ALTERATIONS IN RENAL INNER MEDULLARY LEVELS OF AMINO NITROGEN DURING ACUTE WATER DIURESIS AND HYPOVOLEMIC OLIGURIA IN RATS

Levels of total amino compounds (ninhydrin-positive substances, n.p.s.) have been measured in the inner medullas of rats during acute water diuresis and following the induction of hypovolaemic oliguria by the injection (i.p.) of 30% polyethylene glycol 20 000 (PEG) in 0.9% saline. Mean medullary fluid n.p.s. concentrations fell from 26.5 mmol to 15.2 mmol Gly equiv/l (-43%) within 2.5 h from the onset of diuresis, while the mean calculated tissue osmolality decreased from 738 mosmol/kg (control) to 369 mosmol/kg H2O. By 24 h n.p.s. and osmolality had returned to control levels. By 0.5 h after injection of PEG the mean concentration of n.p.s. had increased from 26.4 mmol to 32.7 mmol Gly equiv/l (+24%) and by 4 h had reached 60.4 mmol Gly equiv/l (+19%). During this time the calculated mean tissue fluid osmolality rose from 696 to 1037 mosmol/kg H2O. Levels of n.p.s. did not increase further for up to 12 h. It is proposed that losses of amino compounds may make a significant contribution to the overall decrease in medullary cellular osmotic potential accompanying reduced tissue fluid osmolality, and that increased levels of these solutes may provide short-term osmoprotection during antidiuresis of rapid onset, in contrast to the more slowly accumulating methylamines and polyhydric alcohols.