Correlation between the effects of retinoic acid and dexamethasone on liver tyrosine aminotransferase.

A single dose of 50 microg of trans-retinoic acid administered to rats significantly raised the level of hepatic tyrosine after a few hours. This effect was compared with that of dexamethasone and a possible correlation between these effectors was also investigated. An equal increase in enzyme activity level caused by retinoic acid was observed in adrenalectomized rats, leading to the suggestion that the effect of retinoic acid on liver tyrosine aminotransferase is independent of glucocorticoids. However, the study of the binding activity of the liver nuclear glucocorticoid receptors vs dexamethasone showed that this activity is favoured by retinoic acid, whereas no variation was evidenced for retinoic acid receptors caused by dexamethasone. In the adrenalectomized rat, the synergistic effect produced by the association of retinoic acid and dexamethasone at the lowest doses used led us to conclude that retinoic acid is an efficient effector of liver tyrosine aminotransferase. It probably affects tyrosine aminotransferase activity in a dependent and an independent way, regulated respectively by the glucorticoid status and by the provision of retinoic acid.