is possible that islet TRH may serve a similar antagonistic function in the regulation of islet cell secretory activity.