Comparison of binding of ¹²⁵I-iodopindolol to control and desensitized cells at 37° and on ice

Binding of ¹²⁵I-iodopindolol (IPIN) to intact 1321N1 human astrocytoma cell B-adrenergic receptors was measured at 37° and on ice. Control cells showed a single component of IPIN binding on ice with the same total number of receptors as measured at 37°. In desensitized cells (pretreated for 20 min with 1 μM isoproterenol) approximately 40% of IPIN binding on ice exhibited kinetics similar to those observed in control cells. The remaining 60% of receptors were labelled by IPIN at a much slower rate requiring the use of very high concentrations of IPIN. Sucrose density gradient fractionation was used to separately study the labelling of plasma membrane receptors and those associated with a light vesicle fraction. Labelling by IPIN on ice of the plasma membrane receptors of control cells was rapid, labelling of the light vesicle receptors of desensitized cells was slow, and labelling of the plasma membrane receptors of desensitized cells appeared to occur with both rapid and slow components. Selective labelling of the plasma membrane receptors of intact cells thus could be obtained by incubation with IPIN on ice under selected conditions. Similar results were obtained when broken cell preparations from control and desensitized cells were used. The decreased binding of IPIN on ice to B-adrenergic receptors in the light vesicle fraction not only provides further evidence consistent with sequestration of B-adrenergic receptors during desensitization, it also provides a convenient and inexpensive means to assay the sequestration reaction.