Does insulin down-regulate its own receptor on erythrocytes in vitro?

The purpose of this study was to examine the in vitro effect of insulin on its own receptors on erythrocytes. Whole blood from 6 fasting normal and 6 diabetic (type II) subjects was incubated without or with exogenous insulin for 5 hours. The insulin binding to erythrocytes was then evaluated. At a tracer concentration, the percent 125I-insulin specifically bound amounted to 8.6 +/- 0.59% and 8.1 +/- 0.41% in fasting normal subjects and type II-diabetics, respectively. In contrast to in vivo studies, supraphysiologic insulin concentrations in the incubation medium did not alter significantly the insulin binding process. Thus, in vitro we could not demonstrate a down regulation of insulin receptor binding. It is likely that factors other than insulin are involved in the short-term regulation of insulin receptor affinity in man.