1. The mineralocorticoid activity of corticosterone based on acute changes in urinary Na+/K+ ratios in adrenalectomized rats was 1000 times less than that of aldosterone. However, corticosterone had only kaliuretic actions whereas aldosterone had both antinatriuretic and kaliuretic properties. Corticosterone inhibited the antinatriuretic actions of aldosterone.

2. Adrenalectomized rats infused continuously with a physiological dose of corticosterone (1 mg/day) were 5 times less sensitive to the antinatriuretic and 25 times less sensitive to the kaliuretic actions of aldosterone when administered acutely than were control adrenalectomized rats.

3. The long term effects of infusions of physiological doses of aldosterone and corticosterone were assessed in adrenalectomized rats maintained in metabolic cages. Aldosterone lowered plasma renin activity and reduced fluid (0.3% NaCl) intake; these effects were diminished when aldosterone and corticosterone were infused simultaneously. Plasma renin activity and fluid intake were correlated in long term infusion experiments. Both hormones had hypokalaemic effects but these were not additive. Corticosterone, but not aldosterone, increased systolic blood pressure and plasma sodium levels.

4. We conclude that glucocorticoid effects on water and electrolyte metabolism are different from those of mineralocorticoids, that glucocorticoids may antagonize mineralocorticoid effects and that interactions between mineralocorticoids and glucocorticoids may be important in long term blood pressure regulation.

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