Accumulation of a slowly dissociable peptide hormone binding component by isolated target cells

The overall rate of dissociation and the fraction of bound radioiodinated human growth hormone that dissociated from hepatocytes varied with time of association. A smaller fraction of bound hormone was dissociable from isolated target cells with increased receptor occupancy and increased incubation time prior to the onset of dissociation. The inability of bound label to reequilibrate completely with the medium was demonstrated further by preincubating cells with labeled hormone prior to the initiation of saturation experiments. In such experiments, time-dependent changes in the binding properties of bound label were observed in Scatchard plots, as a result of the inability of prebound label to reequilibrate rapidly with the medium over the time course of such experiments. These data suggest that bound hormone may be distributed between at least two kinetic components. This phenomenon could be interpreted in terms of heterogeneity of sites, a slow conformational change in the receptor, or a model incorporating spatial compartmentalization of sites.