Abstract
Endothelial cells isolated from a variety of vascular beds bind and transport insulin but exhibit relatively low insulin degrading activity. Because endothelial cells exhibit heterogeneity and since kidney is a major site of insulin degradation, we studied the processing of insulin by glomerular endothelial cells (GEC). When exposed to 2 x 10(-10) M 125I-labeled insulin, GEC associated with the hormone in a specific manner. This interaction was inhibited by insulin but not by a number of unrelated peptide hormones. Over a 90-min period, GEC degraded 42 +/- 3% of the 125I-insulin, as measured by solubility in trichloroacetic acid (TCA). Degradation was inhibited 90% by an excess of insulin or adrenocorticotropic hormone (10(-6) M) and 57% by glucagon, whereas growth hormone and calcitonin were without effect. Separation of plasma membrane bound from internalized insulin was achieved by decreasing extracellular pH. In the steady state, 43% of cell-associated insulin was membrane bound and 57% internalized. The fate of the internalized 125I-insulin was examined by incubating acid-washed cells at 37 degrees C for 60 min. Over this time 18% of the radioactivity was released as TCA insoluble- and 72% as TCA-soluble radioactivity. Release was increased by insulin (10(-6) M) but not by unrelated peptide hormones. In the presence of chloroquine, 125I-insulin release increased by one third while degradation fell. High-performance liquid chromatography revealed that GEC released both intact insulin and large intermediates and that chloroquine inhibited intermediate formation.
