A study has been made of the concentrations and contents of ninhydrin-positive substances (n.p.s.), presumed to be predominantly but not exclusively amino acids, in the cells of rat renal papillary slices incubated in variously modified Krebs phosphate-bicarbonate Ringer solution. 2. When the medium osmolality was increased from 710 (control) to 2000 mosmol/kg H2O by additional NaCl and urea, the steady-state cellular n.p.s. concentration rose from 42.3 +/- 0.6 (mean +/- S.E. of mean; n = 36) to 105 +/- 2 (n = 68) mmol/l (glycine equivalent). Cell fluid content fell from 5.11 +/- 0.09 (n = 36) to 4.16 +/- 0.11 (n = 68) microliter/mg solute-free dry weight. Hence cell n.p.s. content increased from 211 +/- 4 (n = 36) to 421 +/- 10 (n = 68) nmol/mg solute-free dry weight. 3. A comparable loss of cell fluid was observed when urea was replaced by sucrose or sorbitol. No increase in cell n.p.s. occurred, and there was a marked cell Na+-for-K+ exchange. 4. The extent of the increase in cell n.p.s. in the presence of 2000 mosmol/kg H2O (NaCl + urea) was sensitive to the presence of external anions in the sequence acetate less than Cl- less than NO3- less than or equal to SCN-. 5. Cell n.p.s. concentration increased progressively as the medium osmolality was increased by the addition of urea, but Na+ at a concentration above 330 mmol/l had an inhibitory effect. The increase in n.p.s. concentration was also significantly reduced in hyperosmotic media in which Na+ was replaced by choline. 6. The increase in cell n.p.s. content due to hyperosmotic NaCl + urea was completely inhibited by pre-incubation in control medium containing trimethylamine N-oxide. 7. On transference of slices from control to hyperosmotic media (NaCl + urea) the steady-state increase in cell n.p.s. concentration was complete within 20 min and followed a time course similar to that for cell fluid loss. The n.p.s. concentration and cell fluid content returned to control levels, with similar time courses, following re-immersion in control medium. 8. Efflux of alpha-amino[1-14C]isobutyric acid (AIB) from slices pre-loaded in control medium containing 1 mmol AIB/l was slightly but significantly slower into AIB-free hyperosmotic NaCl + urea than into AIB-free control medium. The rate of efflux was greatly increased by the presence of hyperosmotic sucrose or very high Na+ (935 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)
