Organic osmolytes in the developing kidney of the Australian brush-tailed possum, Trichosurus vulpecula

Developmental changes in the plasma and urine composition of 120 immature possums, Trichosurus vulpecula, were investigated and correlated with changes in the osmolyte composition of the kidneys. In early life, when the animal is confined in the pouch, the urine is more or less isosmotic with the plasma (300 ± 70 mOsm kg− 1), but rises steeply in animals more than 90 days old, up to 667 ± 53 mOsm kg− 1 in the oldest age group. In parallel with this change there are corresponding increases in medullary osmolytes. Measured as mmol kg− 1 wet weight, sorbitol was found to increase from 2.1 ± 0.8 to 9.7 ± 2.2, myo-inositol from 10.9 ± 6.2 to 33.6 ± 11.7, while the methylamines glycerophosphorylcholine and betaine rose from 6.2 ± 0.5 to 15.3 ± 3.1 and 3.9 ± 2.2 to 9.4 ± 2.5 mmol kg− 1 wet weight respectively. Medullary taurine showed no significant changes with age in young possums, while urea increased from 11 ± 4.6 to 49 ± 7.1 mmol kg− 1 wet weight. These values are similar to, but less than, those found in adult kidneys, but probably further refinement of the concentrating ability of the kidney occurs after the young animal becomes independent.