N-terminally extended substance P is released together with substance P from rat spinal cord

The release of different forms of substance P-like immunoreactivity (SP-LI) from superfused slices of rat spinal cord was studied. The released SP-LI was characterized by reverse-phase high-performance liquid chromatography and radioimmunoassay with two antisera directed to the C- and N-terminal parts of SP, respectively. The SP-LI detected in the superfusates with the C-terminally directed antiserum was found to consist of (undeca) SP, SP-sulfoxide and a late eluting component which was not detectable with the N-terminally directed antiserum. This component was also found in neutral extracts of the spinal cord. Upon trypsin digestion, it produced SP-LI detectable with both C- and N-terminally directed antiserum which also coeluted with SP. From these results we conclude that this form of SP-LI most likely corresponds to an N-terminally extended form of SP. An increase of the potassium concentration in the superfusion fluid from 5 to 50 mM evoked an increased overflow of both SP and the N-terminally extended SP. The present results indicate that N-terminally extended SP is released by a calcium-dependent mechanism together with SP from terminals in the spinal cord in response to potassium stimulation.