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Conversion of Neuropeptide K to Neurokinin A and Vesicular Colocalization of Neurokinin A and Substance P in Neurons of the Guinea Pig Small Intestine

Abstract: The highest concentration of neurokinin A‐like immunoreactivity and substance P‐like immunoreactivity in the guinea pig small intestine was associated with the myenteric plexus‐containing longitudinal muscle layer. Chromatographic analysis of extracts of this tissue demonstrated the presence of neurokinin A and neuropeptide K but the probable absence of neurokinin B. A fraction of synaptic vesicles of density 1.133 ± 0.003 g/ml was prepared from the myenteric plexus‐containing tissue by density gradient centrifugation in a zonal rotor and was enriched 29 ± 12‐fold in the concentration of neurokinin A‐like immunoreactivity and 43 ± 13‐fold in the concentration of substance P‐like immunoreactivity. This fraction was separated from the fraction of vasoactive intestinal peptide‐containing vesicles (density, 1.154 ± 0.009 g/ml). Chromatographic analysis of lysates of the vesicles indicated the presence of neurokinin A but not neuropeptide K. It is postulated that β‐pre‐protachykinin is processed to substance P, neurokinin A, and neuropeptide K in the cell bodies of myenteric plexus neurons but that conversion of neuropeptide K to neurokinin A takes place during packaging into storage vesicles for axonal transport. The data are consistent with the proposal that neurokinin A and substance P are stored in the same synaptic vesicle, but the possibility of cosedimentation of different vesicles of very similar density cannot be excluded.