Identification of insulin binding sites in isolated cells from rat submaxillary gland.

Isolated cells from rat submaxillary gland bound 125I-labelled insulin in a time-dependent process that reached a maximum at 30-40 min at 25 degrees C. The radioactivity bound to cells could be dissociated by dilution of the binding site-hormone complex with the incubation buffer. The presence of unlabelled insulin in the incubation buffer inhibited 125I-labelled insulin degradation according to the amount of hormone added. After 10 min of incubation at at 25 degrees C, radioactivity associated to cells was almost exclusively identified as intact 125I-labelled insulin. With increasing times, a greater contribution of final products of degradation in total radioactivity bound to cells was observed; nevertheless, in the presence of unlabelled insulin the radioactivity associated to low molecular weight products markedly decreased. Equilibrium binding data analysis gave rise to a non-linear Scatchard plot, whose high affinity component showed a dissociation constant of 6.6 +/- 0.4 nM. These observations are consistent with the presence of insulin binding sites in rat submaxillary gland cells which are similar in their characteristics to those identified in other tissues.