<p> Reduced oxygen tension (i.e. hypoxia) regulates gene expression in various chromaffin cell types that synthesize catecholamines. In this study, the effect of chronic hypoxia on tyrosine hydroxylase (TH) mRNA and protein expression was investigated in the adrenomedullary chromaffin MAH cell line. RT-PCR results indicated that TH mRNA was expressed in MAH cells both during normoxia (20% 0 2) and hypoxia (5% 02). However, TH mRNA expression during chronic hypoxia was significantly higher than that during normoxia, increasing by approximately 2- fold after 16 hour exposure to chronic hypoxia. Western Blot analysis of the regulation of TH gene expression by chronic hypoxia indicated that TH protein initially decreased during 10 hr exposure to hypoxia and this was followed by a rapid increase in expression over the next 10 hr, and then by a slower increase (up to 1.3x initial control) after 72 hr exposure. Therefore, TH mRNA and protein levels were changed in MAH cells by hypoxia in a time-dependent manner. Surprisingly, cobalt treatment of MAH cells, expected to mimic the effects of chronic hypoxia, had little effect on TH gene expression. Interestingly, the decrease in TH expression protein after 10 hr exposure to hypoxia was prevented by nifedipine, an L-type calcium channel blocker. These results suggest that MAH cells represent a useful model system for examining hypoxia-induced gene regulation in an 02-sensitive cell line. Additionally, preliminary studies on HIF-1a expression in MAH cells showed that HIF-la mRNA was expressed and remained stable under both hypoxic and normoxic conditions. </p> / Thesis / Master of Science (MSc)