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Title: Post-translational modulation of corticosteroid feedback inhibition in adenohypophysial corticotrophs
Author: Lim, Min Chin
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2000
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Adrenal corticosteroids play a major role in the negative feedback control of the hypothalamic-pituitary-adrenal axis, which ensures that corticosteroid levels are optimal for homeostatic adaptation. The objective of this thesis was to investigate the validity of a model of early glucocorticoid (GC) feedback inhibition of adrenocorticotrophic hormone (ACTH) secretion developed through the study of mouse corticotroph tumour (AtT20) cells. Early GC inhibition was found to require the synthesis of new mRNA and proteins. Previous work indicated that calmodulin is one of the GC-induced proteins that may mediate the inhibitory action of GC on ACTH secretion. Therefore, AtT20 cells that constitutively over-expressed chicken calmodulin were established to test if elevated calmodulin level could mimic GC action. Corticotrophin releasing factor (CRF)-stimulated ACTH secretion by stably transferred AtT20 cells that showed elevated calmodulin mRNA and protein levels and wild type AtT20 cells was found not be significantly different. Events downstream to the de novo synthesis of proteins in GC feedback inhibition were studied in cultured rat anterior pituitary cells. Membrane depolarisation elicited by a combination of a depolarising concentration of KC1 with 8-(4-chlorophenylthio)adenosine-3',5'-cyclic-monophosphate (CPT-cAMP) or CRF antagonised the GC inhibition of stimulated ACTH secretion. Since the control of membrane potential involves K+-channels, the effect of K+-channel blockers on GC inhibition was tested. The BK-channel inhibitor, charybdotoxin, had no significant effect on GC inhibition, nor did other common K+-channel blockers (tetraethylammonium, apamin and 4-aminopyridine). Clofilium (IsK-type K+-channel blocker) and astemizole (an antihistamine with anti-HERG-type K+-channel properties) were found to significantly reduce the inhibition of the CRF response by GC. More specific blockers of IsK-type K+-channels (chromanol 293B, WAY-123398) or HERG-type K+-channels (E4031), dofetilide) had no significant effect on GC inhibition. Previous studies have also suggested that control of intracellular cAMP levels via Ca2+-dependent feedback is important in GC inhibition. Hence, it was tested if clamping of cAMP at high levels could alter GC inhibition. High cAMP levels were induced and sustained using arginine vasopressin (AVP) or rolipram (inhibitor of cAMP-dependent cyclic nucleotide phosphodiesterase) in combination with CRF.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available