A radioreceptor assay of Epidermal Growth Factor (EGF), which uses as binder plasma membranes prepared from target tissues, instead of specific antibodies, is described. The amount of the polypeptide hormone present in the homogenate has been measured in various tissues. Submaxillary gland and parotid are confirmed to possess the highest levels of the factor. Results obtained incubating sliced tissues with or without pilocarpine, a drug which stimulates the hormone release, suggest that the tissues under investigation can be classified in two groups: a - "target tissues" (i.e. epidermis and corneal epithelium) b- "synthetizing" tissues (i.e. submaxillary gland, parotid, liver), which release the factor under pilocarpine stimulation.
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