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Thyrotropin‐Releasing Hormone‐Induced Down‐Regulation of Pyroglutamyl Aminopeptidase II Activity Involves l‐Type Calcium Channels and Cam Kinase Activities in Cultures of Adenohypophyseal Cells

Released thyrotropin‐releasing hormone (TRH) is inactivated by a narrow specificity ectopeptidase, pyroglutamyl aminopeptidase II (PPII), present in brain and lactotrophs. Various hypothalamic/paracrine factors, including TRH, slowly (in hours) regulate the activity of PPII on the surface of adenohypophyseal cells. TRH‐induced down‐regulation was mimicked by protein kinase C (PKC) activation but was not affected by inhibition of PKC. Adenylate cyclase activation can also down‐regulate PPII. The purpose of this study was to identify elements of the transduction pathway used by TRH to regulate PPII activity. In primary cultures of female adenohypophyseal cells, activation of the stimulatory G protein or adenylate cyclase produced an effect additive to that of TRH; inhibition of protein kinase A activity did not interfere with TRH action. However, regulation of PPII activity by TRH was inhibited by a phospholipase C β inhibitor or chelation of intracellular calcium. l‐type calcium channels (LCC) agonists mimicked TRH action and their effect was not additive with that of TRH. Antagonists of LCC channels and inhibitors of calmodulin or calcium/calmodulin‐dependent protein kinase blocked TRH action. Therefore, TRH‐induced calcium entry through l‐type calcium channels and the activity of calcium/calmodulin‐dependent protein kinase are required for TRH effect on PPII activity in primary cultures of adenohypophyseal cells. This pathway may coregulate PPII and prolactin biosynthesis in response to TRH.