Hormone binding by cells and cell fragments as visualized by fluorescence microscopy

Fluorescent-labeled hormones offer an alternative approach to radio-labeling in studying the binding of hormones to intact cells or cell fragments. The binding of fluorescent-labeled hormones may be followed quantitatively by measurement of the polarization or the binding may be directly visualized in the fluorescence microscope. The binding of both fluorescein labeled prolactin and estradiol to a variety of whole cells or to microsomal fragments has been observed by fluorescence microscopy. No staining was observed with fresh cells whereas all cell types investigated, after freeze-thawing, stained at physiological levels (10-9M) of either hormone. Microsomal preparations from the mammary tissue of mid-pregnant rabbits likewise stained at low levels of prolactin. Inhibition of staining was not produced even by 10-6 M unlabeled hormone.