Adipose cell size-function relationships: insulin binding and degradation.

125I-insulin binding and degradation have been studied in isolated rat adipose cells of increasing size. Binding at 24 degrees C reaches a steady state by 40-60 min in the absence of significant degradation. At 37 degrees C, binding reaches only a transient maximum by 10-15 min because insulin is rapidly degraded. Cellular enlargement is associated with increasing steady-state binding per cell at 24 degrees C, and increasing maximum binding and degradation per cell at 37 degrees C, in spite of increasing plasma insulin concentrations in the rats from which cells were prepared. Detailed steady-state studies at 24 degrees C, however, fail to delineate the mechanism of these alterations. Although dissociation experiments are consistent with the presence of a small degree of negatively cooperative binding site interaction, its magnitude is unchanged with increasing cell size. Furthermore, binding levels per unit cellular surface at 24 degrees C, at least at those insulin concentrations eliciting a biological response, remain relatively constant. None of the alterations in insulin binding observed here can explain the enlarged adipose cell's markedly decreased metabolic response to insulin.