Insulin Receptor Assays Used in Human Studies: Merits and Limitations

Abstract

The establishment of insulin receptor assays about 10 yr ago made it possible to study the regulatory role of the insulin receptor in a variety of physiologic and pathophysiologic conditions in man. Although it would be desirable to examine insulin receptors on muscle and liver cells, due to the ethical and technical limitations most clinical research in insulin receptor binding in man has been confined to accessible cell models, that is, fat and blood cells. The two major components of the receptor assay are (125I)-labeled insulin and isolated cells. Most iodoinsulin preparations used have differed with respect to purity, homogeneity, and distribution of (125I) within the molecule. This makes comparison of insulin binding data between laboratories difficult. Comparing monocytes, erythrocytes, and adipocytes as cell models each cell type possesses several limitations. The human adipocyte, however, has the advantage in that it allows simultaneous measurements of insulin receptor binding and biologic insulin effects under similar conditions of physiologic pH and temperature. Scatchard analysis has been widely used in the interpretation of insulin binding data. The assumptions and prerequisites required for the application of this mathematical approach are, however, not fulfilled. Furthermore, whether the nonlinear Scatchard plot is caused by multiple receptor classes or negative cooperativity in one homogeneous receptor class is an unsolved problem. In general there is a tendency to give values of total receptor concentration and affinity constants based on the negative cooperativity model. The informative value of these binding quantities is questionable, however, due to bias and lack of precision in data generation. Finally, generalization of insulin receptor results from one cell type to other insulin-binding tissues within the same individual should be avoided since the available literature suggests that the insulin receptor status and its changes seem to be tissue specific.