Acute in vivo effects of ACTH on the expression of steroidogenic acute regulatory protein in rat adrenal

We have studied the effects of adrenocorticotropin (ACTH) on the expression of steroidogenic acute regulatory protein (StAR) in rat adrenals in vivo. Following ACTH stimulation, the level of StAR mRNA was increased within Ih in zona glomerulosa (ZG) and zona fasciculata-reticularis (ZFR), with a max imum increase at 3 h. The increase in StAR protein was delayed when compare d to its mRNA. The increase in the mitochondrial StAR protein at 3 h was co ncomitant with that of the homogenate indicating that the entry of StAR int o mitochondria might not be necessary to increase steroidogenesis during th e early stimulatory phase. In conclusion, we showed that ACTH increases StA R mRNA followed, after a delay, by an increase in the level of StAR protein ; this suggests that post-translational modifications of StAR precursor occ ur during the early stimulatory phase and this occurs before the apparent t ranslation of the newly formed mRNA.