Mammotroph autoregulation: intracellular fate of internalized prolactin

As part of a study concerning the mechanism of mammotroph autoregulation, the intracellular fate of [125I]iodo rat PRL, internalized by mammotrophs, was determined by quantitative electron microscope autoradiography. Simple grain density (GD) analysis of mammotroph autoradiograms showed that 5 cellular compartments (plasma membrane, Golgi apparatus, secretory granules, nucleus and mitochondria) were significantly labeled and that the labeling pattern in these compartments changed with time. Inclusion of a 25-fold excess of unlabeled PRL inhibited the binding of radiolabeled hormone to mammotrophs by approximately 85%. At no time were any grains found associated with lysosomes and there was no evidence of degradation of the radiolabeled PRL as judged by SDS-polyacrylamide gel electrophoresis or cold perchloric acid precipitation of cell extracts or media samples. Delivery of internalized PRL to a number of specific intracellular organelles suggests that these organelles may serve in the autoregulation of PRL secretion.