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 maximum increase at 3 h. The increase in StAR protein was delayed when compared to its mRNA. The increase in the mitochondrial StAR protein at 3 h was concomitant with that of the homogenate indicating that the entry of StAR into mitochondria might not be necessary to increase steroidogenesis during the early stimulatory phase. In conclusion, we showed that ACTH increases StAR mRNA followed, after a delay, by an increase in the level of StAR protein; this suggests that post-translational modifications of StAR precursor occur during the early stimulatory phase and this occurs before the apparent translation of the newly formed mRNA.