DISTRIBUTION OF INSULIN BINDING-SITES ON LEYDIG-CELLS OF RAT TESTES USING INSULIN-COATED GOLD PARTICLES

The distribution of insulin binding sites in Leydig cells dispersed wi th collagenase from rat testes was studied using insulin-coated gold p articles as an electron opaque ligand. Using electron microscope is co nvenient to distinguish Leydig cells among a variety of cells in crude preparations by their ultrastructural characteristics. Leydig cells w ere shown to possess insulin-binding sites on their plasma membranes. Initial binding sites of insulin were located to the microvillous surf aces. Following binding, receptor-ligand complexes seemed to move to t he intermicrovillous plasma membrane, then to be internalized. Two mod es of the internalization were confirmed. Most of the receptor-ligand complexes on Leydig cells appeared to be internalized via large, uncoa ted plasma membrane invaginations, while the remainder became internal ized via small pits into vesicles. The receptor-ligand complexes were subsequently transferred to large subsurface vacuoles with electron-lu cent lumens believed to correspond to endosomes. The reason why IGCs o n the postendosomal pathway moving toward lysosomes was also discussed .