[Tissue-specific hormonal regulation of the level of tyrosine aminotransferase mRNA in rats].

Tyrosine aminotransferase (TAT) displays a higher activity in rat liver than in cardiac, renal and cerebral tissues. In the latter three tissues the enzyme is not activated by hydrocortisone. In order to investigate the nature of TAT tissue specificity and to understand how its activity is regulated by hormones in different tissues, the content of TAT mRNA in rat liver, kidney, heart and brain was measured by dot-hybridization using TAT cRNA. TAT mRNA was found in all tissues under study, although in the liver its amount was 10 times as high as in kidney, heart and brain, which is consistent with different levels of TAT enzymatic activity. Upon hydrocortisone administration the amount of TAT mRNA in the liver showed a constant increase reaching a maximum (7-fold) thereafter. The TAT mRNA levels in cardiac and cerebral tissues remained unchanged. These data suggest that the tissue-specific expression and regulation of TAT activity are effectuated by changes in the mRNA contents which may be due to the level of TAT gene transcription.