Characteristics of insulin receptor binding to various rat tissues

The first step in insulin action is its specific binding to alpha-subunits of the receptor in the plasma membrane. Rats of Wistar strain are commonly used as laboratory animals but there are no data comparing insulin binding by various rat tissues. We studied the insulin binding kinetics in plasma membranes isolated from hearts, livers, brains, skeletal muscles, adipose tissue, thymus and testes in order to compare their ability to bind 125I-insulin and to test which membrane preparation is most useful and convenient for such a study. The dissociation constant (Kd) and the quantity of receptors measured as a binding capacity were determined by the Scatchard method using the LIGAND computer program whereas the binding potency of insulin was calculated as IC50 using the ALLFIT computer program. We also introduced the product of Ka x R50 (affinity constant multiplied by binding capacity) as an index which describes the functional features of insulin receptors taking into account both number of insulin receptors and their affinity. Taking all the parameters of insulin binding tested together we can conclude that the liver and, to some extent, adipose tissue may provide a useful model for studying hormone-receptor interaction. By contrast, to the group of rat tissues responding rather poorly to insulin belong thymus and testis.