Peptide YY luminal processing and axial heterogeneity of basolateral binding in renal proximal tubules

This study investigates the definite location of peptide YY (PYY) binding sites on the basolateral membranes in proximal tubules. S1, S2, and S3 segments were dissected, perfused in vitro, and exposed to [125I-Tyr36]monoiodo-PYY either in the bath fluid or in the perfusate. S1 segments exposed to [125I-Tyr36]PYY in the bath fluid were fixed and prepared for electron microscope autoradiography. The results demonstrated a high degree of axial heterogeneity of basolateral binding of PYY, since only S1 bound PYY, 0.59 +/- 0.09 pg/mm after 15 min; 89.1% could be displaced with unlabeled PYY. PYY was not internalized, 90% of the grains were associated with the basolateral membranes, and no accumulation of grains was observed over the vacuolar apparatus. After luminal perfusion with PYY, 79.3 +/- 7.2% was processed, 61.7 +/- 6.3% was degraded at the brush border, and no tubular accumulation was detected. Thus PYY is not taken up by endocytosis. Unexpectedly, a very large fraction of processed PYY was transported from lumen to bath as trichloroacetic acid (TCA)-precipitable label constituting 41.6 +/- 4.7%. There was no axial heterogeneity in the luminal handling of PYY. In conclusion, this study reveals a high density of PYY binding sites at the basolateral membranes from S1 segments, indicating a selective function of S1 segments on stimulation with PYY. In contrast to other proteins PYY was not internalized from the basolateral membranes.