Evidence of high affinity, stereoselective binding sites for [3H]-aldosterone in the spinal cord

Cytosol from the spinal cord (SC) of adrenalectomized rats was incubated with a range of [3H]-aldosterone (ALDO) concentrations and the results analyzed according to Scatchard. High affinity (Kd less than or equal to 1 nM) as well as low affinity (Kd 30-195 nM) sites were measured; the low affinity site was abolished by co-incubation with the pure antiglucocorticoid RU 28362. [3H]-ALDO in concentrations shown to bind to the high affinity site only, was completely displaced by the spirolactone, RU 26752, but not by RU 28362; however, the latter compound competed for 40% of the sites when incubated with a higher (10 nM) concentration of [3H]-ALDO, binding approximately one-half to each receptor type. The high affinity site was distributed uniformly in four sections of the SC, with significantly lower levels in the region corresponding to the filum terminale and horse tail and slightly higher in the cervical and lumbar enlargements. The high affinity site acquired increased affinity for DNA-cellulose after heat-induced transformation, with reduced capacity to bind to DEAE-cellulose. These results suggest that the SC contains, in addition to glucocorticoid receptors, binding molecules of high affinity and stereoselectivity for mineralocorticoids, and that, under appropriate conditions, present increased affinity for DNA acceptor sites.