CHARACTERIZATION OF ORGANIC OSMOLYTES IN AVIAN RENAL MEDULLA - A NONUREA OSMOTIC GRADIENT SYSTEM

We measured the organic osmolytes present in the renal cortex and medu llary cones of adult female domestic fowl before and after 48 h of wat er deprivation. Urine osmolality increased from 198 +/- 82 to 569 +/- 42 mosmol/kgH2O after water deprivation. In water-deprived birds, the major organic osmolytes, myoinositol, betaine, and taurine, in the med ullary cones increased by 40, 100, and 24%, respectively, compared wit h control birds. No sorbitol was detected, and glycerophosphorylcholin e (GPC) content was not affected by water deprivation. In the renal co rtex, only betaine content increased significantly (4.8 +/- 0.6 vs. 3. 1 +/- 0.3 mmol/kg wet wt) after water deprivation. In this study, we d emonstrated that birds, like mammals, accumulate organic osmolytes in response to the increased interstitial osmolality that occurs during a ntidiuresis. Because urea is nearly absent in the avian medullary inte rstitium, our observation that GPC is not osmoregulated in the avian k idney supports the idea that GPC is the ''counteracting osmolyte'' for urea in the mammalian kidney. Furthermore, the organic osmolytes pres ent in avian medullary cones are remarkably similar to those of the ma mmalian outer medulla. This similarity may be relevant to the morpholo gical analogy of the two regions.