Effects of confluence on phosphate transport capacity in cultured renal cell lines

The LLC-PK1 cell line transports phosphate (Pi), glucose, and amino acids using carriers similar to those in proximal tubular cells. Others have reported that when monolayers reach confluence, hexose transport increases and activity of the A-amino acid transporter falls. The present study evaluates Pi uptake by two continuous cell lines derived from renal proximal tubule, and demonstrates that phosphate uptake falls sharply upon reaching confluence in LLC-PK1 cells but not in cultured opossum kidney (OK) cells. The fall in Pi uptake in LLC-PK1 cells at confluence represents a halving in Vmax for Na-dependent phosphate uptake (2.33 vs. 5.00 nmol/mg protein/5 min) without a change in Km (82 vs. 94 microM). Suppression of phosphate transport in confluent monolayers of LLC-PK1 cells is completely reversed by bringing the cells into suspension. As has been shown for the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), exposure of monolayers to serum stimulates phosphate uptake, but unlike phorbol ester, serum does so without stimulating alanine uptake. OK cells differ from LLC-PK1 in that no change occurs in Pi uptake at confluence, although they resemble LLC-PK1 cells in that sugar uptake rises and alanine uptake falls at confluence. The different temporal patterns for Pi uptake in the two cell lines indicates that developmental change in the uptake of Pi is not linked to that of glucose or alanine.