1. The diurnal patterns of plasma aldosterone, plasma renin activity (PRA), Cortisol and adreno-corticotrophic hormone (ACTH) in the supine and in the upright position have been studied in fourteen patients with primary aldosteronism, five with adenoma and nine with bilateral hyperplasia. Blood samples were drawn at intervals from 6 h to 30 min.

2. Supine patients with an adenoma showed marked diurnal variations of aldosterone, with maximal values at 08.00 hours and minimal values at 18.00 hours and secretory spurts beginning after 02.00 hours. Plasma Cortisol paralleled aldosterone, and ACTH seemed to anticipate aldosterone and Cortisol variations; PRA remained unchanged. In patients with hyperplasia, aldosterone was signicantly lower than in the adenoma group at 08.00 hours, and its decline during the day was less marked; fluctuations rather than secretory episodes were seen.

3. After patients assumed the upright posture, aldosterone remained unchanged or decreased in patients with adenoma, whereas it significantly increased in hyperplasia; PRA remained low, although a slight increment was seen in the latter group. The different response of aldosterone in the two groups was not modified by the administration of propranolol, apparently excluding a renin-dependent mechanism. On the other hand, dexamethasone seemed to affect the response of aldosterone to the upright posture in both groups; in adenoma there was a slight but significant increase, and in hyperplasia the usual rise was partially suppressed.

4. It is concluded that ACTH has a predominant role in regulating aldosterone secretion in primary aldosteronism due to adenoma, whereas its action in bilateral hyperplasia is only permissive.

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