Regulation of aldosterone in the 7-day-old rat

Little is known about mineralocorticoid regulation in the neonate. Here, adrenocortical function in 7-day-old Sprague-Dawley rats was studied by measuring the effects of angiotensin II (Ang II) and ACTH on serum aldosterone (ALDO), corticosterone, and cytochrome P-450 ALDO synthetase and 11 beta-hydroxylase messenger RNA (mRNA) levels with and without dexamethasone (DEX) treatment. In the absence of DEX, serum ALDO was unchanged after 5 micrograms/kg, but increased 2- to 12-fold after 50 micrograms/kg, Ang II and 9- to 36-fold after 5 U/kg ACTH. After 4 days of exposure to exogenous ACTH, basal and Ang II-stimulated ALDO were markedly decreased. Basal plasma corticosterone was near or below the assay detection limit and did not change after Ang II, but increased significantly after ACTH administration. After treatment with 200 micrograms/kg DEX, basal serum ALDO fell to below the assay detection limit at 1 h, the responses to 50 micrograms/kg Ang II were attenuated at 1 and 4 h and were undetectable at 18 h. Preincubation of 7-day-old dispersed adrenal glomerulosa cells with 100 nM DEX for 2 h did not decrease basal or stimulated ALDO production. In situ hybridization studies revealed that cytochrome P-450 ALDO synthetase mRNA was confined to the subcapsular zona glomerulosa, whereas cytochrome P-450 11 beta-hydroxylase mRNA was present only in the zona fasciculata-reticularis. DEX caused a time-dependent decrease in P-450 ALDO synthetase mRNA (91 +/- 3%, 77 +/- 6%, 60 +/- 13%, and 38 +/- 19% of the control value at 1, 4, 8, and 16 h, respectively), an effect that was not prevented by ACTH replacement. Only minimal decreases in P-450 11 beta-hydroxylase mRNA levels were observed 18 h after DEX treatment. Hence, the sensitivity of ALDO responses to Ang II in the 7-day-old rat was markedly reduced in vivo, but not in vitro. In addition, DEX markedly reduced ALDO secretion, an effect that was associated with a decrease in cytochrome P-450 ALDO synthetase mRNA.