Type I (mineralocorticoid) receptors in the guinea pig

The affinity, capacity, and specificity of type I receptors (mineralocorticoid receptors, MR) in the guinea pig are indistinguishable from similar values determined in parallel studies in the rat. In both epithelial (kidney, colon) and nonepithelial (hippocampus, heart) cytosol preparations, aldosterone binds with a dissociation constant at 4 degrees C of 1-2 nM in both species; for both guinea pig and rat the tissue concentrations of MR are an order of magnitude higher in hippocampus and colon than in kidney or heart. In both species, aldosterone and cortisol appear to have equivalent affinity for MR, and corticosterone appears to have two- to fourfold higher affinity. Given the wide variety of differences between the guinea pig and other species in other components of the pituitary-adrenal axis (superagonist adrenocorticotropic hormone, high circulating cortisol, low levels of transcortin, low-affinity glucocorticoid receptors) the unexpected finding of pristine type I receptors in the guinea pig suggests powerful specificity-conferring mechanisms to allow aldosterone occupancy of MR in epithelia and the possibility of further definition of the roles of such type I receptors in nonepithelial tissues.