RESUMEN
In rats, chronic treatment with high doses of ACTH (10-40 micrograms/100 g body weight per day) leads to a marked reduction in aldosterone synthesis by adrenal capsules. The possibility that this inhibition is secondary to a decrease in plasma potassium levels or in renin angiotensin system (RAS) activity has been explored. The effects of chronic ACTH treatment were compared in (I) animals in which the endogenous RAS activity was stimulated by restricting dietary sodium intake, (II) animals in which plasma angiotensin II was increased by infusion from implanted osmotic minipumps and (III) animals which received supplementary potassium and in which hypokalaemia was prevented. In all cases, rates of aldosterone biosynthesis in vitro by adrenal capsules were decreased in ACTH-treated animals to an extent similar to those in untreated controls. In addition, ACTH treatment of hypophysectomized rats resulted in a similar inhibition of aldosterone biosynthesis to that found in sham-operated controls. It may be concluded that the ACTH-induced reduction of aldosterone biosynthesis is independent of the secretion of other pituitary hormones, and cannot be simply ascribed to either a reduction in RAS activity or in plasma potassium levels. The results are consistent with the view that the effects of chronic ACTH treatment are mediated by a direct action on the zona glomerulosa cell, which leads to its transformation into a zona fasciculata-like form.