Studies on renal growth regulation by urea and ammonia in normal rat kidney cells

The hypothesis that protein metabolites regulate renal growth has been considered for decades, and the overall objective of this work was to test this hypothesis in vitro using an established line of renal tubular epithelial cells, NRK-52E. The addition of urea to the medium of confluent cultures stimulated DNA synthesis which was dependent on the concentration of urea, but independent of the presence of serum. Urea enhanced proliferation of subconfluent cultures of NRK cells in the presence of dilute serum to a similar degree as epidermal growth factor (EGF). Either deletion of serum from the medium or allowing the cultures to achieve confluence prevented the proliferative response to urea, but not EGF. Ammonium chloride stimulated the uptake of 3H-thymidine but did not increase cell number. Ammonium chloride increased the uptake of 3H-leucine and total cellular protein/DNA, while urea had no effect on these markers of growth. The rate of hydrolysis of urea to ammonia in vitro was not altered by the presence of NRK cells. These results suggest that urea and ammonia may regulate renal mass, and that their actions are separate and different.