Are ninhydrin‐positive substances volume‐regulatory osmolytes in rat renal papillary cells?

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Abstract
1. 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) .mu.l/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 < Cl- < NO3- .ltoreq. 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). 9. The results are discussed in terms of (i) their applicability to the intact kidney during antidiuresis, (ii) the metabolic and membrane transport phenomena regulating the cell n.p.s. pool, and (iii) the possibility that n.p.s. may act as volume-regulatory osmolytes during antidiuresis.