Direct visualization of binding, aggregation and internalization of human growth hormone in cultured human lymphocytes

The distribution of human growth hormone (hGH) receptor complexes on IM-9 cultured lymphocytes was studied using fluorescein and ¹²⁵I-labelled hGH (FhGH) and [¹²⁵I]hGH). The cells labelled with F-hGH were visualized with a sensitive video intensification microscopic system which permitted direct observation of the location of the fluorescent hormone on the surface of the living lymphocytes. At 4°C F-hGH bound diffusely to the cell surface and remained dispersed but following incubation for 30 min at 37° C the hormone receptor complexes aggregated into patches on the cell surface and formed a single cap on one pole of the cell. Progressive internalization into the cell was demonstrated at 37°C with [¹²⁵I]hGH. It is hypothesized that the aggregation and internalization of the hGH receptor complexes are associated with the action and degradation of the hormone and probably also with the mechanism of down-regulation of the receptors.