On the mode of action of ACTH on the isolated perfused adrenal gland.

1. Isolated cat adrenal glands were perfused with Locke solution, and the corticosteroid outputs in response to adrenocorticotrophin (ACTH) were studied.2. Steroid outputs varied with the ACTH concentration, as well as with the duration of exposure to a given ACTH concentration.3. Omission of calcium from the perfusion medium markedly depressed ACTH-evoked steroid release. The steroid output was directly related to the extracellular calcium concentration up to 0.5 mM.4. During a constant exposure to ACTH, steroid output was maintained for at least 2-3 hr, provided that calcium was present in the perfusion medium.5. Strontium, but not barium or magnesium, replaced calcium in maintaining the secretory response to ACTH.6. Magnesium depressed ACTH-evoked secretion in the presence of calcium, and this depression of secretion was antagonized by increasing the calcium concentration.7. Prolonged perfusion with sodium-free or potassium-free solutions did not markedly inhibit steroid output in response to ACTH. Excess potassium (56 mM) did not produce a consistent or marked increase in spontaneous steroid output and did not affect the response to ACTH.8. The steroid content of adrenal glands perfused with Locke solution and exposed to ACTH was about 10% of the amount which was secreted. By contrast, adrenal glands perfused with calcium-free media and exposed to ACTH contained much higher amounts of steroid, despite the negligible amount secreted.9. These data suggest that calcium plays a critical role in the mechanism of corticosteroid secretion from the adrenal cortex.