IN-VIVO AND IN-VITRO EFFECTS OF DEXAMETHASONE ON PITUITARY THYROTROPIN-RELEASING HORMONE-LIKE PEPTIDE CONCENTRATIONS IN THE RAT

The novel peptide, pyroglutamyl-glutamyl-proline amide (pGlu-Glu-ProNH(2); EEP), which has structural and immunological similarities to TRH (pGlu-His-ProNH(2)) has recently been shown to contribute to total TRH-like immunoreactivity (t-TRH-LI) detected in the rabbit prostate and rat and porcine anterior pituitary. In this study, the effects of dexamethasone (DEX) on rat pituitary TRH-like peptide levels in the rat were determined. TRH-like immunoreactivity (TRH-LI) was separated by ion exchange chromatography and detected by TRH RIA. Anion exchange chromatographic analysis suggested that EEP-like immunoreactivity (EEP-LI) accounted for 15.0 +/- 1.2 pmol t-TRH-LI/g (70.4 +/- 3.9%) in the control anterior pituitary with the remaining t-TRH-LI being due to TRH-LI. Following DEX treatment pituitary EEP-LI and TRH-LI increased by 200% and 400% (P<0.001) respectively, constituting a 2.5-fold increase in t-TRH-LI in the pituitary. TRH-LI now accounted for 45.7 +/- 5.3% of t-TRH-LI compared with 29.6 +/- 4.1% in the controls. TRH-LI, but not EEP-LI, was detected in the hypothalamus and posterior pituitary, suggesting that EEP-LI is synthesised within the anterior pituitary. DEX also caused a 2.6-fold rise (P<0.001) in t-TRH-LI in dispersed, cultured anterior pituitary cells. Chromatographic analysis of cultured pituitary cell extracts revealed that the majority of t-TRH-LI (>98%) was due to TRH-LI. A possible explanation for the change in EEP-LI and TRH-LI levels in the in vivo and in vitro pituitary samples is that hypothalamic influences are necessary for the continued production of EEP-LI and are not present in vitro. Alter natively, the dissociation of the cell-cell interactions and/or the accumulation of cell products, particularly pituitary hormones in vitro, may result in a loss of the in vivo paracrine influences or the introduction of factors which inhibit EEP-LI and stimulate TRH-LI.