Ontogeny of Renal Mineralocorticoid Receptors and Urinary Electrolyte Responses in the Rat

We examined the ontogeny of cytosol mineralocorticoid receptors in renal inner medulla-papilla (IM-PAP) and outer medulla-cortex (OM-CX) and of a biological response to administered corticosteroids (urinary K+/Na+ ratio). Rats at early (7-9 days), intermediate (13-15 days), and late (23-30 days) stages of maturation were used. Young rats were found to be essentially insensitive to aldosterone, in contrast to rats aged 13-15 days and older. Scatchard analysis of IM-PAP and OM-CX cytosol showed equivalent high-affinity aldosterone binding sites at all ages. The affinity of these sites was constant (Kd, 4 degrees C approximately 0.5 nM) and their concentration varied only slightly over maturation. In a series of in vivo nuclear transfer studies, the hyposensitivity to aldosterone in young rats did not seem to be due to a difference in the nuclear binding of aldosterone-receptor complexes. The resistance of the immature (7-9 days) rat kidney to aldosterone thus appears to be a postreceptor phenomenon. The maturational events allowing expression of mineralocorticoid-inducible genes in the kidney remain to be determined.