Different characteristics of solubilized lactogen receptors from livers of pregnant and non-pregnant female rats

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Abstract
Receptors specific for lactogenic hormones were solubilized by 1% (v/v) Triton X-100 from the crude particulate membrane fraction of livers of pregnant and non-pregnant female rats and the characteristics of both preparations were compared. Human 125I-labelled somatotropin was used for binding studies of lactogenic hormone. The solubilized receptor retained most of the characteristics noted in the particulate fraction. The binding of human 125I-labelled somatotropin to the solubilized receptor is a saturable process, depending on temperature and time. Scatchard analysis of displacement curves revealed similar affinity constants ranging from 1.02 × 109 to 1.20 × 109 1/mol, while the binding capacity was 4.5 times greater in the pregnant rat livers than in the non-pregnant female rat livers. The receptors for human 125I-labelled somatotropin from livers of non-pregnant and pregnant female rats were equally adsorbed onto a concanavalin-A-Sepharose column and were dissociated from the column with α-methyl-d-glucoside or α-methyl-d-mannoside in the same manner. By gel filtration on Sepharose 6B, however, the molecular sizes of the hepatic receptors were found to be different. The apparent Mr value was approx. 270000 with a Stokes‘ radius of 5.5nm in the non-pregnant female rats and approx. 330000 with a Stokes’ radius of 5.5nm in the pregnant rats. Furthermore, isoelectric-focusing experiments showed that a major part of the receptor from the non-pregnant female rat livers had a neutral pI (7.0—8.5), whereas that from pregnant-rat livers had an acidic pI (4.2—4.7). These data suggest that the increase in the lactogenic binding capacity in rat liver membranes during pregnancy may be associated with marked changes of the physicochemical properties of the receptors.