| Abstract: | The receptors for the polypeptide hormones, insulin and growth hormone, are located on the cell surface. Since the cytoplasmic microtubules and microfilaments are involved in the mobility and distribution of surface receptors for immunoglobulins and lectins, we investigated the role of these structures in the binding of insulin and human growth hormone to their receptors on cultured human lymphocytes (IM-9). Cells preincubated with microfilament modifiers, cytochalasin A, B, and D (10 mug/ml), had decreased binding of insulin (30%) and human growth hormone (60%) under steady state conditions, which was not reversed by removing the cytochalasins from the medium and was due entirely to a reduced number of receptor sites on the cell surfact. The lost receptors were not detected in the medium, suggesting a redistribution within the cell. The cytochalasins failed to alter the affinity of the hormones for their receptors or the negative cooperativity of the insulin receptor. The anti-microtubule agents (vincristine, vinblastine, colchicine) had no effect on the binding of insulin and growth hormone to their receptors. Deuterium oxide, a stabilizer of microtubules and other proteins, decreased the affinity (40%) of insulin for its receptors under steady state conditions and accelerated moderately the spontaneous dissociation of 125I-insulin from its receptors. Since cytochalasin decreases the number of available insulin and human growth hormone receptor sites, cytochalasin-sensitive microfilamentous structures appear to modulate the exposure of cell surface hormone receptors, while microtubules do not seem to be involved. |
