Stepwise degradation of NT in pars convoluta and recta of rabbit proximal tubules: evidence of axial heterogeneity.

Reabsorption and degradation of the neuropeptide neurotensin (NT) in rabbit proximal pars convoluta (PC) and pars recta (PR) nephron segments were characterized. Brush-border membrane vesicle fractions (PC or PR) were incubated with [3H]NT, and the extent and pattern of peptide hydrolysis were determined by reversed-phase high-pressure liquid chromatography (rHPLC). Furthermore, isolated rabbit PC and PR segments were perfused with [3H]NT, reabsorption of [3H]NT was quantified, and the collected perfusate was analyzed by HPLC. Metabolites were characterized. Finally, rabbit proximal tubules were microinfused in vivo with [3H]NT to follow the tubular uptake by electron microscope autoradiography. Degradation increased with time in both vesicle fractions. The main difference was an extensive cleavage of NT in PR, as revealed by a higher proportion of end metabolites. This was also visualized as a higher proportion of the large degradation product in rHPLC fraction 39 [NT-(1-11)] in PC as compared with PR after 30 min of incubation. The isolated perfused proximal tubular segments processed NT with large efficiency. PC segments processed 90% of the perfused amount, and PR processed 88%. Only 13% in PC and 10% in PR of the processed NT were found in the bath and the tubule. The main part of processed NT was in the collected perfusate, and rHLPC profiles revealed that NT-(1-11) was the only metabolite in both PC and PR. Electron microscope autoradiography demonstrated autoradiographic grains over invaginations and over the apical part of the proximal tubule cell in endocytic vesicles and vacuoles 10 min after microinfusion of [3H]NT.(ABSTRACT TRUNCATED AT 250 WORDS)