Characterization of cytosolic female rat liver receptors of the lactogenic type.

Some properties of cytosolic receptors of the lactogenic type from female rat liver were studied and compared with those of membrane-bound (microsomal) receptors. The association constant between the cytosolic receptors and human growth hormone was 2.2 l/nmol, which was not significantly different from the value obtained for the microsomal receptors (3.6 l/nmol). Since unlabeled hGH and human prolactin, but not bovine growth hormone, displaced [125I]hGH bound to receptors from both sources, the cytosolic receptors, like the microsomal receptors, must be lactogenic. Furthermore, the cytosolic receptors were recognized by a monoclonal antibody raised against microsomal receptors from female rat liver. However, covalent cross-linking of cytosolic receptors to [125I]hGH and subsequent sodium dodecyl sulphate electrophoresis gave a single band corresponding to a molecular weight of 42,200 (after subtraction of the molecular weight of hGH), which differs significantly (p less than 0.01) from the values determined for the two distinct bands given by the microsomal fraction. Moreover, upon molecular sieve chromatography the receptor activity in the two fractions appeared at significantly (p less than 0.05) different elution volumes. These results show that the cytosolic and microsomal receptors have some structural features in common but are definitely not identical.