Developmental changes in adrenocorticotrophin (ACTH)-induced expression of ACTH receptor and steroid acute regulatory protein mRNA in ovine fetal adrenal cells.

Objectives: Adrenocorticotrophin (ACTH) plays an important role in mediating the increase in cortisol output in the late gestation sheep fetus. At the adrenal itself, heightened expression of ACTH receptor (ACTH-R) and steroid acute regulatory protein (StAR) appear to be important parallel changes. This study examined how ACTH affects ACTH-R and StAR mRNA expression, and cortisol production in adrenocortical cells isolated from fetuses of varying gestational age (dGA). We hypothesized that the ability of ACTH to stimulate its receptor and StAR mRNA expression would be greater close to term than earlier in development.

Methods: Adrenals were obtained from fetuses (100-105, 120, or 135-139 dGA), and the cortical cells were dispersed. After 3 days of culture, cells were stimulated with ACTH(1-24), and the cells and medium were collected at different time points (0, 3, 6, 9, 12, and 24 hours) for measurement of cortisol and ACTH-R and StAR mRNA.

Results: Cortisol secretion was increased after ACTH treatment in all three age cohorts. Cells from the 135-139 dGA group secreted the most cortisol, followed by the 100-105 and then the 120 dGA groups (P <.05). ACTH-R mRNA levels before and after ACTH were higher in the late compared to both earlier groups. StAR mRNA levels before and after ACTH were higher in the 100-105 and 135 than in the 120 dGA group. The time to peak ACTH-R mRNA response was age-dependent, with the 100-105 dGA cells taking longer to attain maximum levels. Maximal StAR mRNA levels were not age-related.

Conclusion: The data suggest that ACTH-R and StAR are indeed key mediators of fetal adrenocortical responsiveness, and that ACTH is able to up-regulate responsiveness, and hence cortisol production, by increasing their expression.