Thyroid status and the regulation of thyrotropin-releasing hormone synthesis in rat pancreatic islets: comparison with insulin regulation.

Thyrotropin-releasing hormone (TRH), originally characterized as the first hypothalamic hormone, is also synthesized in the insulin-containing cells. TRH stimulates the glucagon secretion and attenuates exocrine pancreatic secretions. We have previously reported, using whole pancreatic homogenates, that TRH content increased in hypothyroid rats, associated to a loss of TRH-degrading activity. The present study was undertaken on purified islets, an appropriate model to examine thyroid status-dependent regulation of two islet hormones, TRH and insulin. The islets from hypothyroid rat pancreas had increased TRH content (4x) without any change in insulin content. Likewise, the Northern blot analysis revealed that the steady-state concentrations of TRH mRNA increased (4x) while those of insulin remain unchanged. These data therefore suggest that TRH gene transcription is under the negative control of T3. This study also provides insight into islet response to impaired thyroid function.