Pomc knockout mice have secondary hyperaldosteronism despite an absence of adrenocorticotropin

Abstract

Aldosterone production is controlled by angiotensin II, potassium, and ACTH. Mice lacking Pomc and its pituitary product ACTH have been reported to have absent or low aldosterone levels, suggesting that ACTH is required for normal aldosterone production. However, this is at odds with the clinical finding that human aldosterone deficiency is not a component of secondary adrenal insufficiency. To resolve this, we measured plasma and urine electrolytes, together with plasma aldosterone and renin activity, in Pomc(-/-) mice. We found that these mice have secondary hyperaldosteronism (elevated aldosterone without suppression of renin activity), indicating that ACTH is not required for aldosterone production or release in vivo. Exogenous ACTH stimulates a further increase in aldosterone in Pomc(-/-) mice, whereas angiotensin II has no effect, and the combination of angiotensin II and ACTH is no more potent than ACTH alone. These data suggest that aldosterone production and release in vivo do not require the action of ACTH during development or postnatal life and that secondary hyperaldosteronism in Pomc(-/-) mice is a consequence of glucocorticoid deficiency.

Figure 1

Agarose gel electrophoresis of PCR products of tail genomic DNA derived from Pomc^−/− (−/− lane), Pomc^+/+ (+/+ lane), and Pomc^+/− (+/− lane) animals. A 600-bp band represents the knockout (KO) allele, and a 317-bp band represents the wild-type (WT) allele. PCR without added DNA (H₂O lane) yielded only a primer band (at bottom of gel), as judged by the DNA ladder fragments (marker lane).

Figure 2

Urine and serum electrolytes in Pomc^−/− and Pomc^+/+ mice under basal, unstressed conditions at 0800 h. A, Urine Na⁺/K⁺ ratio after a urine collection for 24 h (n = 7 animals per experimental group); B, serum Na⁺ (n = 7 animals per experimental group); C, serum K⁺ (n = 7 animals per experimental group).

Figure 3

Plasma aldosterone and renin activity in Pomc^−/− and Pomc^+/+ mice. A and B, Plasma aldosterone (A, n = 8–11 animals per experimental group) and plasma renin activity (B, n = 3 animals per experimental group); C and D, plasma aldosterone (C, n = 4–7 mice per experimental group) and renin activity (D, n = 3 animals per experimental group) levels after ip injection of 20 μg/kg dexamethasone on 3 consecutive days. Groups that are statistically different from each other in the same or different panels are denoted by the same lowercase letter.

Figure 4

Plasma aldosterone in Pomc^−/− and Pomc^+/+ mice after ACTH and angiotensin II stimulation. Plasma aldosterone was measured 30 min after injection of ACTH or angiotensin II and 30 min after combined angiotensin II and ACTH injection. n = 8–11 animals in the basal group; n = 7–10 animals in angiotensin II groups; n = 4 in the ACTH group; and n = 5 in the angiotensin II plus ACTH group. Among animals within the same genotype, treatment groups that are statistically different from each other are denoted by the same lowercase letter.