Specific localization of plasma corticosteroid-binding globulin immunoreactivity in pituitary corticotrophs

The prevailing concept is that steroids are bound to specific receptors inside target cells, whereas extracellular and especially plasma binding of these steroids are due to specific transport proteins. The purpose of this study was to investigate the presence of corticosteroid-binding globulin (CBG) in guinea pig pituitary by immunohistochemical methods. Both immunofluorescence and an immunoperoxidase (unlabeled antibody-peroxidase-antiperoxidase) method using antiserum to guinea pig CBG gave identical results. CBG immunoreactivity was found inside cells of the pituitary gland (intermediate lobe and some cells of the anterior lobe). Control experiments with non-immune serum or anti-CBG serum previously immunoadsorbed with pure CBG did not show fluorescent or immunoreactive cells. Comparison of the immunoreactions obtained with anti-CBG serum and with antisera against the different pituitary hormones showed that "CBG-like" antigen was only found in the corticotrophs. In contrast, other plasma proteins (albumin and immunoglobulins) were not detected in these cells. The presence of CBG immunoreactivity inside pituitary cells is, thus, cell specific and protein specific. The biological role and the origin (uptake from plasma or local synthesis) of the pituitary CBG-like protein are presently not understood.