Occurrence of an insulin-like peptide in extracts of peripheral nerves of the cat and in extracts of human vagal nerves

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Abstract
Biopsies from various peripheral nerves were collected from living cats. The biopsies were extracted with acid ethanol and the insulin-like immunoreactivity (ILI) content of the extracts determined with radioimmunoassay. The vagal, sciatic and radial nerves contained on the average 90, 3 and 28 ng of ILI per nerve tissue (wet wt), respectively. In the sympathetic trunk no ILI was found. In order to partly purify and characterize the nerve ILI extracts these were run on an HPLC [high power liquid chromatography] system. The ILI coeluted with standard bovine insulin. The same amounts of ILI were found in the nerve extracts whether run on the HPLC system or not. All of the immunoreactive material of the nerve extracts corresponds to insulin or to a peptide very similar to insulin. In 2 cats the radial and sciatic nerves were ligated for 24 h. Two to 10 times more ILI occurred in biopsies taken proximal to the site of ligation than in those from corresponding distal biopsies, indicating that the insulin-like material is transported distally within the nerves. ILI was found in extracts from vagal nerves taken from 3 days old human autopsy material. Also this human ILI coeluted with the bovine insulin standard in the HPLC system. Insulin or a very similar peptide is synthesized in peripheral nerves of cats and humans and apparently this neurogenic insulin might act as a neurotransmitter or neuromodulator.