Ontogeny of immunoreactive and bioactive microsomal steroidogenic enzymes during adrenocortical development in rats

The functional development of the neonatal rat adrenal cortex is characterized by a triphasic response to adrenocorticotropic hormone (ACTH), with a nadir in responsiveness around neonatal day 10 (d10). In this study, the hypothesis was tested that hyporesponsiveness to ACTH partly results from deficiencies in steroidogenic enzyme content. Immunoreactive (ir) levels of mitochondrial cytochrome P450 enzymes (side chain cleavage (P450scc) and 11 beta-hydroxylase (P450c11)) did not change during neonatal development. Immunoreactive levels of microsomal 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD), however, were significantly and comparably lower in both day 1 (d1) and d10 neonates compared to adult rats. Activity of 3 beta-HSD did not parallel changes in ir 3 beta-HSD content. Enzyme activity was low on d1 (approximately 39% of adult activity), but by d10 was statistically equivalent to that of microsomes from adult adrenal glands. Immunoreactive levels of microsomal cytochrome P450 21 alpha-hydroxylase (P450c21) were significantly lower in d1 glands than in adult glands (by approximately 50%), but by d10 were statistically indistinguishable from adults. On the other hand, P450c21 activity was equivalent on d1 and d10 and both were significantly lower compared to adults (approximately 62% of adult activity). ACTH injections from d3-d10 facilitated the adrenocortical steroidogenic response to ACTH on d10. This treatment increased levels of ir 3 beta-HSD, but not ir P450c21. The results suggest that rat adrenocortical 3 beta-HSD and P450c21 are developmentally and differentially regulated, and that ir levels of the proteins are not correlated with enzyme activity during the neonatal period. One possible explanation for these observations is that multiple isoforms of the two enzymes, with different antigenic and enzymatic properties, may be expressed during development at different times. In addition, the combined decreased activities of these two enzymes can almost entirely account for the decreased steroidogenic output of rat adrenocortical cells on d1, but not during the later neonatal period.