Nonuniform distribution and grouping of insulin receptors on the surface of human placental syncytial trophoblast

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Abstract
Cytologic localization and topographic distribution of insulin receptors in human placental villi were studied. Biochemical studies showed placental villi to specifically bind 125I-insulin. Radioautographic studies showed the specific binding to be localized to the surface of the syncytial trophoblast. Topographic distribution of insulin binding was determined with ferritin-insulin. Initial studies using ferritin-insulin containing some oligomers of ferritin revealed insulin receptors to be specifically associated with the glycocalyx region of the surface membranes of microvilli. No insulin receptors were detectable in association with the intermicrovillous plasma membrane, even though its glycocalyx is in direct continuity with the clycocalyx of microvilli. Monomeric ferritin-insulin showed the same nonuniform distribution of the insulin receptor, which suggests that there is not complete freedom of lateral mobility of the insulin receptors in the surface membrane of this tissue. Insulin receptors apparently occurred as singletons or in groups of 2 or more. Incubations with monomeric ferritin-insulin at 4.degree. or with tissue prefixed with formaldehyde showed that the groups of insulin receptors were naturally occurring, i.e., they were present prior to and independent of insulin binding and thus not secondary to ligand-induced aggregation. The physiologic meaning of the nonuniform distribution and groups of insulin receptors was unclear.